First, what is a hot climate?
It is a climate where daytime temperatures in the summer reach 30 degrees Celsius or more, and it is located in the Northern Hemisphere between latitudes 15 and 30 north.
Hot climates are divided into three categories:
1- Hot and dry climates
2- Extremely hot and dry climates
3- Hot and humid climates
First, an extremely hot and dry climate is when the difference between minimum and maximum temperatures is greater than 20 degrees Celsius, and the humidity during the summer is less than 30%.
This includes the governorates of Aswan, Qena, Assiut, and the oases.
The rate of evaporation in concrete per hour is greater than 1 kg per square meter.
Second, hot climates
The difference between the maximum and minimum temperature is less than 20 degrees Celsius.
Relative humidity is from 30 to 70%.
Includes the governorates of the Middle Delta, Cairo, and the cities of northern Upper Egypt.
The evaporation rate in concrete is approximately 1 liter per square meter per hour.
Humid climate.
The difference between the maximum and minimum temperature is less than 10 degrees.
Relative humidity is greater than 70%.
Includes the governorates of the Mediterranean coast, the Red Sea coast, and South Sinai.
The evaporation rate in concrete is less than 1 liter of water per square meter per hour.
What is relative humidity?
It is the ratio between the water vapor pressure and the saturated water vapor pressure at the same temperature. It expresses the ability of the air in contact with the concrete surface to Water vapor carrying capacity. The lower the relative humidity, the greater the air's ability to carry water vapor. So, what is the evaporation rate in concrete? It is the weight of water evaporated per unit area per unit time (kg/m²/h).
The evaporation rate depends on the following:
1- The difference between dry and humid air temperatures. 2- Relative humidity. 3- Air speed and direction. 4- Solar radiation. 5- Concrete surface temperature. Evaporation at the concrete surface goes through three basic stages.
The first stage: This is the stage immediately after pouring the concrete, where the water rises to the top, forming a thin film known as bleeding. The evaporation rate in this stage is the same as the evaporation rate of water under the same conditions. The water continues to rise until it evaporates. Initial Setting
Stage Two
This stage begins immediately after initial setting. It occurs when the evaporation rate is higher than the permeation rate (when uncured). It is accompanied by the appearance of white spots on the concrete surface. This stage continues until the concrete surface becomes dry. This stage is called surface hardening. During this stage, the concrete temperature rises and is capable of converting free water into steam, which rises to the top of the concrete and then evaporates.
Stage Three
This stage begins later when the water level in the concrete falls below the exposed surface to a distance that stops the evaporation rate. What are the main problems of concrete production in hot climates? Shrinkage Cracks Concrete is negatively affected by high temperatures during pouring, the most important of which are: Decreased resistance, elasticity parameters, porosity, increased volumetric changes, and increased durability over time.
/> First, regarding fresh concrete
1- Increasing the dryness of the concrete consistency and consequently decreasing slump for the same components, which requires increasing the water content, which reduces resistance and increases porosity. 2- Increasing the rate of slump loss. This is concrete that is difficult to pour and compact (with the same ingredients, even the additions). 3- Hot weather accelerates the setting of cement and hardening of concrete, making it difficult to transport it over longer distances and finish its surface. 4- The increased likelihood of drying shrinkage cracks appears as a result of the rate of water evaporation exceeding the rate of permeation. This appears on large surfaces and also depends on relative humidity, as it is almost invisible at humidity levels above 80%. 5- If temperature, low humidity, and wind speed combine, plastic shrinkage will occur. 6- Difficulty controlling the proportion of entrapped air. 7- All of these problems are compounded when pouring concrete with a high cement content and a low water-to-cement ratio, as the temperature increases the heat of hydration. Second: The Effect of Concrete Temperature on Hardened Concrete 1- Decreased Strength Pressure despite increased resistance at early ages due to higher ambient temperature. 2- Decrease in elasticity after 28 days and subsequent ages. 3- Increased drying shrinkage and increased likelihood of cracking. 4- Failure to take necessary precautions in high-strength concrete leads to disastrous results in reduced resistance, cracking, etc.
Effect of temperature on mixing water
High water temperature leads to higher concrete temperature. A 10-degree increase in temperature above the normal range leads to a decrease in slump of approximately 3 cm. Use of reducing additives in mixing water. Exceeding the normal rate, which increases the cost. The specific heat of water ranges from 4 to 5 times the specific heat of the aggregate, so water temperature has the greatest impact on concrete temperature.
Effect of temperature on admixtures
1- High temperatures significantly affect the performance of admixtures. 2- The effect of retarding admixtures is significantly reduced. 3- Increased slump loss. 4- The harmful effects of improper storage are increased.