Myths and misconceptions are rampant in the concrete construction business. After a while, some of these myths take on a life of their own and everyone starts to take them for granted. In this article, we take a revealing look at ten popular, but completely wrong, myths that still make the rounds in concrete construction.
Myth 1: Adding water to the mix is the only way to increase slump.
Fact: There are other, more effective ways to increase concrete slump besides adding more water.
Adding excessive amounts of water at the jobsite will increase slump, but will also reduce strength significantly. The added water dilutes the paste and increases the water-to-cementitious materials ratio (w/cm). Too much water can also reduce concrete's resistance to freeze-thaw cycles, increase drying shrinkage, and lead to other service-related problems.
ASTM C 94 “Specification for Ready Mixed Concrete” and ACI 301 “Specifications for Structural Concrete,” state that when a load of concrete does not have adequate workability, water may be added to increase the workability of the concrete provided that the maximum slump and w/cm have not been exceeded. As a rule of thumb, adding 1 gallon of water to a cubic yard of concrete will increase the slump about 1 inch. The proper way to add water on-site is to hold back a specific amount of water from the mix and record the amount withheld on the batch ticket. That amount of water can then be added on-site and you can be assured that the concrete still meets the w/cm.
Many specifications forbid any onsite addition of water. Even so, there are other ways to increase the slump and workability of concrete. Aggregate gradation and the maximum size of the aggregate both greatly influence cement and water requirements, which affect mix workability. Water reducers and superplasticizers can also be used to increase the slump while maintaining the water-to-cement ratio. And air-entrainment can increase workability. Adding water to a mix that contains chemical admixtures will change the properties of the mix and can cause excessive slump loss, inconsistent setting, and changes in air content.
Myth 2: Specify a concrete mix by the number of bags of cement.
Fact: Mixes should be specified based on performance requirements, not just cement content.
Some in the concrete business still call out concrete mixes based on the number of bags of cement (a 6-bag mix, or a 7-bag mix), but bag counts don't accurately describe the desired properties of the concrete. Although most cement today is delivered to the ready-mix producer in bulk, portland cement can still be measured as if it came in 94-pound bags, the traditional standard, in accordance with ASTM C 150 “Standard Specification for Portland Cement.” But how much cement is necessary to get high-quality concrete? It depends on the intended use. To maintain economy and avoid adverse effects on workability, shrinkage, and internal temperature rise, high cement contents should be avoided. Minimum cement contents are often specified to improve durability, finishability, wear resistance, or appearance (of vertical surfaces). The most important property of concrete starts with the water-to-cementitious materials ratio.