When water is added to dry mixture, chemical reactions would occur. Water content declines as a result of chemical reactions and consequently concrete workability reduced with time.
So, concrete should be discharged, placed, and compacted before it lost its workability. That is why engineers should be aware about concrete discharge time requirements. This will ensure that hardened concrete would meet requirements set at design stage.
In this article, concrete discharge time requirements are discussed in addition to consequences of exceeding concrete discharge time limit.
What are the Time Requirements for Concrete Discharge?
- All codes of practice have established time limits for concrete discharge for example ACI 301 and ASTM C94. These codes state that, concrete should be discharge within time limit ranges from 1 hour to 1.5 hours after water is added to the mixture.
- It is possible to violate the above limitation if the concrete is still workable and suitable to be placed after 1.5 hour of adding water to the mixture.
- Nonetheless, one should be aware that reasons which allow concrete discharge time limit violation are not strong and have not be been understood properly.
- It is reported by American Society of concrete contractor and Lobo and Gaynor that, 1 to 1.5-hour time limit is a conservative range and can be exceeded in most cases without causing any detrimental effects on concrete.
- If it is necessary to increase concrete workability, then high range water reducing admixtures can be introduced.
- If concrete lost its workability and not suitable for placement upon its arrival, then it must be rejected.
- According to ASTM C94, the concrete should be checked for air content, slump, and specified temperature if 1.5 hour limit is exceeded and the drum has revolved 300 revolutions.
- Finally, it is advised to discuss concrete discharge time limitations in preconstruction meetings to make sure that all parties involved in the construction understand conditions under which concrete discharge time limit can be violated and when such violations are permitted to be employed.
Possible Consequences of Concrete Discharge Time Limit Violations
- High permeability
- Cold joints
- Probably low concrete strength
- ACI Committee 301, 2011. Specifications for Structural Concrete. American Concrete Institute.
- ASCC, N.A. Concrete Discharge Time Requirements. St. Louis: American Concrete Institute American Society of Concrete Contractor.
- ASTM Committee, 2000. Standard Specification for Ready-Mixed Concrete.