​What are the disadvantages of curing by ponding and polythene sheets?

CIVIL_ENGINEERING

What are the disadvantages of curing by ponding and polythene sheets? 

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. 

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. 

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. 

​Is it desirable to use concrete of very high strength i.e. exceeding 60MPa? What are the potential problems associated with such high strength concrete?

CIVIL_ENGINEERING

Is it desirable to use concrete of very high strength i.e. exceeding 60MPa? What are the potential problems associated with such high strength concrete? 

To increase the strength of concrete, say from 40MPa to 80MPa, it definitely helps in 

improving the structural performance of the structure by producing a denser, more durable 

and higher load capacity concrete. The size of concrete members can be significantly 

reduced resulting in substantial cost savings. However, an increase of concrete strength is 

also accompanied by the occurrence of thermal cracking. With an increase in concrete 

strength, the cement content is increased and this leads to higher thermal strains. 

Consequently, additional reinforcement has to be introduced to control these additional 

cracks caused by the increase in concrete strength. Moreover, the ductility of concrete 

decreases with an increase in concrete strength. Attention should be paid during the design 

of high strength concrete to increase the ductility of concrete. In addition, fire resistance of high strength concrete is found to be less than normal strength concrete as suggested by 

Odd E. Gjorv (1994). 

Though the tensile strength of high strength concrete is higher than that of normal concrete, 

the rate of increase of tensile strength is not proportional to the increase of compressive 

strength. For normal concrete, tensile strength is about one-tenth of compressive strength. 

However, for high strength concrete, it may only drop to 5% of compressive strength. 

Moreover, owing to a low aggregate content of high strength concrete, creep and shrinkage 

increases. 

COMPRESSION TEST

CIVIL_ENGINEERING

In concrete compression test, normally 150mmx150mmx150mm concrete cube samples is used for testing. Why isn’t 100mmx100mmx100mm concrete cube samples used in the test instead of 150mmx150mmx150mm concrete cube samples? 

Basically, the force supplied by a concrete compression machine is a definite value. For 

normal concrete strength application, say below 50MPa, the stress produced by a 

150mmx150mmx150mm cube is sufficient for the machine to crush the concrete sample. 

However, if the designed concrete strength is 100MPa, under the same force (about 

2,000kN) supplied by the machine, the stress under a 150mmx150mmx150mm cube is not 

sufficient to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete cubes 

are used instead to increase the applied stress to crush the concrete cubes. 

For normal concrete strength, the cube size of 150mmx150mmx150mm is already 

sufficient for the crushing strength of the machine.

​What is the function of shear keys in the design of retaining walls?

CIVIL_ENGINEERING

What is the function of shear keys in the design of retaining walls? 

In determining the external stability of retaining walls, failure modes like bearing failure, 

sliding and overturning are normally considered in design. In considering the criterion of 

sliding, the sliding resistance of retaining walls is derived from the base friction between 

the wall base and the foundation soils. To increase the sliding resistance of retaining walls, 

other than providing a large self-weight or a large retained soil mass, shear keys are to be 

installed at the wall base. The principle of shear keys is as follows: 

The main purpose of installation of shear keys is to increase the extra passive resistance 

developed by the height of shear keys. However, active pressure developed by shear keys 

also increases simultaneously. The success of shear keys lies in the fact that the increase of 

passive pressure exceeds the increase in active pressure, resulting in a net improvement of 

sliding resistance. 

On the other hand, friction between the wall base and the foundation soils is normally 

about a fraction of the angle of internal resistance (i.e. about 0.8φ ) where φ is the angle of 

internal friction of foundation soil. When a shear key is installed at the base of the retaining 

wall, the failure surface is changed from the wall base/soil horizontal plane to a plane 

within foundation soil. Therefore, the friction angle mobilized in this case is φ instead of 

0.8φ in the previous case and the sliding resistance can be enhanced. 

​If on-site slump test fails, should engineers allow the contractor to continue the concreting works?

CIVIL_ENGINEERING

If on-site slump test fails, should engineers allow the contractor to continue the concreting works? 

This is a very classical question raised by many graduate engineers. In fact, there are two 

schools of thought regarding this issue. 

The first school of thought is rather straightforward: the contractor fails to comply with 

contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could 

order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) – (d), the 

contractor is not entitled to any claims of cost which is the main concern for most engineers. 

This is the contractual power given to the Engineer in case of any failure in tests required by the contract, even though some engineers argue that slump tests are not as important as 

other tests like compression test. 

The second school of thought is to let the contractor to continue their concreting works and 

later on request the contractor to prove that the finished works comply with other 

contractual requirements e.g. compression test. This is based upon the belief that 

workability is mainly required to achieve design concrete compression strength. In case the 

compression test also fails, the contractor should demolish and reconstruct the works 

accordingly. In fact, this is a rather passive way of treating construction works and is not 

recommended because of the following reasons: 

(i) Workability of freshly placed concrete is related not only to strength but also to 

durability of concrete. Even if the future compression test passes, failing in slump 

test indicates that it may have adverse impact to durability of completed concrete 

structures. 

(ii) In case the compression test fails, the contractor has to deploy extra time and 

resources to remove the work and reconstruct them once again and this slows down 

the progress of works significantly. Hence, in view of such likely probability of 

occurrence, why shouldn’t the Engineer exercise his power to stop the contractor 

and save these extra time and cost? 

​What is sucker deck principle for variable depth bridge decks?

CIVIL_ENGINEERING

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 

20-40m. 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).

​How do engineer determine the number of cells for concrete box girder bridges?

CIVIL_ENGINEERING

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 are the shortcomings of grillage analysis which is commonly used in structural analysis of bridges?

CIVIL_ENGINEERING

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: 

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

twists may differ in orthogonal directions. 

(ii) Moment in any beams is mainly proportional to its curvature only. However, moment 

in an element depends on the curvatures in the beam’s direction and its orthogonal 

direction. 

​Under what situation should engineers use pot bearings instead of elastomeric bearings?

CIVIL_ENGINEERING

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 an 

scenario as suggested by David J. Lee. 

​What are the functions of grout inside tendon ducts?

CIVIL_ENGINEERING

What are the functions of grout inside tendon ducts?

Grout in prestressing works serves the following purposes: 

(i) Protect the tendon against corrosion. 

(ii) Improve the ultimate capacity of tendon. 

(iii) Provide a bond between the structural member and the tendon. 

(iv) In case of failure, the anchorage is not subject to all strain energy.