Getting best results from concrete with admixtures

Press, 6 August 1987, Page 26

Getting best results from concrete with admixtures

There are a great many admixtures on the market, all intended to emphasise certain characteristics of your concrete. There is no doubt that if they are used as intended they can offer substantial benefits.

Unfortunately if they are incorrectly used the results may well be disastrous.

In most cases the handyman has neither the understanding nor the sophisticated equipment to enable him to exercise the care necessary, and if he wants to use admixtures at all he must proceed with caution. There is something like half a century in the development of the admixture technology. Most of the manufacturers today have been in the business for a very long time. /

They employ skilled scientists and technicians in modern laboratories. They provide precise instructions on how to use

their products to achieve particular results. Against that background it is illogical to add a squirt of something which has been specially formulated for washing dishes. Remember the chap down the road who ‘reckons it is pretty good stuff probably knows a lot less about concrete than you do — but he will never admit it.

Any concrete technician will give the same advice regarding admixtures — and this will include any responsible admixture supplier:

1. Adjust your mix proportions, and your mixing and handling operations so as to get the best you can from the materials you are using — before you start looking to admixtures.

2. If you then require some further improvement, identify the effect you are wanting and select an admixture designed for that purpose.

3. Follow the instructions meticulously. 4. Under-dose rather than over-dose. That advice is valid whether you are building the country’s largest hydro-electric dam or setting out on the smallest home handyman project. Types of admixtures Assuming that you have conscientiously followed the four steps listed you may wish to consider the types of admixture you could be using. A very general summary of the main types is as follows: Workability aids

All concrete contains about twice as much water as is necessary for the chemical reaction with the cement. This surplus water is needed as a lubricant during mixing and placing, but the voids left behind as it ultimately evaporates will cause a significant loss of quality, reduced strength and durability etc. If the quantity of surplus water can be reduced a better concrete will always result. This may mean an adjustment to the mix proportions, the use of a vibrator to compact a drier concrete, or the use of a waterreducing admixture. Workability aids may

be sold as water reducers or plasticisers. Their benefits lie in the fact that they offer greater workability without increasing the water content, or they allow a reduction in total water without loss of workability. You should not get involved with super-plas-ticisers. These are very special products with quite striking effects but they are right outside the scope of the handyman. Likewise air-entraining admixtures can give very impressive benefits but call for sophisticated controls which are beyond the handyman. Accelerators

As the name suggests these are intended to speed up the reaction. In general there will be little genuine need for them in handyman applications. It may be felt for instance that they would have advantage where an early frost is expected, but better planning of the job may allow you to start concreting early in the morning instead of after lunch. However, they do have benefit in providing an early strength gain, rather than a fast set. The strength level which would normally require say seven days, may be reached in perhaps three or four days. This means boxing can be removed and the concrete brought into service earlier.

This is one admixture which calls for particular care. Many accelerators contain calcium chloride, and reinforcing steel does not like chlorides. If the recommended dosage rate is not exceeded, and if all other principles of good concreting are followed, there should be no problem. If there is insufficient concrete cover to reinforcing, or if the concrete is of inferior quality or is not fully compacted, you will have a much greater chance of experiencing rusting problems when calcium chloride is also present. Retarders These are intended to delay the setting rate and can be particularly useful in certain circumstances.

The most common need for a retarder would be when using ready mixed concrete with a small team to handle the placing and finishing. Asking the concrete supplier to incorporate a retarder can save a Jot of panic on the job. He will know how much chemical to use to give the degree of retardation you require. You should remember that while you are waiting for the retarded concrete to stiffen sufficiently for trowelling, it can be losing moisture to evaporation. On a hot day, particularly with a breeze, this early moisture loss can lead to plastic cracking. Use hessian screens to protect the concrete from the wind, cover it with polythene, or have a fine mist sprayer operating alongside it. Waterproofers

The best waterproofer available is a good dense concrete. You must use good clean, well-graded aggregate, with adequate cement content and minimum mixing water. It must be thoroughly compacted without segregation or honeycombing, and then must be cured with water for as long as possible — a minimum of three days and preferably seven days.

If you want a further improvement you are advised to spend your money on something which will enhance the quality of the concrete. A water-reducer is recommended in preference to a water-proofer. Mortar plasticisers It is very difficult to work with a straight sand/ cement mortar for brick or block-laying. In earlier times lime was added, but today a proprietary mor-tar-plasticiser is used. Cocktails There should be no need to ever mix different types of admixture for the handyman projects. It is important to identify the characteristics you want to enhance and select a single material to suit. If for instance yoii want an accelerator with water reduction, it is possible to buy a product formulated for those twin objectives.

Do not attempt to use an accelerator in combination with a separate water reducer since the individual chemicals may not be compatible. The recommended dosage rate is usually given as litres of the liquid per 100 kg of cement. The home handyman can

gauge the size of his mix from the fact that a four litre paint tin filled with cement would be approximately 6kg. If the recommended volume of admixture for 100 kg of cement is diluted with water to 10 litres, one milk bottle (600 ml of that solution will have the

correct dose for a 4 litre, paint tin full of cement One milk bottle of this solution would be added to the mix, then sufficient fresh water is gradually added to give the desired workability. If your mix has two paint tins of cement you will need two milk bottles of solution.