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slump loss and setting of concrete

Concrete will lose slump, and this is normal due to the hydration process, concrete will lose consistency and stiffen as the time elapsed, water in concrete mix consumed by hydration process to produce hydration products such as ettringite and calcium silica hydrate, also evaporation of concrete water will contribute in reducing concrete consistency. Slump loss should be controlled, especially when delivering concrete required long time's, slump loss can be reduced by using setting retarder. 

The concrete has two setting time, initial and final setting time. Initial setting time of concrete defined as the time at which concrete start losing plasticity,  final setting time of concrete defined as the onset point of strength. The measurement of setting time for cement is different for concrete. Setting time for cement measured using Vicat needle and the sample prepared by mixing of 0.85 of water content determined by consistency test with cement, then measuring the depth of penetration for a Vicat needle under constant load, for concrete setting time, the sample obtained from concrete then seived to get mortar sample. The resistance of penetration for a standard needle is measured for concrete at a regular time

Figure 1

Figure one showing the setting time test apparatus, we can see a container full of mortar, and a standard needle fitted to load cell, this needle will penetrate the sample at regular times, and the load cell will measure the resistance encountered by a rod. This data will be used to draw a curve to determine the initial and final setting time, the initial setting time is the time corresponding of penetration resistance of 500 psi(3.5MPa), the final setting time is the time corresponding of penetration resistance of 4000psi(27.6MPa). 


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