How is the water-cement ratio associated with the strength of concrete?

Water-Cement Ratio

The water-cement ratio is the weight of water provided in a mix divided by the weight of cementitious materials. The total weight of the water includes all the batch water & free water from the surface of aggregates. Cementitious materials includes portland cement, blended cements & supplementary cementitious materials such as fly ash, silica fume & slag. Because of this, the water-cement ratio may be referred to as the water-to-cementitious materials ratio (w/cm).

Strength of Concrete

Water-cement ratio is one of  the largest single factor on which the strength of well compacted concrete depends. Cured concrete strength depends on the two main factors i.e.

  1. Water-cement ratio
  2. Degree of compaction

Air voids in the concrete depends on the water-cement ratio. There is an increase in the air voids with increase in the  weight of water. When this situation happens, then strength of the concrete drops down. Hardened concrete contains about 1% of air voids. In the hardened state concrete, strength is inversely proportional to water/cement ratio.

Above figure shows that validity range of the water-cement ratio is very limited. Compressive strength is at peak, when water-cement ratio is low. Beginning of the curve depends on available means of the compaction (i.e. either done with vibrators or manual-hand compaction). If the large size aggregates is used with the low water-cement ratio & high contents of cement then it exhibit retrogression of concrete strength.
A conclusion can be made that if there is a low water-cement ratio in a fresh mix than after hardening, water/cement will not be able to lead higher strength of the concrete. These conditions happen because of the development of the tensile stresses due to shrinkage & creep. This leads to cracking of the cement or to the loss of bonds (that is between cement and aggregates) as if aggregates try to restrain the tensile stresses.

lets consider two cases;

Case 1 : When the water/cement ratio is high.

If the w/c ratio is high, a large amount of water is available per unit weight of cement in the concrete mix. So if a fixed volume of concrete is poured into a cube of form-work, there are a large number of water-filled voids in the cube. So when the hydration reaction starts on the surface of the cement particle, the gel-like products of hydration get precipitated in the water – away from the surface of cement particles.

The gels formed in such a case are termed as Outer Products of Hydration. Now there are two reasons why the strength of concrete is low in this case, namely –

  1. Since there is a large space for them to develop, the outer products of hydration are large in size. And we know, from the size effect, that larger sized particles have lower strength compared to particles of smaller size.
  2. No matter how high the water consumption rate of the reaction is, due to the large amount of water present in the mix, some water will still be left when the concrete hardens and is ready for use. This trapped water will gradually evaporate, leaving some voids in the concrete block. The presence of voids results in greatly reduced strength.

Case 2 : When the water/cement ratio is low.

In this case, there will be very less amount of water in the block of formwork when the concrete is poured into it, and hence lesser voids. When the hydration reaction proceeds, the gels formed do not have enough space to migrate out and precipitate in the voids. So they get deposited on the surface of the cement particle itself. Such products of hydration are termed as Inner Products of Hydration. The space available for the crystals to grow is limited, so they remain much smaller in size compared to the outer products of hydration.

The strength of concrete is more when the w/c ratio is low, because of the following reasons –

  1. As per the law of size effect, the smaller sized gels formed in this case have much greater strength as compared to those formed when the w/c is high.
  2. Since the water available for hydration is very less, almost all of it is utilized during the reaction. So no water is left to get evaporated later, and hence the strength-reduction due to subsequent void formation is also much lower when the w/c ratio is low.


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