In large concrete members such as thick rafts, big footings and wide walls, the reaction between cement and water generates a lot of heat that the concrete cannot lose quickly. The core heats up over the first days after placing, while the surface cools much faster.
What is mass concrete?
Any volume of concrete with dimensions large enough to require measures to deal with the heat of hydration and the volume change that comes with it. A member is often treated as mass concrete when its smallest dimension exceeds about one metre, but the project specification is the reference.
Why monitor temperature?
- The difference between the hot core and the cooler surface creates tensile stresses that can cause thermal cracking.
- An excessively hot core can harm the long-term properties of the concrete.
- The readings show when insulation and formwork can be removed without thermal shock.
Many specifications, such as ACI 301, set a maximum concrete temperature of about 70 °C and a core-to-surface difference of no more than about 19 °C, unless the project specification sets other limits.
How is it monitored?
- Temperature sensors are fixed to the reinforcement before placing: at the core, near the top surface, at the edges and the bottom, plus one for air temperature.
- The sensors are cabled to a data logger mounted on a stand outside the pour area and shielded from the sun.
- The logger records readings at regular intervals day and night, and they are followed continuously.
- Monitoring continues until the temperature peaks, falls and the core-to-surface difference settles, which can take from several days to two weeks or more depending on the thickness.
The photos show our team setting up the monitoring system on the reinforcement of a member before placing.
What if readings approach the limits?
Measures include adding insulating blankets to the surface to reduce the core-to-surface difference, and delaying the removal of insulation and formwork. Before placing, good planning lowers the heat from the start: low-heat cement or partial replacement with fly ash or slag, cooled mix ingredients, and pouring at cooler times.
The report
We deliver a report with the temperature curves for every sensor and the core-to-surface difference over time, compared with the required limits, together with the necessary recommendations.