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How to mix concrete

Date posted: 8 January 2015

If you're planning to create a garden wall, path or shed base out of concrete, and you don't have previous experience of working with concrete, it's important that you work out in advance how to achieve the right concrete mix ratio for your specific application.

Concrete ingredients and a guide to ratios

Concrete is a versatile material made from a mix of cement, fine aggregate (sand), coarse aggregate (gravel or small stones) and water – although the exact mix depends on the application.

The water:cement ratio represents the ratio of the weight of water to the weight of cement. This ratio impacts the quality of the concrete with a lower water-cement ratio creating greater strength and durability. As a general rule of thumb, for every pound (or other unit of weight) of cement, you need approximately 0.25 pounds (or 0.25 of whatever unit of weight you are using) of water. This is a minimum, however, and can prove very hard to mix properly, with limited flowability. As a result, more water is usually added – but should be done conservatively (aiming for water-cement ratios of between 0.4 and 0.6). For most DIY concreting projects this will be strong enough – but if you are trying to make higher strength concrete you will have to stay with the lower water:cement ratio then add plasticizer to boost its flowability.

For most everyday concrete the cement:sand:stone ratio would be 1:2:4. For DIY projects around the garden – from fence posts to garden paths – this kind of general purpose mix will usually work well. This is available ready-mixed in bags to make your job even easier (but make sure you do not pick up 'mortar mix' by mistake). This produces concrete of a medium strength and is known as a "C20 mix" - i.e. with a compressive strength of 20 newtons per square mm after 28 days. If you plan to pour the concrete more than about three quarters of an inch thick, however, you will need to add coarse aggregate (stones / gravel) to the mix yourself, as these are not included with these ready-mix bags. Before you buy and mix your concrete, it is well worth investing some time in researching the right ratio of ingredients for your specific application to ensure you achieve the best results possible.

Concrete mixing tips

The mixing process is not complex, but should be done carefully to make sure you don't add too much water – otherwise you could end up with weakened concrete that will be more prone to cracks and deterioration.

For very small quantities of concrete you could mix by hand, but this is not advisable for larger quantities because it is difficult to distribute the cement evenly throughout the mix – and this will produce weak concrete. To achieve a well-mixed batch every time it is far better to hire a concrete mixer from a tool hire shop.

Some experts recommend the order in which you should add the ingredients to the mixer should start with 75% of the water, followed by 50% of the aggregates. Next add all of the cement, then the remainder of the aggregates, and then the final 25% of the water.

In terms of how long to mix, aim for 2-4 minutes from when all the material (including the water) has been added. If you over-mix this can reduce the strength of the concrete and cause segregation of any coarser aggregates.

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