Summer concrete work: risks, mistakes and practical solutions

Why does concrete – whose properties have been carefully selected in the laboratory and which fully meets the requirements at the concrete plant – sometimes develop cracks or fail to achieve the required strength just a few days, or even hours, after being laid? In most cases, the cause lies not in the quality of the concrete mix, but in the conditions under which it is laid and cures.

Summer is traditionally considered the best time for carrying out concrete works. However, high temperatures, direct sunlight and wind can significantly affect the setting and hardening processes of concrete. Under such conditions, the mix loses its workability more quickly, water evaporates more rapidly, and the risk of cracking and reduced structural durability increases significantly.

• That is precisely why high-quality concrete work in summer involves not only selecting the right concrete mix, but also careful planning of the work, adherence to placing techniques, and proper curing of the concrete in the first few hours and days after it has been laid.

Risks of Concreting under Elevated Temperatures

At first glance, it might seem that summer is the perfect time for concreting. There is no need to worry about the concrete freezing, and warm weather should facilitate rapid work progress. However, high temperatures are often the root cause of problems that can manifest either immediately after concreting or years into the structure's operation.

Air temperatures between 10 °C and 25 °C are considered the best conditions for concreting. If the thermometer rises above this range, the concrete mix begins to behave quite differently.

• Reduction of Setting Time
  While under normal conditions a concrete mix retains its properties long enough for transport, placement, and compaction, this period is significantly shortened in hot weather. Consequently, construction workers have much less time to perform the work.
• Rapid Change in Concrete Mix Consistency
  Due to intensive water evaporation, the concrete becomes less workable, making it harder to distribute within the structure and compact. Under such conditions, the risk of voids, honeycombs, and other defects increases significantly.
• Shrinkage Deformations of Concrete
  Also, due to intensive water evaporation from the surface of freshly placed concrete, plastic shrinkage cracks can form even before the concrete begins to gain strength. A particularly high risk arises when high temperatures are combined with low relative humidity and strong winds.
• Occurrence of a Temperature Gradient
  During hydration, cement releases heat, and solar radiation increases the concrete's temperature even further. Due to this uneven temperature distribution within the structure—especially in massive elements—internal stresses can arise between layers, which can lead to the appearance of thermal cracks.

Requirements of Regulatory Documents

Concreting in hot weather is regulated by normative documents that define the requirements for the concrete mix, the execution of concrete works, and the curing of freshly placed concrete during the hardening process.

• DSTU EN 206:2022 "Concrete. Specification, performance, production and conformity"
  This standard establishes requirements for concrete and fresh concrete mixes at all stages of their production and application. The document defines approaches to ensuring the quality and durability of concrete, and provides for the control of concrete mix properties during its production, transportation, and placement.
• DSTU EN 13670:2023 "Execution of concrete structures"
  This standard regulates the execution of concrete works directly on the construction site. The document requires taking necessary measures to protect concrete from adverse weather conditions, including the effects of high temperatures, intensive drying, and overheating during hardening. Special attention is paid to the curing of freshly placed concrete, aiming to create favorable temperature and moisture conditions that ensure the normal course of cement hydration processes.
• DSTU EN 12390-2:2024 "Testing hardened concrete. Part 2. Making and curing specimens for strength tests"
  The standard establishes requirements for making concrete test specimens and the conditions for their curing during testing, including during the mix design selection. The test results of these specimens form the basis for assessing concrete conformity with specified requirements. According to the standard, normal curing conditions are considered to be a temperature of (20 ± 2) °C and a relative air humidity of at least 95%, or keeping the specimens in water at the same temperature. It is under these conditions that concrete most effectively realizes its potential to achieve its proper properties. Therefore, during summer concreting, it is especially important to provide conditions for the structure that are as close to the normative ones as possible, allowing for a reduction in the risk of premature concrete drying, cracking, and strength deficit.

Simply put, modern regulatory documents require not only high-quality placement of concrete but also providing it with proper conditions for hardening. After all, even the best concrete mix will not be able to fully realize its properties without proper curing in the first hours and days after pouring.

Practical Recommendations for Summer Concreting

Hot weather is not an obstacle to executing concrete works if the necessary organizational and technological measures are planned in advance. The main task is to ensure that the concrete mix does not lose its properties during placement, and that the concrete has enough moisture for normal hardening after installation.

• Plan Work for Cooler Times of the Day
  Whenever possible, concreting should be carried out in the morning or evening hours, when the air temperature is lower and the surfaces of the structures are not overheated by the sun. This helps reduce the rate of water evaporation and provides more time for placing and compacting the concrete mix.
• Prepare the Base and Formwork
  Before starting concreting, the base must be cleaned of dust and contaminants. If concrete is placed on soil, a crushed stone base, or into formwork, it is advisable to pre-wetted them. This will help prevent moisture from being drawn out of the concrete mix in the first hours after placement.
• Do Not Allow Overheating of the Concrete Mix
  In hot weather, it is especially important to reduce the time between concrete production and its placement. If necessary, producers can use chilled water, special chemical admixtures, or other technological solutions to maintain the mix workability. At the same time, adding water to the concrete mix directly on the construction site is not recommended. This practice can lead to a deterioration in the strength and durability of the concrete.
• Ensure Quality Compaction
  The concrete mix must be placed without long interruptions and immediately compacted using appropriate equipment. Quality compaction allows for the removal of air voids and ensures the formation of a dense concrete structure.
• Protect Concrete from Rapid Drying
  Immediately after placement, concrete must be protected from direct sunlight, wind, and intensive moisture evaporation. For this purpose, plastic sheeting, damp geotextiles, mats, or special film-forming concrete curing compounds are used. The first hours after concreting are particularly critical, as the structure of the cement paste is being formed; during this period, premature loss of moisture can negatively affect the strength and durability of the future structure. In hot weather, the concrete surface can heat up to high temperatures, so wetting must be done gradually and evenly. Using water that is too cold can cause a sharp temperature drop and increase the risk of microcracking.
• Keep the Surface Moist
  After the concrete surface gains its initial strength, regular wetting must be provided. The frequency of watering depends on weather conditions, but the main goal remains unchanged—to prevent the surface from drying out.
• Do Not Rush to Remove Formwork and Load the Structure
  Even if the concrete surface looks strong enough, the strength gain process continues for many days. Premature striking of formwork or loading of the structure can lead to cracking and reduced concrete durability.
• Keep a Freshly Placed Concrete Curing Log
  On critical projects, a concrete curing log must be maintained, recording the date and time of concreting, weather conditions, air temperature, methods used to protect the concrete from drying out, the frequency of wetting, and other important data. Such a log allows for monitoring compliance with technology, timely identification of potential violations, and confirmation of proper work execution in case of disputes regarding the quality of the structure.

Innovative Technologies

The modern concrete industry is constantly evolving, offering new solutions to ensure the quality and durability of concrete structures even under challenging weather conditions. While previously the main way to protect concrete from heat was merely regular surface wetting, today there are significantly more technological possibilities.

• Application of Modern Chemical Admixtures
  Modern chemical admixtures allow the concrete mix to retain its workability for significantly longer without adding water, while having no negative impact on the early strength of the concrete. This is especially important when transporting concrete over long distances or performing concrete works in hot weather.
• Application of Special Concrete Mixes
  For concreting under elevated temperatures, it is advisable to use specially formulated concrete mixes with reduced heat generation, a low water-cement ratio, and a dense structure. These are produced using active mineral components (limestone, granulated blast-furnace slag, fly ash, pozzolans, etc.) that help increase durability and reduce the risk of cracking.
• Application of Special Materials for Internal Concrete Curing
  The principle behind this technology is that an internal water reserve is created within the concrete mix, which is gradually used for cement hydration processes. Thanks to this, the risk of shrinkage deformation and cracking is reduced, and the concrete gains strength more evenly.
• Application of Digital Temperature Monitoring Systems
  Special sensors are installed inside the concrete of massive structures, allowing for real-time monitoring of the structure's temperature, evaluation of the rate of strength gain, and timely detection of the risk of thermal cracking.

Conclusion

Summer concreting requires special attention to work organization, as high temperatures, solar radiation, and wind can significantly affect concrete hardening processes. However, most risks can be successfully minimized through proper work planning, the use of modern concrete mixes and chemical admixtures, as well as timely curing of freshly placed concrete.

To briefly summarize all the stated recommendations, the main task during summer concreting is to prevent rapid moisture loss from the freshly placed concrete to ensure the normal course of cement hydration processes. Particular attention should be paid to the first 7 days; during this period, the concrete must remain continuously moist, as its future strength and durability depend on this most of all.