Ever Wonder Why Concrete Gets Hot? Here’s Why

Concrete Trowel Mix

Have you ever walked barefoot on concrete under a scorching summer sun? It can get quite hot. That’s because a great amount of energy is created by the ground as the sunlight shines onto it. When our bare skin comes into contact with the concrete, we immediately feel it.

The ingredients of concrete generate a thermal mass that absorbs sunlight and heat that falls upon it. This generates energy, which is given back off into the air in the cool of the night. This happens either with direct sunlight or other forms of heat absorbed by the concrete.

Concrete is utilized around the world to pave much of the ground we tread on. What is the reasoning behind the temperature changes it goes through depending on the weather above it? It all has to do with the ingredients of the concrete itself.

Why Does Concrete Get Hot?

Concrete gets hot due to its contents: water, cement, and aggregate (sand, stones, and/or gravel). When sunlight shines on the surface, this combination of ingredients goes through a chemical reaction that generates a thermal mass, absorbing the heat given off by the sun.

The heat of concrete is also dependent upon the stage in which the contents are setting and curing.

Ingredients of Concrete

Concrete is made up of the following components:

  • Water
  • Portland cement (a general type of cement made up of materials such as limestone, sandstone, marl, shale, iron, clay & fly ash)
  • Aggregate (sand, stones, and/or gravel)

When concrete is made, a chemical reaction occurs between the Portland cement and water. This is the basis for the reason concrete in its completed state absorbs heat.

How Concrete is Made

Concrete needs to set and harden before reaching its optimal state of function that we see as our everyday sidewalks. As mentioned above, a chemical reaction occurs in the making of concrete, when Portland cement and water are mixed. The cement is added to water, and then hydroxyl ions are released from the dissolving cement.

When water and cement mix, this process is called hydration. When the cement dissolves, it increases the levels of calcium and silicon in the solution. New solid products are then formed through a precipitation reaction.

The new strength obtained by the mixture and water is determined by how much space can be filled up in between the cement and hydration products that were created. Though concrete hardens within a few hours after this process, hydration may continue for weeks, and even years afterward.

How Hydration Affects the Heat of Concrete

“Hydration is an exothermic process which generates heat through chemical reactions.” Source: HOW IT WORKS: Concrete Hydration produces heat as it continues setting and curing.

A common misconception is that the concrete is drying–it is not. It is actually continuously going through chemical reactions that alter the water molecules, creating solids. This increases the overall temperature of concrete.

How to Reduce Heat in Concrete

Depending on the building design or structure type being created with concrete, there may be different steps when it comes to heat regulation. With normal-sized structures, like sidewalks, the concern may not be too great, as heat can dissipate through the soil or air.

With much larger structures, such as dams, the process requires some extra measures. If temperatures get too high, the structures may crack as the heat causes consistent internal expansion that it cannot maintain.

With such structures, “the maximum temperature differential between the interior and exterior concrete should not exceed 20 degrees Celsius (36 degrees Fahrenheit) to avoid crack development.” – Source: Portland Cement, Concrete, and Heat of Hydration

Why Hot Weather Can Cause Problems for Cement

When it comes to setting concrete, hot weather causes water evaporation. It is, therefore, necessary to mix in more water in order to enable the cement to react with it to create ample, sturdy, and durable concrete.

The rate of temperature rise, aggregate type, and stability can all influence how concrete functions at higher temperatures. Abrupt temperature changes can lead to the concrete developing cracks due to thermal shock. Distress can also arise in the concrete.

When working on a concrete set, it is important to keep track of water evaporation due to environmental heat and sunlight. Without ample water, the cement cannot bind, and the paste-aggregate bond is weakened.

Ways to Cool Down Concrete for Construction

According to this article on the USDA website, here are some ways that construction workers can use to reduce concrete temperatures that “decrease water demand, slow slump loss, increase setting time, and reduce the chance of plastic shrinkage cracking”:

  • Use fly ash and water reduces as cement replacement to minimize cement content
  • Use retarders to control fast setting, also to be used in cases where long haul times are unavoidable
  • Use third-generation high-range water reducers instead of water
  • Substitute Type II moderate heat cement for Type I normal cement
  • Shade aggregate stockpiles and wetting of cement to promote evaporation and cooling
  • Use chilled batch water in extremely high temperatures
  • Avoid long truck waiting periods
  • Use 100 drum revolutions when mixing, set at the speeds put in place by the mixer manufacturer
  • Delay mixing until the truck reaches the jobsite (primarily for long hauls in hot, humid weather)
  • Paint mixer drums white, so they don’t absorb much heat
  • Schedule deliveries to avoid the hottest times of the day
  • Avoid adding extra water at the job site

Building Designs Made of Concrete

There are a number of different types of buildings and structures around the world that are made from concrete. Some of these include:

  • Walkways
  • Bridges
  • Towers
  • Dams
  • Some ancient architecture

Those listed above are just a few of the many types of structures made out of concrete. As mentioned, concrete gets hot not only during the setting and curing process but also from the accumulated and absorbed heat from the surrounding weather conditions and direct sunlight.

Shrinkage and Cracks in Concrete

The next time you take a walk down a street made of concrete, take a look closer look at it. You might see quite a few cracks–some larger, some smaller.

This is a normal phenomenon, not one to be worried about. But when you consider larger structures, there has to be more consistent care and maintenance to make sure the concrete is intact and sturdy.

Shrinkage

Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water.” (Source: Drying Shrinkage) As the concrete slab continues to cure as time moves on, it loses moisture in the process, causing the piece to get smaller.

Again, with regular concrete walkways, it is not too significant, especially if the resulting cracks from the shrinkage are relatively small or average in size.

But with larger buildings or structures made out of concrete, it is important to consider what other structures are holding the concrete in place (such as stabilizing joints, for example).

Shrinkage in such buildings requires routine maintenance. An example, in detail, of how to maintain the concrete of masonry walls can be found here.

Cracks

Cracks in concrete often happen when too much water is added to the mix. The reason why more water is added is generally to make residential work a bit easier to accomplish. That said, the additional water does not necessarily make for a more stabilized and crack-free structure.

Shrinkage is the main cause of cracking, and when shrinkage occurs, the slabs of concrete are pulled apart, causing cracks to form.

So, Why Does Concrete Get Hot?

You may be wondering what cracking and shrinkage have to do with the temperature of concrete. They are actually closely related. Too much heat causes rapid water evaporation in the concrete, which leads to shrinkage and then cracking.

In areas with higher heat and humidity, the maintenance of concrete structures may require more regular attention for this reason. Now you know what is happening beneath your feet as you stroll to the park or sprint to school.

The heat on the ground is not only being generated by the sunlight, but also by the chemical reactions taking place within the concrete itself, as hydration takes form to create a sturdier structure.