10.11
Hot Weather Concreting
Concreting at elevated temperatures accelerates the hydration process, leading to quicker setting but potentially reducing the long-term strength of t…
In dry and hot conditions, placing fresh concrete at high temperatures accelerates the setting and leads to rapid evaporation of water from the concrete, which can cause cracking.
Preferably, the concrete's temperature in hot weather conditions should not exceed 60 degrees Fahrenheit.
This is achieved by reducing the cement content, using Sulfate-free aggregates with less moisture absorption, incorporating set-retarding admixture, adding ice to the mixing water, and ensuring its complete melting before the concrete mixing is completed.
Additionally, on hot weather sites, precautions include shading the aggregate stockpiles from direct sunlight, spraying them with water to facilitate cooling by evaporation, burying the water pipes, and painting concrete handling equipment and tanks white to reflect sunlight.
Formwork is sprayed with water before the concrete is placed, and the placement is done during the cooler evening hours.
After placement, the concrete may be covered with white-colored plastic sheeting to protect it from the sun and rapid evaporation.
Lastly, in dry weather conditions, wetting the concrete and allowing evaporation of water facilitates good curing and cooling.
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Q1: Why does concrete crack in hot weather conditions?
High temperatures accelerate concrete hydration and setting, while low humidity causes rapid water evaporation from the surface. This moisture loss creates plastic shrinkage and crazing, leading to cracking. Additionally, accelerated setting reduces the concrete's long-term strength development, making it more susceptible to types of non-structural cracks in concrete.
Q2: What is the ideal concrete temperature for hot weather placement?
Concrete temperature should not exceed 60 degrees Fahrenheit in hot weather conditions. This target is achieved through multiple strategies: reducing cement content, using sulfate-free aggregates with low moisture absorption, incorporating set-retarding admixtures, and adding ice to mixing water. Ensuring complete ice melting during mixing is critical for temperature control.
Q3: How do you cool aggregate stockpiles on hot weather construction sites?
Shade aggregate stockpiles from direct sunlight to prevent solar heating. Spray the stockpiles with water to facilitate evaporative cooling. Additionally, bury water pipes and paint concrete handling equipment and tanks white to reflect sunlight rather than absorb heat, reducing overall concrete temperature.
Q4: What precautions should be taken after placing concrete in hot weather?
Cover freshly placed concrete with white-colored plastic sheeting to protect it from sun exposure and rapid evaporation. Ensure the evaporation rate does not exceed 0.25 kg/m² per hour on the concrete surface. In dry conditions, wet the concrete to promote both cooling and effective curing while controlling moisture loss.
Q5: How does reducing cement content affect concrete workability in hot weather?
Lower cement content reduces heat generated during hydration but decreases workability. This reduction can be compensated by selecting aggregates with low absorption rates and minimal impurities. Set-retarding admixtures also help adjust both workability and setting time, maintaining proper concrete consistency despite reduced cement.
Q6: When should concrete be placed during hot weather construction?
Place concrete during cooler evening hours on dampened formworks to minimize temperature rise. Concrete should be placed within 45 to 60 minutes after mixing to prevent excessive setting acceleration. Spraying formwork with water before placement further reduces concrete temperature and helps control the hydration process.
Q7: What role does ice play in hot weather concrete mixing?
Adding ice to mixing water helps control concrete temperature by absorbing heat during the melting process. However, complete ice dissolution during mixing is essential to ensure uniform temperature distribution and proper hydration. This method provides an economical way to maintain concrete within the recommended 60-degree Fahrenheit temperature limit.