Concrete Mix Design

Concrete mix design is very important as concrete is one of the most adaptable and versatile materials used in the construction industry and is one of the most widely used materials in construction projects. In a previous article on concrete in this newsletter, we discussed the fact that the estimator must consider each of the following major components individually: formwork, reinforcing steel, concrete mix design, placement of the concrete mix, finishing of the surface, and curing. Subsequent articles will deal with each of these topics in turn. This article will deal with the subject of concrete mix design.

Concrete is a construction material composed of:

Portland cement and sometimes supplementary cementitious materials (SCM) known as pozzolans (fly ash, slag cement, silica fume, metakaolin)
Coarse aggregate — usually crushed limestone or granite
Fine aggregate — usually sand
Water
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'sACI 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.

Placeability — pertains to the characteristics of the wet or plastic concrete that allow it to be placed and finished. We want concrete that doesn't segregate during transport or placement and that can be easily consolidated. Another consideration is the ability to be pumped, sometimes to great heights. The most common measure of placeability is slump, although two different mixes with the same slump can behave differently depending on the choice of aggregates, air content and admixtures in the mix.

Strength — pertains to the ultimate 28-day compressive strength of the cured concrete rated in pounds per square inch (psi). In structural concrete applications the strength of concrete is very critical because the structural engineer is depending on the designed strength to hold up buildings and to carry anticipated loads.

Durability — pertains to the ability of the concrete to resist freeze-thaw cycles and to prevent chlorides from penetrating to the reinforcing steel and causing subsequent corrosion.

Density — pertains to the weight or mass of the cured concrete. Normal concrete has a density of 150 pounds per cubic foot, and is used in structural applications such as footings, walls, columns, beams and slabs. Sometimes there is the need for lightweight concrete for roof decks.

Appearance — pertains to the color and surface characteristics of the cured concrete.

Subsequent articles on concrete mix design will look in depth at each of these characteristics and how they can be affected by the choice of mix ingredients.

So, how do we get it right? The best way is to have a great relationship with your ready-mix supplier. If you furnish him with the structural engineer's specifications for strength and slump, and where, when and how you plan to place the mix, he can develop the ideal concrete mix design for that application.

Concrete is one of the most adaptable and versatile materials used in the construction industry and is one of the most widely used materials in construction projects. In a previous article on concrete, we discussed the fact that the estimator must consider each of the following major components individually: formwork, reinforcing steel, concrete mix design, placement of the concrete mix, finishing of the surface, and curing. Subsequent articles will deal with each of these topics in turn. This article will continue with the subject ofconcrete mix design.

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 weakerconcrete 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.