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Concreting in Warm Weather: Additional Precautions for Contractors

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Northern Ireland is known for its temperate and often damp climate, but as we saw during the recent spell of great weather, conditions can occasionally shift. While these warm, sunny days are welcome for most, they can present real challenges on construction sites - particularly when it comes to working with concrete.

When temperatures rise, it's not just about staying cool on site. The behaviour of concrete can change significantly in warm weather, impacting everything from workability and setting time to strength development and surface finish. It’s critical that contractors understand these risks and take appropriate measures to ensure quality and durability are maintained.

Why Warm Weather Matters

Hot weather concreting is not just about temperature. It's the combination of high ambient temperatures, high solar radiation, low relative humidity, and high wind velocity that accelerates the hydration of cement. This can result in:

  • Reduced workability
  • Accelerated setting times
  • Increased risk of plastic shrinkage cracking
  • Greater permeability due to surface crazing
  • Lower long-term strength development


These effects can significantly impact both the fresh and hardened states of concrete, and proper planning is essential to mitigate the risks.

Temperature Considerations

Two key temperatures must be monitored when concreting in warm conditions:

1. Air Temperature

  • Up to 20°C: Generally manageable, especially in humid or damp conditions.
  • Above 20°C: If accompanied by low humidity and drying wind, more efficient curing is required.
  • Thermal shock: Rapid temperature drops at night after a hot day can cause cracking in exposed concrete.


2. Concrete Temperature

  • BS EN 206 specifies a maximum delivery temperature of 30°C.
  • Typically, concrete temperatures can be 5°C higher than the mean ambient temperature.
  • In rich mixes or large pours, internal temperatures may spike, increasing the risk of early age thermal cracking.


Concrete Mix Adjustments

To manage temperature and control setting times, consider the following steps:

  • Time your pour: Schedule early morning or evening pours to avoid peak heat.
  • Admixtures: Water reducing and set retarding admixtures can help maintain workability (seek technical advice).
  • Modify slump class: Use a higher slump class to accommodate faster slump loss in hot conditions (seek technical advice).


Best Practices for Placing and Curing

  • Minimise transport delays: Keep concrete moving efficiently from batching plant to site.
  • Fast action: Place, compact, and finish concrete quickly.
  • Surface protection: Use curing membranes, mats, or covers to reduce moisture loss and solar gain.
  • Do not add water on site: This reduces strength and durability.
  • Moisten surfaces: Especially important for dry or absorbent substrates and construction joints.
  • Coordinate logistics: Ensure delivery volume and pace align with site resources to avoid placing delays.


Testing and Quality Control

  • Cube curing: Store cubes in moist conditions at 20°C ±5°C, and transfer demoulded cubes to a curing tank at 20°C ±2°C.
  • Consider maturity methods: Use temperature matched curing and maturity calculations for accurate in-situ strength tracking in critical elements.


Conclusion

Even though hot weather may be rare in Northern Ireland, being unprepared can lead to long term issues with concrete performance. By understanding the risks and implementing the right procedures from mixing to placing and curing, contractors can maintain high standards, even in the heat.

For more technical advice or to discuss your next concreting project, get in touch with the RTU team. We’re here to support you with expert knowledge and high quality concrete solutions.

DISCLAIMER: These guidelines have been prepared alongside the MPANI Concrete Development Group. Every care has been taken to ensure that the information contained herein is correct and accurate at the date of publication. However, the RTU cannot accept any responsibility or liability for any errors, inaccuracies or omissions which may have occurred inadvertently.