Concrete cooling: chillers, ice and cooling pipes for mass concrete

Concrete cooling is used in large pours to keep under control the heat released when cement reacts with water (hydration). The temperature of fresh concrete is lowered by cooling the mixing water with a chiller or by replacing part of the water with ice; after the pour, chilled water is passed through pipes embedded in the concrete. The aim is to limit the concrete's internal temperature and the difference between core and surface, and so prevent thermal cracking. In dam bodies, bridge piers, thick raft foundations and energy structures, this is often a clause in the specification.

Why does temperature matter in mass concrete?

Cement hydration gives off heat. In a thin slab this heat soon dissipates into the air; in a block several metres thick, it stays inside. Within days, the core temperature can rise tens of degrees above the placing temperature, while the surface, in contact with the air, cools faster. As the core expands, the surface wants to contract; the difference creates tensile stress at the surface, and concrete that has not yet reached sufficient strength cracks.

The second risk comes during cooling. As the core cools and contracts over the following weeks, it is held back by the foundation or by the layer poured beneath it; this restrained contraction can now open cracks that run through the section. A very high internal temperature can also lead to chemical problems that impair the concrete's long-term durability, such as delayed ettringite formation (DEF).

Mass concrete specifications therefore usually set two limits: about 70 °C for the maximum internal temperature, and about 20 °C for the difference between core and surface. The exact values are set by the project specification and its thermal analysis; the cooling system is designed to meet these limits.

Cooling is not the only measure; in most projects the mix design and the pour plan come first. Using low-heat cement, replacing part of the cement with fly ash or ground granulated blast-furnace slag, dividing a large volume into lifts and avoiding the hot hours by pouring at night all lower the internal temperature. The cooling system closes the gap these measures leave, which is why the mix design also determines how much cooling is needed.

Cooling fresh concrete: chilled water and ice

The most direct way to lower the placing temperature is to cool the materials that go into the mix. The specific heat of water is more than four times that of aggregate and cement, so although water is a small share by mass, its temperature has a marked effect on the concrete. Ice is even more effective: one kilogram of ice absorbs about 334 kJ as it melts. That is enough energy to heat the same amount of water by 80 degrees.

Example: assume that 1 m³ of concrete contains 1,900 kg of aggregate (30 °C), 350 kg of cement (50 °C) and 170 kg of water. Take the specific heat of aggregate and cement as 0.92 kJ/kg·K, and ignore the moisture in the aggregate and the heat generated during mixing. A simple heat balance gives:

• with water at 25 °C, the fresh concrete is at about 31 °C,
• with the water chilled to 5 °C, it drops to about 26 °C,
• with 50 kg of the water as ice and the remaining 120 kg as water at 5 °C, it drops to about 19.5 °C.

The effect of chilled water is limited, because the amount of mixing water is fixed by the mix design. For larger reductions, ice, shading and sprinkling the aggregate, and on big projects cooling the aggregate with cold air or using liquid nitrogen are applied. Cement arriving hot from the silo also enters the calculation. Which method is chosen depends on the target placing temperature and the daily volume of concrete.

Embedded cooling pipes

To remove heat from inside the concrete after the pour, steel or polyethylene pipes are laid between the lifts. Once the concrete is placed, water is passed through these pipes for days, and in large structures for weeks. The best-known early application of the method is the Hoover Dam, built in the 1930s: the body was poured in blocks, and first river water, then water chilled in a refrigeration plant, was circulated through pipes embedded in the blocks.

Supplying the pipe water as cold as possible is not the right approach. If the difference between water and concrete is too large, local cracks can open around the pipes, and the cooling rate can exceed the limit the specification allows. That is why:

• the water temperature is lowered in steps according to the measured concrete temperature,
• the flow direction is reversed at intervals so the block cools evenly,
• the chiller set point is adjusted using readings from temperature sensors embedded in the concrete.

In arch dams, pipe cooling has a second task: before the joints between the blocks are grouted, the concrete must be cooled to its permanent operating temperature. In this phase, the aim is not to clip the peak temperature but to bring the whole body down to the target temperature in a controlled way.

The pipe spacing, water temperature and cooling period come from the structural engineer's thermal analysis. Our part of the job is to select the unit that will produce water at the temperature and flow the analysis calls for, without interruption.

Cooling in ground and stone works

Cooling is not limited to concrete pours. When tunnels, shafts or deep excavations are made in soft, water-saturated ground, the ground freezing method is used: brine or glycol, usually below −20 °C, is circulated through pipes placed in the ground, the water in the soil freezes, and a load-bearing, watertight wall of ice forms around the excavation. What this application needs is a low-temperature circuit that runs without interruption for weeks; the low-temperature version of our industrial chiller units goes down to −30 °C with glycol. In such a project, the suitability of the unit is assessed against the temperature and duration the geotechnical engineer requires.

In screed, coating and stone laying work done in hot weather, the problem is that the mortar or concrete loses water too quickly and stiffens early. Surface cracks and poor bond often start here. The first measure, again, is to cool the mixing water and to protect the surface being worked from sun and wind.

How is a chiller sized for a construction site?

For mixing water cooling, the load is calculated from the hourly concrete output. Example: the batching plant produces 60 m³ of concrete per hour, and each cubic metre takes 170 kg of water. The power needed to cool 10,200 kg of water per hour from 25 °C to 5 °C follows from Q = m · c · ΔT: 10,200 / 3,600 kg/s × 4.18 kJ/kg·K × 20 K ≈ 237 kW. When ice is used, there is less water to cool; with 120 kg of water per cubic metre as in the mix above, the load falls to about 167 kW, and ice production is calculated separately.

The unit does not have to be sized for the peak flow of the pour. An insulated chilled water tank lets the chiller run in the hours before the pour as well and store the water; a smaller unit running for longer does the same job. For embedded pipe cooling, on the other hand, capacity is chosen for the highest heat load the thermal analysis gives for the first days; after that period, the need falls quickly.

Site conditions also affect the selection:

• during summer pours the outside air can exceed 40 °C; the capacity of an air-cooled unit should be checked at that temperature,
• water below +5 °C needs a glycol circuit and an intermediate heat exchanger,
• conditions such as dust, transport and generator supply should be stated at the quotation stage,
• in long pipe cooling campaigns, a unit stopping disturbs the concrete's temperature curve; a standby unit or a two-circuit design should be considered from the start.

We have collected the general calculation method and the other selection criteria in our chiller selection article. The story of how the technology developed, including the Hoover Dam, is told in our article on chiller history.

For your project

For concrete or ground cooling, the unit is selected from our chiller units according to the project's daily concrete volume, target temperatures and site conditions. Share a summary of the thermal analysis, the pour schedule and the site details through our quotation form, and we will work out the capacity and circuit layout together.