A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU r = 0.084 An 12 o C 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 o C produces heat of hydration twice as fast when compared with concrete placing at 20 o C. Hence, high concrete placing temperature has significant effect to the problem of early thermal movement. 21. What are the differences in method to seal moving cracks and non-moving cracks in concrete? 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. 22. What is the difference in functions between internal waterstop and external waterstop? (J4) 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. 102
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 23. 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). 103
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A Self-Learning Manual - Institution of Engineers Mauritius

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