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 9. Is curing compound suitable for all concrete? (C1) For concrete structures with low water-cement ratio (i.e. less than 0.4), it may not be suitable to use curing compound for curing. When hydration takes place, the relative humidity of interior concrete drops which leads to self-desiccation and drying-out. With no external supply of water, the cement paste can self-desiccate in such an extent that the hydration process stops. As such, curing compounds may not be sufficient to retain enough water in the concrete. In this case, wet curing is a better choice which serves to provide an external source of water. 10. What is the difference between curing compound and sealing compound? (C1) Curing compound is primarily used for reducing the loss of moisture from freshly-placed concrete and it is applied once after concrete finishing is completed. Sealing compounds is adopted to retard the entrance of damaging materials into concrete and they are normally applied after the concrete is placed for 28 days. The harmful substances include water, deicing solutions and carbon dioxide which eventually cause freeze-thaw damage, steel corrosion and acid attack respectively. 11. Curing time in summer is less than that in winter. Why? While concrete sets, it gains hardness and strength as the process of hydration slowly spreads the entire body of material. Curing should be allowed to continue for several days before subjecting the new concrete to significant stress. The period of curing depends on the temperature because the rate of all chemical reactions is dependent on temperature. Therefore, in summer the rate of reaction (hydration) is faster so that a shorter curing time is required. On the contrary, in winter the rate of reaction (hydration) is slower so that a longer curing time is required. 12. Why does plastic sheet cause discolouration to freshly placed concrete? (C2) Plastic sheets are commonly used in curing to prevent moisture loss from concrete surface. However, it is not uncommon that discolouration occurs on the concrete surface. When plastic sheeting is spread over concrete surface and in direct contact with concrete, it tends to leave colour streaks on concrete surface. The problem of discolouration becomes even worse 108
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU when calcium chloride is used in concrete mixes. 13. What are the disadvantages of curing by ponding and polythene sheets? (C2) 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 section. Moreover, curing serves to reduce of the loss water from freshly placed concrete to the atmosphere. Ponding: This method of thermal curing is readily affected by weather condition (cold wind). Moreover, a large amount of water used has to be disposed off the construction sites after curing. Polythene sheet: This method of curing is based on the principle that there is no flow of air over the concrete surface and thereby no evaporation can take place on top of the freshly concreted surface by provision of polythene sheets. However, it suffers from the demerit that polythene sheets can be easily blown off in windy condition and the performance of curing would be affected. Moreover, for water lost due to self-desiccation, this method cannot replenish these losses. 109
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A Self-Learning Manual - Institution of Engineers Mauritius

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