Covering insulating blanket on the surface of high strength concrete foundation can greatly reduce the cooling rate of surface concrete.
Engineers often use high-strength concrete in the design of heavy-duty structures. Because the strength of this kind of concrete is relatively high, its component size is smaller than that of traditional concrete. The hydration heat of mass concrete (whether high strength concrete is used or not) and the temperature rise it produces will lead to thermal expansion and shrinkage problems. If it is not monitored, the expansion of temperature difference in concrete will cause the internal tensile stress to exceed its tensile strength, which will lead to concrete cracking. This paper introduces a temperature difference monitoring method for mass high strength concrete foundation adopted by a project contractor.
Mass concrete foundation
The Tennessee Valley Authority is installing priority catalytic reduction equipment for coal-fired power plants in its jurisdiction, including a structural unit first located in northeastern Alabama. The new equipment requires mass concrete foundation to bear its huge gravity and instantaneous load. The foundation consists of four huge caps and connected beams. The cap area is 2.7 m2, the thickness is 2.4 m, the floor beam width is 1.2 m, the depth is 1.2 m, and the 28-day strength of the foundation concrete is C40 (40MPa). The standard mixtures to meet the project design specifications are limestone aggregate, Portland II cement with 350 kg ASTM C150 specification, and F-grade fly ash with 47kg ASTM C618 specification.
ACI 207.1R defines mass concrete as "any volume of concrete is large enough to take into account the measurement of heat generated by cement hydration, and the accompanying volume changes will lead to the slightest cracking." ACI207 goes on to point out that the huge tensile stress and strain may develop further with the volume change caused by temperature rise and fall in mass concrete.
The foundation of this project is densely reinforced with restrained bars around it. However, these reinforcing bars can not ensure that concrete does not crack, let alone prevent concrete from generating heat. These high strength concrete foundations, if exposed to cold climate for maintenance, will inevitably be due to the huge temperature difference between the center of the foundation and the exposed surface, and many problems. However, if the maximum temperature difference of concrete is controlled, the heat dissipation of mass concrete foundation is uniform and the temperature difference in foundation is avoided, these problems can be avoided. The method chosen for this project is to minimize the temperature difference and the maximum temperature rise of concrete, so as to prevent concrete cracking and potential internal damage.
Measuring equipment and monitoring method
When the external surface temperature of concrete continues to drop (due to heat dissipation), it will continue to rise with the internal temperature of mass concrete (due to hydration), so that the possibility of temperature cracks will increase. In addition, because of the mix proportion design, the amount of cement and the size of the pouring scale, the internal temperature of concrete will easily exceed the maximum safety limit temperature of 70 C. The setting of the limit temperature comes from the long-term durability of concrete, which is concerned by the current construction practice in the concrete industry and related to delayed ettringite reaction. External temperature (ambient temperature) is quite different from the internal temperature of concrete (this situation is quite serious in actual construction). If the external temperature is further reduced, the external concrete will prevent the thermal expansion of the internal concrete, which is constantly warming up, and the result will lead to concrete damage.
The foundation of the project was laid in November, when the average outdoor temperature was between 4 and 10 degrees Celsius. Especially the two caps located in the north have three sides exposed in the air environment, and will suffer cold currents when pouring. However, the two caps and beams located in the south are less likely to suffer from extreme temperature difference because they are partially buried in the soil on the same level as the ground surface and only exposed to the North side.
Initially, the plan to avoid the expansion and shrinkage of temperature difference was to use the existing standard insulation blanket to insulate concrete and minimize the temperature difference. It is estimated that it will take 14 days to dissipate the heat generated during the hydration of foundation concrete. Due to the tight lifting schedule of steel structure, the contractor attaches great importance to 14-day heat preservation and maintenance of the foundation. They decided to place thermocouples in concrete to determine when the temperature inside the concrete had dropped enough to lift the blanket. This is a simple temperature monitoring program, which can provide the highest internal temperature data in concrete.
The thermocouple is placed inside the cap in the northeast and southwest directions (to represent the central temperature of concrete) and in the center of the outer surface of the same cap (to represent the surface temperature of concrete). The depth of the thermocouple is 50 mm from the surface of the formwork. The temperature measurement of the cap in the southwest direction is carried out to judge the influence of the land on the temperature change and cooling of concrete.
In order to control cracking, the Portland Cement Association's "Design and Control of Concrete Mixtures" code considers that good techniques are as follows: 1) continuous and one-time pouring of all concrete projects, 2) avoiding external constraints from adjacent concrete components, 3) preventing coagulation. Excessive temperature difference between interior and exterior of soil controls the change of interior temperature. The foundation of this project is continuous one-time pouring. It is not restrained by adjacent concrete members.
Implementation of specific measures
The internal hydration heat can cause temperature rise. In order to predict the peak temperature rise inside the foundation, we use two original documents as reference basis, namely Portland Cement Association document and ACI 207.2R specification. The factors causing the peak temperature rise include the initial temperature of concrete, the ambient temperature, the mix ratio of mixtures (total amount of cementitious materials), the size of concrete components and the amount of steel used.
In hot climatic conditions, the most commonly used method is to use cooling water or ice to cool concrete. Other cooling methods include spraying aggregate with water or injecting liquid nitrogen into fresh concrete.
The initial temperature of foundation concrete is estimated to be about 16 C. It is based on the temperature of aggregate and other materials measured by the mixing station. Concrete producers say that cooling concrete below 16 degrees Celsius in a special way will increase production costs. The initial temperature of the concrete, together with the predicted temperature rise of the concrete, is then used to predict whether the concrete will exceed the ultimate temperature of 70 degrees Celsius.
Section 5.3 of ACI 207.1R can be used to approximate the maximum temperature rise of concrete without cooling loss. According to the formula of ACI 207.2R, the predicted adiabatic temperature of concrete with 40 MPa strength is 60 C (i.e. no heat loss).
According to the formula of "Design and Control of Concrete Mixtures", 350 kg cement, 47 kg fly ash, equivalent to 23.5 kg cement can be calculated (generally 1/2 of the weight of fly ash is regarded as the weight of cement that generates hydration heat). The maximum temperature rise of concrete is estimated to be about 59 C (without cooling measures).
The highest temperature of concrete is approximately 77 C (without any cooling loss) when the higher peak temperature is 60 C and the initial temperature of concrete is 16 C. It is predicted that the maximum temperature of concrete will be between 68 and 70 degrees Celsius through normal exposure or cooling in the atmosphere. Therefore, it is not necessary to add ice to reduce the initial temperature of concrete.
In order to control surface cracks, the PCA document recommends that the temperature difference between inside and outside should not exceed 20 degrees Celsius. The document also points out that the maximum temperature difference of concrete with limestone aggregate should be limited to 31 C. The temperature difference of concrete used in this project may exceed this limit temperature. Therefore, thermal insulation blanket should be used to reduce the temperature difference until the internal temperature is the same as the ambient temperature through heat dissipation. The limit temperature difference between inside and outside of concrete selected in this project is 28 C.
In this project, double-layer thermal insulation blanket is chosen. Its total heat conduction is about 422.h.f/Btu (0.70M2.K/W), which can prevent rapid cooling of concrete surface. The concrete was laid on November 22, 2002. Thermal blankets were immediately covered on all exposed concrete surfaces and wood formwork surfaces. Temperature difference between interior and exterior surface of concrete is measured by thermocouple. It is estimated that it will take 14 days to reduce the internal temperature of the concrete foundation to the ambient temperature (28 C). The thermocouple is monitored regularly to confirm when the covered blanket can be removed.
In the cold climate, the soil around the foundation is a good thermal insulation material to reduce heat dissipation. However, the exposed concrete must be insulated by thermal blanket.
The ambient temperature is within 28 degrees Celsius.
The contractor uses thermocouples to regularly monitor the temperature of the concrete and lift the insulation blanket when the internal temperature of the concrete reaches the ambient temperature of 28 degrees Celsius. It is interesting to observe that when the foundation is partially buried under the soil layer, such as the foundation located in the southwest direction, the internal and external temperatures of concrete decrease faster than those of the cap located in the Northeast direction. The reason is that soil is a good thermal insulation material. The temperature difference of the ground beam is not obvious. The reason is that the width of the beam is only 1.2m, while that of the cap is 2.7m. On December 3rd (10th day after pouring), the temperature inside the concrete of the Northeast cap is 27 C, which is basically equal to the expected average ambient temperature of 28 C. At this time, the insulation blanket is removed. After the foundation formwork was removed, no cracks were found on the exposed concrete surface. After a few weeks, there were still no surface cracks.
Limit temperature difference
Surface cracks will not only affect the beauty of concrete structure, but also damage its life. Although the surface cracks are very small, engineers, contractors and owners are still worried about them. Internal cracks and too high concrete temperatures can also cause problems and are even more worrying. Because if delayed ettringite reaction occurs, it will bring many unpredictable effects on the integrity of concrete. In the design of concrete structures, as the use of high-strength concrete is becoming more and more common, designers must understand and adopt the recommended methods of PCA and ACI in order to avoid similar cracks and high temperature problems.
Thermocouple or other temperature measuring device is a method used to determine the temperature difference. In the cold climate, the thermal insulation on the surface of concrete can minimize the temperature difference between inside and outside of mass concrete in the course of curing. Moreover, the thermal insulation on the surface has little effect on the maximum temperature inside. Relying on soil heat dissipation can help to reduce the maximum temperature, while keeping the external temperature of concrete within the acceptable range of internal temperature. Compared with using thermal blanket to insulate exposed concrete, soil can better reduce the temperature difference of mass concrete by a large margin.
Using the technical guide compiled by PCA and ACI, the maximum temperature of concrete can be calculated and checked to prevent concrete from overheating and internal damage. Relevant regulations should be adopted, such as modifying the amount of cementitious materials used in mixtures, reducing the initial pouring temperature, or providing internal cooling measures to avoid high temperature in concrete according to the strength requirements specified in the project.