Recent questions tagged concrete-structures

Description : Can grout replace concrete in normal structure?

Last Answer : The mixture of cement and water alone cannot replace concrete (Longman Scientific and Technical (1987)) because: (i) Shrinkage of grout is several times that of concrete with the same mass. (ii) ... of cement with water is more than normal concrete and this leads to the problem of severe cracking.

Description : If a contractor proposes to increase concrete cover beyond contractual specification (i.e. 40mm to 70mm), shall engineers accept the proposal?

Last Answer : In contractual aspect, based on the requirement of General Specification of Civil Engineering Works (1992 Edition), the tolerance of concrete cover is between + ... larger cover. Therefore, with an increase of concrete cover, the crack width will increase which is undesirable.

Description : What is the function of rebate in a typical construction joint?

Last Answer : 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 ... After the other pour is concreted, it will hide the uneven joint inside the rebate.

Description : In carrying out compression test for concrete, should test cubes or test cylinders be adopted?

Last 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 ... are lower than that of cubes. Reference is made to Longman Scientific and Technical (1987).

Description : In erection of falsework, for a rectangular panel inside a falsework should it be braced along the two diagonals?

Last Answer : When a rectangular panel is subject to an eccentric load or a lateral load, it tends to deform into a parallelogram with one diagonal shortening and the other elongating. Theoretically, it is sufficient to ... may come from both sides of the panel and hence it should be braced in both diagonals.

Description : What is the indication of shear slump and collapse slump in slump tests?

Last 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 ... concrete. Collapse slump indicates that concrete mix is too wet ad the mix is deemed to be harsh and lean.

Description : If concrete compression test fails, should Schmidt hammer test be adopted as an alternative test to prove the concrete strength?

Last Answer : The Scmidt 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 ... strength of concrete. Therefore, it may not be considered a good substitute for concrete compression test.

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

Last 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 ... would be affected. Moreover, for water lost due to self­desiccation, this method cannot replenish these losses.

Description : 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?

Last Answer : 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 ... Moreover, owing to a low aggregate content of high strength concrete, creep and shrinkage increases.

Description : What are the major problems in using pumping for concreting works?

Last Answer : 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 ... other hand, a proper selection of aggregate grading helps to improve the pumpability of concrete.

Description : 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?

Last Answer : 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 ... cube size of 150mmx150mmx150mm is already sufficient for the crushing strength of the machine.

Description : In designing concrete structures, normally maximum aggregate sizes are adopted with ranges from 10mm to 20mm. Does an increase of maximum aggregate size benefit the structures?

Last 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 ... requirement can offset the disadvantages brought about by discontinuities as suggested by Longman Scientific and Technical (1987).

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

Last Answer : 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 ... instead of 0.8φ in the previous case and the sliding resistance can be enhanced.

Description : If on­site slump test fails, should engineers allow the contractor to continue the concreting works?

Last Answer : 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 ... t the Engineer exercise his power to stop the contractor and save these extra time and cost?

Description : What are the functions of different components of a typical expansion joint?

Last Answer : In a typical expansion joint, it normally contains the following components: joint sealant, joint filler, dowel bar, PVC dowel sleeve, bond breaker tape and cradle bent. Joint sealant: it seals the ... the joint . Cradle bar: It helps to uphold the dowel bar in position during construction.