Question No. 181
What are the three major types of reinforcement used in pre-stressing?
- Spalling reinforcement Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with Poisson’s effects or by the shape of stress trajectories.
- Equilibrium reinforcement Equilibrium reinforcement is required where there are several anchorages in which pre-stressing loads are applied sequentially.
- Bursting Reinforcement Tensile stresses are induced during pre-stressing 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.
Question No. 182
How many Lbs are there in a yard?
Answer: A pound (Lb) is a unit of measurement for weight while a yard is a unit of measurement for distance.
If you are referring to common measurements used on a construction site, a "yard" is a common abbreviation for a "cubic yard", which is a unit of measurement of volume. (A cubic yard equals 27 cubic feet.) Since dirt weighs about 110 pounds per cubic feet, a cubic yard of dirt weighs about 2970 Lbs. Since concrete is heavier (150 PCF), a cubic yard of concrete weighs about 4050 Lbs.
Question No. 183
Sometimes the side of concrete bridges is observed to turn black in colour. What is the reason for this phenomenon?
Answer: In some cases, it may be due to the accumulation of dust and dirt. However, for the majority of such phenomenon, it is due to fungus or algae growth on concrete bridges. After rainfall, the bridge surface absorbs water and retains it for a certain period of time. Hence, this provides a good habitat for fungus or algae to grow. Moreover, atmospheric pollution and proximity of plants provide nutrients for their growth. Improvement in drainage details and application of painting and coating to bridges help to solve this problem.
Question No. 184
What are the components of a waterproofing system in the roof of a typical pumping station?
Answer: In the design of a waterproofing system at the roof of a pumping station, normally the following components are:
- Above the structural finish level of the concrete roof, a screed of uniform thickness is applied to provide a smooth surface for the application of waterproofing membrane. (Screed of varying thickness can also be designed on the roof to create a slope for drainage.) The screed used for providing a surface for membrane should be thin and possess good adhesion to the substrate. Moreover, the screed aids in the thermal insulation of the roof.
- Above the screed, waterproofing membrane is provided to ensure water-tightness of the roof.
- An insulation board may be placed on top of waterproof membrane for thermal insulation. In cold weather condition where the loss of heat at the roof is significant, the insulation board helps to reduce these losses. On the contrary, in summer the roof is heated up by direct sunlight and the insulation layer reduces the temperature rise inside the pumping station.
Question No. 185
If the contractor is liable for defective works for 12 years with contract under seal (6 years with contract not under seal), then what is the significance of Maintenance Period?
Answer: Defective works constitute a breach of contract in accordance with Limitation Ordinance An action founded on simple contract (not under seal) shall not be brought after expiration of 6 years while an action founded with contract under seal shall not be brought after expiration of 12 years. For construction works, the date of counting these actions should be the date of substantial completion.
To answer the above question, one should note that under the contractual requirement, the contractor during Maintenance Period has the right to rectify the defects and the employer has also the right to request the contractor to make good defective work. However, after the expiry of Maintenance Period, in case of any arising of defects, the employer has to employ others to rectify these works and bring the action to court to claim the contractor for the costs associated.
Question No. 186
What is the biggest dam in America?
Answer: The Grand Coulee Dam Grand - Coulee, Washington
- Length 5,223 ft
- Height 550 ft
Question No. 187
In the design of elastomeric bearings, why are steel plates inserted inside the bearings?
Answer: For elastomeric bearing to function as a soft spring, the bearing should be allowed for bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates are inserted. After the addition of steel plates, the freedom to bulge is restricted and the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates.
However, the presence of metal plates does not affect the shear stiffness of the elastomeric bearings.
Question No. 188
What is the indication of shear slump and collapse slump in slump tests?
Answer: There are three types of slump that may occur in 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 implies that the concrete mix is deficient in cohesion. Consequently, it may undergo segregation and bleeding and thus is undesirable for durability of concrete.
- Collapse slump indicates that concrete mix is too wet and the mix is deemed to be harsh and lean.
Question No. 189
Is the material of formwork (timber or steel) helps to reduce thermal cracks in concreting operation?
Answer: To answer this question, one must fully understand the effect of formwork on the temperature of concreting structure. Without doubt, with better insulation of structure by timber formwork, the overall rise of temperature and hence the peak hydration temperature is also increased. However, for a well-insulated structure, the temperature gradient across concrete element is reduced. Therefore, the use of well-insulated formwork (like timber) increases the maximum temperature and reduces the temperature gradient across the structure at the same time. Hence, whether steel or timber formwork should be used to control thermal cracking is dependent on the restraints and the size of section. If the section under consideration is thick and internal restraint is the likely cause to thermal cracking, then timber formwork should be used. On the other hand, if external restraint is the main concern for thermal cracking, then steel formwork should be used instead.
Question No. 190
What is the importance of critical steel ratio in calculating thermal reinforcement?
Answer: The fulfilment of critical steel ratio means that in construction joints or planes of weakness of concrete structure, steel reinforcement will not yield and concrete fails in tension first. This is important in ensuring formation of more cracks by failure of concrete in tension; otherwise failure in steel reinforcement would produce a few wide cracks which are undesirable.
Question No. 191
What is the purpose of skin reinforcement for deep beams?
Answer: In BS8110, it states that secondary reinforcement should be provided for beams exceeding 750 mm deep at a distance measured 2/3 depth from the tension face. Experimental works revealed that at or close to mid-depth of deep beams, the maximum width of cracks arising from flexure may be about two to three times larger than the width of the same crack at the level of surface where the crack originally forms.
The presence of crack is undesirable from aesthetic point of view. Moreover, it poses potential corrosion problems to reinforcement of deep beams. To safeguard against these crack formation, skin reinforcement is designed on the sides of deep beams to limit the formation of flexural crack widths. Though the principal function of skin reinforcement is to control crack width, it may be employed for providing bending resistance of the section.
Question No. 192
What is the average density of soil?
Answer: For purposes of civil engineering, an average soil will have a density of about 100 to 110 pounds per cubic foot. (This can vary a little depending upon how well compacted the soil is.)
Question No. 193
What is the difference between normal bolts and high friction grip bolts?
Answer: High friction grip bolts are commonly used in structural steelwork. They normally consist of high tensile strength bolts and nuts with washers. The bolts are tightened to a shank tension so that the transverse load across the joint is resisted by the friction between the plated rather than the bolt shank’s shear strength.
Question No. 194
In the design of watermain, the normal practice is to use ductile iron for pipe size less than 600 mm and to use steel for pipe size more than 600 mm. Why?
Answer: For watermain pipe size less than 600 mm, ductile iron is normally used because internal welding for steel pipes below 600 mm is difficult to be carried out. Moreover, it requires only simple jointing details which allow for faster rate of construction. For watermain pipe size above 600 mm, steel pipes are recommended because steel pipes are lighter than ductile iron pipes for the same material strength and therefore the cost of steel pipes is less than that of ductile iron pipes. In addition, in areas of difficult access the lighter mild steel pipes pose an advantage over ductile iron pipes for easy handling.
Question No. 195
In carrying out compression test for concrete, should test cubes or test cylinders be adopted?
Answer: Basically, the results of compression test carried out by using cubes are higher than that by cylinders. In compression test, the failure mode is in the form of tensile splitting induced by uniaxial compression. However, since the concrete samples tend to expand laterally under compression, the friction developed at the concrete-machine interface generates forces which apparently increase the compressive strength of concrete. However, when the ratio of height to width of sample increases, the effect of shear on compressive strength becomes smaller. This explains why the results of compression test by cylinders are lower than that of cubes. Reference is made to Longman Scientific and Technical (1987).
Question No. 196
Should design life be the same as return period for design conditions?
Answer: Design life means the minimum duration a structure is expected to last. The longer is the design life; the higher is the cost of a project. Therefore, in choosing the design life for a structure, engineers should consider the design life which generates a economical project without sacrificing the required function.
In selection of return period of certain design conditions, winds, waves, etc., one should consider the consequences of exceedance. In fact, there are normally no extreme maximum values of these design conditions and its selection is based on the probability of exceedance which is related to return period.
Therefore, design life may not be equal to return period of design conditions because their selections are based on different considerations.
Question No. 197
How are freeway bridges built?
Answer: After calculating the anticipated traffic for the bridge, cement/reinforced-with-rebar stanchions are spaced over the freeway to accommodate the bridge. An 'off-ramp' from the freeway to the bridge is constructed, as is an 'on-ramp' to the subsequent road. Cement/rebar slabs are built and lifted with cranes to form the platform, and voila! Drive carefully.
Although the bridge deck/roadway is almost always a concrete slab, the structure that holds up the bridge deck can be reinforced concrete, structural steel, or a combination of steel and concrete.
Question No. 198
What is the difference between sureties and security?
Answer: In construction contracts, if a contractor fails to perform the works, the employer would suffer from severe financial loss and therefore some forms of protection has to be established in the contract.
For surety bond, the contractor obtains a guarantee from a third party i.e. a bank or an insurance company, which in return for a fee, agrees to undertake the financial responsibility for the performance of contractor’s obligations. This third party will pay to the employer in case there is a contractor’s default.
For security, a sum of money is deposited in the employer’s account and upon satisfactory fulfilment of contractor’s obligations; the sum will be repaid to the contractor.
Question No. 199
What is the difference between air chamber and surge tank?
Answer: Air chambers and surge tanks are normally installed in watermain to ease the stress on the system when valves or pumps suddenly start up and shut down. A surge tank is a chamber containing fluid which is in direct contact with the atmosphere. For positive surge, the tank can store excess water, thus preventing the water pipes from expansion and water from compression. In case of down-surge, the surge tank could supply fluid to prevent the formation of vapour column separation. However, if the surge pressure to be relieved is very large, the height of surge tank has to be designed to be excessively large and sometimes it is not cost-effective to build such a chamber. On the contrary, an air chamber can be adopted in this case because air chamber is a enclosed chamber with pressurized gases inside. The pressure head of gas inside the air chamber is the component to combat the hydraulic transient. However, air chamber has the demerits that regular maintenance has to be carried out and proper design of pressure level of gas has to be conducted.
Question No. 200
What is Gravity flow?
Answer: Gravity flow is fluid flowing due to the forces of gravity alone and not to an applied pressure head. In the Bernoulli equation, the pressure term is omitted, and the height and velocity terms are the only ones included.
Question No. 201
How do rock sockets take up loads?
Answer: The load transfer mechanism is summarized as follows:
When a socketed foundation is loaded, the resistance is provided by both rock socket wall and the socket base and the load distribution is a function of relative stiffness of foundation concrete and rock mass, socket geometry, socket roughness and strength. At small displacements the rock-socket system behaves in an elastic manner and the load distribution between socket wall and socket end can be obtained from elastic analysis. At displacements beyond 10-15 mm, relative displacement occurs between rock and foundation and the socket bond begins to fail. This result in reduction of loads in rock-socket interface and more loads are transferred to the socket end. At further displacements, the interface strength drops to a residual value with total rupture of bond and more loads are then distributed to the socket end.
Question No. 202
What is made from large rocks, which protects the base of cliffs?
Question No. 203
In designing the lateral resistance of piles, should engineers only use the earth pressure against pile caps only?
Answer: In some design lateral loads are assumed to be resisted by earth pressure exerted against the side of pile caps only. However, it is demonstrated that the soil resistance of pile lengths do contribute a substantial part of lateral resistance. Therefore, in designing lateral resistance of piles, earth pressure exerted on piles should also be taken into consideration.
In analysis of lateral resistance provided by soils, a series of soil springs are adopted with modulus of reaction kept constant or varying with depth. The normal practice of using a constant modulus of reaction for soils is incorrect because it overestimates the maximum reaction force and underestimates the maximum bending moment. To obtain the profile of modulus of sub-grade reaction, pressure-meter tests shall be conducted in boreholes in site investigation. Reference is made to Bryan Leach (1980).
Question No. 204
What is the density of most of construction materials?
Answer: If it floats it is less dense than water, 62.4 lbs/cubic ft. Wood is about 40 lbs/ cu.ft.it floats. Concrete is 150 lbs. / cu.ft. It does not float.
Question No. 205
What are the differences in function between rock anchors and rock sockets?
Answer: Rock anchors are used primarily for resisting uplift forces. On the contrary, rock sockets serve three main purposes:
- Rock socket friction and end bearing to resist vertical load;
- Passive resistance of rock sockets contribute to resistance of lateral load; and
- Socket shaft friction is also used for resisting uplifting forces. But only 70% of this capacity should be used because of the effect of negative Poisson ratio.
Note: Rock anchors, which may consist of a high tensile bar or a stranded cable, are provided for tension piles when there are insufficient soil covers to develop the required uplifting resistance.
Question No. 206
In designing concrete structures, normally maximum aggregate sizes are adopted with ranges from 10 mm to 20 mm. Does an increase of maximum aggregate size benefit the structures?
Answer: To answer this question, let’s consider an example of a cube. The surface area to volume ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to volume ratio decreases with an increase in volume. Therefore, when the size of maximum aggregate is increased, the surface area to be wetted by water per unit volume is reduced. Consequently, the water requirement of the concrete mixes is reduced accordingly so that the water/cement ratio can be lowered, resulting in a rise in concrete strength.
However, an increase of aggregate size is also accompanied by the effect of reduced contact areas and discontinuities created by these larger sized particles. In general, for maximum aggregate sizes below 40 mm, the effect of lower water requirement can offset the disadvantages brought about by discontinuities as suggested by Longman Scientific and Technical (1987).
Question No. 207
Why nautical mile is different from statute mile?
Answer: One nautical mile is defined by one latitude minute of arc (there are 60 such minutes to a degree). This equals 1852 meters, and roughly (but coincidentally) 2000 yards or 6000 feet. (Edit: actually, a standard nautical mile is 6076 feet, 6000 feet and 2000 yards are commonly used approximations, but produce an error of about 1%).
The statute mile had a little fuzzier definition to start with, as one mile was the same as 1000 roman Paces/steps. The definition has since changed, but one statute mile equals about 1609 meters.
Question No. 208
What is the function of followers in driven H-piles?
Answer: A follower is an extension between the pile head and the hammer that transfers the blow to the pile in which the pile head cannot be reached by the hammer or is under water .For construction of driven piles, the piling frame and hammer are normally erected on existing ground level but not at the base of pile caps. However, H-piles are designed to be terminated near the base of pile caps. If piles are driven at ground level, a certain length of H-piles is wasted and cut when constructing pile caps. In this connection, pile followers are used so as to save the wasted section of H-piles because followers can be removed during subsequent construction of pile caps.
Question No. 209
In deep excavation, adjacent ground water table is drawn down which may affect the settlement of nearby buildings. What is the remedial proposal to rectify the situation?
Answer: One of the methods to control settlement of nearby buildings due to excavation work is by recharging. Water collected in wells in deep excavation is put back to the top of excavation in order to raise the drawn-down water table. The location of recharge should be properly selected to ensure the soil is sufficiently permeable to transfer the pumped water back near the affected buildings.
Question No. 210
Why are vibrators not used in concrete compaction in piling works?
Answer: Concrete for piles should be a high-slump self-compacting mix which is capable of flowing between reinforcement cage with ease. Since concrete is designed to be self-compacting, vibrators are not used for providing further compaction. Moreover, the concrete in piles is compacted by energy derived from free falling. However, if vibrators are used, the vibrated concrete may be compacted to the sides of the concrete casings and hinders the lifting up of casings. Reference is made to GEO (1996).
Question No. 211
Why are the bolts that hold steel rails together in oval holes?
Answer: It has to do with the design of the bolts. The reason that head of the bolt is oval. The head of a bolt fit flush into the bar and set in opposing sets is so that a piece of dragging equipment will not shear off all the bolts in a joint but rather the just the nuts one side. As a result, if the hole were not oval on one side there would be no way to tighten or loose the bolts in a joint.
Question No. 212
What is the difference between capping beams and ground beams for piles?
Answer: Capping beams for piles aim at transferring loads from closely spaced columns or walls into a row of piles. On the other hand, ground beams are beams provided between adjacent pile caps and they perform as compression struts or ties in an attempt to prevent lateral displacement or buckling of piles under uneven distribution of loads on pile caps. Both of them have to be specially designed to cater for differential settlement of piles. Capping beam performs the same functions as pile caps. However, ground beams are structural elements to connect adjacent pile caps to improve the stability of foundation.
Question No. 213
It is not necessary to design nominal reinforcement to piles. Is it true?
Answer: In BS8110 and BS5400 Pt.4, they require the provision of nominal reinforcement for columns. However, for pile design the requirement of nominal reinforcement may not be necessary. Firstly, as piles are located underground, the occurrence of unexpected loads to piles is seldom. Secondly, shear failure of piles is considered not critical to the structure due to severe collision. Moreover, the failure of piles by buckling due to fire is unlikely because fire is rarely ignited underground.
However, the suggestion of provision of nominal reinforcement to cater for seismic effect may be justified. Reference is made to J P Tyson (1995).
Question No. 214
What are the head details of H-piles under compression and subject to bending moment?
Answer: For steel sections referred to in BS5950, universal bearing pile is characterized by having equal flange and web thickness while universal column has different flange and web thickness. Universal columns can also be used as bearing piles.
In the design of the head details of H-piles, there are three typical cases to be considered, namely compression piles, tension piles and piles with bending moment at the head in addition to tension or compression. The design of these piles recommended by G. M. Cornfield (1968) is listed below:
- Compression piles: For this type of piles, H-piles should be embedded 150 mm in concrete pile caps and it is not necessary to use any dowels and capping plates in their connection.
- Tension piles: A number of hook-ended bars are welded to the top of H-piles.
- Piles with bending moment at their head (tension or compression): The depth of embedment of piles into pile caps is substantially increased and loads are transferred by horizontal bars welded to piles’ flanges.
Question No. 215
What are some facts about concrete?
Answer: Actually, drying is not directly linked to hardness in concrete. After concrete is poured, putting it in a wet environment by spraying it constantly with water will hasten its hardening and its curing. After concrete cures, it is hard.
Question No. 216
What are the problems associated with pre-stressed concrete piles (Daido)?
Answer: The origin of Daido piles comes from Japan where these pre-stressed concrete piles are used as replacement plies. Holes are pre-formed in the ground and Daido piles are placed inside these pre-formed holes with subsequent grouting of void space between the piles and adjacent ground. However, in Hong Kong Daido piles are constructed by driving into ground by hammers instead of the originally designed replacement method. Since the installation method of Daido piles is changed, construction problems like deformation of pile tip shoes, crushing of concrete at pile tip etc. occur. Reference is made to B. W. Choy (1993).
Question No. 217
Which one is a better choice, a large diameter piles or a system of several smaller piles with the same load capacity?
Answer: The choice of a large diameter pile suffers from the disadvantage that serious consequences would occur in case there is setting out error of the pile. Moreover, in terms of cost consideration, for the same load capacity the cost of a group of small diameter piles is generally lower than that of a large diameter pile. On the other hand, for small diameter piles i.e. mini-piles, they are advantageous in site locations with limited headroom and space. In addition, in some structures with only a few piles, it is uneconomic because of its high mobilization cost. Reference is made to Dr. Edmund C Hambly (1979).
Question No. 218
In some codes, they limit the ratio of weight of hammer to weight of pile for pile driving. What is the reason behind this?
Answer: When a hammer with initial motion collides with a stationary pile, the transfer of energy is most efficient when the two masses are comparable. That is the reason why some codes limit the ratio of weight of hammer to the weight of pile to be more than 0.5. If the weight of hammer is too low, most of energy during hammer driving is distributed to the hammer and this causes tension induced in hammer and results in inefficient transfer of energy.
Question No. 219
What is the significance of driving sequence of driven piles?
Answer: For basement construction, if piles are driven from the centre to the perimeter, there is a tendency of soils to move outwards. Such lateral movement of soil may cause damage to nearby structures and utilities.
However, if piles are driven from the outside perimeter inwards, there are little soil lateral movements. This results in a well-compacted centre with an excess pore water pressure built up to resist the loading of piles. Consequently, shorter pile lengths than the original designed ones may result. However, some time after the pile driving operation, the excess pore water pressure is dissipated and the shorter driven piles may not be able to take up the original design loads. In this situation re-driving is required to drive the piles to deeper depths after dissipation of excess pore water pressure.
Question No. 220
During concreting of diaphragm walls, three tremie pipes are used in one time.
However, only one concrete truck is available. How should the concreting works be carried out?
Answer: The most ideal situation is to supply each tremie pipe with a single concrete truck. However, if only one concrete truck is available, all the fresh concrete in the truck should not be placed in one single tremie pipe. With all fresh concrete placed in one single tremie pipe while the others left void, then due to the huge supply of concrete to the tremie pipe, a small concrete hump may form at the base of the tremie pipe and it is likely that it may collapse and trap the slurry inside the diaphragm walls. Therefore, the fresh concrete should be evenly shared among the tremie pipes to avoid such occurrence.
Question No. 221
For a rigid pile cap with vertical piles at the middle and raking piles at the sides, what is the pattern of load distribution of piles in such arrangement?
Answer: Due to the effect of interaction of individual piles, the central piles tend to settle more than the edge piles when the pile cap is under a uniform load. Therefore, raking piles at the edge take up a higher fraction of total loads and are subject to higher axial and bending loads in case the pile cap is stiff. In the extreme case, failure of these raking edge piles may occur.
Question No. 222
What is the difference between direct circulation drilling and reverse circulation drilling?
Answer: For direct circulation drilling and reverse circulation drilling, the major difference in drilling method is related to the direction of movement of drilling fluid. For direct circulation drilling, the drilling fluid is circulated from the drill stem and then flows up the annulus between the outside of the drill stem and borehole wall. The drilling fluid that carries the drill cuttings flows to the surface and the subsequent settlement pits. Pumps are employed to lift the cuttings free fluid back to the drill stem.
For reverse circulation drilling, the direction of flow of drilling flow is opposite to that of direct circulation drilling. Drilling fluid flows from the annulus between the drill stem and hole wall to the drill stem. The drilling fluid is pumped to a nearby sump pits where cuttings are dropped and settled.
Question No. 223
Should engineers rely solely on Hiley’s formula in the design of H-piles?
Answer: About 90% of H-piles adopt Hiley’s formula for design. However, this formula is only applicable to pile lengths less than 30m and is suitable for course-grained materials (not suitable to fine-grained soils) as suggested by GEO (1996). In Hiley’s formula, by observing the penetration of piles after the hammer impact, the pile capacity could be readily obtained from the response of the impacting force. Therefore, the individual pile capacity could be obtained by this dynamic method.
However, in normal foundation, groups of H-piles are present and the soil foundation may not be able to support these H-piles simultaneously even though individual piles are proven to have sufficient capacity by using dynamic method. In this case, static method should be employed to ascertain if the soil foundation could support these H-piles.
Question No. 224
What is the difference between routing maintenance and major maintenance in school facilities?
Answer: The routine maintenance is the minor and consistent repairs that are engaged in the school premises. Examples could include Painting, replacement of fixtures, louver blades, furniture repairs, and patching cracks in the school buildings.
The major maintenance could also be a total rehabilitation of the school, either school buildings or any other project within the school community. It could be total restructuring of the lighting system in the school or sanitary system.
Question No. 225
What are the uses of alloys in daily life and how are alloys made?
Answer: Alloying is not always done to produce a 'superior' material, but to produce materials having a desired requirement in the industry or elsewhere. A classic case is of lead solder (containing lead & tin), in which the melting point of the constituent elements are lowered, not necessarily a desirable property.
Alloying can be carried out using hot press method (a sheet of material is sandwiched with alloying material), rolling the heated base metal in a drum full of alloying metal powder, using hot spraying, galvanizing (dipping the base in a molten solution of alloying material) etc. Sometimes the alloying material is added in small proportions to the molten base metal (e.g., in production of different types of steel).