Temperature Effects on Reinforced Concrete

Concrete Mix Design

• Concrete manufacturers design concrete mixes with various ratios of Portland cement (lime), water, stone aggregate, chemical admixtures and, sometimes, steel reinforcement. Hydration is a chemical process involving water that binds the cement and aggregate together. During the hydration process, the concrete heats up before it slowly cools and solidifies over a 28 day curing process. Various admixtures can accelerate the chemical bonding process and reduce the curing time. In hot climates, the contractor may use a set-retarder admixture to speed up the curing process and reduce the amount of water required in the concrete mix.

Hot Weather Effects

• Hot weather causes many problems in working with concrete. This includes complications from heat, high concrete temperature, low humidity, and solar radiation. Any of these variables can cause the too-rapid evaporation of water from the concrete resulting in shrinkage, cracking and decrease in the concrete strength. Engineers sometimes use ice in the mix design to keep the temperature down.

Thermal Conductivity

• Several of the temperature-dependent characteristics of concrete rely on the type of aggregate used and the composition of the concrete mix. Thermal conductivity is directly related to the ambient temperature. As the conditions heat up, the thermal conductivity of the concrete falls. During the early curing stage, the concrete temperature rises quickly with the hydration process, and it may dry out rapidly, resulting in thermal cracking.

Thermal Expansion

• Reinforced concrete contains steel fibers and steel rods (rebars) in the structure for strength. This allows the steel to take up the tension stress in the concrete structure; however the steel does not interfere with thermal expansion of the concrete. Many different aggregates can be used in the concrete mix design to add strength to the structure as it cures. Crushed rock is used in most concrete mixes because it has fewer quartz crystalline structures. If a crystalline aggregate is added to the mix, the thermal expansion increases with temperature up to a maximum temperature of 700 degrees Celsius. As the quartz crystals expand and crack with the heat of hydration, the concrete disintegrates and the concrete begins to flake off in small sheets. In nature, this occurs primarily in granite. The peeling off of layers is called exfoliation.