Concreting in hot weather is one special situation every civil engineer might encounter in their carrier. Even though “hot weather” implies high ambient temperature, the real situation might be severe due to low humidity, excessive evaporation rate, and high wind conditions.

Effect of hot weather on concretes

Both the properties of fresh and hardened concrete are affected by hot weather conditions.

Fresh concrete

Slump loss

Hot weather most significantly affects the slump of fresh concrete. It is natural to lose the consistency or the slump soon after the mixing of concrete. It is the beginning of cement hydration cause this gradual reduction of consistency. However, hot weather conditions accelerate the slump loss.  Even you expect the workability to remain for a few hours, and accelerated slump loss will reduce the workability drastically.

Workability & Flowability

This results in difficulties in pumping, consolidating & surface finishing works. There can be blockages in the pipeline and pump breakdowns due to the workability loss.

Slump loss will reduce the flowability. Your concrete may not flow through reinforcement and bind well. It will limit the performance of the concrete. Consolidation difficulties make it harder to control entrapped air content causing vulnerable to freeze-thaw attacks.

Shrinkage cracks

Low humidity and dry wind will evaporate the topwater and dry out the top surface fast. You will need more labor to finish the surfaces within a short time. Further evaporation can cause shrinkage cracks. These are not structurally significant, but it will increase permeability as well as abrasion possibility. Shrinkage effect propagates as a bunch of shallow cracks weakening the surface layer.

Thermal cracks

Thermal cracks are another story compared to shrinkage cracks. The risk of thermal cracking is developed with hydration heat. 1 to 3 days after the placement is the most critical time period for thermal cracks. The environmental condition throughout time can affect thermal cracks. Placing temperature, peak hydration temperature, and the environmental temperature has a significant effect on thermal cracking.

These cracks propagate from the center and spread through the lift. Hydration heat is dissipated by these cracks separating concrete into pieces. Therefore the thermal cracks are harmful structurally as well as durability aspects.

Hardened concrete

Concrete poured in hot weather may be defective even after hardening. Excessive evaporation rate will impact the hardening process, failing to achieve target strength at 20 days. Keeping in mind all the challenges you have to face, proper planning is necessary prior to work. The ideal range of concrete placing temperatures is 10°C – 15°C. If the average temperature at your site is above that during the concreting time, follow these precautions.

Guide to hot weather concreting

Prior planning

  • Do some weather forecast and make sure what are the environmental conditions you are facing
  • Use the evaporation chart to estimate the evaporation rate. Evaporation rate between 1 – 0.5 (kg/m2/h) is considerable. Shrinkage cracks may happen.
  • Perform trial tests. Have some experience with the mix. Try to synchronize the batching, transporting, and placing times.
  • Calculate placing temperature & arrange pre-construction meetings to discuss possible ways of reducing placing temperature. Estimate fresh concrete temperature using the equation

 Mcem = Mass of cement in kg/m3

 Mscm = Mass of supplementary cementitious material in kg/m3

  • Try to estimate the peak temperature. Use the PCA method for concrete having cement content of 300-600 kg/m3 and the least dimension of 1.8 m.
  • Use chill water to reduce the fresh concrete temperature. You can even use crushed ice, but it should fine enough to completely melt during the mixing.
  • Adjust the setting time by using retarding admixtures.
  • Try to use low heat or pozzolanic Portland cement. These will lower the peak hydration temperature and the hydration rate.
  • When placing thick sections, it is harder to maintain the internal temperature controlled. Arrange some post-cooling measures. We used 1m by 1m post-cooling grid in dam constructions with 20°C water having a flow rate of 20 liter/min.
  • Add supplementary cementitious materials to the mix. Fly ash and ground granulated blast-furnace slag are common materials. These will increase workability as well as hydration time, which is favorable in hot weather concrete construction.

Site arrangements

  • Prepare concreting surfaces. Do not place concrete on a hot surface. This can induce cracks at the bottom. Sprinkle some water and wipe it with pressurized air.
  • Cool the aggregates
    • Cover the aggregates
    • Do frequent moistening
    • Use aggregate coolers
  • Use more of steel forms. Wet the outside frequently to control the temperature
  • Erect some wind barriers to slow down the wind speed. Typically 3 meters is enough. But it can vary with the situation.
  • Arrange sunshades to cover the concreting area.
  • Arrange the concrete pouring pattern to cover the area in a short time.
  • Start curing as early as possible
  • Keep wetting and cooling forms for 2-3 days until the heat dissipates
  • Try to maintain cooler environmental temperatures than concrete. But do not let the difference more than 20°C.

Summery

It is critical to control the fresh concrete temperature and environmental temperature when concreting in hot weather. Make sure the concrete is not dry out before finishing the surface. Use the above list of recommendations and refer ACI 305R-10 for more details.

kalhara

Kalhara Jayasinghe is a civil engineer currently engage with hydropower construction works in Sri Lanka. He has completed his bachelor's degree & master's in structural engineering from the University of Peradeniya and achieved chartered engineer title in 2019 from the Institute of Engineers Sri Lanka.

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