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 16. Is the requirement of crack width limitation (
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Crazing is the formation of a network of fine cracks on concrete surface caused by early shrinkage of surface layer. The pattern of crazing cracks is in the form of irregular hexagon. The cause of crazing cracks is the shrinkage of concentrated dense cement paste at concrete surface. A wet mix tends to depress the coarse aggregates and form a highly concentrated cement paste and fines on surface. Hence, the difference between plastic shrinkage cracks and crazing cracks lies in the fact that crazing cracks arise from the shrinkage of weak material such as laitance in concrete surface while plastic shrinkage cracks appear even in normal concrete surface. 19. What are the reasons of occurrence of dusting and scaling of concrete surface? (C2) Dusting refers to the formation of loose powder arising from the disintegration of concrete surface. For dusty concrete surface, they can be easily scratched by nails. The cause of dusting can be related to finishing works carried out before completion of concrete bleeding. The working back of bleeding water to concrete surface produces a low strength layer with high water cement ratio. It may also arise owing to inadequate curing and inadequate protection of freshly placed concrete against rain, wind and snow. Scaling is the occurrence of peeling of hardened concrete surface as a result of freezing and thawing effect. Scaling occurs in non-air-entrained concrete as air entrainment is normally adopted to protect concrete against freezing and thawing. Spaying of sodium chloride deicing salts is also another common cause of scaling. 20. What is the effect of concrete placing temperature on early thermal movement? 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 o × 2 r( T −T1) Ho = Rate of heat production at a reference temperature T = Temperature where rate of heat production H T1 = Temperature where rate of heat production Ho 101
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A Self-Learning Manual - Institution of Engineers Mauritius

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