Quality Management and Avoiding Common Defects – Seasonal working with concrete

Winter working

Two different temperatures have to be considered when working with concrete in cold weather, firstly the ambient air temperature and secondly the concrete temperature at time of delivery. The definition of cold weather is a period during the day or night when the ambient air temperature will fall below 2°C. The BS and EN standard states that the temperature of the concrete should not fall below 5°C until the concrete achieves a strength of 2 N/mm² and that the temperature of fresh concrete shall not be less than 5°C at the time of delivery.

Therefore general good practice is to only place concrete when the ambient air temperature is 2°C and rising. However, following best practice and through early engagement with the supply chain, concrete could be placed down to 0°C with not unforeseeable risk.

• Ensure batching facilities have heated water and heated aggregate bins.
• Review sequence, logistics, materials and methodologies to ensure best practice is followed.
• Remove frost from rebar and formwork before pouring.
• Monitor the concrete temperature as it arrives on site. Reject any that is below 5°C.
• Temperature must be 2°C and rising (sometimes 4°C and rising, check specification).
• Apply frost blankets quickly at the end of the pour; avoid thermal shock when removing.
• Concrete will be frost damaged unless 2 N/mm² or stronger.
• Measure the temperature gradient for large pours; thermal gradient control (insulated shutters, quilts etc.) must be designed to control the thermal gradient.
• Concrete will ‘go off’ slower. Allow more time for slabs and beams to cure, and measure strength before striking.
• In wall pours control the rate of rise to keep shutter pressures as designed. Masonry and Render.
• Temperature must be 2°C and rising (sometimes 4°C and rising, check specification).
• Provide frost protection for minimum 24hrs.
• Avoid retarded mortar.
• Special admixtures can be used to accelerate the set.
• For rendered elevations, ensure the cement particle board, insulation and render are applied in quick succession; the CPB and insulation will deteriorate in a day or less if exposed to bad weather.
• Maintain moisture in wet mortar.
• Cover completed work with polythene to avoid desiccation.

Summer working

Again with concrete pours that take place in warm weather the same two different temperatures need to be considered, the ambient air temperature and the concrete temperature at time of delivery. Ambient temperatures up to about 20°C should not on their own cause significant problems, especially in damp or humid conditions but when the ambient temperatures of 20°C and above are in partnership to low humidity and drying winds consideration of a more efficient curing regime is needed.

The target temperature of concrete on delivery from the chute should not be more than 30°C.

• Make appropriate modifications to concrete mixtures to manage rate of slup loss, setting time and other characteristics. Retarders, extended set control admixtures, synthetic fibers or other proven local solutions may be helpful.
• The consistence of the material is affected. The slump/flow/slump flow of concrete reduces more rapidly. If there is water added to improve the consistence it decreases the concretes compressive strength, potentially increases permeability and ultimately affects the durability of the structure.
• The hot weather will accelerate the loss of moisture from the surface and therefore increase the risk of plastic shrinkage cracking. Consideration of some form of protection is needed. If using hessian and water, you will need to rewet the hessian frequently (up to four times per hour) to ensure that it is effective.
• On hot or dry days when the conditions are conducive to plastic shrinkage consider dampening the sub-grade, pans and forms and reinforcement prior to pouring concrete, do not allow water to pond however during and after this process.
• Check the weather forecast to pick the best time to pour. Consider starting large pours at 6am or earlier avoiding working during the hottest part of the day, often between noon and mid-afternoon.
• As the concrete temperatures increase the setting time reduces and impacts the time to place, compact and finish the concrete. Consideration of more labour is needed to achieve the work and schedule the rate of concrete delivery to avoid overloading the labour and/or equipment.
• Consideration of how to maintain the labour in hot conditions. On a hot day you need to have access to drinking water at all times to avoid exhaustion during manual work. Also, sunscreen should be applied before starting the pour.
• Keeping equipment out of direct sunlight until it is required to lay the concrete is a small but effective measure. This will stop the equipment from heating the concrete while it is being poured.
• Concrete will go off quicker. Begin final finishing operations as soon as the water sheen has left the surface. Apply curing measures immediately on finishing an area; on a large slab this will be while pouring is still in progress in other areas.
• In large elements or rich mixes faster hydration of cementitious materials down to high ambient temperatures can result in higher maximum concrete temperatures and thermal cracking.
• Changes in temperature of the concrete may also result in cracking particularly where concrete is placed on a hot day followed by a cool night.
• With the increased rate of hydration the surface of the concrete will dry quicker which leads to premature finish being applied, trapping of bleed water and possible debonding of the top surface with subsequent flaking/de-lamination
• Delays in transport should be minimised although in numerous cases this is a difficult task.

None the less when it comes to planning any pour with any amount of concrete you will need the knowledge and advice of an expert. Whether that is you, a contractor or your concrete supplier depends on your project, what you are doing and how much risk it entails. To avoid pouring concrete incorrectly careful consideration and forethought is a must.