Factors Affecting the Concrete Strength: Water-Cement Ratio

Water-Cement ratio refers to the ratio of the weight of water to that of cement in a concrete mix. For instance, a volume of concrete with w/c ratio of 0.5 means that for every 100 kg of cement used in the concrete, 50 kg or liter of water is present.

Water-cement ratio affects especially in two aspects of the concrete: work-ability and strength.
When the w/c ratio is less, the concrete mix becomes hardly workable and even results in the formation of honeycomb—formation of hollow spaces and cavities inside concrete mass or on the surface where concrete mix cannot reach. However, honeycomb may also form due to various other reasons, like improper or insufficient vibration, compaction.

And, when the w/c ratio is higher, the strength of the concrete reduces along with other various effects. Higher w/c ratio means that the mix is dilute, which not just results in the reduction of strength of the concrete but also reduces the flexural, torsional, and tensile strength of concrete. In addition, upon dehydration, the concrete has a higher amount of voids which increases its porosity and shrinking of concrete.

Figure shows that the compressive strength of the concrete is reducing with increase in the w/c ratio.

The figure shows the variation of the compressive strength of concrete with the change in w/c ratio--higher the w/c ratio, lower the strength of the concrete. It also shows that the higher strengths of concrete are achieved even at lower w/c ratio when the concrete mix is compacted using vibrator rather than compacted manually by hand.

Generally, the w/c ratio of 0.35–0.65 are used in normal concrete; however, the ratio is increased or decreased depending upon nature of structures. Lower w/c ratio is usually used along with the addition of plasticizers, whereas, higher w/c ratio is used in the structures, like sidewalks, walkways, and so on.

The strength of concrete, therefore, is inversely proportional to w/c ratio. While lower w/c ratio makes concrete unfit for work, forms honeycomb, higher w/c ratio reduces the strength of concrete. In general, w/c ratio of 0.35–0.65 is used; however, the ratio is adjusted according to mix designs and applications.

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