The strength and longevity of a structure are determined by the concrete hardening, also known as setting, phase of construction. Temperature is one of the most important factors that determines how long it takes for concrete to harden. Comprehending the impact of temperature on concrete setting times can guarantee a seamless construction project that yields a robust and enduring structure.
The concrete sets more quickly at higher temperatures. This is due to the fact that heat speeds up the chemical reactions that result in the hardened concrete—a mixture of cement and water. This can be advantageous for projects that require quick turnaround times, but there’s a chance that the concrete will set too quickly, increasing the chance of cracks and decreased strength. To reduce these risks, the concrete mix and hydration process must be carefully managed.
On the other hand, the setting process is slowed down by lower temperatures. Construction schedules may be delayed by the longer time it takes for concrete to reach the appropriate strength in colder weather. The water in the concrete mix may freeze and expand, weakening the concrete if the temperature drops too much. Special precautions can be taken to avoid this, such as insulating the concrete or covering it with heated blankets to create the ideal curing environment.
The key to controlling concrete setting times in varying temperatures is to balance the concrete mix and keep an eye on the weather forecast. To ensure that the concrete sets at a desired rate regardless of external conditions, the hardening process can be controlled by adjusting the mix by adding accelerators or retarders. Builders can achieve optimal concrete performance and guarantee the longevity of their structures by comprehending and adjusting to temperature variations.
- Stages of a set of strength of concrete structure
- Stage of grasping
- Hardening stage
- Standard curing time for concrete
- Dependence of the time of strength gain on the grade of concrete mix
- Special additives
- Strength gain of concrete depending on temperature
- At high temperatures
- In cool weather
- At negative temperatures
- Reduction in the viscosity of the solution
- Dependence of the strength gain level on the temperature indicators of the material
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Stages of a set of strength of concrete structure
The chemical reaction between cement and water is what gives cement-based solutions their gravity and hardness. During different phases of curing, Portland cement’s silicates, aluminums, and aluminoferes contribute to an increase in strength.
Temperature and the presence of catalysts, or unique additives, affect how quickly chemical reactions occur.
Stage of grasping
Tricalcium aluminate (3CaO*Al2O3), dicalcium silicate (belite, 2CaO*SiO2), tricalcium silicate (alite, 3CaO*SiO2), and aluminoferrite are all present in the cement powder. The majority of Portland cement’s bulk, alite, is used in both hardening stages. It releases heat when combined with water at the start of the setting stage, speeding up the reaction rate.
Nevertheless, tricalcium aluminate is the more active ingredient in cement during the setting phase. It forms primary bonds in concrete by intensely reacting with water within 24 hours of mixing. The aluminate has no further effect on the cement’s strength after setting is complete.
The first few hours after the formwork is poured are when the setting stage takes place. The mixture’s composition and the temperature of the surrounding air determine how quickly the reaction starts and how long it takes to complete. Concrete sets in 2.5–3 hours at room temperature (+18–+22°C). Of these, the reaction takes place between 1.5 and 2 hours, and the setting process takes 1 hour.
The reaction can take up to 15-20 hours to fully develop when the temperature drops, with a 4–8 hour delay.
Setting happens more quickly and actively in a hot environment. The reaction takes 15-20 minutes, and the entire process can be completed in less than 1-2 hours.
Hardening stage
After setting is finished, the concrete stone formation stage starts. The removal of free water causes the material to harden. A portion of the liquid evaporates into the surrounding air, while the remainder attaches itself to silicate and aluminate molecules to form stable complexes. It’s important to maintain ideal environmental humidity and temperature to avoid upsetting the balance between bound and evaporated water.
Alite is the primary reagent during the hardening phase. Belit guarantees that the material will gradually strengthen while in use; as a result of these properties, the material’s strength can reach up to 250% of its hardened strength after two to three years.
Standard curing time for concrete
For concrete, the typical hardening period is 28 to 30 days. The ideal temperature range for curing is +15–+22°C with a humidity level of 60–100%. The quality of the concrete, the process parameters, and the existence of extra additives in the mixture all affect how long the concrete takes to harden.
Dependence of the time of strength gain on the grade of concrete mix
An increase in the mix’s viscosity is correlated with an increase in concrete’s compressive strength. This indicates that the setting and hardening times decrease as material grade increases.
Reaction times for various grades of concrete
Grade of material | Setting time, hours | Hardening time, days |
M100 | 3-3.5 | Up to 30 |
M200 | 2-2.5 | 14-25 |
M300 | 1.5-2 | 7-14 |
M400 | 1-2 | 4-7 |
M500 | 2-4 |
The material, humidity, ambient temperature, and mix composition all affect how long strength gains last.
The critical strength of the concrete stone is also determined by the grade and purpose of the solution. This is the point at which the structure will maintain its operational properties even after freezing and continue to solidify. This indicator is grade-dependent in the following ways:
- for concrete M100 and M150 it corresponds to 50%;
- for M200, M250, M300 and M350 – 40%;
- for M400, M450 and M500 – 30%;
- for loaded structures (regardless of grade) — 70%.
Temperature variations won’t significantly affect the sample’s strength if it possesses the proper amount of compressive strength when it freezes. The strength of the completed structure decreases by at least 50% when freezing during the early stages of hardening without the use of antifreeze additives. For instance, the critical strength point for grade M200 is 80 kgf/cm², or 8 MPa.
M300 concrete grades are typically utilized for loaded structures and foundations. In standard structures, formwork removal is permitted after four to five days provided that there are spaces between the form panels and the concrete. The curing period is extended to 14 days for floors and stairs up to 6 meters in length, and up to 28 days for stairs that are longer. For up to ninety days, bridges, dams, and other significant, heavily laden structures are maintained in their current form.
Special additives
Concrete loses strength when the mixture sets and hardens too quickly or too slowly. In addition, slow hardening raises the structure’s maintenance costs. To modify the curing rate, additives that control the process kinetics are employed.
The solution’s hardening process is regulated by two different kinds of additives:
- Accelerating. Reagents of this type reduce the time before the onset of setting by 30-40%, accelerate hardening and improve the strength properties of the material. They are added to the mixture during industrial stamping of concrete products, pouring foundations, ceilings and other building structures at low temperatures. The cheapest accelerating additives are calcium chloride and potash (potassium carbonate). The list of popular building compounds for accelerating hardening includes: Relaxor, Addiment B3, Fort-UP2, Pozzolit-100, Concrete-F, etc.
- Retarding. Plasticizers and set retarders have a positive effect on the workability and mobility of the mortar. They are used when delivering concrete in mobile mixers, delays in construction and pouring structures at temperatures above +25…+30°C. The plasticizing properties of retarders make it possible to avoid vibration compaction when laying concrete with low mobility. The most common retarding additives are NTP acid, sodium citrate and gluconate, Linamix, SikaPlast 520 N, Frem Linas 200, etc.
Antifreeze reagents are used when pouring at low temperatures. They keep water from going through phase transitions at 0…+4°C by lowering its freezing point.
At temperatures as low as -15 to 25°C, you can work with concrete mortar depending on the kind and concentration of additives. Reagents that are resistant to frost include urea, calcium nitrate-nitrite, and sodium nitrite.
Strength gain of concrete depending on temperature
The rate at which the reactions that form concrete stone occur depends on the surrounding temperature. Low air temperature slows down the hydration processes in the solution, whereas high air temperature tips the scales in favor of liquid evaporation.
At high temperatures
Water evaporation is accelerated in dry and hot air, so there might not be enough liquid left over for complete hydration. Consequently, the structure’s dependability declines and its compressive strength exhibits notable variations in both the upper and central layers.
Retarding additives are added to the concrete, and the completed structure is wetted during the hardening process to prevent unevenness and rapid drying.
Standard concrete products are produced in autoclaves at high temperatures and humidity levels. These circumstances guarantee quick setting and maximum structure hardening.
In cool weather
In comparison to the grade strength, the solution takes a long time to set at low temperatures and then stays brittle for a long period of time. Chemical reactions take place up to the temperature at which water undergoes phase changes.
At negative temperatures
The water freezes and the solution stops being hydrated when the outside temperature falls below 0°C. The curing process restarts when the air warms, but the structure’s strength may decline following a pause.
Concrete’s strength increase at various temperatures
Curing time, days | Share of 28-day strength achieved under optimal curing conditions | |||||
At -3°C | At 0°C | At +5°C | At +10°C | At +20°C | At +30°С | |
1 | 3 | 5 | 9 | 12 | 23 | 35 |
2 | 6 | 12 | 19 | 25 | 40 | 55 |
3 | 8 | 18 | 27 | 37 | 50 | 65 |
5 | 12 | 28 | 38 | 50 | 65 | 80 |
7 | 15 | 35 | 48 | 58 | 75 | 90 |
14 | 20 | 50 | 62 | 72 | 90 | 100 |
28 | 25 | 65 | 77 | 85 | 100 |
The M200 and M300 grades’ strength gains are displayed in the table.
Temperature has a major impact on how long it takes for concrete to harden, including setting and curing. Concrete’s chemical reactions are accelerated by higher temperatures, which shortens the time it takes for setting and curing. Lower temperatures, on the other hand, slow down these processes, which can cause construction schedules to be delayed and have an impact on the concrete’s ultimate strength. It is essential to comprehend how temperature affects concrete hardening in order to plan construction projects and guarantee the longevity and structural integrity of buildings.
Reduction in the viscosity of the solution
The concrete solution keeps its flexibility while it sets. The mixture exhibits the property of thixotropy—a decrease in composition viscosity under continuous dynamic load—when it moves in a stationary or mobile concrete mixer.
Overmixing causes the concrete to "overcook," which reduces the final structure’s structural strength. Plasticizers are added to the mixture in order to keep the solution mobile and prevent bad outcomes. They prolong the times for setting and hardening.
At the hardening stage, the mixture’s viscosity cannot be decreased. The formation of defects and cracking of the structure are caused by mechanical impact on the hardening concrete stone. It is recommended to shield the hardening concrete from impacts, vibration, and other external factors until it reaches the minimum permissible strength level.
Dependence of the strength gain level on the temperature indicators of the material
The performance properties of the concrete stone are adversely affected by low ingredient temperature. The grade strength cannot be ensured by subsequent structural maintenance if cold water and filler are used for mixing.
It is advised that the water used for making be heated when the temperature drops below 10°C. The fine filler (river sand) needs to be heated if the thermometer reads -5…0°C or lower.
The components are heated as high as is permitted in order to minimize the amount of time the concrete needs to set and the cost of heating the concrete inside the formwork. Portland cement’s grade and composition determine its maximum value. The final mixture will react less intensely when heated above this point, which will reduce the structure’s strength.
Limiting the temperature of the ingredients in concrete mortar
Type of cement | Maximum temperature of water for mixing, °C | Limiting temperature of filler, °C | Maximum temperature of concrete mortar after mixing, °C |
Aluminous | 40 | 20 | 25 |
Portland cement grade M400 and higher |
Temperature (°C) | Setting Time (Hours) |
5 | 12-18 |
10 | 8-12 |
20 | 4-8 |
30 | 2-4 |
40 | 1-2 |
Concrete hardening times are crucial for construction projects because they impact both the final product’s quality and the project’s overall timeline. A better understanding of how temperature affects this process can aid in the planning and execution of concrete pours. Concrete generally sets and hardens more quickly at higher temperatures while setting and hardening more slowly at lower temperatures.
Concrete may set too quickly in hot weather, which could cause problems with strength and workability. Retarding admixtures and maintaining a cool concrete temperature are crucial for mitigating this. In contrast, concrete may take too long to set in cold weather, increasing the chance of freezing-related damage. Maintaining the intended hardening rate can be facilitated by the use of heaters, insulation, and accelerating admixtures.
Concrete is guaranteed to reach its desired strength and durability through careful monitoring and adjustments based on the surrounding temperature. Engineers and builders need to be aware of these temperature effects in order to prevent common problems like delays, reduced strength, or cracking. Regardless of the weather, building projects can continue to move forward without interruption if they are proactive and adaptable.
Using optimal temperature control techniques during the concrete hardening process not only improves construction quality but also makes the most use of time and resources. This information guarantees the durability and security of concrete structures while enabling professionals to produce better results.