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After the truck mixes the concrete to the designed workability, you should not request any additional water in the concrete. If you think you will need a very soft (flowing) concrete when casting your structure, this should be communicated to Jamaica Pre-Mix’s sales representative when placing the order.
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Honeycombing is caused by a number of different reasons which are:
- Insufficient compacting or vibrating
- Segregation of the concrete when it is allowed to free fall from heights over three feet
- When the steel reinforcement is too close to the formwork (boxing)
- When the formwork is not nailed tight and the grout from the concrete leaks out
- When too stiff a concrete is used without the proper vibrating equipment
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Curing is the term given to the act of keeping the placed concrete moist from as early as one hour after placing for as long as possible or at least seven days. This can be done by applying a sprayed-on curing compound to the concrete, encasing it with polythene, continually wetting the concrete or keeping it wet by flooding it.
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Concrete develops strength ONLY in the presence of moisture therefore, it is absolutely important that your concrete is kept moist after casting. If your concrete is allowed to dry out, it will not develop the strength for which it was designed.
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Surface cracks can be of two types ‘shrinkage’ and ‘settlement’ cracks.
Shrinkage Cracks: These are short semi-circular cracks which appear on a slab when the surface of the freshly placed concrete is allowed to dry. When the surface becomes dry, stresses build up within the concrete. These stresses subject the concrete to tensile forces, and since concrete is very weak in tensile strength, the concrete cracks. The pattern of cracking is very irregular.
Settlement Cracks: These are cracks which occur directly over deep sections of concrete. They usually adopt a pattern similar to that of the reinforcement directly under it. Since all concrete settles within few hours after being placed, the deeper the section the greater the settlement will be therefore, settlement cracks occur mainly over beams.
Both types of cracking are influenced by additional water in the concrete, improper curing, improper placing, insufficient compaction. Shrinkage cracks are also influenced by high winds, direct sunlight, hot and humid weather, which all speed up the drying of the surface of freshly placed concrete.
