The many variables involved in the manufacture, placing and finishing of concrete mean that a good awareness of the major controlling factors is important. We have developed this guide to help provide a broad view of what needs to be considered. For more in-depth or specific information please contact your Holcim Concrete representative.
Ordering & Despatching
For every pour check the following:
- Firm and approved access is available for mixer trucks
- A wash out area is available for cleaning chutes
When ordering we require:
- Site address and access information
- The method of payment.
- The volume, mix, and slump required
- Truck timings (depends on the method of placement; barrow, skip or pump)
For large or difficult pours concrete pumps may be desirable. These can be organised through our despatch team.
Placing
We recommend using a member of the Master Concrete Placers Association (MCPA). Members of the association understand the correct steps required to provide a quality finish for long-term satisfaction. These steps are:
- Vibrate the concrete to ensure there are no voids in the concrete
- Screed the concrete to the correct levels
- Bull float to close the surface
- Use an evaporation retardant if required
- Power float the surface to increase durability
- Finish the surface as required eg broom, wooden float, exposed, polished.
- Cure the concrete
- Cut the concrete to reduce possible cracking
Finishing
The finishing process involves creating a dense hard surface to the concrete then applying the desired surface profile. For surfaces that will be subject to a high degree of wear this process is extremely important. Hand or mechanical methods can be utilised depending on the situation. An experienced placer will have the necessary equipment to produce the required finish.
Cutting to control cracking
It is a property of concrete that as it hardens it also shrinks slightly. Without cutting the concrete unsightly and uncontrolled cracking can develop. By cutting the concrete the cracks are induced where the cuts are, and hence are controlled. The general rule is that the cut should be 1/3 of the depth of the concrete
Curing
After the concrete has been finished, the most important and yet often the most ignored phase of the process must begin. Curing the concrete is the process of maintaining as much moisture as possible in the mix to fully hydrate the cement and thus give the best long term result. The cement-water reaction takes weeks to fully complete and maintaining the water molecules in the concrete for as long as possible is desirable.
The best method is wet curing ie keeping the concrete wet, for 7 days. On many projects this can be done with ponding. Other wet methods are keeping the slab wet with sprinklers or using plastic sheeting or sacking to reduce evaporation.
Where a wet system is impractical, curing compounds can be used which place a thin membrane on the surface and stop the moisture from evaporating.
Ponding
Build low wall and fill with water to cover slab
Spraying
Keep continuously wet to avoid crazing
Use low pressure supply to avoid any surface damage
Covering
Use wet straw, hessian, sand or sawdust (avoid staining)
Impermeable covering such as plastic sheet will trap moisture on concrete surface
Curing Compounds
Apply after finishing when bleedwater disappears
Apply twice at right angles to each other to form continuous film
Hot weather concreting
The summertime effect of wind, temperature and air humidity can collectively have a detrimental impact on the performance of the concrete.
Precautions to be taken:
- Organise with haste in mind and have sufficient staff to avoid delay
- Wherever possible protect the job with shade, wind breaks or construct the roof prior to laying the floor.
- After screeding use an evaporation retardant to reduce evaporation
- After finishing use one of the wet curing methods described earlier on in this brochure.
Cold weather concreting
The expansion of water as it freezes can cause serious damage to partly-set concrete. In cold weather cement hydration slows and hence the concrete takes longer to gain strength.
Precautions to be taken:
- Order low slump concrete with air-entrainer.
- Consider using higher strength concrete or accelerators
- Protect concrete from frost with covers or pond with water
