A Concrete Plan

By Dr Bill Price, National Commercial Technical Manager, Tarmac Cement

We’ve been packing cement and unpacking myths for years, which make us pretty expert on what makes the perfect concrete mix. But not everyone has as many years’ experience, and knows the pot-holes and pitfalls to avoid. Mixing together knowledge, experience and some professional myth-busting, we’re here to give you guidance on how to make a solid batch.

Wetter is not better

When making the perfect concrete, less is always more when it comes to water. A common misconception is that the best way to get more workable concrete is to add more water – this is untrue. When too much additional water is added, the mixture becomes unstable and the positive effects are temporary. Adding too much water will weaken the concrete, oversaturate it, and make it porous and hard to work with after it has settled. Too much water can lead to concrete cracking, which can cause a multitude of problems on building sites. There are a few fail-safe ways to prevent it happening. Firstly, cracking can be avoided by adding only just enough water, when mixing, to make it workable enough to place and tamp down. Secondly, if your concrete remains too hard to mix after the initial addition of water, use an admixture (plasticiser). This is a material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar. This will help to control setting and early hardening.

Concrete always cracks (or does it?)

Those that believe that concrete always cracks, need a few hints and tips! In addition to ensuring that the right amount of water is used, covering and protecting concrete after it has been mixed, known as ‘curing’, will allow it to gain strength and withstand shrinking without cracking. Curing plays an important role in the strength development and durability of concrete taking place immediately after concrete placing and finishing. But you can also avoid cracking by placing in joints before the concrete has been poured. Each bay should be no more than 1-2 times as long as the width. If a bend in the path is present, place another joint. After you have poured the concrete, then push appropriately spaced thin wood strips vertically into it. When the concrete has hardened, remove the strips. Now if cracks will appear they will be hidden at the bottom and out of sight.

All washed up!

For the perfect mortar, don’t fake it while you make it. Avoid the temptation of reaching for the bottle of washing up liquid that sits perched on your kitchen cabinet! There is a common myth in the production of mortar for laying bricks which claims that there is an advantage to adding washing up liquid to your mix. Although believed to be a viable alternative, dish-liquid is not a substitute for a specialised plasticizer and will end up doing more harm than good. Adding washing up liquid will also add large air bubbles to your mixture, reducing the strength and hindering frost resistance. Make sure you grab a mortar plasticiser to ensure it will mix well, stand strong, and last as long as the China plates in your cabinet.

When making your own concrete and mortar, remember: less water means more strength, take extra steps to ensure less cracking, and never substitute quality for anything, especially not washing up liquid!