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    Interview Questions and Answers for Civil Engineering - Part 07

    Question No. 251
    For installation of silt curtains, why is it not desirable to design the curtain to touch the seabed?
    Answer: Silt curtains are impermeable vertical barriers extending from the seawater surface to its designed depth. The curtains are held in a vertical position by the carrier float on their top and a curtain weight at their bottom. A tension cable is designed at the carrier float to resist stresses incurred by currents. Moreover, the silt curtains are anchored to the seabed to hold them in the designed configuration.

    In essence, the depth of silt curtains should not be so long and touch the seabed because the bottom segment of the silt curtains would be trapped inside the newly accumulated sediment, thus resulting in sinking of the curtain. Consequently, it is difficult to remove these sunken curtains. Moreover, reversal tidal and current actions may cause the movement of bottom region of curtains which stir up the settled suspensions and induce additional turbidity.

    Question No. 252
    What are the disadvantages of curing by ponding and polythene sheets?
    Answer: The purpose of curing is to reduce the rate of heat loss of freshly placed concrete to the atmosphere and to minimize the temperature gradient across concrete cross section. Moreover, curing serves to reduce of the loss water from freshly placed concrete to the atmosphere.
    1. Ponding: This method of thermal curing is readily affected by weather condition (cold wind). Moreover, a large amount of water used has to be disposed off the construction sites after curing.
    2.  Polythene sheet: This method of curing is based on the principle that there is no flow of air over the concrete surface and thereby no evaporation can take place on top of the freshly concreted surface by provision of polythene sheets. However, it suffers from the demerit that polythene sheets can be easily blown off in windy condition and the performance of curing would be affected. Moreover, for water lost due to self-desiccation, this method cannot replenish these losses.

    Question No. 253
    What are different approaches for reclamation in deep water region and shallow water region?
    Answer: To illustrate the different approaches adopted for reclamation in deep water and shallow water region, the following example is used:

    In deepwater region, consider the seabed level is -8.5 mPD. After laying of geo-textiles and 1.5 m thick sand blanket, the top level of sand blanket is about -7 mPD. Split barges are deployed for dumping public fill to -2.5 mPD. Afterwards, end dipping of public fill by trucks will be carried out up to +2.5 mPD which is the designed reclamation level. Between level -2.5 mPD and +2.5 mPD, it is too shallow for split barges to enter the water, thus the method of end dipping is used instead.

    For shallow water region, the seabed level is taken as -5.5 mPD in this example. With the laying of geo-textiles and 1.5 m sand blanket into position, the top level of sand blanket is about -4 mPD. In this case, split barges are also used for reclamation work between the level -4 mPD and -2.5 mPD. After that, if end dipping is used for reclamation work above -2.5 mPD, then in considering the relative thin layer of fill above seabed (1.5 m sand blanket + 1.5 m sand blanket), it stands a high chance that mud wave would occur in seabed. Therefore, half-loaded derrick barges are employed for reclamation up to level 0 mPD. With a thicker layer of public fill now, end dipping can then be used for reclamation between 0 mPD and +2.5 mPD.

    This above reclamation sequence is just an example to demonstrate the different considerations for reclamation in deep water and shallow water region.

    Question No. 254
    Given a 1 m high staircase resting on solid concrete, would it be adequate to design nominal reinforcement for the staircase?
    Answer: For the design of staircase, there are three main scenarios:
    1. Stairs spans longitudinally:  This kind of stairs refers to stairs spanning between landings only without any side supports. In this case, the staircase should be designed as a beam between two end supports (i.e. landing) and the main reinforcement is provided at the bottom of staircase slabs.
    2. Stairs spanning transversely:  This kind of staircase is supported by sidewalls only and it may also be supported by stringer beams. For the case of sidewalls, it acts as a cantilever beam and the main reinforcement are provided the top surface of slab. For the case of staircase supported sideways by both sidewall and stringer beam, it should be designed transversely with end supports as sidewall and stringer beam and reinforcement is provided at the bottom of the staircase.
    3. Stairs resting on solid support:  For stairs resting on solid supports, only nominal steel reinforcement is provided to control thermal and shrinkage cracking.

    Question No. 255
    What is the importance of air void content in bituminous pavements?
    Answer: The air void content of bituminous materials is an important control parameter for the quality of bitumen being laid and compacted. If the air void content is too high, it allows for intrusion of air and water. Moreover, it also increases the rate of hardening of binders which produce premature embrittlement of pavements. In addition, too high a void content will also lead to differential compaction subject to traffic loads and result in formation of ruts and grooves along the wheel track.

    However, a minimum amount of air void should be maintained to avoid instability during compaction process and to provide space for bitumen flow in long-term consolidation under traffic loads. A sufficient amount of air voids should be designed to make room for expansion of binder in summer and compaction by road traffic as suggested by National Association of Australian State Road Authorities (1968), otherwise bleeding and loss of stability may occur and the pavement will deform readily under severe loads.

    Question No. 256
    What are the pros and cons of using timber fenders, plastic fenders and rubber fenders?
    Timber fenders: They are low in strength and are subject to rotting and marine borer attack. Moreover, they have low energy absorption capacity and the berthing reaction depends on the point of contact. The contact pressure between fender and vessels are high. They are considered to be environmentally unfriendly because they consume tropical hardwoods in their production.

    Plastic fenders: Their strength is similar to that of timber fenders but they have relatively high abrasive resistance. They are resistant to chemical and biological attack. Their energy absorption capacities are moderate and the berthing reactions are also dependent on the point of contact. The reaction is lower when compared with timber fenders for a given energy absorption. They are considered to be environmental friendly because they are manufactured from recycled material.

    Rubber fenders: They possess high abrasive resistance and are also resistant to most biological and chemical attacks. They have moderate to high energy absorption capacity and the energy absorption performance is independent of the point of contact. Similar to plastic fenders, they are also environmental friendly products.

    Question No. 257
    What are the functions of the following features observed in a typical manhole? (i) Groove near benching, (ii) R.S.J. (iii) double seal manhole cover and (iv) U-trap with rodding arm.
    1. The groove is used to facilitate the maintenance of manholes and sewer/drain pipes. Shutoff boards are erected on the grooves during maintenance operation so that water flow coming from upstream is terminated in the manhole and backwater from downstream is also blocked. In addition, the groove also facilitates water flow diversion for routine maintenance operation.
    2. R.S.J. is a small-scale size of universal beams and is used for resisting the high stresses incurred by heavy traffic loads acting directly on the upper narrow projected section of manholes.
    3. Double seal terminal manhole covers are used for sealing off gases emitted inside sewer/drains and prevent them from releasing out of the manhole.
    4. U-trap with rodding arms is also used for sealing off unpleasant gas smell by the trapped u-shaped water columns. Rodding arm is normally closed with rubber rings during normal operation. However, during maintenance operation, the rubber ring is removed and rodding can be carried out through the rodding arm.

    Question No. 258
    Why are concrete profiles barriers designed with curved surface profiles?
    Answer: Safety fencings are designed to contain vehicles in the carriageway in which they are travelling and prevent them from rebounding into the road and causing hazards. For normal fencing design, when vehicles crash into safety fencings, it will give way so as to absorb as much energy as possible, thus reducing the impact forces on the vehicles. Moreover, it serves to realign the vehicles along the carriageway when vehicles hit on them. However, for concrete profile barriers they are not designed to absorb energy during vehicle crashing, but to hold the vehicles hitting on them. In this connection, concrete profile barriers are designed with curved profiles so that vehicles can mount and go up partly on them, and yet they will not cause overturning of vehicles. Reference is made to Arthur Wignall, Peter S. Kendrick and Roy Ancil.

    For shallow-angle crashing of cars, they would climb on the lower slope face of concrete profile barriers. On the other hand, when a car hits at a large angle to the barrier, the bumper collides with the upper sloping face of concrete profile barrier and the car rides upwards. This provides the uplift of the car whose wheels move up the lower sloping face of the barrier. It is not intended to lift the car too high, otherwise it may result in rolling. Since the friction between the wheels and barriers provide extra lifting forces, it is undesirable to design rough finish on these faces. In essence, the kinetic energy of the car during collision is transformed to potential energy during its lifting up on profile barrier and finally converted back to kinetic energy when the car returns to the road.

    Note: For details of concrete profile barriers, reference is made to HyD Standard Drawing No. H2101A.

    Question No. 259
    What is the significance of direction of approaching velocities of ships during berthing operation?
    Answer: One of the major effects of angle of approaching velocities of ships is its influence of the energy to be absorbed by the fender system. Consider several ships berth on the same pier at the same speed but with different angle of approach, though their kinetic energies are the same, the amount of energy absorbed by fender differs. The amount of energy absorbed by fender is:
    W = 0.5mv² (k² + r²cos²φ)/ (k² + r²)
    Where W= energy absorbed by the fender
    m = mass of the ship
    v = velocity of the ship
    k = radius of gyration of the ship
    r = distance of centre of gravity of the ship to the point of contact of the fender
    φ = direction of velocity
    Hence, when the direction of approaching velocity of a ship is normal to the fender system
    (i.e. φ = 90°), the amount of energy absorbed is smaller when compared with that of a ship whose velocity is tangential to the shoreline. Reference is made to F. Vasco Costa (1964).

    Question No. 260
    When branch pipelines are connected to main pipelines, sometimes Y-junctions or fitting branched pipelines to main pipelines by formation of holes in main pipelines are used. Which one is a better choice?
    Answer: By using standard precast units of Y-junction branch pipelines, it is beyond doubt that joints between branched pipelines and main pipelines are properly formed and the quality of joints is relatively less dependent on workmanship. However, it suffers from the problem that with fixed precast units of Y-junctions, sometimes it may be difficult for contractors to determine the precise orientation of specific angles of Y-junctions with respect to gullies. (e.g. gullies are connected through side branches to carrier drains)

    By forming elliptical holes in main pipelines and fitting the side branches into them with cement mortar, the quality of pipe joints is highly dependent on workmanship. It is commonly found that in subsequent CCTV inspections side branches are projected inside main pipes. This is undesirable because the projected side branches reduce the cross sectional area of main pipes locally and affect their hydraulic performance. Moreover, the projected side pipes may trap rubbish and dirt in the vicinity. On the other hand, cement mortar may not be properly applied at connection joints because these areas are hidden from view and are difficult to be inspected by engineers. Therefore, in selecting between the two available methods, engineers should make their own judgments based on the above considerations.

    Question No. 261
    What is the consideration in selecting the orientation of wing walls in the design of bridge abutments?
    Answer: There are three common arrangements of wing walls in bridge abutments based on Dr.
    Edmund C Hambly (1979):
    1. Wing walls parallel to abutments:  This is the simplest and shortest time to build but is not the most economical design. This design has the advantage that it has least disturbance to existing slope embankment.
    2. Wing walls at an angle to abutments: This is the most economical design among the three options in terms of material cost.
    3. Wing walls perpendicular to abutments: Though it is not the most economical design, the wing walls provide a continuous alignment with bridge decks which provide supports to parapets. However, they cause disturbances to adjacent structures and utility services during construction. Moreover, if the bridge is curved, the wing walls may hinder the road curvature.
    One the other hand, when the wing walls are structurally connected to the abutment, then structural advantage can be taken by the stability of box structure.

    Question No. 262
    What is the difference among cement plaster, cement render and cement screed?
    Under what situations should each of the above be used?
    Answer: The purpose of plastering, rendering and screeding is to create a smooth, flat surface to receive finishes like paint, wallpaper etc.

    Plastering is the intermediately coating of building materials to be applied on the internal facade of concrete walls or block-walls.
    Rendering is the intermediate coating for external walls only.
    Screeding is the coating laid on floors to receive finishes like tiles, carpet, and marble.

    Hence, these terms differ basically from the locations at which they are applied. Due to different locations of application of plasterwork, the proportion of material component for plaster and render is different. For example:
    • Cement plaster
               Undercoat- cement : lime : sand (by volume) = 1 : 4 : 16
               Finishing coat - cement : lime : sand = 1 : 12 : 30
    • Cement render
              Undercoat- cement : lime : sand (by volume) = 1 : 2 : 6
              Finishing coat - cement : lime : sand = 1 : 3 : 6

    Question No. 263
    How to determine the size of elastomeric bearings?
    Answer: For elastomeric bearing, the vertical load is resisted by its compression while shear resistance of the bearing controls the horizontal movements. The design of elastomeric bearings is based on striking a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.

    The cross sectional area is normally determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates. In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.

    Question No. 264
    Why is the slump specified in concrete carriageway comparatively low (30 mm) when compared with normal concrete (75 mm)?
    Answer: The slump of concrete carriageway is purposely specified to be a relatively low value, i.e. 30mm. For concrete carriageway, traffic loads directly act on concrete pavement surface and therefore the surface strength is detrimental to its future performance. In freshly placed concrete, segregation (may be in the form of bleeding) occurs within the mixture of cement paste and aggregates. The degree of resistance to segregation is related to workability of concrete. If substantial segregation is allowed to take place, then the relatively porous and weak laitance layer will be formed on the carriageway surface and the aggregates will concentrate in the bottom. Hence, concrete which has insignificant bleed possesses a stronger surface layer and is more abrasion resistant. Consequently, a small slump value is specified to increase the wearing resistance of concrete and to achieve a suitable surface texture of concrete pavements.
    Moreover, a low-slump concrete facilitates the use of slip-forms when constructing the concrete pavement. With concrete of a low slump value, it still remains its compacted shape and is not liable to deform when the paving machines go away. However, if a high slump concrete is used instead, the pavement surface would drop and the edges may deform readily.

    Question No. 265
    Should emulsified asphalts or cutback asphalts be selected as tack coat in bituminous road-works?
    Answer: Emulsified asphalt is a suspension of asphalt in water by using an emulsifying agent which imposes an electric charge on asphalt particles so that they would be join and cement together. On the other hand, cutback asphalt is simply asphalt dissolved in petroleum. The purpose of adding emulsifying agent in water or petroleum is to reduce viscosity of asphalt in low temperatures.

    The colour of emulsion for tack coat is brown initially during the time of application. Later, the colour is changed to black when the asphalt starts to stick to the surrounding and it is described as “break”. Finally, when water has all evaporated, the emulsion is said to have “set”. Similarly, for cutback emulsion, it is described to “cure” when the solvent has evaporated. However, there are several problems associated with cutback asphalts:
    1. Emulsified asphalt can be diluted with water so that a low application rate could be achieved.
    2. The evaporation of petroleum into atmosphere for cutback asphalt poses environmental problem.
    The cost of production of petroleum is higher than that of emulsifying agent and water.

    Question No. 266
    What is the difference between epoxy grout, cement grout and cement mortar?
    Answer: 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. 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. They are used as bedding for concrete kerbs in roadwork.

    Question No. 267
    How can unreinforced concrete pavement function without mesh reinforcement?
    Answer: For concrete carriageway, it is normally classified into two types: reinforced and unreinforced concrete pavement. The reinforcement in reinforced carriageway (in the form of mesh) is used for controlling cracking. Then one may query how unreinforced pavement can control cracking without the use of mesh reinforcement. To answer this question, one should pay attention to the features of unreinforced concrete pavement. In accordance with Highways Standard Drawing No. H1109, an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness is designed with a regular spacing (normally 5m). The grooves are designed to be too narrow for stones to fall into when the cracks are open due to concrete contraction. The sectional area in which the groove is located is a plane of weakness and thus this groove acts a potential crack-inducing device in which any potential cracks due to shrinkage and thermal contraction may form. Consequently, the cracks are formed at the base of the groove and thus it would not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement.

    Question No. 268
    What are the differences in applications between pipe culverts and box culverts?
    Answer: Basically, a culvert means a covered hydraulic structure which conveys fluid. Therefore in a broad sense, pipe culverts in a small scale represent normal pipes like precast concrete pipes.

    In terms of hydraulic performance, circular section is the best geometrical sections among all. Therefore, for relative small discharge, precast concrete pipes and ductile iron pipes are normally used which are circular in shape. But for applications of very large flow, precast concrete pipes and ductile iron pipes may not be available in current market. In this connection, cast-in-situ construction has to be employed. It is beyond doubt that the fabrication of formwork for circular shape is difficult when compared with normal box culvert structures. However, circular shape is the most hydraulic efficient structure which means for a given discharge, the area of flow is minimum. Therefore, it helps to save the cost of extra linings required for the choice of box culverts.

    However, box culverts do possess some advantages. For example, they can cope with large flow situation where headroom is limited because the height of box culverts can be reduced while the size of pipe culverts is fixed. Secondly, for some difficult site conditions, e.g. excavation of structure in rock, for the same equivalent cross-sectional area, the width of box culverts can be designed to be smaller than that of pipe culverts and this enhances smaller amount of excavation and backfilling.

    Question No. 269
    In General Specification for Civil Engineering Works (1992 Edition), the design of road-base material is based on recipe approach. Why?
    Answer: The design of road-base material is based on recipe approach (David Croney and Paul Croney (1992) because Hong Kong government follows the traditional British practice by adopting recipe design in which the aggregate grading envelope, the quantity and grade of bitumen are specified in the bituminous mix. This recipe of bituminous mix is derived based on past experience and good workmanship during construction. In fact, many countries nowadays adopt special design mix of road-base which proves to produce satisfactory bituminous mixes to suit different site and design conditions.

    In fact, recipe specification of bituminous materials does suffer from several drawbacks. Firstly, the conditions of traffic and climate of newly constructed bituminous road may differ from the conditions on which the recipe design is based. In case adjustment has to be made to the recipe design, it is very difficult to determine and assess the modifications required. Secondly, it poses problem to site engineers to assess the effects of minor non-compliance if recipe specification is adopted. Finally, the recipe mix may not be the most economical design which is dependent on site conditions.

    Question No. 270
    In incremental launching method of bridge construction, what are the measures adopted to enhance sufficient resistance of the superstructure during the launching process?
    1. During the launching process the leading edge of the superstructure is subject to a large hogging moment. In this connection, steel launching nose typically about 0.6-0.65 times span length is provided at the leading edge to reduce the cantilever moment. Sometimes, instead of using launching nose a tower and stay system are designed which serves the same purpose.
    2. The superstructure continually experiences alternative sagging and hogging moments during incremental launching. Normally, a central pre-stress is provided in which the compressive stress at all points of bridge cross section is equal. In this way, it caters for the possible occurrence of tensile stresses in upper and lower part of the cross section when subject to hogging and sagging moment respectively. Later when the whole superstructure is completely launched, continuity pre-stressing is performed in which the location and design of continuity tendons are based on the bending moments in final completed bridge condition and its provision is supplementary to the central pre-stress.
    3. For very long span bridge, temporary piers are provided to limit the cantilever moment.

    Question No. 271
    Poly-tetra-fluoro-ethylene (PTFE) is commonly used in sliding bearings. Why?
    Answer: The choice of sliding surface of bearings is of vital importance because the sliding surfaces generate frictional forces which are exerted on the bearings and substructure of the bridge. For instance, PTFE and lubricated bronze are commonly choices of sliding surfaces for bearings. 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 (David J. Lee). 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 of dirt and rubbish. Lubricants are sometimes introduced to reduce the friction between the PTFE surface and the upper stainless steel plate. Hence, the PTFE may be designed with dimples to avoid the lubricant from squeezing out under repeated translation movements.

    Question No. 272
    What is the difference between working stress approach and limit state approach?
    Answer: For working stress approach, service loads are used in the whole design and the strength of material is not utilized in the full extent. In this method of design, stresses acting on structural members are calculated based on elastic method and they are designed not to exceed certain allowable values. In fact, the whole structure during the lifespan may only experience loading stresses far below the ultimate state and that is the reason why this method is called working stress approach. Under such scenario, the most economical design can hardly be obtained by using working stress approach which is now commonly used in the design of temporary works.

    For limit state approach, for each material and load, a partial safety factor is assigned individually depending on the material properties and load properties. Therefore, each element of load and material properties is accurately assessed resulting in a more refined and accurate analysis of the structure. In this connection, the material strength can be utilized to its maximum value during its lifespan and loads can be assessed with reasonable probability of occurrence. Limit state approach is commonly used for the majority of reinforced concrete design because it ensures the utilization of material strength with the lowest construction cost input.

    Question No. 273
    In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25 L bridge segment is constructed in the first phase of construction?
    Answer: Basically, there are mainly three reasons for this arrangement:
    1. The permanent structure is a statically indeterminate structure. During construction by using span-by-span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two-span permanent structure. To avoid such occurrence, 0.25 L of bridge segment is extended further from the second pier which provides a counteracting moment, thereby reducing the mid-span moment of the partially completed bridge.
    2. The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two-span bridge is uniformly loaded) in which the bridge moment is about zero in the event of future loaded bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the bridge.
    3. In case of a pre-stressed bridge, pre-stressing work has to be carried out after the construction of first segment of the bridge. If the pre-stressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the pre-stressing location at 1.25 L from the first pier where there is relatively more space to accommodate pre-stressing works.
    Note: Span-by-span construction means that a bridge is constructed from one bridge span to another until its completion.

    Question No. 274
    Why should acetylene gas cylinders used for gas welding be erected in upright position?
    Answer: Acetylene gas is commonly used for gas welding because of its simplicity of production and transportation and its ability to achieve high temperature in combustion (e.g. around 5,000°F). Acetylene is highly unstable and flammable and would explode in elevated pressure when reacting with oxygen in air. Storing acetylene gas in cylinders under pressure is very dangerous. Hence, for welding purpose, gas acetylene is stored in cylinders of liquid acetone contained in porous material (like firebrick) to enhance there is no free space left for acetylene gas and for cooling purpose in the event of thermal decomposition. It also prevents the formation of high pressure air pockets inside the cylinder. Dissolved acetylene in acetone will no longer in contact with oxygen and is not subject to decomposition. On the other hand, acetone is used because it is capable of dissolving large amount of acetylene gas under pressure without changing the nature of the gas.

    The cylinders for gas welding i.e. oxygen cylinders and acetylene cylinders, when not in use should be stored separately because any mixture of these gases resulting from accidental leakage can be highly explosive. When in use, acetylene cylinders should always be kept in upright position because acetone liquid will be drawn from the cylinders with the gas if they are kept horizontally. Consequently, significant leakage of acetone liquid will result.

    Note: Oxygen and acetylene gas cylinders are commonly used in construction sites for gas welding.

    Question No. 275
    If there is a delay of bituminous laying on top of sub-base, should tack coat be applied on the top surface of sub-base?
    Answer: When there is a delay between bituminous laying of different bituminous layers (i.e. road-base, base course etc.), a tack coat is applied on top of the bituminous layers because it helps to enhance better bonding between bituminous materials. If there is insufficient bonding between adjacent bituminous layers, they behave as separate independent layers which can hardly resist the traffic loads. When applying the tack coat, it should be sprayed uniformly on the bituminous surface and allowed for sufficient curing. The hot bituminous material laid on top of the coat would soften it, enabling the tack coat to partly fill voids in the bituminous materials. For emulsified asphalt type tack coats, they are normally diluted with water in order to achieve a more uniform application without excessive usage of asphalt. After the subsequent compaction is carried out, the coat would be interlocked with the bituminous materials. On the other hand, care should be taken to ensure that excessive coat would not be laid, otherwise slippage or shear cracks in the bituminous material would occur due to the relative thick layer of the tack coat applied.

    However, for sub-base surface, priming coat instead of tack coat may be applied in the event of a delay in laying of bituminous layer on top of the sub-base layer. The function of the primer serves to maintain the existing surface condition for a longer period and it also provides an impermeable surface to prevent ingress of water or water loss by evaporation. Moreover, it fills the surface voids and protects the sub-base from adverse weather conditions. In addition, it also helps to promote adhesion between adjacent road layers and to harden the surface.

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