I know many of you guys know (and over the years have learnt!!!) the answer to this question. Nonetheless Dr Concrete often gets asked how much CSH should be used on an installation and the reasons why. So, I apologise in advance that this Dr Concrete may seem a little bit long winded. (But I can assure you, not as long winded as I could make it!)

The two factors that will affect the amount of CSH that will be / needs to be used are:

• Surface wetness /softness of the concrete. (mainly the wetness)
• Prevailing weather conditions.Consequently, wet concrete on a wet day will require more colour than dry concrete on a dry day to achieve the same surface consistency and colour.

Using too little CSH in relation to the above two points will create three potential issues:

• Colour will not be true. e.g. a red colour may look pink, or a black colour may look grey.
• Using insufficient CSH will not ‘dry up’ the surface. There will then be a poor water / cement ratio at the surface and consequently the long term abrasion resistance and freeze thaw and de icing salt resistance will be compromised.
• Thin coloured wearing surface.

The ‘poor water cement ratio’ and ‘thin coloured wearing surface’ issues will manifest themselves further down the line regarding long term durability of the paved area.

However the colour not being true is an instant issue. This can (and does) create problems, particularly on multi pour installations. Sure, on one pour installations, if too little CSH is used and the colour is not true and / or is a bit patchy, since there is nothing to compare it with, this may not be noticed by the end user and the installation company will ‘get away with it’ as the area still looks attractive and aesthetically pleasing.

HOWEVER on multi-pour installations, colour differences between individual pours are noticeable, and it is important that areas are coloured appropriately to avoid any heartache, which is very often difficult to resolve.

Therefore two points are recommended as good standard practice:

• Most importantly, always ensure that the concrete is coloured generously either side of a day joint. This will greatly reduce / eliminate a potential colour difference either side of the day joint, which is always the most noticeable area on any installation.
• In addition to the above point, why not use a bit more CSH on the whole of the slab so that it’s all a truer colour and a more durable surface is created.

So, the wetter concrete, the more CSH that is required.

But, how do you know when enough CSH has been applied?

A very good guide is that if the CSH is very easy to float in, then probably the surface is still a bit too wet / not enough CSH has yet been applied. In these situations another application is recommended. As soon as an application of CSH becomes a bit harder work to float in then that is an indication that the surface has been ‘dried up’ and the resultant surface is more likely to be a true reflection of the intended colour. This guide, when followed, not only addresses potential colour variances, but also produces a good water cement ratio which in turn produces a more durable and long lasting surface with a thicker wearing layer.

So, how much CSH to order for a job? All jobs will vary, depending on concrete and climatic conditions. When asked, Dr Concrete always recommends:

Always take to site 1 bag of CSH per 10m2, plus 50%.

So, for a 60m2 job, take 6 bags, plus 50% = 9 bags. Dr Concrete would be surprised if only 6 bags were used and equally surprised if all 9 bags were used. Most times it will be around 7 or 8 bags to produce a quality slab which is true in colour and has good wearing characteristics.

Remember. You can always take a left over bag back with you to use another day. However, if the concrete comes a bit wet and / or climatic conditions are damp, and you need a bit more CSH then it is difficult to get hold of an extra bag when it’s needed. Look at the extra CSH as an insurance policy.

Finally, even if you do use a bit more CSH than the minimum guide of 1 bag per 10m2 and it costs you a bit extra, what’s one or two bags on a 60m2 pour? Nothing! (or should I say very little!!!)

In fact, if it means one less return visit to site for a remedial (or worse still a dig up), then you’ll be ‘quids in’ AND have better looking, more durable work and happier customers.

Thank you for reading it all,

Dr Concrete.