Not all aggregates are suitable for producing “High strength concrete” because the strength of aggregates may control the ultimate strength of concrete once they are crushed apart before the failure of cement paste.

Recycled aggregates and recycled concrete aggregates are normally not recommended in producing “High strength concrete”. Owing to their intrinsic effect of reduced compressive strength, it requires increased cement content to counterbalance this effect in normal concrete situation. However, in the case of “High strength concrete”, the very high cement level has already been adopted which offers little scope for further increase in cement content.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Within a few hours after the placing of fresh concrete, plastic concrete may experience cracking owing to the occurrence of plastic shrinkage and plastic settlement. The cause of plastic settlement is related to bleeding of fresh concrete. Bleeding refers to the migration of water to the top of concrete and the movement of solid particles to the bottom of fresh concrete. The expulsion of water during bleeding results in the reduction of the volume of fresh concrete. This induces a downward movement of wet concrete. If such movement is hindered by the presence of obstacles like steel reinforcement, cracks will be formed.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In some occasions no fines concrete is used in houses because of its good thermal insulation properties. Basically no fines concrete consists of coarse aggregates and cement without any fine aggregates. It is essential that no fines concrete should be designed with a certain amount of voids to enhance thermal insulation. The size of these voids should be large enough to avoid the movement of moisture in the concrete section by capillary action. It is common for no fines concrete to be used as external walls in houses because rains falling on the surface of external walls can only penetrate a short horizontal distance and then falls to the bottom of the walls. The use of no fines concrete guarantees good thermal insulation of the house.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Measurement of constituents for concrete is normally carried out by weight because of the following reasons :

(i) Air is trapped inside cement while water may be present in aggregates. As such measurement by volume requires the consideration of the bulking effect by air and water.

(ii) The accuracy of measurement of cement and aggregates by weight is higher when compared with measurement by volume when the weighing machine is properly calibrated and maintained. This reduces the potential of deviation in material quantity with higher accuracy in measurement for the design mix and leads to more economical design without the wastage of excess materials.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In designing protective coating on concrete structures, stoppage of water ingress through the coating is normally required. Since chloride ions often diffuse into concrete in solution and cause deterioration of concrete structures, the prevention of water transmission into the coating certainly helps to protect the concrete structure. However, if water gets behind the coating from some means and becomes trapped, its effect may not be desirable. Firstly, vapour pressure would be developed behind the surface treatment and this leads to the loss of adhesion and the eventual peeling off of the coating. Moreover, the water creates a suitable environment for mould growth on concrete surface.

In fact, the surface treatment should be so selected that it is impermeable to liquid water but it is permeable to water vapour. This “breathing” function enhances the concrete to lose moisture through evaporation and reject the uptake of water during wet periods.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.