Essential Civil Engineering Interview Questions For Freshers Part – 4
After the construction of watermain, prior to hydrostatic pressure test, swabbing is carried out. What is the purpose of swabbing?
Pipelines should be tested before commissioning to check the strength of watermain and the absence of leak. Before carrying out hydrostatic pressure test, swabbing is conducted to clear out rubbish and dirt left inside the pipeline during construction. Swabbing is required for pipes less than 600mm diameter because for larger size of pipes, they can be inspected internally to ensure cleanliness.
After carrying out of hydrostatic pressure test, test for water sterilization is then conducted which involves collecting water sample from the pipeline. The purpose is to check the water quality like colour, turbidity, odour, pH value, conductivity etc. and is compared with the quality of water drawn from water supply point.
What are the functions of grout inside tendon ducts?
Grout in pre-stressing works serves the following purposes:
Protect the tendon against corrosion.
Improve the ultimate capacity of tendon.
Provide a bond between the structural member and the tendon.
In case of failure, the anchorage is not subject to all strain energy.
Is the procurement of third party insurance necessary to be incorporated in contract for construction works?
The purpose of third party insurance is to protect contractors from bankruptcy in case there are severe accidents happened to the third party due to the construction work. Therefore, in government contracts, contractors are requested contractually to procure third party insurance from the commencement of contract until the end of Maintenance Period. If contractors have the financial capability to handle the claims due to accidents to third party, the client is not bound to include this requirement in the contract.
What is absolute pressure?
Absolute pressure is simply the addition of the observed gage pressure plus the value of the local atmospheric pressure.
In the design of service reservoirs, horizontal reinforcement in walls of reservoirs is placed at the outer layer. Why?
Since service reservoirs are designed as water-retaining structures with stringent requirement of crack width control, the design of reinforcement of service reservoirs is under the control of serviceability limit state. For the walls of service reservoirs, contraction and expansion of concrete are more significant in the horizontal direction of walls because of their relatively long lengths when compared with heights.
In this connection, in order to minimize the usage of reinforcement, horizontal bars are placed at the outmost layer so that the distance of reinforcement bars to concrete surface is reduced. Since the shorter is the distance to the point of concern, the smaller is the crack width and hence with such reinforcement arrangement advantages are taken if the reinforcement bars in the critical direction are placed closest to concrete surface.
Can grout replace concrete in normal structure?
The mixture of cement and water alone cannot replace concrete (Longman Scientific and
Technical (1987)) because:
Shrinkage of grout is several times that of concrete with the same mass.
The effect of creep of grout is far more than that of concrete.
Heat of hydration of cement with water is more than normal concrete and this leads to the problem of severe cracking.
Under what situation should engineers use pot bearings instead of elastomeric bearings?
In the event of high vertical loads combined with large angle of rotations, rubber bearings are undesirable when compared with pot bearings. For instance, elastomeric bearings require large bearing surfaces so that compression can be maintained between the contact surfaces between the bearings and piers. Moreover, it also leads to uneven distribution of stress on the piers and some of these highly induced stresses may damage the piers. Consequently, pot bearings are better alternatives than elastomeric bearings in such a scenario as suggested by David J. Lee.
How high should the water tank be to provide a 60 PSI at base of this water tower?
138.60 ft
2.31 ft of water = 1 PSI
What is sucker deck principle for variable depth bridge decks?
For a variable depth bridge deck, the depth of continuous multi-span bridge deck is increased in pier supports and this absorbs sagging moments in the mid-span with the consequent increase in hogging moments in pier supports. As a result, the mid-span depth can be significantly reduced due to the reduction in sagging moment. In essence, this sucker deck principle is applied in locations where headroom requirement is of great concern.
Moreover, in terms of structural performance, sucker decks are effective in reducing dead loads than voided slab of equivalent uniform depth for span length between 20-40 m. In terms of aesthetics point of view, the public tends to appreciate the structural form of arches and curved soffit rather than boring uniform deck alignment. Reference is made to Brian Pritchard (1992).
What is a projection line?
Projection line is the way, in which the earth is shown on a flat piece of Paper.
In pre-cast concrete jacking pipes, sometimes grout holes are designed inside these pre-cast pipes. Why?
Grout holes are present in pre-cast jacking pipes for the following reasons:
They serve as the locations for injection of bentonite or other lubricant. Lubricant is used for both granular soils and cohesive soils to trim down the frictional resistance. For cohesive soils, the soils cannot get onto the pipes by the presence of lubricant and the shearing plane lies within the lubricant as suggested by R. N. Craig (1983). On the other hand, for granular soils, the lubricant mixes with soils with a significantly reduced friction. With the use of lubricant, longer pipe lengths can be jacked without the use of intermediate jacking station.
They provide the inlet locations for subsequent grouting works after completion of pipe jacking to fill completely the void space between the pipes and surrounding soils.
They are used as lifting holes when placing the pre-cast jacking pipes into rails inside the jacking pits.
In designing reservoirs, the indirect tensile strength of the concrete mix is specified to be less than a specific value (e.g. 2.8 N/mm²) for potable water. Why should engineers put an upper limit of indirect tensile strength?
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 structures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have an indirect tensile strength at 7 days not exceeding a certain value (e.g. 2.8N/mm² for potable water). Reference is made to R. D. Anchor, A.W. Hill and B. P. Hughes (1979).
How many kilo-Newtons equals 1000 kilogram?
When the 1000 kg are at rest, on Earth with gravitational acceleration 9.81 m/s2, 1000 kg equal 9.81 kN.
What are the shortcomings of grillage analysis which is commonly used in structural analysis of bridges?
Grillage analysis suffers from the following shortcomings based on E. C. Hambly:
For coarse mesh, torques may not be identical in orthogonal directions. Similarly, twists may differ in orthogonal directions.
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.
How many Pyramid in Egypt?
About 80
What is the crack pattern induced by hydration due to internal restraint?
Let’s take a circular column as an example to illustrate this.
When the temperature is rising, the inner concrete’s temperature is higher than outer concrete’s temperature and the inner concrete is expanding. This induces pressure to the outside and the induced compressive stress will result in 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.
Which type of bar reinforcement are more corrosion resistant, epoxy-coated bars, stainless steel bars or galvanized bars?
V-Based on the experiment conducted by the Building Research Establishment, it was shown that the corrosion resistance of galvanized steel was the worst among the three types of bar reinforcement.
For galvanized steel bars, corrosion started to occur when certain chloride content in concrete (i.e. 0.4% by cement weight) was exceeded. However, for epoxy-coated bars, they extended the time taken for cracking to occur when compared with galvanized steel bars.
The best corrosion resistant reinforcement among all is stainless steel. In particular, austenitic stainless steel stayed un-corroded even there was chloride contamination in concrete in the experiment. Reference is made to K. W. J. Tread away (1988).
How many feet are in 4 square yards?
There are 9 square feet in a square yard therefore there are 36 square feet in 4 square yards.
In the design of a simply supported skew bridge, which direction of reinforcement should be provided?
In the conventional design of steel reinforcement for a simply supported skew bridge, a set of reinforcement is usually placed parallel to free edge while the other set is designed parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement.
The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure. In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by L. A. Clark (1970). In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.
What materials were used to build the Golden Gate Bridge?
Its materials were steel and concrete.
What is the function of washers when using bolts?
The purpose of installing washers in a typical bolting system is to distribute the loads under bolt heads and nuts by providing a larger area under stress. Otherwise, the bearing stress of bolts may exceed the bearing strength of the connecting materials and this leads to loss of preload of bolts and creeping of materials. Alternatively, flanged fasteners instead of using washers could be adopted to achieve the same purpose.
How much does the leaning tower of Pisa lean?
About 14 feet
What is the significance of quality of bentonite slurry in the construction of diaphragm walls?
The quality of slurry plays an important role in the quality of diaphragm walls. Firstly, if a thick slurry cake is formed in the interface between slurry and in-situ soil, it has a tendency to fall off during concreting works and it mixes with freshly placed concrete. Moreover, large thickness of slurry cake would reduce the concrete cover and affect the future durability performance of diaphragm walls.
What are the differences in design between normal pre-cast concrete pipes and pipes used for pipe jacking?
For pipes used for pipe jacking, they should possess the following characteristics:
Pipes should have high concrete strength to withstand the stress induced during the jacking process.
There is tight tolerance in pipe dimension and the pipe joints are specially designed to provide trouble-free joint details. Two commonly available joints are rebated joint and butt end joint.
Pipes preferably should have smooth external concrete finishes to reduce the friction between the pipes and surrounding soil.
Shall reversible moisture movement be taken into account in estimating movement for movement joints?
The size of concrete is affected by changes in atmospheric humidity: moisture causes expansion while drying causes shrinkage. Such moisture movement is reversible. This is totally different from drying shrinkage in which concrete slowly loses moisture during hardening, thus causing irreversible shrinkage.
In fact, the variation of humidity and the estimated reversible moisture movement is not significant (about 30%) and therefore, its contribution to movement does not justify for movement joints as suggested by MN Bussell & R Cather (1995).
Which is stronger solid steel rod or hollow steel pipe?
On a per pound basis, steel pipe is stronger in bending and is less likely to buckle in axial compression.
For long slender structures like beams, propping is required after removal of formwork. Why?
After concreting, the time at which striking of formworks should not be too long, otherwise it would affect the colour of concreted structures. For long span concrete structures, when they have attained sufficient strength to support their self-weight, creep deflection may occur in these structures if propping is not provided after the removal of formwork. Therefore, re-propping is carried out after removing formwork and these props should not be allowed to stand too long because creep loads may overstress them.
Note: Propping refers to provision of false-work to support slabs and beams during their gain in concrete strength after concreting.
In joints of pre-cast concrete bridge segments, what are the functions of applying epoxy adhesive?
Epoxy adhesive is applied in these joints for the following purposes according to
International Road Federation (1977):
It seals up the joints completely between pre-cast concrete segments to protect the pre-stressing tendons;
By filling voids and irregularities along the segment joints, it helps to reduce stress concentrations otherwise it will be developed; and
It helps in transferring of shear between the joints in case a large single shear key is used.
What is aggregate?
Aggregate is the component of a composite material used to resist compressive stress.
Can all utility detectors detect the depth of utilities?
In Hong Kong, underground utility detectors are normally divided into two types: electromagnetic detector and ground penetrating radar (GPR). For electromagnetic detector, it can detect the signals emitted by metallic utilities themselves by passive mode. While in active mode, the detector has to pick up the signals through a transmitter and sonda connected directly to the non-metallic utility.
Both the alignment and depth can be found in active mode while only alignment can be found in passive mode. The electromagnetic detector available in market can detect utilities up to depth of 3 m.
For ground penetrating radar, it sends radio waves into the ground and receives signals from reflections from utilities. It has the advantage of locating both the depth and alignment of utilities. More importantly, it can detect both metallic and non-metallic utilities. However, it suffers from the disadvantage that it is quite expensive and interpretation of data is not simple. Reference is made to LD, DSD (2000).
What are some structures that may be subjected to fatigue?
Bridges, hydraulic presses, burners trains
In concrete pavement, why is the requirement of 95% by mass of quartz grains are specified in contract?
In General Specification of Civil Engineering Works (1992 Edition), in Clause 10.09 it states “Fine aggregate for concrete shall be natural river-deposited sand consisting of at least 95% by mass of quartz grains”. The aim of such specification is to control the quality of river sand. As quartz is a durable and hard material, using a high percentage of quartz in aggregates of concrete can enhance the strength and durability of the surface texture of concrete carriageway.
In addition, such specification requires a high percentage of quartz content, thereby reducing the probability of presence of impurities like shell.
What are moment of inertia and its importance in civil engineering?
The moment of inertia measures the opposition any kind of body will have against a certain momentum (along that same axis) trying to rotate that body.
What is the difference in application of surface-protecting fenders and energy-absorbing fenders?
Surface-protecting fenders are fenders that induce high reaction forces to berthing structures for the energy absorbed while energy-absorbing fenders are fenders which transmit low impact to berthing structures for the energy absorbed (Carl A. Thoresen (1988)). In fact, the principal function of fenders is to absorb the berthing energy and transmit a force to the structures without damaging them.
Therefore, in open berth structures, it is desirable to use energy-absorbing fenders to reduce the loads acting on the relatively flexible structures. On the other hand, for solid berth structures the usage of surface-protecting fenders is adequate because they are capable of taking up large berthing loads.
What is the absolute pressure scale?
Absolute pressure is calculated from a vacuum (0 PSI) and atmospheric pressure is 14.7 PSIa or 14.7 PSI above a vacuum 1 PSI on a tire pressure gauge is called 1 PSIg = 14.7 PSIa, 10PSIg = 24.7PSIa, 100PSIg = 114.7PSIa etc.
In conducting triaxial test to determine shear strengths for soil samples, what consolidation pressures should be specified?
It appears that the selection of consolidation pressure is independent of in-situ soil stress theoretically. However, this may not be correct because the actual shear strength envelopes for soils are non-linear over a wide range of stresses. Therefore, consolidation pressure corresponding to the range of stresses relevant to site condition should be adopted.
What is the difference between standpipe, standpipe piezometer and piezometer?
A standpipe normally contains plastic pipes with perforated holes at the base. The annular space between the perforated tube and casing is filled with gravel or sand backfill. Under such an arrangement, standpipe is used to measure water level of a certain region. A standpipe piezometer is a type of piezometer which measures pore water pressure at a certain level. It consists of plastic pipes without holes.
The tip of the standpipe piezometer is perforated and the annular space between the tip of the piezometer and soil is filled with sand while the annular space between other parts of plastic tube and soil is filled with cement/bentonite grout to seal off water from entering the region of piezometer tip. This enables the pore water pressure in the region of piezometer tip to be measured.
In essence, standpipe piezometers are installed to study the pore water pressure of a specified depth below ground. However, it suffers from the disadvantage that the response time is relatively slow in clayey soils. Reference is made to Marius Tremblay (1989).
How does siphoning work?
Pretty good if the outside hose is a lot lower than the inside hose. The siphon works by a hose stuck in a fluid container higher than the receiving container. Suction is shortly placed on the lower side of the hose until the fluid is flowing down the down line. Gravity will cause a continued flow.
In pumping stations one of the choices for the material of water tanks is fibre-reinforced plastic (FRP). What are the advantages associated with this kind of material?
There are two main advantages for FRP water tanks:
It possesses high strength to weight ratio and this leads to the ease of site handling.
It is highly resistant to corrosion and thus it is more durable than steel water tank.
For rigid pavement, what are the advantages of using lean concrete sub-base instead of traditional granular sub-base?
There are several shortcomings of using granular sub-base in concrete carriageway:
Since sub-base is permeable, water can seep through sub-base and soil particles will be pumped out through contraction/expansion joints when subject to traffic load. Consequently, voids are formed underneath the pavement structure and the concrete pavement may crack under severe traffic loading.
Lean concrete increases the strength and renders the roads capable of carrying heavy traffic loads (David Croney and Paul Croney (1992).
Due to workmanship problem, it may have uneven distribution of sub-base and this result in cracking of concrete carriageway when subject to severe traffic loading.
For marine pile type of steel tubular piles with reinforced concrete infill, minimum toe level is often specified in contract drawings. What is its purpose?
The purpose of minimum toe level is two-fold:
In detailed design stage, ground investigation should be conducted and the approximate level of rock-head is known. Therefore, to avoid the marine piles to be founded prematurely on boulders, minimum toe levels of marine driven piles are specified in contract.
It provides sufficient length of soils for lateral and uplift resistance.
Note: Minimum toe level refers to the minimum level that a marine driven pile should be driven into seabed.
How do you compute volume of airflow?
Orifice meter is used for measuring flow rate air.
Soil nails are mainly designed for stabilization of major slips. How should designer cater for the stability of minor slips?
There are some methods to treat minor slips:
Adoption of smaller diameter size bars at closer spacing;
Installation of tie beams at the same horizontal levels;
Provision of steel wire meshes in-between soil nails; and
Provision of short soil nails in combination of long soil nails.
In terms of pumping performance, how should engineers determine the use of radial flow pumps and axial flow pumps?
Specific speed is usually defined for a pump operating at its maximum efficiency. In order to minimize the cost of future operation, it is desirable to operate the pumps as close to the maximum efficiency point as possible.
The specific speed for radial flow pumps is relatively small when compared with that of axial flow pumps. This implies that radial flow pumps tend to give higher head with lower discharge while axial flow pumps tend to give higher discharge with lower head.
What is the formula for figuring out how many gallons of water are in a pipe by the foot?
0.61 × d², where d is the diameter of the pipe in inches.
What is difference in purpose for conducting surface regularity test and sand patch test (test on texture depth)?
The purpose of surface regularity test is to measure the riding quality of pavements and the same requirement and standard is applied to both concrete and bituminous carriageway. On the other hand, sand patch test is used for checking the skid resistance of road pavements. Moreover, it is related to traffic noise because the intensity of noise generated from road traffic is related to texture depth of carriageway.
For typical pile bents in marine piers, how is vertical loads related to horizontal capacity of the pile bents?
Let’s consider a pile bent with a top slab supported by two ranking piles, each inclining at an equal angle to the pier slab. In designing such a system, truss action is normally adopted to analyze the pile bent. When the reaction forces of these piles, horizontal forces (e.g. due to berthing and de-berthing of vessels) and vertical forces (e.g. superimposed deck loads) are analyzed by drawing a force polygon, it is noted that lateral resistance of the pile bent is dependent on the vertical load, i.e. lateral resistance is small when vertical loads are high.
What is kn measurement?
kn (both letters lowercase) is knots, which is a speed nautical miles per hour.
In determining the effective stress parameters of a soil sample, which test is preferable, consolidated un-drained test or consolidated drained test?
The effective stress parameters of a soil sample can be obtained from both consolidated un-drained test and consolidated drained test.
However, consolidated un-drained test is normally selected because of the following reasons:
Time taken for consolidated un-drained test is shorter than that of consolidated drained test. It is because consolidated drained test requires the full dissipation of excess pore water pressure of the soil during testing and it takes long time when soils of low permeability are tested.
Useful information can be obtained from the stress path of consolidated un-drained test.
Failure occurs in lower stress level when compared with consolidated drained test.
What are the design considerations for dowel bars in joints of concrete carriageway?
The behavior of dowel bars resembles that of piles in soils subject to lateral loads. Failure of the joint occurs by yielding of steel under bending action or by crushing of concrete due to bearing stresses.
In general, the spacing of dowel bars should not be too close which may pose problems during construction. However, it should be not too wide to allow the occurrence of bending between the dowel bars. On the other hand, regarding the length of dowel bars, it should not be too long because the induced stress at the end of long dowel bars is insignificant and is not effective in transferring loads between adjacent concrete panels. However, if the dowel bars are too short, the stress at the face of joint is increased resulting in concrete crushing. Reference is made to Ministry of Transport (1955).
What is the function of a sand layer underlying paving slab/interlocking blocks?
Normally after the laying of sub-base layer of the paving slab/interlocking blocks, a 30 mm thick sand bedding is screeded and tamped over the pavement area. Then paving slabs are laid horizontally with joints of 2-3 mm wide and are laid in uphill direction. After completing the laying of paving slabs, sand used for filling joints is spread over the surface of the units and brushed into the joints such that all joints are completely filled. The paving slabs are then bedded into final position by using plate vibrators.
The sand layer serves the same purpose of normal blinding layer under concrete structure:
Provide a level and flat surface for the paving slab/interlocking blocks to lay on;
Protect the foundation (i.e. underground sub-grade and sub-base layer) against adverse outside conditions (e.g. bad weather) during construction of the laying work of paving slab/interlocking blocks.
How do we calculate absolute pressure?
Absolute is equal to gauge pressure plus atmospheric.
What is the relation of bearing pressure on soil nail head to the ratio La /Lb, where La is the length of soil nail before the potential slip circle while Lb is the length of soil nail beyond the potential slip circle?
The unstable soil mass before the potential circular slip is resisted by two components: soil nail head bearing pressure and friction of soil nail in the unstable soil mass. Therefore, the longer is the length of soil nail before potential slip circle La, the higher is the proportion of forces being resisted by frictional forces and hence the smaller amount is to be resisted by soil nail head. Hence, the smaller the ratio La/Lb, the greater is the resistance provided by soil nail head.
In dredged reclamation, what are the considerations in selecting between trailer suction hopper dredgers and grab dredgers?
Trailer suction hopper dredgers are vessels which remove material off the seabed through hydraulic suction by using pumps. During the dredging operation, a mixture of soil and water is transported through suction pipe to storage hoppers. Significant turbulence inside the hoppers keeps the dredged mixture in suspension and this should be minimized to enhance the material to settle swiftly prior to the process of overflowing.
Trailer suction hopper dredgers are mounted with drag-head or dragarm pumps which increases the dredging depth and trims down the occurrence of cavitation as suggested by John B. Herbioh (1992). This machine is limited to dredging relatively low-strength material. Moreover, they cannot be deployed in very shallow waters and instead grab dredgers should be used. However, its dredging capacity is higher than that of grab dredger and it can be mobilized in relatively deep-water region.
Trailer suction hopper dredgers are renowned for their mobility, versatility and capability to operate in unfavourable sea conditions.
Can a sub-base layer serve as a drainage layer to remove water from pavement?
Besides providing load distribution in post-construction stage and working platform in construction stage, sub-base can also serve as a drainage layer to remove water coming from the pavement. For thick layers of road pavement, it is likely that the water leakage from pavement is insignificant and therefore the sub-base layer mainly serves to support the traffic stresses. However, for thin pavement layers, water penetration is quite substantial and therefore sub-base may also act as a drainage layer to remove this water.
What is the origin of name Railway Sleepers?
When sleepers were added below the railway, lines the trains ran more smoothly and people fell asleep on their journeys.
What is the mechanism of formation of soil plugs in marine driven steel piles with reinforced concrete infill?
During initial driving process, open-ended steel piles are driven through the soils at their bases. However, shaft friction will gradually develop between the steel piles and soils inside piles at some time after pile driving. The hitting action of driving hammers induces forces to the soil and later it comes to a stage when the inertial forces of inside soils, together with the internal frictional forces exceeding the bearing capacity of soils at pile toes. Consequently, the soil plug formed is brought down by the piles. Reference is made to M. J. Tomlinson (1977).
Note: A soil plug is a column of soil formed at the bottom portion of marine pile type of steel tubular piles with reinforced concrete infill.
What is the design level of landings in piers?
Landings are designed as resting place for passengers during berthing and de-berthing of vessels. In general, landings are provided near mean high and mean low water levels to facilitate embarking and disembarking of passengers (BS6349: Part 2: 1988). Therefore, the level of landing steps should be different from place to place because of different mean high and mean low water levels in different locations.
What are the advantages of assigning the central pier and the abutment as fixed piers?
For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force; the bending moment induced is smaller.
For the central pier to be selected as the fixed pier, the bridge deck is allowed to move starting from the central pier to the end of the bridge. However, if the fixed pier is located at the abutment, the amount of movement to be incorporated in each bearing due to temperature variation, shrinkage, etc. is more than that when the fixed pier is located at central pier. Therefore, the size of movement joints can be reduced significantly.
What are the major problems in using pumping for concreting works?
In pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure.
The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to improve the pumpability of concrete.
How long does it take concrete to harden?
It takes 28 days to cure.
For unreinforced concrete carriageway, what is the sequence of closing and opening of expansion joints and contraction joints?
Let’s take an example to illustrate the sequence of closing and opening of joints (Ministry of Transport (1955). For instance, an unreinforced concrete carriageway is constructed in winter. When temperature rises in the following summer, the section between expansion joints will expand as a whole single element resulting in the closure of expansion joints.
This section of concrete pavement will move outwards from the mid-point between the expansion joints. In the next winter, each bay (i.e. concrete pavement between adjacent contraction joints) of concrete contracts about the midpoint of its length with opening of contraction joints.
At what point does a circle start losing area when being flattened?
The area ratio decreases slowly at first then more rapidly.
The area of an ellipse is A = π × a × b, ‘a’ and ‘b’ are the ½ major and ½ minor axis. If a = r = b the area is A = π × r², which is a circle. Not hard to graph this but you will have to write the area of the ellipse in terms of the circumference.
For underwater concreting, tremie pipes are normally used with the aid of hoppers. Sometimes tubes are inserted inside the hoppers. Why?
In placing concrete by tremie pipes, hoppers are connected to their top for receiving freshly placed concrete. However, air may be trapped inside the tremie pipes if there is rapid feeding of fresh concrete. To release the trapped air inside the tremie pipes, hoses (called ventilation tubes) are inserted and lowered down through the hoppers. Reference is made to Carl A. Thoresen (1988).
What are the functions of bedding under storm-water drains?
Bedding, which is normally made of granular materials or concrete, serves four main functions as suggested by O. C. Young and J. J. Trott:
To enhance a uniform support under pipes in order to reduce the bending moment longitudinally;
To increase the load-supporting strength of the pipes;
For pipes with spigot and socket joints, it enables pipes to be supported along pipe lengths instead of pipe sockets. Otherwise, uneven stress may be induced and it may damage the pipes;
To provide a platform for achieving correct alignment and level during and after construction.
What are the strongest shapes in building?
The pillars
In a curved pre-stressed bridge, how should the guided bearings in piers of the curved region be oriented with respect to the fixed bearing in abutment?
To determine the orientation of guided bearings, one should understand the movement of curved region of a pre-stressed bridge. Movement of pre-stress and creep are tangential to the curvature of the bridge (or along longitudinal axis) while the movement due to temperature and shrinkage effects are in a direction towards the fixed pier. If the direction of guided bearings is aligned towards the fixed bearing in the abutment, the difference in direction of pre-tress and creep movement and the guided direction towards fixed bearing would generate a locked-in force in the bridge system.
The magnitude of the lock-in force is dependent on the stiffness of deck and supports. If the force is small, it can be designed as additional force acting on the support and deck. However, if the force is large, temporary freedom of movement at the guided bearings has to be provided during construction.
If concrete compression test fails, should Schmidt hammer test be adopted as an alternative test to prove the concrete strength?
The Schmidt hammer test is based on the elastic rebound of hammer which presses on concrete surface and it measures the surface hardness of concrete. Since the test is very sensitive to the presence of aggregates and voids at the concrete surface, it is necessary to take more than 10 readings over the area of test. However, it should be noted that Schmidt hammer test measures surface hardness only but not the strength of concrete. Therefore, it may not be considered a good substitute for concrete compression test.
What is the advantage of sliding bearings over roller bearings?
In roller bearing for a given movement the roller bearing exhibit a change in pressure centre from its original position by one-half of its movement based on David J. Lee. However, with sliding bearing a sliding plate is attached to the upper superstructure and the moving part of bearing element is built in the substructure. It follows that there is no change in pressure center after the movement.
How do you maintain water pressure?
If you have water pressure and wish to maintain it, do not cause flow in the line, which will reduce pressure due to friction.
To keep pressure up, reduce friction by increasing the line size or eliminating some other restriction.
What is the difference in arranging pumps in series and in parallel?
For identical pumps with similar functions, if the pumps arranged in series, the total head is increased without a change to maximum discharge. On the other hand, for pumps arranged in parallel to one another, the discharge is increased without any changes to maximum head.
What is the function of rebate in a typical construction joint?
Construction joints are created on sites to facilitate the construction process. However, if improperly constructed, the completed construction joints will leave an uneven scar on the concrete surface and affect significantly its appearance. To avoid this, a rebate is formed during the first pour of one side of construction joint. After the other pour is concreted, it will hide the uneven joint inside the rebate.
Shall a layer of wearing course or additional thickness be designed on the surface of piers?
In the design of piers, consideration should be given to the effect of wearing action caused by passengers, other traffics and even sometimes vehicles. In maritime environment, the durability and integrity of concrete is detrimental to the servicing life of piers because it acts an essential barrier to chloride attack. However, in view of these gradual wear and tear generated by the loading traffic, some forms of surface protection should be provided on top of pier surface like wearing course or additional increase in concrete cover.
The spacing of manholes in straight sections for different pipe sizes is stated in Storm-water Drainage Manual. How are these figures arrived at?
For pipe size < 300 mm, rodding is usually adopted in which workers place about 1 m long rods through the pipes to the location of blockage and manually operate the rod to clear the blockage.
For pipe size < 700 mm, water-jetting is normally employed in which water is supplied from nearby fire hydrants and pressurized water jet is used for clearing blockage.
Winching method is adopted for all sizes of pipes.
For instance, for pipe size exceeding 1050 mm, it is stated in Storm-water Drainage Manual that maximum intervals between manholes along straight lengths should be 120 m. This is because for sizes over 1050 m, the main method of pipe maintenance is by winching whose maximum length of operation is 120 m. Similarly, the maximum intervals of manholes for other straight pipes are derived from their corresponding maintenance methods.
What is unit weight of normal concrete?
145 PCF
General Specification for Civil Engineering Works (1992 Edition) Clause 15.09 specifies that tying wires for reinforcement adjacent to and above Class F4 and F5 finishes should be stainless steel wires. Why?
If plain steel tying wires are used for reinforcement adjacent to Class F4 and F5 finishes, it poses the problem of rust staining which may impair the appearance of exposed concrete surfaces. The rate of corrosion of plain steel tying wires is similar to normal steel reinforcement.
However, for tying wires with very small diameter, upon long exposure it stands a high chance of rusting completely and this rust will stain the formwork and significantly affect the concrete finish. Therefore, stainless steel tying wires are specified for locations in the vicinity of high quality of finishes to avoid rust staining by corroded typing wires.
Note: Tying wires are wires used for fixing and connecting steel reinforcement bars.
Which architectural movement is also known as Jazz Moderna?
Art nouveau
Should raking piles of a bridge abutment be placed under an embankment?
For a bridge abutment to be supported on raking piles with different orientations, the movement between the ground and the pile group is difficult to predict. For instance, if some of the raking piles of the bridge abutment are extended beneath an embankment, then the settlement of embankment behind the abutment may cause the raking piles to experience severe bending moment and damage the piles as recommended by Dr. Edmund C Hambly (1979).
In designing of access ramps for drainage channels, why should the direction of access ramps be sloping down towards downstream?
In the design of access ramps, the direction is normally specified to be sloping down towards downstream so as to avoid the occurrence of over-shooting of flowing water for supercritical flow in case of aligning the ramps in the reverse direction of channel flow.
Note: Access ramps refer to ramps used for maintenance vehicles during routine maintenance of channels.
What is the difference in application between open stirrups and closed stirrups in concrete beams?
Open stirrups are provided principally to resist shear forces in concrete beams and they are applied in locations in which the effect of torsion is insignificant. U-shaped stirrups are placed in the tension side of concrete beams in which shear cracks would occur. However, when concrete beams are designed to resist a substantial amount of torsion, closed stirrups should be used instead.
How do you convert wind speed into force?
Thermometer
Why are voids filled with lightweight infilling material in raft foundation of pumping stations?
To reduce the dead load and hence to reduce the settlement of pumping stations, the voids inside the raft foundations are filled with light material. If instead concrete is placed inside these voids, it poses severe thermal cracking problem and drastically increases loads to pumping stations.
The use of general fill is also not desirable because its self-weight is comparable to that of concrete. On the contrary, if these voids are left vacant, water may penetrate into these voids during future operation and increases the dead load of pumping stations during its normal operation. Therefore, lightweight infill material, which is non-water-absorbing and non-biodegradable, is designed inside these voids to avoid ingress of water and to reduce the dead load of the structure.
Nowadays, most flap valves are made of HDPE. What are the advantages of using HDPE when compared with cast iron?
It has no reaction with sewage and seawater and does not suffer from the corrosion problem associated with cast iron.
No protective coating is required and it is almost maintenance-free.
HDPE flap valves require very low opening pressure in operation (like 5mm water level difference). For cast iron flap valves, due to its own heavy self-weight, the required opening pressure of cast iron flap valves is higher than that of HDPE flap valves. This criterion is essential for dry weather flow conditions.
However, the pressure resistance of HDPE flap valves is not as good as cast iron flap valves. For instance, a typical 450 mm wide HDPE flap valve can only withstand about 5 m water column.
Does brick grows bigger every year.
Usually bricks do not grow, but since stuff gets all over, it looks like it grows.
Are diaphragms necessary in the design of concrete box girder bridges?
Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, 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 based on Jorg Schlaich & Hartmut Scheef (1982). This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult.
In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.
How do engineer determine the number of cells for concrete box girder bridges?
If 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 choice as suggested by Jorg Schlaich & Hartmut Scheef (1982). 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.
What is the quantity of cement in Mix M20 concrete?
For nominal Mix Cement consumption for M25 will be 570 kg per cum. and for Design Mix it can be optimized to 510 kg per cum.
When a drainage system (i.e. u-channels with catch-pits) is connected to a main drainage channel, a segment of short pipe is used. What is the reason of such arrangement?
There are three scenarios of such connection arrangement:
A new drainage system is connected to an existing drainage channel
An existing drainage system is connected to a new drainage channel
A new drainage system is connected to a new drainage channel.
For all scenarios, what engineers consider is the total amount of differential settlement or lateral movement to be encountered between the drainage system and main drainage channel.
For scenario (b) and (c), it is very likely that substantial differential settlement will occur and this will cause damage to the connecting concrete pipes. Therefore a segment of short pipes are designed so that they serve to provide flexibility to the pipes in case of uneven settlement occurring between drainage system and main drainage channels.
What glue is least sticky?
Elmer’s glue is least sticky.
For column reinforcements, why is helical reinforcement sometimes designed instead of normal links?
The use of links for column design in Britain is very popular. However, in U.S.A. engineers tend to use helical reinforcement instead of normal links because helical reinforcement has the potential advantage of protecting columns/piles against seismic loads.
Moreover, when the columns reach the failure state, the concrete outside hoops cracks and falls off firstly, followed by the eventual failure of the whole columns. The peeling off of concrete outside helical reinforcement provides a warning signal before the sudden failure of columns as suggested by G. P. Manning (1924). In addition, it can take up a higher working load than normal link reinforcement.
For instance, helical reinforcement is adopted in the design of marine piles in Government piers.
Note: Helical reinforcement refers to shear reinforcement which is spiral in shapes.