Concrete mixed or delivered in a truck mixer should be completely discharged while the concrete still has sufficient work ability to respond properly during the placing and finishing operations. After arriving at the site,the time for the concrete to work properly worked will generally ranges from 45 minutes to 2 hours
, depending on the weather and the concrete proportions. Prolonged mixing accelerates the rate of stiffening and can greatly complicate placing and timing of finishing operations.
When concrete arrives at the point of delivery with a slump below that which will result in the specified slump at the point of placement and is unsuitable for placing at that slump, the slump may be adjusted to the required value by adding water up to the amount allowed in the accepted mixture proportions unless otherwise permitted by the architect/engineer. Addition of water should be in accordance with ASTM C 94. The specified water cementitious material ratio or slump should not be exceeded. After adding plasticizing or high-range water-reducing admixtures to the concrete on the site to achieve flowable concrete, do not add water to the concrete. Water should not be added to concrete delivered in equipment not acceptable for mixing. Testing samples should be taken after any necessary adjustment.
Concrete for floor and slab placement can be delivered directly from a truck mixer chute to the formwork, or by pump, belt conveyor, buggy, crane and bucket, or a combination of these methods. It is important that delivery of concrete be at a consistent rate appropriate to the size of the placement, which is deposited as close as possible to its final location. Concrete should not be moved horizontally by vibration, as this contributes to segregation.
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