Effects of curing conditions and insulation on permeability, microstructure and mechanical properties of high strength concrete

Effects of the humidity and temperature on concrete are defined as curing conditions. There should be humidity in the air for cement to continue hydration process. In the absence of humid, hydration becomes slow until it stops and consequently, concrete will not gain strength. For this reason, appropriate curing conditions should be provided in order to get concrete having desired properties. 7 days for cast-in-place concrete, more than 7 days of curing in humid environment for concrete made of pozzolana cement are advised. However, in prefabricated concrete industry, where production and assembly processes are faster, applying long curing times for concrete and concrete elements slows production. In these situations, temperature can be increased to increase hydration rate and so strength gaining rate. The most common method for both maintaining appropriate humidity and for increasing temperature is to apply steam curing. Applying curing by increasing temperature in isolated environment is applied instead of standard humid curing under normal conditions while producing ultra-high strength concrete which have spread in the recent years. Steam curing processes can be summarized as pre-waiting, controlled heating, holding at constant temperature and controlled cooling. In case of these processes are not applied correctly and timely, lack of strength of the concrete or cracking at the surface of the concrete can be seen. For example, in case of fast dropping of temperature in the environment cause the surface of the concrete to cool quickly while inside of the concrete is still warm. This causes high temperature difference between inside and surface of the concrete which will lead different deformations and so cracking. It is obvious that a cracked concrete is affected negatively, by means of permeability and mechanical properties. For instance, in case of presence of cracking on the concrete spacer, transportation of chloride ions will happen very quickly and thus Chloride concentration at the cracking area will be much higher than other places. This high-level chloride concentration causes corrosion to happen in much shorter time. Besides, because of fast passage of other harmful effects and water through these cracking, these effects may harm the concrete much more easily. Physical influences like freeze-thawing would create faster damage. For these reasons, cracking in concrete elements reaching steel reinforcement should be repaired appropriately. Additionally, depending on environmental effects faced by concrete elements, cracking smaller than 0.2 mm or web-shaped cracking should be insulated. For this, it is advised to cover the cracked concrete surface to be covered with water-repellent materials. However, there are few studies in the literature concerning life-time of these cover materials and influences of environmental effects like sun-light, temperature, freeze-thawing on these materials. In order to achieve structures with long service lives, the quality of concrete cover should be taken into account and to obtain a crack-free cover with low permeability, care should be given to the curing conditions. With sufficient cover thickness, the reinforcement can be protected from corrosion and long service life can be ensured. However, studies in the literature related to the permeability of the interfacial zone between the cover material and the surrounding concrete is not enough. The main objective of the presented work is to study the effect of curing conditions on the physical and mechanical properties of concrete. In the experimental work; concretes in three different classes including normal strength, high strength and ultra high strength were cast. The concretes were cured in three different curing conditions which were; water curing at standard conditions, steam curing and high temperature curing in an isolated environment. Compressive strength, chloride permeability and capillary water absorption of the samples were determined. The experimental results demonstrated that curing conditions significantly affect the properties of concrete. The effect of early age cracks, occurring due to deficiencies in steam curing or the conditions after casting, were also investigated in the study. Cracks were formed on some of the concrete samples and the cracked concrete surfaces were coated with the water repellent material. Permeability tests were made by on these samples to investigate the effectiveness of this material. In addition, the effects of different types of cover spacers materials, such as concrete or plastic, on the permeability of concrete were also investigated. In this study, for normal strength concrete 300 kg/m³ cement and 0,52 water/cement ratio; for high-strength concrete 500 kg/m³ cement and 0,32 water/cement ratio and for ultra-high-strength concrete 1000 kg/m³ cement and 0,17 and 0,22 water/cement ratio are used. Applying curing at high temperature causes decrease in compressive strengths of normal strength and high strength concrete. Whereas, applying curing at high temperature has positive impact on compressive strength of ultra-high strength concrete, in other words, increase in strength. Decrease in strength because of curing is more obvious for normal concrete than high strength concrete. Internal structure of normal and high-strength concrete are harmed by the effects of curing at high temperature that also cause increase in permeability. On the other hand, internal structure of ultra-high strength concrete is healing and becoming denser under curing at high temperature. For all concrete classes, samples with plastic spacers have higher capillary water absorption and rapid chloride permeability compared to those with concrete spacers. The increase in the chloride permeability is higher for the ultra high strength concretes compared with normal and high strength concretes. When capillary water absorption and rapid chloride ion permeability of the concretes with and without spacers are analyzed, it is seen that the use of spacers increases the permeability for all concrete classes. It is seen that applying water repellent material to cracked concrete samples decrease both capillary absorption and rapid chloride permeability values. These decreases are seen at all types of concrete, however, mostly at normal strength concrete, then high-strength concrete and the least at ultra-high strength concrete. Permeability values of cracked samples are decreased by using water repellent materials. Aging tests are applied to both water repellent material applied samples and blank samples not having cracking. Aging test creates web-shaped cracking even on the blank samples. As a result of these test capillary absorption values of all samples are increased, however, increase in water repellent material applied sample is less than the other samples. These cycles damage all concretes that the water repellent material had been applied. However, this damage is higher in normal strength concrete than those in the high and ultra high-strength concretes. The efficiency of the water repellent material is reduced significantly in normal strength concrete after aging and freeze-thaw cycles. After the aging and freeze-thaw cycles, the water repellent material on normal concretes is lost but those on ultra-high strength concrete is still observed. For ultra high strength concretes, the rapid chloride permeability values are negligible even after aging and freeze-thaw cycles. This result shows that ultra high-strength concrete is not affected by this aging. The internal structure of the concrete is also effective for this result.