. WATER/CEMENT RATIO OF CONCRETE MIXES
The first essential criterion when mixing concrete is to ensure that the water is clean, as any impurities will affect the strength of the finished concrete. The term ‘drinkable water’ is often used to define the quality of clean and uncontaminated water that is fit for use in concrete. Water fit for drinking is also referred to as being ‘potable’.
The quantity of water used in mixing concrete is a very important consideration as the amount of water in the mix has a direct bearing on the properties of the finished concrete.
The real job of water is to make the cement set hard – to change it from a powder to a solid mass (in the process of hydration), which binds all the aggregates together. The process actually uses only a small part of the water added to concrete although insufficient water will not chemically ‘activate’ the cement and/or will reduce the ability to compact the concrete properly. The rest of the water is there to make the concrete workable enough to be transported, placed and compacted properly. All the extra water will evaporate out as the concrete sets and each drop will leave behind a tiny air pocket in its place, which renders the concrete a little like a solidified sponge. A higher volume of excess water leads to more holes, and a weaker concrete that will be more susceptible to frost damage. In any severely cold weather that follows, water inside those
air pockets will expand on freezing and cause the concrete to crack and/or the surface to spall.
Excessive water results in excessive workability and elongated setting times but can also dilute the cement paste to the point where it drains away from the aggregate, causing the whole mix to segregate. Also, the more excess water present, the greater the tendency for it to rise to the surface of newly placed concrete. This ‘bleeding’ forms fine, open channels, which remain after the concrete has set, and reduce its durability and frost resistance.
On average, the water content should be 50 per cent of the weight of the cement in the mix to give the best balance between workability and strength. This is referred to as the water/cement ratio. It must be remembered that, when adding water to the concrete mix, an allowance must be made for any water that might already be present in the aggregates. This is why it is always best to store aggregates in such a way that they stay dry.
Fig. 10 Concrete strength related to water:cement ratio; the strength of finished concrete decreases as the water content of the mix is increased.