Why is my new cement crumbling?

People have used cement for building since Ancient Egyptian times. When mixed with water and an aggregate such as sand, gravel or crushed stone, cement acts as a powerful binding agent. The resulting mix, known as "concrete", can be poured and moulded while wet, but will dry to a hard, durable finish. You must consider a number of factors when mixing concrete, or your end result will "spall" or crumble.

Concrete spread over a poor foundation will quickly deteriorate. To make a firm foundation, dig out any area of ground you plan to cover with concrete, and replace the earth with a layer of crushed stone or gravel. Use a heavy garden roller on this layer, or compact it all over by bashing or tamping it with a heavy wooden post.

Concrete will crack and crumble as it sets if you fail to use the dry elements of the mix in their correct proportions. The traditional formula is one part Portland cement, two parts sand and three parts gravel. Check the information on the cement bag if you are unsure. Measure out the correct amounts of cement and aggregate, in spadefuls or bucketfuls, depending upon how much concrete you need.

Crumbling concrete is often caused by someone adding too much water to the mix. Water reacts with the cement to start the hardening process. The label on the cement bag usually tells you how much water to add. Typically, this will be a two-to-one ratio, two measures of dry mix to one measure of water. Measure out the exact amount of water you need before adding it to the dry materials, to avoid making your mixture too wet.

Unevenly mixed concrete will crumble. The dry elements of the mix must be evenly distributed. Mix small amounts of cement and aggregate in a bucket or a wheelbarrow, using a spade. Use a concrete mixer for larger amounts. Add the water and thoroughly mix it in. Any pockets of dry powder will produce weak places in the finished concrete, so churn and combine the mixture for six to ten minutes, until you are sure it is wet throughout.

Any hold-ups in the mixing process can affect the setting, or curing, of new concrete, so it is important to have all the tools and materials ready. Aim to complete the mixing and pouring of the concrete in one continuous process, without interruptions.

Concrete starts to harden as soon as you add water. It normally remains workable for around 20 minutes, a period builders call the "pot life". For the best results, pour the concrete mix into its final position within this time. Spread the mix evenly with a shovel or concrete rake, then level the surface with a trowel. If faced with a large job, mix and pour the concrete in batches, so you do not run out of time.

Set concrete is porous. It will absorb rainwater, expanding as it does so. If sections of concrete do not have room to expand, they will crack. To avoid this, allow a large area of poured concrete to cure for around 45 minutes, then use a thin straight blade to cut neat lines, or "expansion joints" through the solidifying mix. Use the joints to divide your curing concrete into slabs, each about 1m square.

Ideally, your poured or moulded concrete should cure gradually. The longer this curing time, the more opportunity there is for the water and cement to react, and this produces stronger concrete. Cover the poured concrete with a close-fitting tarpaulin for three days to extend the curing time. This should help to prevent a weak and crumbling finished result.

Avoid mixing and pouring concrete on very hot or very cold days. Hot conditions speed up the curing process and this will result in weak concrete. In cold conditions, if wet concrete is subject to freezing and thawing, the surface will dry out flaky and cracked. Pick a mild, dry day to work with concrete, if at all possible.