Worktop High-Strength Wet-Pour Concrete Mix
The figures I'm quoting below are based on typical batch weights used by the Ready Mixed Concrete (RMC) Industry for a 5,000 psi concrete based on good quality 10mm aggregate (gravel, limestone or granite) and sharp sand.
It's important when creating concrete worktops that you design your concrete mixture with this application in mind. Too many people think that 'concrete is concrete' and that it is only the finishing process that makes the concrete suitable for use as a worktop material.
That assumption is quite simply - wrong.
It is absolutely essential to design the concrete mixture to give you a high-density, high-strength product in order to create truly great concrete worktops. If you design your concrete properly it will be stronger, more stain and water resistant and polish far more easily and to a higher standard. If on the other hand, you simply slap it together, then you will start to see problems within the first couple of weeks of use – that can be a very time-consuming and costly error. Get it right the first time.
The first factor to take into consideration is the cement type. Most bagged cement types, especially those found at your local DIY store, are General Purpose Cements containing limestone filler, these are much weaker than those used by the RMC industry. The General Purpose Cements are mostly (not all) classified as 32.5 CEM II/L or LL (the '32.5' refers to the nominal strength of the cement, 32.5 is the lowest strength cement type available and is really designed as a mortar or render cement). The cement used by the Ready Mix industry will generally be 42.5 for CEM II/B or CEM III/A (Ordinary Portland Cement blended with PFA or GGBS) or 52.5 for CEM I (100% Ordinary Portland Cement). Those products which carry the 52.5 specification are considered 'high-strength' cement and are the most suitable for use in creating concrete worktops.
Bagged 52.5 cement are available from larger builders merchants (most probably by special order). These products include "Procem" from Lafarge or "High Strength" from Hanson, Rugby and Dragon Alfa. Most white cement such as "Snowcrete" from Lafarge also carry the 52.5 specification and are often far more easily found than high-strength grey cement.
If you do purchase your cement from a DIY centre, check the date on the bag. Cement, especially those packed in paper sacks only has a shelf-life of six months. I have come across cement in some of the national DIY chains that have been a year out of date so buyer beware. Out of date material has absorbed moisture from the air and tends to clump, these balls of cement will not break-up during mixing and as a result, your concrete will have voids containing these unmixed balls. If you encounter these during the polishing process you will end up with sizable voids in your concrete which will spoil the finish as they will be difficult, if not impossible to fill.
Typical batch weights for a medium workability concrete are as follows (dry weight):
Cement 52.5 400 kg/m3
10mm aggregate 1000 kg/m3
Sharp sand 800 kg/m3
White Microsilica 28 kg/m3
PXR-Max Superplasticiser - 4.0Kgs (A 5.0litre jerry can of PXR-Max weighs about 5.5Kgs)
Water (mains) 160 - 175 litres/m3 (same as kg/m3) Please note that this quantity will depend on just how wet your sand and aggregate is when used.
Ideally, to minimise shrinkage/plastic cracking/surface dusting, it's best to incorporate a high-range water reducing plasticiser such as Cemcraft's PXR-Max (an absolute must if you plan on using Microsilica or fibres as without, the mix may well be unworkable). If used, this may also permit a slight reduction in cement content, say 5-10%. The water content will be reduced by about 20-25% (follow the admixture dosage guidance given by Cemcraft). Do not use a mortar plasticiser as they contain an air entraining agent as well as a plasticiser. The air will result in a lower concrete strength.
Notes on the materials listed above:
The above aggregate and sand weights will need to be increased slightly to allow for the 'as delivered moisture content' (how wet the material is when it comes to you) – If the material is delivered wet you will need to increase it approximately 2% for gravel (limestone and granite are generally delivered dry) and 6% for sand. Deduct the corresponding amounts from the water content. Note that the water content is to be used as a guide only and depends on the type/shape of aggregate, grading of sand, actual workability required etc. For instance, some types of sand and aggregates are smoother, more rounded and these types tend to aid workability as they will flow better (dredged sands tend to contain more rounded grains whereas quarried sand tends to contain more angular grains).
However, avoid the temptation to use aggregates such as pea gravel, it's cheap and easily found but the regular, smooth shape makes for a weaker concrete as the particles do not 'mesh' together as firmly. It's the irregular shape of the different size particles of sand and aggregate 'locking together' which makes for a stronger concrete. You can add another dimension to your worktops by utilising a granite sand with a high mica content as the particles of mica will reveal themselves during the polishing stages adding a little sparkle to the finished product
The aggregate and sand weights may also need to be adjusted slightly to achieve the finish required. If you deduct 50 kg/m3 from the aggregate add the same amount to the sand to keep the yield (volume) the same - personally I wouldn't adjust it by more than about 50 kg/m3.
I must stress that the only way to be absolutely certain of the concrete strength is to make test cubes and have them tested in compression (crushing) after 28 days. For obvious reasons, the strength cannot be specifically guaranteed.
Other factors that must be taken into consideration are:
Curing - keep the concrete moist after placing / demoulding using polyethene sheeting for at least 7 days.
Reinforcement - Regularly used is something called 'ladder wire'. It is available in a number of sizes although we have found that the 4.0 x 100mm x 2700mm size works very well and is usually pretty easy to find. This material is usually used to reinforce brick and block work. It is available in galvanised and stainless steel versions, either of which can be used for worktops although we would suggest stainless for anything used outdoors. It is quite easy to cut into sections for both length-wise and cross-wise placement and the sections can be fastened together with cable ties.
You could also mesh or fibres etc but seek advice as they are dependent on unit thickness and overall dimensions. Fibres WILL be visible if you plan to machine polish your worktop so we do not recommend either poly or AR glass fibres.