There are fundamentally two different approaches to creating a concrete worktop, a 'wet-pour' where you create a mould and fill it with concrete, and the GRC/GFRC - glass fibre reinforced method. You will find formulas for both methods here.
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. So far we have found one shop which will ship single bags of either Hanson White Cement or Hanson High Strength which is a grey 52.5 cement. If you cannot find it locally, try Build and Plumb Materials Online the link at left will take you to their online shop but you may also need to call them as not all products appear on their web shop. Ring them on 01228 635-022.
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 1 cubic metre of a medium workability concrete are as follows (dry weight):
Cement 52.5 - 400Kgs
10mm aggregate - 1000Kgs
Sharp Sand - 800Kgs
Grey or 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. If you cannot find this locally, try online at ReadyFix Online, Product No: K411-250.
I recommend that you watch this video by Jeff Girard of the Concrete Countertop Institute regarding the correct placement of reinforcement. Video Link. Many people think they should place the reinforcement in the centre of the slab, however, that is incorrect. Reinforcement needs to be placed relative to the stresses placed on the concrete so, for instance, in a worktop, the ladder-wire needs to be placed about 6mm from what will be the underside on the slab when the finished piece is in place. For a tabletop or a breakfast bar it would be placed about 6mm down from what will be the top surface of the finished piece.
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.
Manufacturing Worktops using the GRC/GFRC Method
The special glass fibre needed to reinforce concrete was developed by Pilkington in conjunction with the British Building Research Station. This resulted in the commercialisation of the first ever alkali resistant fibre which was branded Cem-FIL. This is still one of the leading AR fibre for Glassfibre Reinforced Concrete, GRC.
Another AR fibre is manufactured in Japan under the trade name ARG fibre and is sold by NEG. Recently, the Chinese have also developed versions of AR fibre mainly for home consumption but also for export.
Care has to be taken that the minimum quantity of Zirconium is above 16% otherwise long term performance will be affected.
The classic E-glassfibre used with resin systems CANNOT be used for concrete since the lime developed as concrete sets totally destroys this type of glassfibre so it is NOT suitable for Glassfibre Reinforced Concrete.
Glassfibre Reinforced Concrete (GRC), also known as Glass Fiber Reinforced Concrete (GFRC) is generally manufactured by either the "sprayed" process or the "premix" process. Premix GRC can either be vibration compacted, or manufactured using a self compacting GRC mix. The method chosen is normally dictated by factors such as strength requirements, size of mould, architects specification etc.
As a general rule, larger items, such as building cladding panels, are normally "sprayed" whereas small items are manufactured using a "premix" GRC method. Items such as concrete worktops can, however, be manufactured with either method.
Sprayed GRC is generally stronger than premix vibration cast GRC. The reasons for this are firstly that with sprayed GRC it is possible to achieve a fibre content of 5% - 6% whereas premix GRC is limited to around 3% - 3.5%. Secondly, sprayed GRC usually has a lower water content than premix GRC.
Typical mix design for Sprayed and Premix GRC, all items measured by weight NOT volume
Water+ 6.3->6.6Kgs (NB. +1.25kg contained in polymer if used)
Latex Polymer+ optional (2.5Kgs)
AR Glass Fibres+ 1->2.7 (AR Glass Fibres+ 4-6% by weight for sprayed GRC and 2-3.5% of weight for pre-mix GRC)
*The sand used should have a particle size not exceeding approximately 1mm and should be well graded. Material passing sieve No. 100 should not exceed 10%. The sand should preferably dry. If more sand is added, the bond to the fibre is reduced resulting in a significant loss of strength
**Dependent on type of fibre and property requirements.
+The water/admixture/polymer content will need to be adjusted according to materials.
The water and admixture (and polymer if used) are placed in a "high shear mixer" and the sand/cement are slowly added until a smooth creamy slurry is achieved. The consistency of the slurry can be checked using a simple slump test kit. Mixing time is about 1 - 2 minutes.
When ready the mix is transferred to a "pump/spray unit". The pump conveys the slurry at a regulated rate of flow to the spray gun. At the spray gun fibre, in the form of a roving, is chopped to a length of approximately 32mm and added to the slurry. The two materials are projected onto the mould surface using an air supply from a compressor.
The GRC material is sprayed and built up in thin layers until the required thickness is achieved - normally 10 - 15mm. Simple hand rollers are used to compact the material between layers.
The product is left in the mould and covered with polythene to prevent moisture loss until the next day. The product is then demoulded.
After demoulding the units are covered with polythene and allowed to cure for approximately 7 days. Alternatively, if a polymer curing compound is used in the mix the units can be exposed to the atmosphere immediately although it is advisable to keep them protected from direct sunlight or severe external conditions for a day or two. Reference should be made to the Polymer Supplier's instructions.
The sand and cement are mixed dry and then the water/admixture and polymer (if used) are added. Generally a two speed slurry/fibre blender mixer is used. With this type of mixer, the fast speed is designed to create a smooth creamy slurry. This takes about 1 - 2 minutes. The mixer is then switched to slow speed and fibre in the form of chopped strand (length approximately 13mm) is added slowly. The fibre is blended into the mix for approximately 1 minute.
Once the mix is ready, it is poured into moulds which are vibrated using a vibrating table.
The product is left in the mould and covered with polythene to prevent moisture loss until the next day. The product is then demoulded.
After demoulding the products are cured under polythene sheets to maintain moist conditions for approximately 7days. Alternatively a polymer curing compound can be used as described for the sprayed process.
Blend the water, polymer and plasticiser (if used) together. Add the sand. Start mixer, add cement and blend until lump free.
Blending in the fibre
The fibres are added depending on the production process being used. The 2 common methods are:
· Premix using pre-chopped 6 or 12mm fibre
· Chopping/Spraying process with purpose built equipment.
Premix The chopped strands are stirred into the morter mix until they are dispersed with no dry clumps visible.
The wet mortar is pumped to the spray head and atomised by air pressure.The continuous strands are fed into a chopper gun, cut to 35mm and simultaneously sprayed out with the wet mortar directly onto the mould.
Moulds For GRC
The appearance of the finished product is influenced by the mould material and the quality of the mould itself.
Moulds can be made from various materials such as:-
* Plastic (including melamine surfaced chipboard)
* Glassfibre reinforced resins
* GRC itself
* Rubber moulding compounds
A combination of materials is frequently necessary in order to give the desired stiffness, shape and surface finish.
All corners should have fillets, chamfers or rounded corners and mould release compounds to match the mould material being used.
Finishes For GRC
Basic finishing techniques:
* Mist Coats
* Face mixes
1-3 mm layer of the sand/cement mix [no fibre] is first sprayed into the mould and allowed to ‘firm-up’ before the fibrous GRC mix is applied. The appearance can be enhanced by acid etching or light sanding or abrasive blasting after demoulding.
A 4-6mm. thick cement/sand or crushed stone, pigmented mix is laid into the mould before the GRC backing is applied. The facing mix can be left ex-mould or, more usually, exposed by either:
* using chemical retarders placed on the mould then washing after demoulding.
* by acid etching after demoulding
* by mechanical exposure through abrasive blasting
* honing and polishing with diamond disks.