External waterstops are applied externally on the structures and they proved to be effective when installed on the face with a net clamping pressure. For instance, external waterstops can be placed on the outer face of a basement to guard against water entry into the basement.

Internal waterstops are applied internally within the thickness of concrete and it is usually adopted when water pressure can act in both ways. They proved to be effective measures to guard against water flow in both directions but its success lies on the proper installation of waterstops inside the concrete structure. For instance, the installation of waterstops inside concrete slab encounters the problem of improper compaction of concrete around the waterstops.

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 devising a suitable method to seal up cracks detected on concrete surface, it is of paramount importance to determine if further movement would be expected for the cracks. If the crack is not expected to move further, it is sufficient to brush cement grout into it. For wider cracks, other materials like latex-cement mixture may be considered for sealing the crack.

When further movement is expected for the crack, seals wider than the cracks are recommended to be applied over the crack in order to reduce the strain around it to an acceptable level. Moreover, it is desirable to apply the treatment when the cracks are widest so that the sealing material is not subject to further extension. Care should be taken to prevent bonding of sealing material with the bottom of the crack to ensure that only direct tension forces are experienced in the sealing material.

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.

The rate of hydration of cement paste is related to the placing temperature of concrete. The rate of heat production is given by the empirical Rastrup function:

H= H_ox 2^r(T-T1)

H_o = Rate of heat production at a reference temperature
T = Temperature where rate of heat production H
T1 = Temperature where rate of heat production H_o
r = 0.084

An 12^oC increase in placing temperature doubles the rate of reaction of hydration. Hence, concrete placed at a higher temperature experiences a higher rise in temperature. For instance, concrete placed at 32^oC produces heat of hydration twice as fast when compared with concrete placing at 20^oC. Hence, high concrete placing temperature has significant effect to the problem of early thermal movement.

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.

There are two principle mechanisms in transferring shear forces across a reinforced concrete crack, namely, aggregate interlock and dowel action.

The aggregate interlock refers to the interaction between rough surfaces of the crack. The shear stiffness of aggregate interlock is influenced by the axial tensile stiffness of the reinforcement. When shear displacement occurs, there is a tendency for the crack to widen. In the meantime, the reinforcement restrains the crack widening, which subsequently increases in axial stress of the reinforcement. On the other hand, dowel action refers to the shear resisted by the reinforcement.

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 many standards and code of practice of many countries, the allowable size of crack width is normally limited to less than 0.5mm for reinforced concrete structure to enhance the durability of concrete. The limitation of crack width can serve the aesthetic reason on one hand and to achieve durability requirement by avoiding possible corrosion of steel reinforcement on the other hand. Regarding the latter objective, site surveys and experimental evidence do not seem to be in favor of the proposition. Beeby showed that there was no correlation between surface crack width (<0.5mm) and durability of reinforced concrete structure. In practice, most corrosion problems are triggered by the presence of surface cracks parallel to the reinforcement instead of surface cracks perpendicular to the
reinforcement.

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.