Concrete mix design - one of the most adaptable and versatile materials

Aug, 04 2010

Concrete is a construction material composed of Portland cement and sometimes supplementary cementitious materials, coarse aggregate, fine aggregate, water and chemical admixtures. The correct concrete mix ratio, or mix design, can solve problems — and the wrong mix ratio can create problems.

What is needed is a concrete mix that is designed for placeability, strength, durability, density, and final appearance. The standard for designing a concrete mix is the American Concrete Institute's ACI 211.1, Proportions for Normal, Heavyweight, and Mass Concrete.

 

Another source for concrete mix design is the Portland Cement Association's Design and Control of Concrete Mixtures.

By varying the proportions of the concrete mix ingredients, or by substitution for the cementitious and aggregate components, the finished product can be tailored to its application. This article will deal with the subject of placeability.

Placeability, or workability, pertains to the characteristics of the wet or plastic concrete that allow it to be placed more easily with less consolidating effort. It is also desirable to have concrete that doesn't segregate during transport or placement. Workability depends on the water content, cementitious content, aggregate shape and size gradation, and age of hydration. Concrete workability can be increased by raising the water content or by adding chemical admixtures. It is bad practice to add water on-site which exceeds the water-cement ratio of the mix design as this will lead to a much weaker concrete strength. Also, excessive water in the mix will cause increased surface bleeding and possibly the separation of aggregates leading to concrete with reduced quality.

Workability is most often measured by the concrete slump test — a measure of the plasticity of a fresh batch of concrete. Slump is normally measured by filling an upside-down cone in 3 layers of fresh concrete with each layer being tamped with a steel rod to consolidate each layer. When the cone is lifted off, the cone-shaped concrete will slump a certain amount due to gravity. A relatively drier sample of concrete will slump perhaps one or two inches; a wet concrete sample will slump perhaps as much as eight inches.

The most important chemical admixture for improved placeability or workability of fresh concrete is a water reducer. These come in three ranges: conventional, mid-range and high-range, and are used to maintain a specified slump with less water. Conventional water reducers are used to achieve up to an 8% reduction in water. Mid-range water reducers are used to achieve an 8% to 15% reduction in water; added benefits include a delayed initial set of the concrete and a slower setting rate. High-range water reducers, also known as superplasticizers, are used to achieve a 15% to as much as a 40% reduction in water and can turn concrete into self-consolidating concrete (SCC) that has a soupy mixture that is useful when concrete has to flow easily around embedded objects or congested reinforcing steel.

Water reducers have become so important in Concrete Mix Design that they could be considered the "fifth" ingredient after water, cement, coarse aggregate and fine aggregate