How to Design a Sturdy Bridge with the Best Method and Good Choice of Materials

How to Design a Sturdy Bridge with the Best Method and Good Choice of Materials
Bridge Works Theorical

Advantages of continuous multiple-span deck over simply supported multiple-span deck

Movement joints are normally added to bridge structures to accommodate movements due to dimensional changes arising from temperature variation, shrinkage, creep and effect of prestress.

However, the provision of excessive movement joints should be avoided in design because movement joints always encounter problems giving rise to trouble in normal operation and this increases

the cost of maintenance. Some designers may prefer to add more movement joints to guardagainst possible occurrence of differential settlements. However, the effect of continuity is disabled by this excessive introduction of movement joints.

From structural point of view, the use of continuous deck enhances the reduction of bridge deck thickness. Moreover, deck continuity allows the potential increase in headroom in the mid-span of bridges by using sucker deck principle. Some designers may prefer to employ the use of simply supported multiple-span deck to guard against possible occurrence of differential settlements. However, the effect of continuity is undermined by the introduction of movement joints. In essence, the structural reserve provided by a continuous bridge is destroyed by the multiple-span statically determinate structure resulting from the addition of joints. Moreover, the reduction of joints in bridge structures represents substantial cost savings arising from the construction and maintenance

costs of movement joints. The reduction of deck thickness helps to cut

the cost for both the deck and foundation. In particular, the number of

bearings in each piers is substantially reduced when compared with the

case of simply supported multiple-span deck.


Benefits of using the bridge form of precast prestressed beams

supporting in-situ concrete top slab The potential benefits of using the bridge form of precast prestressed

beams supporting in-situ concrete top slab are :

(i) For bridges built on top of rivers and carriageway, this bridge form provides the working platform by the precast beams so that erection of falsework is not required.

(ii) This bridge form generally does not require any transverse beams or diaphragms (except at the location of bridge supports), leading to reduction of construction time and cost.

(iii) It creates the potential for simultaneous construction withseveral spans.

Coatings at the back faces of abutments There are different views on the necessity of the application of

protective coatings (may be in the form of two coats of paint) to the back faces of bridge abutment. The main purpose of this coating serves to provide waterproofing effect to the back faces of abutments.

By reducing the seepage of water through the concrete, the amount of dirty materials accumulating on the surface of concrete would be significantly decreased. Engineers tend to consider this as an inexpensive method to provide extra protection to concrete. However, others may consider that such provision is a waste of money and is not worthwhile to spend additional money on this.

Step 1 Coatings at back faces of an abutment.

Dimples in Polytetrafluoroethylene (PTFE) PTFE is a flurocarbon polymer which possesses good chemical

resistance and can function in a wide range of temperature. The most important characteristic of this material is its low coefficient of friction. PTFE has the lowest coefficients of static and dynamic friction of any solid with absence of stick-slip movement . The coefficient of friction is found to decrease with an increase in compressive stress. However, PTFE do have some demerits like high thermal expansion and low compressive strength.

In designing the complementary contact plate with PTFE sliding surface, stainless steel plates are normally selected where the plates should be larger than PTFE surface to allow movement without exposing the PTFE. Moreover, it is recommended that the stainless steel surface be positioned on top of the PTFE surface to avoid contamination by possible accumulation of dirt and rubbish on the larger lower plates. Lubricants are sometimes introduced to reduce the friction between the PTFE surface and the upper stainless steel plate. Dimples are designed on PTFE surfaces to act as reservoirs for lubricant and these reservoirs are uniformly distributed over the surface of PTFE and normally they cover about 20%-30% of the surface area. Hence, the PTFE may be designed with dimples to avoid the lubricant from squeezing out under repeated translation movements.

Discontinuity of joint – position of bearing
Expansion joints in a bridge structures cater for movements in transverse, longitudinal, vertical and rotational forms. The layout and position of expansion joins and bearings have to be carefully designed to minimize the future maintenance problem. The position of bearings affects the discontinuity of a joint. If the  location of a bearing is too far away from a bridge joint, discontinuity of the joint would be experienced when there is an excessive angular  rotation at the joint. Hence, by keeping the bearings and movement joints close in position, the discontinuity in the vertical direction can   be avoided.

Step 2  The effect of position of bearing to the discontinuity of joint.
Diaphragms in bridges

The main function of diaphragms is to provide stiffening effect to deck slab in case bridge webs are not situated directly on top of bearings. Therefore, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be directly transferred to the bearings . On the other hand, diaphragms also help to improve the load-sharing characteristics of bridges. In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.

 Step 3 Diaphragm.

Excessive movement joints in bridges Movement joints are normally added to bridge structures to accommodate movements due to dimensional changes arising from temperature variation, shrinkage, creep and effect of prestress. However, the provision of excessive movement joints should be avoided in design because movement joints always encounter problems giving rise to trouble in normal operation and this increases the cost of maintenance.

Some designers may prefer to add more movement joints to guard against possible occurrence of differential settlements. However, the effect of continuity is disabled by this excessive introduction of movement joints. In essence, the structural reserve provided by a continuous bridge is destroyed by the multiple-span statically determinate structure resulting from the addition of excessive joints.

Earth pressure on abutment The magnitude of earth pressure coefficient in calculating the earth pressure on bridge abutment depends significantly on the degree of restraint provided by the abutment . For example, active earth pressure is usually adopted for cantilever abutment because there is Possible occurrence of small relieving movements. However, for abutment founded on piles, the at-rest earth pressure can be assumed in assessing the earth pressure as the abutment is considered to be rigidly supported by piles and is fully restrained against lateral movement.

Effect of bridge piers across a stream The presence of bridge piers across a stream causes constricted flow in the openings because of the decrease of width of stream owing to the presence of the piers. Moreover, it creates the following problems from hydraulic point of view :

(i) Local scouring at the piers and bed erosion may take place. To avoid the damage to the foundation of piers, some protective layers of stone or concrete apron could be provided around the piers.

(ii) The head loss induced by the bridge piers causes the backwater effect so that the water level upstream is increased. Consequently, this may result in flooding in upstream areas. Functions of sleepers in railway

The functions of sleepers in railway works are as follows :

(i) The primary function of a sleeper is to grip the rail to gauge and to distribute the rail loads to ballast with acceptable induced pressure.

(ii) The side functions of a sleeper include the avoidance of both longitudinal and lateral track movement.

(iii)It also helps to enhance correct line and level of the rails.

Step 4 Sleepers.

Joint continuity influenced by inclined bridge deck Bearings are usually designed to sit in a horizontal plane so as to avoid the effect of additional horizontal force and uneven pressure distribution resulting from non-horizontal placing of bearings . For an inclined bridge deck subject to a large longitudinal movement, a sudden jump is induced at the expansion joint and discontinuity of joint results. To solve this problem, an inclined bearing instead of a truly horizontal bearing is adopted if the piers can take up the induced horizontal forces.

Step 5 The effect of inclined bridge deck on joint discontinuity.
 
Knife edge loads – representation of wheel axles?

In BS5400 the traffic loads for HA loading are given by the uniformly distributed loads along the loaded length and a knife edge load. In the code, it is not intended that knife edge loads simulate a wheel axle of vehicles. Instead, it is just a tool to provide the same uniformly distributed loading to imitate the bending and shearing effects of actual traffic loads.

Limitations of grillage analysis In designing the number of cells for concrete box girder bridges, in case the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, then it is recommended to be designed as a single cell box girder bridge. However, if the bridge depth is smaller than 1/6 of the bridge width, then a twin-cell or multiple cell is a better. However, one should note that even for wider bridges with small depths, the number of cells should be minimized because there is not much improvement in transverse load distribution when the number of cells of box girder is increased to three or more.

For structural analysis of bridges, grillage analysis, which involves the structure to be modeled as a series of longitudinal and transverse elements which are interconnected at nodes, is normally adopted.

Grillage analysis suffers from the following shortcomings :

(i) For coarse mesh, torques may not be identical in orthogonal directions. Similarly, twists may differ in orthogonal directions.

(ii) Moment in any beams is mainly proportional to its curvature only. However, moment in an element depends on the curvatures in the beam’s direction and its orthogonal direction.

Grillage analysis cannot be used to determine the effect of distortion and warping. Moreover, the effect of shear lag can hardly be assessed by using grillage analysis. By using fine mesh of elements, local effects can be determined with a grillage. Alternatively, the local effects can be assessed separately and put in the results of grillage   analysis.

Local Scour at obstructions (e.g. bridge piers) in rivers

When the water flow in river is deflected by obstructions like bridge piers, scouring would occur arising from the formation of vortexes. The mechanism of formation of vortices is as follows: the flow hits the bridge piers and tends to move downwards. When the flow reaches the seabed, it would move in a direction opposite to its original flow direction before hitting the bridge piers. Hence, this movement of flow before the bridge piers results in the formation of a vortex. Owing to the formation of this vertical vortex, seabed material is continuously removed so that holes are formed at the seabed and this result in local scour at bridge piers. As the shape of vortices looks like horseshoes, it is sometimes called “horseshoe vortex”.

Multiple-cell box girder: cells connected by top flanges vs cells connected both by top and bottom flanges When the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, it is recommended to be designed as a single cell box girder bridge. However, if the bridge depth is smaller than 1/6 of the bridge width, then a twin-cell or multiple cell is a better choice . However, even for wider bridges with small depths, the number of cells should be minimized because there is not much improvement in transverse load distribution when the number of cells of box girder is increased to three or more.

For multiple-cell box girders, there are generally two arrangements. The first one is that independent cells are connected by their top flanges only while the other one is that the cells are connected both at the top and bottom flanges. From the structural point of view, it is recommended to adopt the second arrangement. For the case of cells connected by top flanges only, their flanges are heavily stressed in the transverse direction owing to flexure which cannot be effectively distributed across the cross section.

 Step 6 Box girder with cells connected by top flanges and cells connected both by top and bottom flanges.

One-way prestressing vs two-way prestressing During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.

In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.

 Overlays on concrete bridge deck

After years of servicing, some overlays may be applied on the top surface of bridges. Overlays on concrete bridge decks achieve the following purposes :

(i) It aims to provide a smooth riding surface. Hence, it may be applied during the maintenance operation to hide the uneven and spalling deck surface and offers a smoother surface for road users.

(ii) The use of overlays can extend the life of the bridge deck.

Preset in bridge bearing “Preset” is a method to reduce the size of upper plates of sliding bearings in order to save cost. The normal length of an upper bearing plate should be composed of the following components: length of bearing + 2 x irreversible movement + 2 x reversible movement.

Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement. However, as irreversible movement normally takes place at one direction only, the bearing is displaced/presetted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + 2 x reversible movement. In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate.

Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.

Step 7 Preset in sliding bearing.

Parasitic forces for prestressing
In statically determinate structures, prestressing forces would cause the concrete structures to bend upwards. Hence, precambering is normally carried out to counteract such effect and make it more pleasant in appearance. However, for statically indeterminate structures the deformation of concrete members are restrained by the supports and consequently parasitic forces are developed by the prestressing force in addition to the bending moment generated by eccentricity of prestressing tendons [53]. The developed forces at the support modify the reactions of concrete members subjected to external loads and produces secondary moments (or parasitic moments) in the structure.

Purpose of dowel bars in elastomeric bearing Elastomeric bearing is normally classified into two types : fixed and free. For fixed types, the bridge deck is permitted only to rotate and   the horizontal movements of the deck are restrained. On the other hand, for free types the deck can move horizontally and rotate. To achieve fixity, dowels are adopted to pass from bridge deck to abutment. Alternatively, in case there is limitation in space, holes are formed in the elastomeric bearings where anchor dowels are inserted through these holes. It is intended to prevent the “walking” of the bearing during its operation.

Reason of loading on alternative spans to obtain maximum positive moment in a span of a continuous beam To acquire a maximum sagging moment in a span of a continuous beam, the general rule is to load the span under consideration and alternative spans on each side of the span. To account for this rule, let’s consider the following example. For instance, loads are applied to the mid-span of a multiple-span continuous beam. It is noticed that this loads induce positive moments near mid-span in all even spans.

Therefore, if all even spans are loaded simultaneously, this will result in the increase of positive moments in all other loaded spans. Similarly, to obtain maximum negative moment at a support, load adjacent spans of the support and then alternative spans on each side.

Shear lag in typical box-girder bridge

For multiple-cell box girders, there are generally two arrangements. The first one is that independent cells are connected by their top flanges only while the other one is that the cells are connected both at the top and bottom flanges. From the structural point of view, it is recommended to adopt the second arrangement. For the case of cells connected by top flanges only, their flanges are heavily stressed in the transverse direction owing to flexure which cannot be effectively distributed across the cross section. In the structural analysis of bridges, shear lag have to be considered in design in some circumstances. Shear lag takes place when some parts of the cross section are not directly connected. For a box-girder bridge,  not all parts of flanges are joined directly to webs so that the connected part becomes highly stressed while the unconnected flanges are not fully stressed. In particular, for wide flanges of box-girder bridges axial loads are transferred by shear from webs to flanges which result in the distortion in their planes. Consequently, the plane sections do not stay plane and the stress distribution in the flanges are not uniform. Moreover, there is a tendency for longitudinal in-plane displacements of bride deck away from the flange/web connection to lag behind those parts of the bridge in close vicinity to the flange/web connection. The effect of shear lag causes the longitudinal stress at flange/web connection to be higher than the mean stress across the flange. Therefore, the effect of shear lag has to be catered for in the design of box-girder bridges, especially for those with wide flanges.

Shear stiffness in elastomeric bearing

For elastomeric bearing, the shear stiffness is an important parameter for design because it influences the force transfer between the bridge and its piers. In essence, elastomers are flexible under shear deformation but it is relatively stiff in compression. However, elastomeric bearings should not be used in tension. Elastomeric bearing should be designed in serviceability limit state only. The cross sectional area is normally determined by the compressive stress limit under serviceability limit state. The shape factor, i.e. plan area of the laminar layer divided by area of perimeter free to bulge, affects the relation between shear stress and the compressive load. In essence, higher capacity of bearings could be obtained with higher shape factor.

The long side of the bearing is usually oriented parallel to the principle axis of rotation because it facilitates rotational movement. The thickness of bearings is limited and controlled by shear strain requirements. In essence, the shear strain should be less than a certain limit to avoid the occurrence of rolling over at the edges and delamination due to fatigue. Hence, it follows that higher rotations and translations require thicker bearing. On the other hand, the vertical stiffness of bearings is obtained by inserting sufficient number of steel   plates. In addition, checks should be made on combined compression and rotation to guard against the possible occurrence of uplifting of corners of bearings under certain load combinations.

Shock transmission unit in bridges

Shock transmission unit is basically a device connecting separate structural units. It is characterized by its ability to transmit short-term impact forces between connecting structures while permitting long-term movements between the structures. If two separate structures are linked together to resist dynamic loads, it is very difficult to connect them structurally with due allowance for long-term movements due to temperature variation and shrinkage effect . Instead, large forces would be generated between the structures. However, with the use of shock transmission unit, it can cater for short-term transient loads while allowing long-term movements with negligible resistance. It benefits the bridge structures by acting as a temporary link between the structures to share and transfer the transient loads.

Step 8 Shock transmission unit.

Spalling reinforcement for prestressing works in anchor blocks

Reinforcement of anchor blocks in prestressing works generally consists of bursting reinforcement, equilibrium reinforcement and spalling reinforcement. Bursting reinforcement is used where tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. Reinforcement is needed to resist these lateral tensile forces. For equilibrium reinforcement, it is required where there are several anchorages in which prestressing loads are applied sequentially.

During prestressing, spalling stresses are generated in the region behind the loaded faces of anchor blocks . At the zone between two anchorages, there is a volume of concrete surrounded by compressive stress trajectories. Forces are induced in the opposite direction to the applied forces and it forces the concrete out of the anchor block. On the other hand, the spalling stresses are set up owing to the strain compatibility relating to the effect of Poisson’s ratio. Stress corrosion of prestressing steel Stress corrosion is the crystalline cracking of metals under tensile stresses in the presence of corrosive agents. The conditions for stress corrosion to occur are that the steel is subjected to tensile stresses arising from external loading or internally induced stress (e.g.prestressing). Moreover, the presence of corrosive agents is essential to trigger stress corrosion. One of the main features of stress corrosion is that the material fractures without any damage observed from the outside. Hence, stress corrosion occurs without any obvious warning signs.

Transition slabs in bridges

In some designs, transition slabs are provided on the approach to bridges. For instance, soils in embankment supporting the roads may settle due to insufficient compaction and sharp depressions would be developed at the junction with the relatively rigid end of bridge decks [53]. This creates the problem of poor riding surfaces of carriageway and proper maintenance has to be carried out to rectify the situation. As a result, transition slabs are sometimes designed at these junctions to distribute the relative settlements between the approaching embankments and end of bridge decks so that the quality of riding surface between these junctions could be significantly improved and substantial savings could be obtained by requiring less maintenance.

Truss with K-bracing In the arrangement of triangulated framework in truss structures, it is more economical to design longer members as ties while shorter ones as struts (e.g. Pratt truss). As such, the tension forces are taken up by longer steel members whose load carrying capacities are unrelated to their lengths. However, the compression forces are reacted by shorter members which possess higher buckling capabilities than longer steel members For heavy loads on a truss structure, the depth of the truss is intentionally made larger so as to increase the bending resistance and to reduce deflection. With the increase in length of the vertical struts, buckling may occur under vertical loads. Therefore, K-truss is designed in such as way that the vertical struts are supported by compression diagonals.

Vierendeel girder

The Vierendeel girder design is sometimes adopted in the design of footbridges. In traditional truss design, triangular shape of truss is normally used because the shape cannot be changed without altering the length of its members. By applying loads only to the joints of trusses, the members of truss are only subjected to a uniform tensile or compressive stress across their cross sections because their lines of action pass through a common hinged joint.

The Vierendeel truss/girder is characterized by having only vertical members between the top and bottom chords and is a statically indeterminate structure. Hence, bending, shear and axial capacity of these members contribute to the resistance to external loads. The use of this girder enables the footbridge to span larger distances and present an attractive outlook. However, it suffers from the drawback that the distribution of stresses is more complicated than normal truss structures.

Step 9 Vierendeel Truss.

Waterproofing for bridge decks Waterproofing materials like membranes are applied on top of bridge deck surface because :

(i) Vehicular traffic (e.g. tanker) may carry dangerous chemicals and the leakage of such chemicals in the absence of waterproofing materials may endanger the life of bridges. The chemicals easily penetrate and cause the deterioration of concrete bridge decks.

(ii) In some countries where very cold weather is frequently encountered, salt may be applied for defrosting purpose. In case waterproofing is not provided, the salt solution penetrates through the concrete cracks of the bridge and causes the corrosion of reinforcement.

(iii)In the event of cracks appearing on concrete deck, water penetrates the bridge deck and brings about steel corrosion.

Warren Truss, Howe Truss and Pratt Truss

A truss is a simple structure whose members are subject to axial compression and tension only and but not bending moment. The most common truss types are Warren truss, Pratt truss and Howe truss.

Warren truss contains a series of isosceles triangles or equilateral triangles. To increase the span length of the truss bridge, verticals are added for Warren Truss.

Pratt truss is characterized by having its diagonal members (except the end diagonals) slanted down towards the middle of the bridge span. Under such structural arrangement, when subject to external loads tension is induced in diagonal members while the vertical members tackle compressive forces. Hence, thinner and lighter steel or iron can be used as materials for diagonal members so that a more efficient structure can be enhanced.

The design of Howe truss is the opposite to that of Pratt truss in which the diagonal members are slanted in the direction opposite to that of Pratt truss (i.e. slanting away from the middle of bridge span) and as such compressive forces are generated in diagonal members. Hence, it is not economical to use steel members to handle compressive force.

Step  10 A typical Howe Truss.

Step 11 Warren Truss and Pratt Truss.
 How  Concrete Works

Bond breaker for joint sealant Joint sealant should be designed and constructed to allow free extension and compression during the opening and closure of joints. In case joint sealants are attached to the joint filler so that movement is prohibited, they can hardly perform their intended functions to seal the joints against water and debris entry. Polyethylene tape is commonly used as bond breaker tape.

To facilitate free movement, it can be achieved by adding bond breaker tape in-between the joint sealant and joint filler. Primers may be applied to the sides of joints to provide a good bond between them. Fig. 2.1 Bond breaker tape for concrete joints.

Bonding performance to concrete: Epoxy-coated bars vs galvanized bars

Based on the findings of CEB Bulletin 211 [11], the bonding of galvanized bars to concrete is lower in early age owing to hydrogen release when zinc reacts with calcium hydroxide in concrete and the presence of hydrogen tend to reduce the bond strength between galvanized bars and concrete. However, bonding will increase with time until the full bond strength of ungalvanized bars is attained.

For epoxy-coated bars, there is a 20% decrease in bond strength for   bars placed at the bottom of concrete sections while for bars placed on the top there is no major difference in bond compared with uncoated bars. Coating on concrete – complete impermeability to moisture? In designing protective coating on concrete structures, stoppage of water ingress through the coating is normally required. Since chloride ions often diffuse into concrete in solution and cause deterioration of concrete structures, the prevention of water transmission into the coating certainly helps to protect the concrete structure. However, if water gets behind the coating from some means and becomes trapped, its effect may not be desirable. Firstly, vapour pressure would be developed behind the surface treatment and this leads to the loss of adhesion and the eventual peeling off of the coating. Moreover, the water creates a suitable environment for mould growth on concrete

surface.

In fact, the surface treatment should be so selected that it is impermeable to liquid water but it is permeable to water vapour. This “breathing” function enhances the concrete to lose moisture through evaporation and reject the uptake of water during wet periods. Crack width limitation (<0.5mm) = control reinforcement corrosion?

In many standards and code of practice of many countries, the allowable size of crack width is normally limited to less than 0.5mm for reinforced concrete structure to enhance the durability of concrete.

The limitation of crack width can serve the aesthetic reason on one hand and to achieve durability requirement by avoiding possible corrosion of steel reinforcement on the other hand. Regarding the latter objective, site surveys and experimental evidence do not seem to be in favor of the proposition. Beeby [6] showed that there was no correlation between surface crack width (<0.5mm) and durability of reinforced concrete structure. In practice, most corrosion problems are triggered by the presence of surface cracks parallel to the  reinforcement instead of surface cracks perpendicular to the reinforcement.

Critical steel ratio – only consider 250mm of concrete from outer face The purpose of critical steel ratio is to control the cracking pattern by having concrete failing in tension first. If steel reinforcement yields first before the limit of concrete tensile strength is reached, then wide and few cracks would be formed. In the calculation of critical steel ratio, the thickness of the whole concrete section is adopted for analysis. However, if the concrete section exceeds 500mm in thickness, only the outer 250mm concrete has to be considered in calculating minimum reinforcement to control thermal and shrinkage cracks .

It is because experimental works showed that for concrete section greater than 500mm, the outer 250mm on each face could be regarded as surface zone while the remaining could be regarded as core. The minimum reinforcement to control cracking should therefore be calculated based on a total maximum thickness of 500mm.

Corrosion protection of lifting anchors in precast concrete units The corrosion of lifting anchors in precast concrete units has to be prevented because the corroded lifting units cause an increase in steel volume leading to the spalling of nearby surface concrete. Consequently, steel reinforcement of the precast concrete units may be exposed and this in turns results in the corrosion of steel reinforcement and the reduction in the load carrying capacity of the precast units. To combat the potential corrosion problem, the lifting anchors could be covered with a layer of mortar to hide them from the possible external corrosion agents. Alternatively, galvanized or stainless steel lifting anchors can be considered in aggressive environment.  Concrete cover to enhance fire resistance In the event of exposing the concrete structures to a fire, a temperature gradient is established across the cross section of concrete structures. For shallow covers, the steel reinforcement inside the structures rises in temperature. Generally speaking, steel loses about half of its strength when temperature rises to about 550oC. Gradually, the steel loses strength and this leads to considerable deflections and even structural failure in the worst scenario. Hence, adequate cover should be provided for reinforced concrete structure as a means to delay the rise in temperature in steel reinforcement.

Differences between epoxy grout, cement grout and cement mortar

Epoxy grout consists of epoxy resin, epoxy hardener and sand/aggregates. In fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc. Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all. On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete structures and can be adopted as primer or bonding agent.

Cement grout is formed by mixing cement powder with water in which the ratio of cement of water is more or less similar to that of concrete . Owing to the relatively high water content, the mixing of cement with water produces a fluid suspension which can be poured under base plates or into holes. Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures.

Cement mortar is normally a mixture of cement, water and sand (typical proportion by weight is 1:0.4:3). It is intended that cement mortar is constructed by placing and packing rather than by pouring.

They are used as bedding for concrete kerbs in roadwork. They are

sometimes placed under base plates where a substantial proportion of load is designed to be transferred by the bedding to other members. Disadvantages of excessive concrete covers In reinforced concrete structures cover is normally provided to protect steel reinforcement from corrosion and to provide fire resistance. However, the use of cover more than required is undesirable in the following ways.

(i) The size of crack is controlled by the distance of longitudinal bars to the point of section under consideration. The closer a bar is to this point, the smaller is the crack width. Therefore, closely spaced bars with smaller cover will give narrower cracks than widely spaced bars with larger cover. Consequently, with an increase in concrete cover the crack width will increase.

(ii) The weight of the concrete structure is increased by an increase in concrete cover. This effect is a critical factor in the design of floating ships and platforms where self-weight is an important design criterion.

(iii)For the same depth of concrete section, the increase of concrete cover results in the reduction of the lever arm of internal resisting force.

Effect of concrete placing temperature on early thermal movement The rate of hydration of cement paste is related to the placing temperature of concrete. The rate of heat production is given by the empirical Rastrup function :

( 1) 2 r T T o H H − = ×

Ho = Rate of heat production at a reference temperature

T = Temperature where rate of heat production H

T1 = Temperature where rate of heat production Ho

r = 0.084

 An 12oC increase in placing temperature doubles the rate of reaction of hydration. Hence, concrete placed at a higher temperature experiences a higher rise in temperature. For instance, concrete placed at 32oC produces heat of hydration twice as fast when compared with concrete placing at 20oC. Hence, high concrete placing temperature has significant effect to the problem of early thermal movement.

Effect of rusting on steel reinforcement The corrosion of steel reinforcement inside a concrete structure is undesirable in the following ways:

(i) The presence of rust impairs the bond strength of deformed reinforcement because corrosion occurs at the raised ribs and fills the gap between ribs, thus evening out the original deformed shape.

In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. The reduction of mechanical locks by corrosion results in the decline in bond strength with concrete.

(ii) The presence of corrosion reduces the effective cross sectional area of the steel reinforcement. Hence, the available tensile capacity of steel reinforcement is reduced by a considerable reduction in the cross sectional area.

(iii)The corrosion products occupy about 3 times the original volume of steel from which it is formed. Such drastic increase in volume generates significant bursting forces in the vicinity of steel reinforcement. Consequently, cracks are formed along the steel reinforcement when the tensile strength of concrete is exceeded.

Formation of pedestrian level winds around buildings When a building blocks the wind blowing across it, part of the wind will escape over the top of the building. Some will pass around the edges of the building while a majority of the wind will get down to the ground. The channeling effect of wind for an escaping path, together with the high wind speeds associated with higher elevations, generates high wind speeds in the region at the base of the building. At the base   level of the building, there are three locations of strong pedestrian level winds :

(i) Arcade passages – wind flow is generated by the pressure difference between the front and the back of the building.

(ii) At the front of the building – high wind is produced by standing vortex.

(iii)At the corners of the building – high wind is induced by corner flow.

GGBS – cement replacement??

From structural point of view, GGBS replacement enhances lower heat of hydration, higher durability and higher resistance to sulphate and chloride attack when compared with normal ordinary concrete. On the other hand, it also contributes to environmental protection because it minimizes the use of cement during the production of concrete. However, it is identified that there are still some hindrances that prevent the prevalence of its usage in local market. Technically speaking, GGBS concrete suffers from lower rate of strength development which is highly sensitive to curing conditions. In this connection, certain site measures have to be introduced to the construction industry to ensure better quality of curing process in order to secure high quality of GGBS concrete. On the other hand, designers have to be cautious of the potential bleeding problem of GGBS concrete.

Another major hurdle of extensive use of GGBS concrete lies in the little source of supply of GGBS. As Hong Kong is not a major producer of steel, GGBS as a by-product of steel has to be imported overseas and this introduces higher material cost due to transportation and the supply of GGBS is unstable and unsteady.

Indirect tensile strength in water-retaining structures The crack width formation is dependent on the early tensile strength of concrete. The principle of critical steel ratio also applies in this situation. The amount of reinforcement required to control early thermal and shrinkage movement is determined by the capability of reinforcement to induce cracks on concrete structures. If an upper limit is set on the early tensile strength of immature concrete, then a range of tiny cracks would be formed by failing in concrete tension .

However, if the strength of reinforcement is lower than immature concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which are undesirable for water-retaining tructures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have a tensile strength (normally measured by Brazilian test) at 7 days not exceeding a certain value (e.g. 2.8N/mm2 for potable water).

Joint filler in concrete expansion joints – a must??

The presence of joint filler is essential to the proper functioning of concrete joints though some may doubt its value. For a concrete expansion joint without any joint filler, there is a high risk of rubbish and dirt intrusion into the joint in the event that the first line of defense i.e. joint sealant fails to reject the entry of these materials. In fact, the occurrence of this is not uncommon because joint sealant from time to time is found to be torn off because of poor workmanship or other reasons. The presence of rubbish or dirt inside the joint is undesirable to the concrete structures as this introduces additional restraint not

catered for during design and this might result in inducing excessive stresses to the concrete structure which may fail the structures in the worst scenario. Therefore, joint filler serves the purpose of space

occupation so that there is no void space left for their accommodation. To perform its function during the design life, the joint filler should be non-biodegradable and stable during the design life of the structure to enhance its functioning. Moreover, it should be made of materials of high compressibility to avoid the hindrance to the expansion of concrete.


Lifting hoops in precast concrete – mild steel vs high yield steel The strength of high yield steel is undoubtedly higher than mild steel and hence high yield steel is commonly used as main steel reinforcement in concrete structures. However, mild yield steel is commonly used in links or stirrups because they can be subjected to bending of a lower radius of curvature. For lifting hoops in precast concrete, it is essential that the hoops can be bent easily and hence mild steel is commonly adopted for lifting hoops because high yield bars may undergo tension cracking when it is bent through a small radius.

Lap length > anchorage length

In some structural codes, the lap length of reinforcement is simplified to be a certain percentage (e.g. 25%) higher than the anchorage length. This requirement is to cater for stress concentrations at the end of lap bars. A smaller load when compared with the load to pull out an anchored bar in concrete triggers the splitting of concrete along the bar because of the effect of stress concentration. A higher value of lap length is adopted in design code to provide for this phenomenon. Longitudinal steel – an enhancement of shear strength In addition to shear resistance provided by shear reinforcement, shear forces in a concrete section is also resisted by concrete compression force (compressive forces enhances higher shear strength), dowel actions and aggregate interlocking. The presence of longitudinal steel contributes to the enhancement of shear strength of concrete section in the following ways :

(i) The dowelling action performed by longitudinal reinforcement directly contributes significantly to the shear capacity.

(ii) The provision of longitudinal reinforcement also indirectly controls the crack widths of concrete section which consequently affects the degree of interlock between aggregates.

Longer tension lap lengths at the corners and at the top of concrete structures In BS8110 for reinforced concrete design, it states that longer tension lap lengths have to be provided at the top of concrete members. The reason behind this is that the amount of compaction of the top of concrete members during concrete placing is more likely to be less than the remaining concrete sections [49]. Moreover, owing to the possible effect of segregation and bleeding, the upper layer of concrete section tends to be of lower strength when compared with other locations.

When the lap lengths are located at the corners of concrete members, the degree of confinement to the bars is considered to be less than that in other locations of concrete members. As such, by taking into account the smaller confinement which lead to lower bond strength, a factor of 1.4 (i.e. 40% longer) is applied to the calculated lap length. Location of lifting anchors in precast concrete units It is desirable that the position of anchors be located symmetrical to the centre of gravity of the precast concrete units. Otherwise, some anchors would be subject to higher tensile forces when compared the other anchors depending on their distance from the centre of gravity of the precast concrete units. As such, special checks have to be made to verify if the anchor bolts are capable of resisting the increased tensile forces.

A typical concrete anchor.

 Location of construction joints

Construction joints are normally required in construction works because there is limited supply of fresh concrete in concrete batching plants in a single day and the size of concrete pour may be too large to be concreted in one go.

The number of construction joints in concrete structures should be minimized. If construction joints are necessary to facilitate construction, it is normally aligned perpendicular to the direction of the member. For beams and slabs, construction joints are preferably located at about one-third of the span length. The choice of this location is based on the consideration of low bending moment anticipated with relatively low shear force. However, location of one-third span is not applicable to simply supported beams and slabs because this location is expected to have considerable shear forces and bending moment when subjected to design loads. Sometimes, engineers may tend to select the end supports as locations for construction joints just to simplify construction.

Measurement of cement and aggregates – by weight vs by volume Measurement of constituents for concrete is normally carried out by weight because of the following reasons :

(i) Air is trapped inside cement while water may be present in aggregates. As such measurement by volume requires the consideration of the bulking effect by air and water.

(ii) The accuracy of measurement of cement and aggregates by weight is higher when compared with measurement by volume when the weighing machine is properly calibrated and maintained. This reduces the potential of deviation in material quantity with higher accuracy in measurement for the design mix and leads to more economical design without the wastage of excess materials.

Movement accommodation factor for joint sealant Movement accommodation factor is commonly specified by manufacturers of joint sealants for designers to design the dimension of joints. It is defined as the total movement that a joint sealant can tolerate and is usually expressed as a percentage of the minimum design joint width . Failure to comply with this requirement results in overstressing the joint sealants.

For instance, if the expected movement to be accommodated by a certain movement joint is 4mm, the minimum design joint width can be calculated as 4÷30% = 13.3mm when the movement accommodation factor is 30%. If the calculated joint width is too large, designers can either select another brand of joint sealants with higher movement accommodation factor or to redesign the arrangement and locations of joints.

Minimum distance between bars and maximum distance between bars In some codes, a minimum distance between bars is specified to allow for sufficient space to accommodate internal vibrators during compaction.

On the other hand, the restriction of maximum bar spacing is mainly for controlling crack width . For a given area of tension steel areas, the distribution of steel reinforcement affects the pattern of crack formation. It is preferable to have smaller bars at closer spacing rather than larger bars at larger spacing to be effective in controlling cracks. Hence, the limitation of bar spacing beyond a certain value (i.e. maximum distance between bars) aims at better control of crack widths.

Minimum area of reinforcement vs maximum area of reinforcement Beams may be designed to be larger than required for strength consideration owing to aesthetics or other reasons. As such, the corresponding steel ratio is very low and the moment capacity of pure concrete section based on the modulus of rupture is higher than its ultimate moment of resistance. As a result, reinforcement yields first and extremely wide cracks will be formed. A minimum area of reinforcement is specified to avoid the formation of wide cracks.

On the other hand, a maximum area of reinforcement is specified to enable the placing and compaction of fresh concrete to take place easily.

Mild steel vs high yield steel in water-retaining structures In designing water-retaining structures, movement joints can be installed in parallel with steel reinforcement. To control the movement of concrete due to seasonal variation of temperature, hydration temperature drop and shrinkage etc. two principal methods in design are used: to design closely spaced steel reinforcement to shorten the spacing of cracks, thereby reducing the crack width of cracks; or to introduce movement joints to allow a portion of movement to occur in the joints.

For the choice of steel reinforcement in water-retaining structures, mild steel and high yield steel can both be adopted as reinforcement. With the limitation of crack width, the stresses in reinforcement in service condition are normally below that of normal reinforced concrete structures and hence the use of mild steel reinforcement in water-retaining structure will suffice. Moreover, the use of mild steel restricts the development of maximum steel stresses so as to reduce tensile strains and cracks in concrete.

However, the critical steel ratio of high yield steel is much smaller than that of mild steel because the critical steel ratio is inversely proportional to the yield strength of steel. Therefore, the use of high yield steel has the potential advantage of using smaller amount of steel reinforcement. On the other hand, though the cost of high yield steel is slightly higher than that of mild steel, the little cost difference is offset by the better bond performance and higher strength associated with high yield steel.

 Mechanism of plastic settlement in fresh concrete

Within a few hours after the placing of fresh concrete, plastic concrete may experience cracking owing to the occurrence of plastic shrinkage and plastic settlement. The cause of plastic settlement is related to bleeding of fresh concrete. Bleeding refers to the migration of water to the top of concrete and the movement of solid particles to the bottom of fresh concrete. The expulsion of water during bleeding results in the reduction of the volume of fresh concrete. This induces a downward  movement of wet concrete. If such movement is hindered by the presence of obstacles like steel reinforcement, cracks will be formed. No fines concrete In some occasions no fines concrete is used in houses because of its good thermal insulation properties. Basically no fines concrete consists of coarse aggregates and cement without any fine aggregates. It is essential that no fines concrete should be designed with a certain amount of voids to enhance thermal insulation. The size of these voids should be large enough to avoid the movement of moisture in the concrete section by capillary action. It is common for no fines concrete to be used as external walls in houses because rains falling on the surface of external walls can only penetrate a short horizontal distance and then falls to the bottom of the walls. The use of no fines concrete guarantees good thermal insulation of the house. Over vibration of fresh concrete For proper compaction of concrete by immersion vibrators, the vibrating part of the vibrators should be completely inserted into theconcrete.

The action of compaction is enhanced by providing a sufficient head of concrete above the vibrating part of the vibrators. This serves to push down and subject the fresh concrete to confinement within the zone of vibrating action. Over-vibration should normally be avoided during the compaction of concrete. If the concrete mix is designed with low workability,   over-vibration simply consumes extra power of the vibration, resulting in the wastage of energy. For most of concrete mixes, over-vibration creates the problem of segregation in which the denser aggregates settle to the bottom while the lighter cement paste tends to move upwards. If the concrete structure is cast by successive lifts of concrete pour, the upper weaker layer (or laitance) caused by segregation forms the potential plane of weakness leading to possible failure of the concrete structure during operation. If concrete is placed in a single lift for road works, the resistance to abrasion is poor for the laitance surface of the carriageway. This becomes a critical problem to concrete carriageway where its surface is constantly subject to tearing and traction forces exerted by vehicular traffic.

Pulverized fly ash as cement replacement – how it works?

Pulverized fly ash is a type of pozzolans. It is a siliceous or aluminous material which possesses no binding ability by itself. When it is in finely divided form, they can react with calcium hydroxide in the presence of moisture to form compounds with cementing properties. During cement hydration with water, calcium hydroxide is formed which is non-cementitious in nature. However, when pulverized fly ash is added to calcium hydroxide, they react to produce calcium silicate hydrates which is highly cementitious. This results in improved concrete strength. This explains how pulverized fly ash can act as cement replacement.

PFA vs GGBS

(i) Similarities

Both GGBS and PFA are by-products of industry and the use of them is environmentally friendly. Most importantly, with GBS and PFA adopted as partial replacement of cement, the demand for cement will be drastically reduced. As the manufacture of one tonne of cement generates about 1 tonne of carbon dioxide, the environment could be conserved by using less cement through partial replacement of PFA  and GGBS.

On the other hand, the use of GGBS and PFA as partial replacement of cement enhances the long-term durability of concrete in terms of resistance to chloride attack, sulphate attack and alkali-silica reaction.

It follows that the structure would remain to be serviceable for longer period, leading to substantial cost saving. Apart from the consideration of long-term durability, the use of PFA and GGBS results in the reduction of heat of hydration so that the problem of thermal cracking is greatly reduced. The enhanced control of thermal movement also contributes to better and long-term performance of concrete.

In terms of the development of strength, PFA and GGBS shared the common observation of lower initial strength development and higher final concrete strength. Hence, designers have to take into account the potential demerit of lower strength development and may make use of the merit of higher final concrete strength in design.

(ii) Differences Between GGBS and PFA

The use of GGBS as replacement of cement enhances smaller reliance on PFA. In particular, GGBS is considered to be more compatible with renewable energy source objectives. The replacement level of GGBS can be as high as 70% of cement, which is about twice as much of PFA (typically replacement level is 40%). Hence, partial replacement of GGBS can enable higher  reduction of cement content. As the manufacture of one tonne of cement generates about 1 tonne of carbon dioxide and it is considered more environmentally friendly to adopt GGBS owing to its potential higher level of cement replacement.

The performance of bleeding for GGBS and PFA varies. With PFA, bleeding is found to decrease owing to increased volume of fines. However, the amount of bleeding of GGBS is found to increase when compared with OPC concrete in the long term. On the other hand, drying shrinkage is higher for GGBS concrete while it is lower for PFA concrete.

In terms of cost consideration, the current market price of GGBS is similar to that of PFA. As the potential replacement of GGBS is much higher than PFA, substantial cost savings can be made by using GGBS. Purpose of setting minimum amount of longitudinal steel areas for columns In some design codes it specifies that the area of longitudinal steel reinforcement should be not less than a certain percentage of the sectional area of column. Firstly, the limitation of steel ratio for columns helps to guard against potential failure in tension. Tension may be induced in columns during the design life of the concrete structures. For instance, tension is induced in columns in case there is uneven settlement of the building foundation, or upper floors above the column are totally unloaded while the floors below the column are severely loaded. Secondly, owing to the effect of creep and shrinkage, there will be a redistribution of loads between concrete and steel reinforcement. Consequently, the steel reinforcement may yield easily in case a lower limit of steel area is not established. In addition, test results showed that columns with too low a steel ratio would render the equation below inapplicable which is used for the design of columns :

N=0.67fcuAc+fyAs Purpose of reducing the seasonal and hydration temperature by one-half in the calculation of crack widths arising from thermal movement In the calculation of thermal movement, the following formula is used in most codes :

wmax=s x a x (T1+T2)/2 where wmax = maximum crack width s = maximum crack spacing a = coefficient of thermal expansion of mature concrete T1= fall in temperature between peak of hydration and ambient temperature T2= fall in temperature due to seasonal variation For T1, it represents the situation when the freshly placed concrete is under hydration process. Since the occurrence of high creep strain to the immature concrete tends to offset the effect of early thermal movement, a factor of 0.5 is purposely introduced to take into account such effect.

For T2, it refers to the seasonal drop in temperature for the mature concrete. Owing to the maturity of concrete in this stage, the effect of creep on concrete is reduced accordingly. Since the ratio of tensile strength of concrete (fct) to average bond strength between concrete and steel (fb) increases with maximum crack spacing, the lower values of fct/ fb in mature concrete leads to smaller crack spacing. Therefore, the increased number of cracks helps to reduce the effect of thermal movement brought about by seasonal variation. Hence, T2 is reduced by one-half to cater for further creep and bond effects in mature concrete.

Relation of pouring rate and temperature with concrete pressure on formwork Freshly placed concrete exerts pressure on formwork during the placing operation. It is influenced by the rate of placing and the air temperature. For instance, if the concrete pouring rate is too slow, setting of concrete starts to take place. As a result, the concrete at the bottom of the formwork sets prior to the placing of fresh concrete at the top and the maximum pressure will be reduced. Temperature affects the rate of hydration of concrete. The higher the air temperature is, the higher will be the rate of hydration reaction. Consequently, fresh concrete tends to set at a faster rate. The pressure exerted on formwork decreases with an increase in temperature. For this reason, formwork is subjected to a higher pressure exerted by fresh concrete in winter than in summer.

Diagram of design concrete pressure diagram on formwork.

Reinforcement of concrete in contact with internal vibrators During concreting, if internal vibrators are placed accidentally in contact with some of the reinforcement bars, some undesirable effects may result. The most obvious one is that the reinforcement bars may become damaged or displaced if loosely tied. Air bubbles tend to move towards the source of vibration. For poker vibrators touching the reinforcement bars, air pockets may be trapped in the vicinity of the reinforcement because the vibration generated by internal vibrators attracts these air bubbles. Consequently, the bond between the reinforcement and surrounding concrete would be impaired.

To produce good surface finish close to densely-packed reinforcement cage, workers may insert the poker vibrators in the gap between the reinforcement cage and formwork because the reinforcement cage tend to damp down the vibration effect when the vibrators are placed at a distance from the formwork. However, concentrated vibration within the cover region causes the migration of finer cement mortar to this region and results in changes in concrete colour. If the concrete cover is small, the chance of getting the poker vibrators jammed within the gap is high and the formwork is likely to be damaged by the vibrators.

Reasons for blockage in pumping concrete Concrete pumping is commonly adopted in highly elevated locations for which access for concrete trucks is difficult. Construction works can be speeded up by using concrete pumping because a larger volume of pours can be achieved within a specified duration when compared with normal concrete placing methods. Blockage may occur during pumping operation for the following two common reasons :

(i) For saturated concrete mixes, the pump pressures may force water out of the concrete resulting in bleeding. The flow resistance is then increased and may contribute to the blockage of pipelines.

(ii) If the cement content (or other components of concrete mixes that increase the frictional forces) is high, a higher frictional resistance to pumping may develop and the concrete may not be pumpable.

Retardation of fresh concrete Retardation of fresh concrete has several advantages as follows:

(i) A rapid hydration process results in loss in concrete strength because the concrete will have a poorer structure with a higher gel/space ratio compared with the concrete with a lower hydration rate.

(ii) During the hydration process, a substantial heat of hydration will be generated. If the hydration process is carried out too swiftly, it will cause a rapid rise in temperature and results in considerable early thermal movement in concrete.

(iii)In hot weather concreting, the loss of workability is substantial. In order to ensure sufficient compaction of fresh concrete, it is necessary to extend the time for fresh concrete to remain plastic.

Standard mixes of concrete In some countries like Britain, specification for concrete does not normally require cube tests for standard mixes of concrete. The quality control of standard mixes in Britain is achieved by checking if the appropriate mix proportions are adopted during the mixing of concrete. However, in Hong Kong the requirement of testing for compressive strength is still required for standard mixes in the specification because it is impractical to inspect and check all constituent materials (e.g. cement, aggregates etc.) for concrete for compliance. As there is high variability in mixing materials owing to variance in the origin of production of constituent materials in Hong Kong, there is a risk that the end-product concrete does not comply with the design requirements even though the mix proportions of standard mixes are followed closely by engineers.

 Shear slump vs collapse slump in slump test

There are three types of slump that may occur in a slumps test, namely, true slump, shear slump and collapse slump. True slump refers to general drop of the concrete mass evenly all around without disintegration. Shear slump indicates that the concrete lacks cohesion. It may undergo segregation and bleeding and thus is undesirable for the durability of concrete. Collapse slump indicates that concrete mix is too wet and the mix is regarded as harsh and lean.

Type of concrete slump

Sealing moving cracks and non-moving cracks In devising a suitable method to seal up cracks detected on concrete surface, it is of paramount importance to determine if further movement would be expected for the cracks. If the crack is not expected to move further, it is sufficient to brush cement grout into it. For wider cracks, other materials like latex-cement mixture may be considered for sealing the crack. When further movement is expected for the crack, seals wider than the cracks are recommended to be applied over the crack in order to reduce the strain around it to an acceptable level. Moreover, it is desirable to apply the treatment when the cracks are widest so that the sealing material is not subject to further extension. Care should be taken to prevent bonding of sealing material with the bottom of the crack to ensure that only direct tension forces are experienced in the sealing material.

Time to remove formwork to cater for early thermal movement

Let us take a circular column as an example to illustrate effect of internal restraint to thick sections. When the temperature is rising, temperature in the core is higher than that at outer zone. The inner core will have a higher expansion and exert pressure to the outside. The induced compressive stress will result in the formation of radial cracks near the surface of concrete. When the temperature drops, the concrete at the outside drops to surrounding temperature while the concrete at the central region continues to cool down. The contraction associated with inner concrete induces tensile strains and forms cracks tangential to the circular radius.

It is beneficial for thick sections (say >500mm) to have late removal of formwork to reduce early thermal cracking. This is to allow more time for the centre of concrete section to cool down gradually to reduce the risk of thermal cracking. This is effective in controlling the  temperature differential across the cross section of the concrete structures and reducing the potential of internal cracking due to early thermal movement. Tension anchorage length vs compression anchorage length Tension anchorage length of steel reinforcement in concrete depends on bond strength. When steel reinforcement is anchored to concrete and is subjected to compressive forces, the resistance is provided by the bond strength between concrete and steel and the bearing pressure at the reinforcement end. Tension lap length is generally longer than compression lap length. In some design codes, instead of permitting the use of bearing pressure at reinforcement ends, the allowable ultimate bond stress is increased when calculating compression anchorage length.

Tension reinforcement leads to increasing deflection in concrete structures?
In BS8110 a modification factor is applied to span/depth ratio to take into account the effect of tension reinforcement. In fact, deflection of concrete structure is affected by the stress and the amount of tension reinforcement. To illustrate their relationship, let’s consider the following equation relating to beam curvature: Curvature = 1/r = e/(d-x) where r = radius of curvature e = tensile strain in tension reinforcement d = effective depth x = depth of neutral axis

Provided that the tensile strain in tension reinforcement remains constant, the curvature of concrete structure increases with the depth of neutral axis. It is observed that the depth of neutral axis rises with tension steel ratio. Therefore, the curvature of concrete section is directly proportional to the tension steel ratio. In addition, the larger value of the depth of neutral axis enhances increased area of concrete compression so that the effect of creep on deflection appears to become significant. Use of primers in joint sealant Most joint sealants applied in concrete joints are adhesive and the recommended joint width/depth of joint sealant is from 2:1 to 1:1 as given by BS6213 and Guide to Selection of Constructional Sealants. When joint sealant is applied on top of joint filler in concrete joints, additional primers are sometimes necessary because :

(i) Primers help to seal the surface to prevent chemical reaction with Water  :

(ii) It provides a suitable surface for adhesion of joint sealant. Fig. 2.5 Primer in joints.

Vibration in structures – resilient bearings When railway tunnels are built close to buildings, ground-borne vibration is transmitted to the building by means of compression and shear waves [36]. When structural members (e.g. wall) of a building have natural frequencies similar to the frequency at the source, the response of the structures would be magnified. This effect is even more significant when the building is designed with small number of movement joints. Consequently, the vibration can be felt inside the building and noise associated with such vibration is produced. To avoid this, vibration isolation can be implemented, sometimes by providing resilient bearings at column heads in buildings.

Step 3 How Drainage Works

Application of embankment condition for drainage design In considering the loads on buried pipeline, there are normally two scenarios: narrow trench condition and embankment (wide trench) condition. For narrow trench condition, when the pipe is laid in a relatively narrow trench with backfill properly compacted, the weight of fill is jointly supported by both the pipe and the frictional forces along the trench walls. For embankment condition, the fill directly above settles less than the fill on the side. Consequently, loads are transferred to the pipeline and the loads on pipeline are in excess of that due to the fill on pipeline.

The narrow trench condition is used where excavation commences from the natural ground surface without any fills above the surface. On the contrary, the embankment condition applies where the pipes are laid at the base of fill. For instance, embankment condition is normally adopted where the pipes are laid partly in trench or partly in fill or poor foundations to pipes are encountered so that the trenches have to be excavated wider than the minimum requirement.

Best hydraulic section

The best hydraulic section of an open channel is characterized by provision of maximum discharge with a given cross sectional area. As such, channels with circular shape is the best hydraulic sections while a rectangular channel with channel width being equal to two times the height of channel is the best hydraulic section among all rectangular sections. In fact, the choice of best hydraulic section also possesses other advantages than hydraulic performance. For instance, for a given discharge rate the use of best hydraulic section could guarantee the least cross sectional area of the channels. Substantial savings could be made from the reduction in the amount of excavation and from the use of less channel linings.

Colebrook White formula suitable for shallow gradient of pipes?

Manning's Equation is commonly used for rough turbulent flow while Colebrook-White Equation is adopted for transition between rough and smooth turbulent flow. For Manning's Equation, it is simple to use and has proven to have acceptable degree of accuracy and is the most commonly used flow formula in the world. When using Colebrook-White Equation, it is observed that for very flat gradient (i.e. <1.5%) it tends to  underestimate the flow because as gradient approaches zero, velocity also approaches zero. Hence, care should be taken when using Colebrook-White Equation for flat gradients. Concrete surround for drainage pipes Concrete surround is normally adopted for rigid drainage pipes to resist high traffic loads (e.g. under shallow covers) and to allow for using pipes with lower strength. Moreover, the use of concrete surround can minimize settlement of adjacent structures. In addition, the highest possible accuracy in levels and gradient can be achieved by using concrete surround as considerable settlement is expected in other types of beddings like granular bedding.

The distribution of reinforcement in concrete pipes may not be uniform owing to the occurrence of tensile stresses in different locations around the circumference of the pipes . For instance, tensile stresses are highest at the inner face of pipes at invert and crown levels and at the outer face of the two sides of pipes. An elliptical cage may be designed in order to optimize the usage of steel reinforcement.

Concrete surround of drains.

Difference between road gullies and catchpits Both road gullies and catchpits are the two basic types of drainage inlets of drainage system. Though they are designed to catch stormwater, road gullies and catchpits are intended to catch stormwater at different locations. Catchpits are designed to receive stormwater from slopes and stream courses. There is no standard design of catchpits and they can take different forms and shapes like inclusion of sand trap to improve the quality of collected stormwater and to prevent the blockage of drains. On the other hand, road gullies are intended to receive stormwater from roads only.

Drainage pipes in reclamation areas

In reclamation areas drainage pipes are usually laid at flatter gradients when compared with upstream stormwater pipes. The fact that the nature of flow in stormwater drain is by gravity makes the downstream pipes in reclamation areas relatively deep below ground surface. It is preferable to have outfall of drains above the tidal influence level and this accounts for the relative flatter gradient of drain pipes in reclamation area.

Attention has to be paid to the possible occurrence of differential settlement in reclamation area. For pavement design, flexible pavement is preferred to rigid pavement to cater for settlement problems. Similarly, in the design of drains flexible joints like spigot Civil Engineering Practical Notes A-Z Vincent T.  . CHU and socket joints and movement joints in box culverts have to be provided to guard against the effect of differential settlement. Effects of sewer sediment on hydraulic performance The presence of sediment in sewers has adverse effects on the hydraulic performance of sewers [13]. For the case of sewage flow carrying sediment without deposition, the presence of sediment in the flow causes a small increase in energy loss.

In case the sewer invert already contains a bed of sediment deposit, it reduces the cross sectional area of sewers and consequently for a given discharge the velocity increases. As such, the head losses associated with this velocity increase. Moreover, the increase in bed resistance induced by the rough nature of sediment deposit reduces the pipe flow capacity of sewers. For sewers which are partially full, the presence of sediment bed enhances higher frictional resistance and results in increasing the flow depths and subsequent decrease of velocity. The reduction of velocity will lead to further deposition of sediment owing to the decrease of sediment carrying capacity if the increase of capacity of sewers generated by the presence of sediment bed does not exceed the reduction in flow caused by the bed roughness.

Energy dissipation at outlets

Flow velocity at outlets is usually high. Without proper control of this energy, the subsequent bank erosion may result in failure of the banks. Therefore, some energy dissipating structures are designed to cope with this problem. Impact energy dissipaters may be provided at outlets by making use of impact walls to dissipate energy. Alternatively, the flows at outlet are dispersed artificially to achieve a significant loss of energy. However, the problem of cavitation may occur in this type of energy dissipating structures.

A typical drainage outlet.

Functions of hydraulic jump The use of hydraulic jump in hydraulic engineering is not uncommon and the creation of such jumps has several purposes :

(i) Its main aim is to perform as an energy-dissipating device to reduce the excess energy of water flows.

(ii) The jump generates significant disturbances in the form of eddies and reverse flow rollers to facilitate mixing of chemicals.

(iii)During the jump formation, considerable amount of air is entrained so that it helps in the aeration of streams which is polluted by bio-degradable wastes.

(iv) It enables efficient operation of flow measuring device like flumes. Full bore flow in drainage design In the design of gravity drainage pipes, full bore flow capacity is normally adopted to check against the design runoff. However, one should note that the maximum flow rate does not occur under full bore conditions. The maximum discharge occurs when the water depth in circular pipes reaches 93.8% of the pipe diameter. Therefore, the use of full bore discharge is on the conservative side though the pipe’s maximum capacity is not utilized.

Similarly, the maximum velocity does not occur in full bore conditions and for circular pipes it occurs when the water depth is 81.3% of the pipe diameter. Hence, in checking for the maximum velocity of flow in pipes to avoid possible erosion by rapid flow, the use of full-bore velocity may not be on the conservative side.

Functions of wetwells

Wetwells are designed to store temporarily water/sewage before it is pumped out. They are usually provided for sewage and stormwater pumping stations and they serve the following functions : (i) They assist in attenuating the fluctuations of flow owing to the diurnal variation of sewage discharge. (ii) The wetwells serve as sump pits where the suction pipes are inserted and the fluid level in the sumps can be employed for the control of opening and closure of pumps. Joints in box culverts and channels - necessity of watertightness The joints for box culverts and channels should be capable of accommodating movements arising from temperature and moisture changes. However, the joints are not necessarily designed as watertight except the following conditions :

(i) There is a high possibility of occurrence of high water table in the vicinity of box culverts/channels. The high groundwater level and rainwater seepage through embankment may cause water passing through the joints and washing in soils. Consequently, the loss of soils may lead to the failure of the structures.

(ii) If the box culvert/channels are designed in such a way that water flow through joints from the structures causes the washing out of bedding materials, the requirement of watertightness of joint has to be fulfilled.

(iii)In cold countries, road salt is sometimes applied on roads abovebox culvert or at crossings of channels to prevent freezing and thawing. The leaching of road salts into the joints may cause corrosion of joint reinforcement.

Manhole covers – triangular halves

Manhole covers are generally made up of two pieces of triangular plates to form a square cover . One may wonder why two rectangular halves are used for a rectangular cover. To understand this, one should note that a triangular cover could simply lie on a plane while a rectangular cover contains a point of redundancy. Hence, the potential problem of rocking produced by vehicular traffic by rectangular traffic could be eliminated by using two triangular halves. Other the other hand, the two pieces of triangular covers should be bolted together. As for a piece of triangular cover, it is easily dropped into the rectangular hole of manhole during routine maintenance. Therefore, from maintenance point of view, some countries prefer another geometrical shape i.e. circular, as this is the only shape that the cover could hardly be accidentally dropped into the manhole. On the other hand, for other geometrical shapes such as rectangle or square, they could still be dropped into their formed hole when inclined into proper angles.

 Different types of manhole covers.

Manhole loss

Manholes are provided in locations where there are changes in size, direction and gradient of gravity pipelines. In normal practice for straight pipelines manholes have to be installed at a certain spacing to facilitate the maintenance of pipes. With the introduction of manholes, there are various reasons which account for the manhole loss :

(i) The sudden expansion of inflow into manholes and the sudden contraction of flow out of manholes lead to significant energy losses.

(ii) It is not uncommon that several pipes may be connected to the same manhole. As such, the intermixing of flow takes place inside the manhole and this leads to head losses.

(iii)Flow inside the manholes may be designed to change directions which contribute to additional losses.

Necessity of reinforcement in precast concrete manhole units Precast concrete manholes are normally constructed by placing the bases of manholes firstly. The walls of precast manholes are formed by placing the precast concrete rings one on top of the other up to the required height. Someone may notice that reinforcement used for resisting the lateral earth pressure and surface loads are not considered in some design. It is discussed in Concrete Pipe Association of Great Britain that analysis of soil pressures shows that standard unreinforced precast units are capable of resisting uniformly distributed pressures (e.g. loading condition in a manhole) down to a depth of 150m. If very severe road traffic and side loads are encountered, an additional concrete surround of about 150mm may be provided.

A precast concrete manhole.

On-line storage vs off-line storage The design of storage pond is commonly divided into on-line storage and off-line storage. The on-line storage concept involves inclusion of storage facilities in series with the pipelines so that overflow at the storage facilities is allowed. One simple application of on-line storage is to enhance a large size of drainage pipes. However, for heavy rainfall situation, the spare capacity of drainage pipes will be rapidly exhausted. On the other hand, off-line storage (e.g. underground storage tank) refers to storage facilities in parallel with the pipeline and the return flow to the main pipeline is only allowed when the outflow pipelines are not surcharged. Possible defaults for precast concrete pipes made by spinning and vertical casting Small diameter precast concrete pipes are normally manufactured by spinning method. The spinning method basically makes use of the principle of centrifugal forces which diminishes towards the centre of precast pipe. Hence, problems like the presence of voids and variation of dimension occur frequently and remedial works like filling of voids by cement mortar has to be carried out depending on the severity of deficiency.

Large diameter precast concrete pipes are commonly produced by vertical casting method. In this method the concrete pipes are normally placed upright with spigot staying on top, resting on socket moulds before the freshly-placed concrete has set. There is a possibility of deformation of pipe spigots to form oval shapes. Purpose of granular bedding for concrete pipes In designing the bedding for concrete drainage pipes, granular materials are normally specified instead of soils containing a wide range of different particle sizes. The main reason of adopting granular material free of fine particles is the ease of compaction as it requires very little tamping effort to achieve a substantial amount of compaction and the crushed aggregates readily move to suitable place around the pipes. However, the use of granular materials has the drawback that a stable support can hardly be provided for the drainage pipes. In particular, it cannot maintain an accurate slope and level for the bedding of concrete pipes. Most pipes are gravity pipes and the accuracy in level is essential to maintain the flow capacity.

 How to Bedding of concrete pipes.

Purpose of carrying out water absorption test for precast concrete pipes Cement will mix with more water than is required to eventually combine during hydration of cement paste. As such, some voids will be left behind after the hydration process which affects the strength and durability of concrete. With the presence of air voids in concrete, it is vulnerable to penetration and attack by aggressive chemicals. Good quality concrete is characterized by having minimal voids left by excess water and therefore, water absorption test for precast concrete pipes is adopted for checking the quality of concrete in terms of density and imperviousness. Reason in checking the ratio (i.e. design flow to full-bore flow > 0.5) in circular pipe design For checking of self-cleansing velocity for pipes, there is another criterion to check design flow Q to full bore flow Qfull> 0.5. If this criterion is met, it can be deduced that the design flow is always greater than self-cleansing velocity.

The reason behind is that from the chart of circular pipes, when Q/ Qfull >0.5, then the ratio of design velocity V to full bore velocity Vfull >1. After confirming Vfull >1m/s, then it leads to V>1m/s. Hence, minimum velocity at full bore flow should be checked. Relation of the angle of contact between pipe invert and bedding material to the load resisting capacity of pipe Minimum crushing strength is a commonly adopted parameter for describing the strength of rigid pipes like concrete pipes. This value is determined in laboratory by subjecting the test concrete pipe to a line load diametrically along the pipe length while the pipe invert is supported on two bearers for stability reason. This test is called three-edge bearing test and the load at failure of pipes is expressed in terms of kN per length of test pipes (called minimum crushing strength).

Bedding factor of a pipe is defined as the failure load for the pipe laid in actual ground with bedding to the failure load under three-edge bearing test. The bedding factor is largely related to the angle of contact between pipe invert and the bedding material. The angle of  ontact between pipe invert and the bedding material increases with the ratio of bending moment at invert (for the case of three-edge bearing test) to the angle under consideration .  Rubber dams – air-filled vs water-filled Most of the existing rubber dams are of air-filled types. Water-filled rubber dams are not preferred for the following reasons :

(i) By giving the same sheet length and dam height, the tensile stress for water-filled dams is higher than that of air-filled rubber dams.

(ii) A significant size of water pond is normally provided for water-filled water dams for filling the rubber dams during the rising operation of dams.

Single-cell box culvert vs double-cell box culvert

The use of double-cell box culverts is preferred to single-cell box culverts for cross-sectional area larger than about 5m2 owing to the following reasons :

(i) Where there is tight headroom requirement, the use of double-cell box culvert can shorten the height of culverts by having a wider base so that the same design flow can be accommodated.

(ii) The invert of one cell can be designed at a lower level to cater for low flow condition so that it reduces the occurrence of sediment deposition and avoid the presence of standing waters.

(iii)The provision of temporary flow diversion can be easily provided for inspection and maintenance of each cell. During routine maintenance operation, water flow can be diverted to one cell and the other one is open for desilting.

If a choice has to be made between a single-cell box culvert and smaller multiple pipes, it is better to select single-cell box culvert because of the lower risk of blockage when compared with smaller size of multiple pipes. In addition, the hydraulic performance of a single-cell box culvert is better than multiple pipes system because of the larger hydraulic radius associated with the box culvert for a given cross-sectional area.

Side clearance of pipes in trenches From the design point of view, it is preferred to minimize the width of pipe trenches because of the following reasons:

(i) Higher cost of excavation is associated with wider pipe trenches.

(ii) The width of trench affects the loads on installed pipelines in consideration of embankment condition and wide trench condition. For minimum pipe trench width, the loads on pipelines can be reduced. However, sufficient space has to be provided to allow for proper compaction. This is helpful to reduce the reaction at critical locations of pipelines under traffic and fill loads. Moreover, consideration should be given to accommodate temporary works for deep trenches where shoring has to be provided during construction.

Significance of tailwater level in culverts

The headwater level and tailwater level of culverts are important parameters in hydraulic design. The headwater level cannot be set too large, otherwise flooding upstream may occur leading to the loss of life and properties. On the other hand, the tailwater level of culverts has to comply with the following requirements :

(i) For low tailwater levels at the outlet of culverts, the small depths of flow may cause significant erosion of downstream channels.

(ii) For high tailwater levels, it may cause the culvert upstream to be flowing full or even under submerged condition. As such, the headwater level is increased in order to flow through the culvert and this in turn increases the flooding risk associated with high headwater level.

How to Tailwater level in culvert.

Stilling basins

Stilling basins are usually introduced to convert supercritical flow to subcritcal flow before it reaches downstream. A typical stilling basin consists of a short length of channels located at the source of supercritical flow (e.g. end of spillway). Certain features are introduced to the basins like baffles and sills to provide resistance to the flow. As such, a hydraulic jump will form in the basin without having conducting significant amount of excavation for the stilling basin if baffles are installed.

Uncompacted bedding for concrete pipes In the middle third of the base of precast concrete pipes, the bedding layers are recommended to be left uncompacted because it helps to reduce the reaction force at the invert of the pipes and intensifies the effect of shear forces. Moreover, the bending moment at pipe invert is increased by the compaction of bedding layer. The general rule for this region of bedding layer is that it should be firm enough for the pipes to rest on.

The sides of haunch and bedding directly under the haunch should be compacted because this will reduce the bending moment at the invert which is the critical failure location for pipes. The compacted haunch helps to resist the pipe load and maintain level and alignment.

Step 4 How Geotechnical works

Bentonite slurry vs polymeric slurry
For the construction of diaphragm walls, bentonite slurry is commonly used to form a filter cake on walls of trenches to support earth pressure. The use of bentonite solely is based on its thixotrophic gel viscosity to provide support. Though the cost of polymer is generally more expensive than bentonite, the use of polymer is increasing because polymer is generally infinitely re-usable and very small amount of polymer is normally required for construction works. The disposal cost of bentonite is quite high while the disposal of polymer can be readily conducted by adding agglomerator.

Bleeding test for grout – an essential requirement?

Bleeding is a form of segregation in which a layer of water migrates to the surface of the grout during the initial stage of cement hydration process. Later on, some of the floating water is re-absorbed into the grout due to further hydration reactions. Even without the problem of bleeding, there is a total reduction of volume of grout after hydration action when compared with the total initial individual volume of cement and reacted water. Bleeding tests should be carried out for grout because of the following reasons :

(i) During bleeding, the upflow of water from grout mixture leads to the formation of channel paths inside the grout mix. These channels act as potential paths for aggressive materials to pass through as these channels would not be closed during further hydration of the grout.

(ii) The loss in volume by bleeding generates voids inside the grout mix which affects the properties and performance of the grout. Moreover, it increases the chance of corrosion of steel elements protected by the grout. (e.g. tendons)

(iii)In bleeding test, there is a usual requirement of total re-absorption of water after 24 hours of grout mixing because for some cold countries, this layer of water may cause severe freezing problem leading to frost damage.

Core-barrel samplers: single tube sampler vs double tube sampler vs triple tube sampler Core barrel samplers are originally designed to sample rock. In single tube sampler, the core barrel of the sampler rotates and this poses the possibility of disturbing the sample by shearing the sample along certain weak planes. Moreover, the cored samples are subjected to erosion and disturbance by the drilling fluid. For double tube samplers, the tube samplers do not rotate with the core barrels and the samplers are not protected against the drilling fluid. The logging of samples presents difficulty for highly fractured rock. The triple core barrel basically consists of a double core barrel sampler including an addition of a stationary liner which is intended to protect the cored samples during extraction. Therefore the quality sample obtained from triple core barrel is the best among the three types of barrels mentioned above.

Continuous Piezocone Penetration Test

Continuous piezocone penetration test basically consists of standard cone penetration test and a measurement of pore water pressure. Three main parameters, namely sleeve friction, tip resistance and pore water pressure measurement are measured under this test. Pore water pressure generated in the soils during penetration of the cone is measured. An electrical transducer located inside the piezocone

behind saturated filter is used for the measurement. By analyzing the results of pore pressure with depth, the stratigraphy of fine-gained soils with different layers is obtained readily.

Diaphragm wall – maintenance of excess slurry head For the construction of diaphragm walls adjacent to buildings, previous experience showed that excess slurry head above groundwater level had to be maintained to limit the ground settlements during the construction of diaphragm walls. In fact, the excess slurry head can be achieved by the following methods. The first one is to construct a ring of well points to lower the piezometric level to achieve a higher excess slurry head in diaphragm walls. Alternatively, guide walls may be raised above ground level to accommodate the slurry column.

Direction of gunning in shotcreting

During the construction of shotcrete, it is aimed at gunning the full thickness in one single operation and this helps to reduce the occurrence of possible delamination and formation of planes of weakness. Moreover, the nozzles should be held about 0.6m to 1.8m from the surface  and normal to the receiving surface. The reason of gunning perpendicular to the receiving surface is to avoid the possible rebound and rolling resulting from gunning at an angle deviated from the perpendicular. The rolled shotcrete creates a non-uniform surface which serves to trap overspray and shotcrete resulting from the rebounding action. This is undesirable because of the wastage of materials and the generation of uneven and rough surface. Function of mortar in brick walls A typical brick wall structure normally contains the following components:

1. (i) a coping on top of the brick wall to protect it from weather;
2. (ii) a firm foundation to support the loads on the brick wall; and
3. (iii)a damp course near the base of the brick wall to avoid the occurrence of rising damp from the ground.

Bricks are bedded on mortar which serves the following purposes :

1. (i) bond the bricks as a single unit to help resist lateral loads;
2. (ii) render the brick wall weatherproof and waterproof; and
3. (iii)provide even beds to enhance uniform distribution of loads.

Brick wall

Formation of frost heave In the past, it was believed that the formation of frost heave was related to the volumetric expansion of soil water which changed from liquid state to solid state. However, the increase of volume of changes in states for water at zero degree Celsius is only about 9％ and the observed heaving is far more than this quantum. In fact, the mechanism of frost heave is best explained by the formation of ice lenses . In cold weather, ice lenses develop in the freezing zone in soils where there is an adequate supply of soil water. Soil particles are surrounded by a film of water which separates the soil particles from ice lenses. The moisture adhered to soil particles gets absorbed to the ice lenses on top of the soils and in turn water is obtained from other soil pores to replenish the loss of water to ice lenses. This process continues and results in pushing up of soils on top of the lenses and subsequently the formation of frost heave.

Functions of diaphragm walls
The functions of diaphragm walls are as follows:

1. (i) It is designed to retain soils during the construction of underground structures.
2. (ii) It helps to control the movement of ground during construction.
3. (iii)It is intended to take up high vertical loads from aboveground structures during construction (e.g. top-down approach). In addition, during the servicing of the completed structures, the diaphragm walls, internal piles and basement raft act together as a single unit to perform as piled raft.

Granular fill or rock fill essential at the base of concrete retaining walls?

It is not uncommon that granular fill layers and rockfill layers are placed beneath the bottom of concrete retaining walls. The purpose of such provision is to spread the loading in view of insufficient bearing capacity of foundation material to sustain the loads of retaining walls. Upon placing of granular fill layers and rockfill layers, the same imposed loads are supported by a larger area of founding material and hence the stress exerted by loads is reduced accordingly. Layers of granular fill and rockfill materials are not standard details of concrete retaining wall. If we are fully satisfied that the founding material could support the loads arising from retaining walls, it is not necessary to provide these layers of granular fill and rockfill materials. “Grout curtain” around excavation When excavation work is carried out in grounds with highly permeable soils, other than the installation of well points to lower down the groundwater table, consideration may be given to the injection of grout to the soils [60]. The purpose of the injection of grout is to fill the pore spaces and cavities of soils with grout and to reduce the permeability of soils. The method of grouting is effective in coarse soils but not for  sands. In essence, “grout curtain” is constructed around the excavation by installation of several rows of injection holes for grouting. Kicker of reinforced concrete cantilever retaining walls located at the position of largest moment and shear force – why?

Normally for reinforced concrete cantilever retaining walls, there is a 75mm kicker at the junction wall stem and base slab to facilitate the fixing of formwork for concreting of wall stems. If a higher kicker (i.e. more than 75mm height) is provided instead, during the concreting of base slab the hydraulic pressure built up at kicker of fresh concrete cause great problem in forming a uniform and level base slab. Despite the fact that the position of kicker in a cantilever retaining wall is the place of largest flexure and shear, there is no option left but to provide the kicker at this position.

Loading and unloading cycles for soil nails

In carrying out pull-out tests for soil nails, it normally requires the loading and unloading of soil nails of several cycles up to 80% of ultimate tensile strength of soil nails. The principal function of soil nail tests is to verify the design assumptions on the bond strength between soil and grout which is likely to exceed the design values based on past experience. In addition, the ultimate bond strength between soil and grout can be determined and this information is helpful as a reference for future design. Then someone may query the purpose of conducting load/unloading cycles of soil nails as it does not provide information on the above two main purposes of soil nails. In fact, loading and unloading soil nails can provide other important information on their elastic and plastic deformation behaviour. However, as stress levels in soil nails are normally low, the knowledge on elastic and plastic performance may not be of significant value. On the other hand, the creep and slippage performance of soils nails can also be obtained which may be useful for some soils.

Typical pull-out test result.

Landslides induced by rainfall

After rainfall, groundwater pressure is built up and this elevates the ground water table. The water inside the pores of soil reduces the effective stress of soils. Since shear strength of soils is represented by the following relations : Shear strength = cohesion + effective stress x tanΦ where Φ is the friction angle of soils Hence, the presence of water causes a reduction of shear strength of soils and this may lead to landslide. On the other hand, the rainfall creates immediate instability by causing erosion of slop surface and results in shallow slope failure by infiltration. In addition, the rain may penetrate slope surface openings and forms flow paths. As a result, this may weaken the ground.

Piston samplers

In sampling clays or silts, Piston sampler is lowered into boreholes and  the piston is locked at the bottom of the sampler. This prevents debris from entering the tube prior to sampling. After reaching the sampling depth, the piston is unlocked so that the piston stays on top of the sample going into the tube. Prior to the withdrawal of the sampler, the  piston is locked to prevent the downward movement and the vacuum generated during the movement of the piston from the sampler’s end aids in retaining the samples recovered. As such, sample recovery is increased by using Piston samplers. Position of shear keys under retaining walls The installation of shears keys helps to increase the sliding resistance of retaining walls without the necessity to widen the their base. The effect of shears keys enhances the deepening of the soil failure plane locally at the keys. The increased sliding resistance comes from the difference between the passive and active forces at the sides of the keys. In case weak soils are encountered at the base level of shear keys, the failure planes along the base of retaining walls due to sliding maybe shifted downwards to the base level of the keys. Shear keys are normally designed not to be placed at the front of the retaining wall footing base because of the possible removal of soils by excavation and consequently the lateral resistance of soils can hardly be mobilized for proper functioning of the shear keys. For shear keys located at the back of footings, it poses a potential advantage that higher passive pressures can be mobilized owing to the higher vertical pressure on top of the passive soils.

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