The water cement ratio in concrete dictates a number of the hardened concrete properties, most notably the strength. The water cement ratio in concrete is simply the ratio of the volume of water and the volume of cement in the fresh concrete mix. A higher water cement ratio indicates a high water content in the concrete leading to a cheaper mix and a weaker concrete. Conversely a low water cement ratio indicates a low water content leading to a more expensive mix and a higher strength concrete.
Water Cement Ratio in Concrete
All concrete mixes must contain sufficient water to facilitate the hydration of all the cement in the mix. Additional water over and above what is strictly required for hydration is also required in order to make the fresh concrete fluid, workable and finishable. Too much water however is detrimental to the strength and permeability of the hardened concrete.
Water Cement Ratio Definition
The water cement ratio in the concrete mix must include all water. This must include any ice or frost, free moisture on the aggregates, any water contained in admixtures and any water added after the initial mix.
The water cement ratio calculation is the mass of the water in the mix divided by the mass of all cements including supplementary cementitious materials. This accepts the fact that the water content requirements are proportional to the cement content of the mix.
Maximum Water Cement Ratio in Concrete
In all cases a maximum water cement ratio in the concrete mix of 0.55 is generally specified in the UK. This is to minimize the free water content of the mix which leads to excessive bleeding of the fresh concrete and more drying shrinkage. The maximum water cement ratio used in the concrete mix will usually be lower than 0.55 to ensure that either strength of durability requirements are met.
Water Cement Ratio for Concrete Strength
This is why any measures which can reduce the water requirements of the mix will also reduce the cement requirements. These can include water reducing admixtures, the characteristics of any supplementary cementitious materials, type and grading of aggregates and the air content.
Anything which improves the workability of the mix without adding water is beneficial as the either the water-cement ratio in the concrete mix can be lowered improving strength and durability, or the cement content can be lowered reducing costs without impacting strength. Further information on the workability of concrete is included in our Workability of Concrete post.
The strength gain of a concrete mix is inversely proportional to the water cement ratio assuming that hydration progresses normally. This is because less water produces more solids and less voids in the hardened concrete. Typical strength to water cement ratio curves for concrete with entrained air and concrete without are shown in the diagram below to illustrate this.
Abrams’ Water Cement Ratio Formula can be used as a rough guide to how concrete strength relates to the water cement ratio in normal concrete mixes. The formula for 7 day and 28 day strengths are shown below with the water-cement ratio as x. A design tool using these water cement ratio formula is included in the CivilWeb Compressive Strength of Concrete spreadsheet. Further information on the strength of concrete is included in our Compressive Strength of Concrete post.
Water Cement Ratio for Durable Concrete
As well as strength the water cement ratio in the concrete mix also affects the durability of the hardened concrete. The fewer voids and pores created by a low water cement ratio concrete mixes reduces the permeability of the hardened concrete, increasing its resistance to chlorides, salts, sulfates and other chemicals which would affect the durability of the pavement. Further information is included in our Permeability of Concrete post.
Where strength is the critical design criteria, the water cement ratio in the concrete mix should be specified to ensure that the concrete reaches the required compressive and flexural strengths. Where durability is the critical criteria, the water cement ratio can be specified based on the permeability requirements as shown in the water cement ratio table below taken from US standards.
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