90% of cases of cracking in mass concrete are due to ignoring this detail!
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90% of cases of cracking in mass concrete are due to ignoring this detail! (Suggested for collection) Having worked in concrete technology on the front lines for over a decade, I have seen numerous cases of cracking in large-volume concrete: basement slabs, bridge footings, and foundations for large equipment. At the time of pouring, their surfaces appeared smooth and normal, but after a few days of curing, numerous cracks would appear. In mild cases, this affected the visual quality; in severe cases, it caused cracks to run through the structure, impacting its durability. In such situations, repairs were necessary, which not only wasted materials and time but also created safety hazards. Many front-line technicians, when faced with cracking, instinctively try to adjust the mix ratio, increase the amount of cement used, and improve curing methods; yet they fail to identify the root cause, which results in the cracking problem recurring repeatedly. In fact, everyone is trapped in a misconception: they pay excessive attention to the mix ratios and curing details, while ignoring the most crucial and yet easiest-to-overlook detail – inadequate control of the temperature difference between the inside and outside. In line with GB50496-2018 \"Code for Construction of Mass Concrete\", a key principle should be established first: cracking in mass concrete occurs essentially because the temperature stress exceeds the tensile strength of the concrete. After concrete is poured, the hydration of cement releases a large amount of heat, causing the internal temperature to rise rapidly, while heat from the surface dissipates quickly, resulting in a temperature difference between the inside and outside ； When the temperature difference is too large, the expansion of the internal concrete is constrained by the surface, resulting in tensile stress. Once this tensile stress exceeds the concrete’s own tensile strength, cracks will appear. And 90% of these engineering failures occur because the detail of \"temperature difference control between the inside and outside\" is overlooked – either no monitoring is carried out or no effective temperature control measures are taken, which ultimately leads to cracking. First, understand: why is the temperature difference between the inside and outside the culprit behind cracking?
First, understand: why is the temperature difference between the inside and outside the culprit behind cracking? Many people think it’s normal for concrete to heat up, and that it will cool down again once that happens. What they don’t realize is that large volumes of concrete dissipate heat very slowly; once the temperature difference between the inside and outside exceeds the specified limits, cracking almost inevitably occurs. In accordance with the requirements of GB50496-2018, after the pouring of mass concrete, the temperature difference between the interior and exterior should not exceed 25°C, the temperature difference between the surface and the surrounding environment should not exceed 20°C, and the cooling rate should not exceed 2°C per day. Take a common example from practice: after the pouring of a shear wall in a basement, the internal maximum temperature reached 75°C, while the surface temperature was only 35°C, resulting in a temperature difference of 40°C – well above the limits specified by the standards. Cracks appeared on the surface after 3 days; this is a typical case of \"temperature difference stress cracking\". Specifically, there are two key issues: one is the concentrated release of heat generated during cement hydration, which causes the internal temperature to soar. Large-volume concrete requires a large amount of cementing materials, and the heat generated by cement hydration cannot be dissipated quickly, resulting in its accumulation inside the concrete. Especially in components with a pouring thickness of over 1.5 meters, the internal temperature can reach 70–80 degrees℃ ； Second, heat is lost from the surface rapidly, creating a temperature difference. The concrete surface is in direct contact with the air, and it is affected by environmental temperature, wind speed, and humidity, which causes rapid heat loss. Meanwhile, the heat generated inside cannot be transferred to the surface in a timely manner, resulting in a sharp increase in the temperature difference between the interior and the exterior. More importantly, during the curing process, concrete has not yet reached its full strength, and its tensile strength is very low (usually only about 1/10 of its compressive strength), making it completely unable to withstand the tensile stresses generated by temperature differences. And the root of all this is the overlooked detail of \"inadequate control of temperature differences between the inside and outside.\"
Key breakdown: 90% of people make mistakes in these 3 areas. Understanding the temperature difference between the inside and outside is crucial; it’s also necessary to figure out where, in actual practice, people overlook this detail Based on cases from daily inspections, there are mainly 3 common mistakes. Error 1: In determining the mix proportions for laboratories, only strength is considered, while the \"temperature control design\" is ignored. Many laboratory technicians, when formulating mix proportions for mass concrete, focus solely on compressive strength and increase the amount of cement used. They fail to realize that the more cement is used, the more heat is generated during hydration, which in turn raises the internal temperature and increases the risk of cracking – this is the most fundamental and fatal mistake. In practical applications, the correct approach is to minimize the use of cement while still ensuring the required design strength and durability, by preferentially using a dual-blending method of \"cement + high-quality admixtures\" (such as fly ash + slag powder) to replace part of the cement. Admixtures can not only reduce the heat of hydration (fly ash can reduce it by 10%~20%), but also improve the workability of concrete and enhance its durability, achieving two benefits at once. For example, in C30 mass concrete, the conventional cement usage is 190 kg/m³; by replacing it with 80 kg/m³ of fly ash and 80 kg/m³ of mineral powder, the heat of hydration is significantly reduced, and the maximum internal temperature can be lowered by 5–8°C, thereby greatly reducing the risk of cracking. Furthermore, many laboratories overlook the impact of \"aggregate gradation\" on temperature control: using aggregates with a continuous gradation and low void ratio can reduce the amount of cementing material needed, thereby indirectly lowering the heat of hydration ； At the same time, giving priority to low-heat cements (such as low-heat slag silicate cement) can also effectively control the rate of internal temperature rise. These details are key to temperature control in mix design, yet they are overlooked by 90% of people. Error 2: In the production process, attention is focused only on the quality of the mixture, while \"cooling measures\" are neglected. Production staff usually pay attention only to the slump and cohesion of the concrete to ensure smooth pumping, but they ignore control of the temperature of the concrete as it leaves the mixer. If the temperature of the concrete exiting the mixer is too high, it will lead to a sharp rise in the internal temperature after pouring, further increasing the temperature difference between the inside and outside. According to the specifications, the temperature of mass concrete upon leaving the mixer should not exceed 30°C, and the pouring temperature should not exceed 35°C. In actual field operations, in many projects carried out during the summer, sand and gravel are exposed to direct sunlight, resulting in temperatures of over 40°C. When used to mix concrete, the temperature of the concrete mixture exceeds 35°C easily; after pouring, the internal temperature can easily rise above 70°C, doubling the risk of cracking. The correct approach is simple, yet few people follow it properly: during construction in the summer, cover the sand and gravel with something to shield them from direct sunlight ； Spray water on the sand and gravel to cool it down when necessary (be careful to control the moisture content and adjust the mix ratio promptly) ； Groundwater or cooled water can be used for mixing; for large-scale raft slabs, ice water is employed in tests, and a thermometer is used to measure the temperature difference between the inside and outside of the mixture, which helps to effectively reduce the temperature of the mixture upon exiting the mixer ； During winter construction, although the ambient temperature is low, it is still necessary to prevent the concrete from having an excessively low temperature upon leaving the mixer (it should not be below 5°C), in order to avoid rapid freezing of the surface and the formation of temperature-induced cracks. Error 3: During construction and maintenance, only focus on \"moisture retention\" while ignoring \"heat preservation and temperature control\" – this is the most common and easily overlooked mistake! Many construction workers believe that for the curing of mass concrete, it is sufficient to water it to maintain moisture and cover it with plastic film; however, they are unaware that \"thermal insulation\" is more important than \"moisture retention\". The key to thermal insulation lies in controlling the temperature difference between the inside and outside, thereby preventing rapid loss of heat from the surface. A common mistake made in practice: after pouring, only a layer of plastic film is used for covering, or no coverage is provided at all, leaving the surface exposed to the air. Especially during hot summers with strong winds, heat dissipates rapidly from the surface, causing the temperature difference between the inside and outside to exceed the specified limits in an instant ； During winter construction, if no insulation measures are taken, the surface temperature becomes too low, resulting in a large temperature difference with the interior, which can also cause cracking. The correct maintenance method relies on \"layered insulation and gradual cooling\": after pouring, cover it with a layer of plastic film first (to retain moisture), followed by 2–3 layers of geotextile or blankets (for insulation); the thickness should be adjusted according to the environmental temperature (it can be increased to 4–5 layers in winter) ； For mass concrete with a thickness of over 2 meters, cooling water pipes can be embedded within it; circulating water is passed through these pipes to force a reduction in the internal temperature, ensuring that the temperature difference between the inside and outside remains within 25 degrees℃ ； At the same time, the curing period must be no less than 14 days; the frequency of watering should be increased in hot weather, and heating measures should be taken in cold weather to ensure that the rate of temperature drop does not exceed 2°C per day. Key reminder: Control the temperature difference; these 2 practical details are essential. 1. Proper temperature monitoring is necessary: Before pouring, install temperature sensors inside the concrete, on its surface, and in the surrounding environment. After pouring, monitor the temperature every 2–4 hours, recording the temperature difference between the inside and outside as well as the rate of temperature drop. Once the limits set by the regulations are exceeded (temperature difference greater than 25°C, or rate of temperature drop greater than 2°C/day), immediate action must be taken (such as adding insulation layers or adjusting the flow rate of the cooling water pipes). This is a crucial prerequisite for controlling the temperature difference – many projects suffer damage due to a lack of proper monitoring, and by the time cracks appear, it’s already too late to fix the situation. 2. The pouring process must be standardized: For mass concrete, pouring should be carried out in layers, with each layer having a thickness of 500–600 mm; this is to avoid pouring too thick a layer at once, which could lead to heat accumulation ； The pouring speed should not be too fast, to ensure that there is enough time for heat to dissipate ； Vibration should be thorough, but over-vibration is strictly prohibited to avoid damaging the internal structure of the concrete, reducing its tensile strength, and exacerbating cracking.
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