What is warm weather?
For the purpose of concreting operations, warm weather can be defined as when one or more of the below conditions exist. These conditions can easily lead to significant permanent impairment of the concrete quality through an accelerated rate of moisture loss and cement hydration
- High ambient temperature
- High concrete temperature
- Low relative humidity
- High solar radiation
- Drying wind
There is no specific temperature which would be considered an action point for warm weather concrete. Ambient temperatures up to about 20°C should not on their own cause significant problems. Ambient temperatures of 20°C and above with lower humidity and drying winds present a high risk to long term concrete quality. Lower ambient temperatures on days with a strong breeze also present a high risk.
The Irish national Annex to EN206 requires that the temperature of fresh concrete does not exceed 30°C unless the user specifies otherwise. Note that the temperature of fresh concrete in the delivery truck or freshly placed concrete onsite may exceed ambient temperatures.
Potential impacts of warm weather on concrete
With fresh concrete the main issues that may arise are:
- Increased rate of slump loss leading to requests to add water to the concrete onsite
- Accelerated setting which may create issues with handling, placing, compacting and finishing the concrete
- Unpredictability with setting time in such conditions and experience of concrete operatives with such factors can lead to unsatisfactory finished concrete elements
- Increased risk of cold joints
- Potential difficulties maintaining specified air content in air entrained concrete mixes
- Very high potential for plastic shrinkage cracking, particularly on flatwork such as slabs and yards
Issues which may arise with hardened concrete in such conditions generally cannot be rectified and include:
- Permanent loss of compressive strength
- Increased potential for drying shrinkage
- Higher potential for thermal cracking either due to cooling of the overall structure, temperature differential or thermal shock due to high differential from daytime to nighttime temperatures
- Increased permeability, reduced durability and reduced water tightness where applicable
- Surface scaling, laitance or delamination due to rigid moisture loss from the surface
Measures to minimise risk
Concreting operations need to be carefully planned and executed when warm weather conditions occur to prevent permanent damage to the concrete. There are a number of practical measures which can be implemented and include:
- Consider placing concrete outside peak temperature times. Critical work should be postponed and carried out in ideal environmental conditions
- Plan and communicate delivery and demand carefully. Concrete should not be sitting in trucks agitating where it will rapidly lose workability
- Do not add water to concrete onsite. Where required order a concrete mix with increased workability suited to the task at hand
- Discharge, place, compact and finish concrete without delay. The two hour ‘rule of thumb’ on the workable life of concrete will not be a suitable measure of usability in these conditions.
- For flatwork, where possible the placement of concrete should be done after roofing and erection of walls to minimise solar gain and drying winds
- When discharged directly onto sub-base materials the sub-base must be fully damp to avoid loss of water from the concrete
- Protect the concrete from rapid moisture loss during any delay between placing and finishing operations
- Cure the concrete immediately and fully. Curing will need to be continuous for a number of days after placement of concrete
- A detailed and planned curing procedure must be in place in accordance with EN 13670 clause 8.5