When the temperature exceeds 30℃, the hydration reaction of concrete is accelerated and the evaporation of water is accelerated, resulting in a sharp decline in working performance, which seriously affects the construction quality and structural durability. We will comprehensively analyze the impact of high temperature environment on concrete and propose countermeasures in this post, hoping to help your projects.
What effects will high temperature environment have on concrete performance?
>> Slump loss and setting time
Slump loss is accelerated. For every 10℃ increase in temperature, the cement hydration rate increases by 2-3 times. Under a 30℃ environment, the slump of ordinary concrete may lose more than 50% within 1 hour. In addition, the evaporation rate of water increases significantly, resulting in serious surface bleeding.
Abnormal setting time. The initial setting time may be shortened by 40-60%. And it is easy to have a “false setting” phenomenon, with the surface hardened but the inside still plastic. The final setting time is difficult to control, affecting the subsequent construction process.
>> The surface water evaporates too quickly, and plastic shrinkage cracks
High temperature accelerates the evaporation of water on the surface of concrete, causing rapid loss of surface water, while internal water has not yet migrated to the surface, forming a humidity gradient. When the evaporation rate exceeds the water seepage rate, tensile stress is generated on the surface, resulting in a significant increase in the risk of plastic shrinkage cracks.
Plastic shrinkage cracks often appear before initial setting, especially in strong winds or low humidity environments. Cracks may extend into the structure, resulting in reduced durability of concrete.
>> Acceleration of hydration reaction and imbalance between strength development
For every 10℃ increase in temperature, the hydration rate of silicate cement increases by about 2 to 3 times. C2S reacts rapidly at high temperatures to form C-S-H gel, but the product is unevenly distributed.
Early hydration products accumulate densely around the particles, hindering later water penetration. Early strength development is too fast, and later strength growth is limited, resulting in a 5% to 15% reduction in 28-day strength. The porosity of high-temperature cured concrete is 3% to 8% higher than that of standard curing.
>> Thermal stress and temperature cracks
The temperature at the center of large-volume concrete can reach 70~80℃, while the surface is cooled to 30~40℃ by the environment. When the temperature difference between the inside and outside is greater than 25℃, tensile stress is easily generated and cracks are formed.
Crack type: Early temperature cracks are mostly perpendicular to the long side of the structure, while later drying shrinkage cracks are cracked. For every 10℃ increase in temperature difference, the strain caused by the thermal expansion coefficient increases by 0.01%.
>> Long-term durability degradation
Increased permeability: High temperature leads to increased connectivity of capillary pores, the chloride ion diffusion coefficient can increase by 50%~100%, and the carbonation depth increases by 30%~60%.
Chemical erosion: In a sulfate environment, high temperature (>40℃) accelerates the conversion of calcium sulfonate to gypsum, and the expansion rate increases by 2~3 times. The rate of alkali-aggregate reaction at 80℃ is 10 times that of 20℃.
Freeze-thaw damage: The deterioration of the pore structure reduces the frost resistance, and the relative dynamic elastic modulus may be lower than 60% after 300 freeze-thaw cycles (the standard requires >80%).
>> Microstructure deterioration
High temperature dehydration: C-S-H gel loses bound water at 100-300℃, portland cement decomposes above 300℃, Ca(OH)₂ dehydrates to CaO at 500℃, and rehydration leads to secondary expansion.
Phase change effect: Quartz aggregate undergoes α-β phase change at 573℃, with a volume expansion of 0.85%, exacerbating the risk of cracking.
Microscopic observation: Above 400℃, the porosity of concrete increases from 10% to 25%, and the pore size distribution migrates to >100 nm.
>> Rebar bonding performance decreases
Temperature effect: The bond strength between steel bars and concrete decreases by about 15% at 100℃ and loses more than 50% at 400℃. The mechanical bite of threaded steel bars is weakened due to the loosening of concrete.
Slip amount: The limit slip amount can increase by 200%-300% at high temperature, resulting in reduced collaborative working ability.
What measures can be taken to reduce the impact of high temperature on concrete performance?
>> Raw material control and cooling
(1) Lowering aggregate temperature
Shading. A sunshade is set up in the aggregate yard to avoid direct sunlight, which can reduce the temperature by 5~10℃.
Water spray cooling. Spray cold water on coarse and fine aggregates before mixing, but pay attention to the amount of water sprayed to avoid excessive moisture affecting the mix ratio.
Use cooled aggregates. Liquid nitrogen or cold air is used to cool aggregates when necessary. This method is suitable for large-volume concrete.
(2) Lowering mixing water temperature
Adding ice to mix. Replacing part of the mixing water with crushed ice can reduce the concrete temperature by 5~8℃.
Cold water mixing. Use cold water of 4~10℃ and avoid using high-temperature groundwater or circulating water.
(3) Cement selection
Preferably use low-heat cement, or add fly ash or slag to reduce the hydration heat.
>> Optimize mix design
(1) Reduce cement dosage
Add fly ash or slag powder to reduce hydration heat temperature rise by 10~15℃.
Use high-strength concrete mix to reduce water-cement ratio and reduce shrinkage.
(2) Use retarding admixtures
The use of polycarboxylic acid water reducer and sodium gluconate retarder can delay the setting time by 1~3 hours and reduce slump loss.
Air entraining agent can improve the resistance to plastic shrinkage.
>> Construction process control
(1) Control the pouring temperature
The mold temperature should be ≤30℃, which can be relaxed to 35℃ in hot areas, but maintenance is required.
Pour at night or during low temperature periods, avoiding high temperature periods.
(2) Shorten transportation and pouring time
Use sunshade or reflective paint on mixer trucks to reduce temperature rise during transportation.
Use pumping agent to improve fluidity and reduce stagnation time.
(3) Layered pouring and cooling
Large-volume concrete is poured in layers, and cooling water pipes are buried to circulate cold water for cooling.
Thin-wall structures speed up pouring and prevent premature water loss on the surface.
>> Maintenance measures
(1) Timely moisturizing maintenance
Immediately cover with wet burlap or geotextile after initial setting, and continue to sprinkle water for more than 7 days.
Spray curing agent to reduce water evaporation.
(2) Windproof and sunshade
Set up windbreaks to reduce evaporation caused by air flow.
Cover with white reflective film to reduce surface temperature by 5~10℃.
(3) Steam curing of prefabricated components
Use low-temperature steam curing to avoid micro cracks caused by high temperature.
>> Monitoring and quality control
(1) Temperature monitoring
Bury temperature sensors to monitor the temperature difference between the core and the surface.
Use infrared thermometers to check surface temperature to prevent local overheating.
(2) Strength and crack detection
Early strength tests are conducted on the first, third and seventh days to assess the impact of high temperature so that timely countermeasures can be taken.
Use ultrasound or crack microscope to check early micro cracks.
Which concrete admixtures can cope with the impact of high temperature?
>> Concrete Retarders
Function: Delay cement hydration rate, offset premature setting caused by high temperature, reduce peak temperature by 5~10℃, and reduce the risk of plastic cracks.
Typical materials:
Sodium gluconate: Delay initial setting time by 1~3 hours, dosage 0.03%~0.1%.
Sodium Lignosulfonate: Has both retarding and water reducing effects, dosage 0.1%~0.3%.
>> High-efficiency water reducer
Function: Reduce water consumption (15%~30%) and improve slump loss at high temperature. At the same time, it can also reduce the water-cement ratio and increase the later strength by 10%~20%.
Typical materials:
Polycarboxylate based superplasticizer: best high temperature resistance, slump can be maintained for more than 2 hours at 30℃, dosage 0.1%~0.3%.
Naphthalene based superplasticizer or melamine series: need to be used in combination with concrete retarder.
>> Air-entraining agents
Function: introduce tiny bubbles to relieve the pressure caused by rapid evaporation of water at high temperature. The dosage is controlled at 0.005%~0.02%, and the air content is controlled at 4%~6%. Excessive air entrainment will reduce strength.