What are the advantages and disadvantages of lightweight aggregate concrete compared with ordinary concrete?

The frost resistance of lightweight aggregate concrete is different from ordinary concrete, and lightweight aggregate plays a great role in the frost resistance of lightweight aggregate concrete. Under the same water-cement ratio, the frost resistance of lightweight aggregate concrete is better than that of ordinary concrete, mainly because lightweight aggregate plays a role. As Shaesto belov and others pointed out in their works, cement mortar with good frost resistance cannot protect aggregate with poor frost resistance and the whole concrete from damage. The advantage of lightweight aggregate concrete ceramsite is its porous structure. Lightweight aggregate is in concrete, and there are solid, liquid and gas phases around the aggregate. The shape of holes determines the frost resistance of concrete. The shapes of holes in aggregate can be roughly divided into three categories: 1. The structure of the hole is closed at one end and open at the other end, which is self-pumping. If it is located on the surface of aggregate, it will generate internal pressure and promote the water in the cement stone wrapped around it to spread inward. At this time, the water-cement ratio in the external cement stone drops sharply, which improves the structure of the interface area of concrete and reduces the porosity of the interface area, thus improving the strength of concrete. 2. The structure of the hole that runs through the aggregate depends entirely on the degree of water filling. A hole is harmful when it is completely filled with water. However, when there is not enough water in the hole, there is still extra space in the hole, which can provide extra space for the expansion of surrounding aggregate besides the expansion of water. It can offset some of the pressure. 3. The structure of the hole is completely closed, which can provide a certain expansion space for the surrounding substances. In addition, regardless of the structure of the hole, its water filling degree is always related to the strength of concrete. Ordinary concrete is a capillary porous body composed of cement mortar and coarse aggregate. In order to obtain the necessary workability when mixing concrete, the mixing water is always more than the hydration water of cement, and this excess water remains in the concrete as free water to form a connected capillary, occupying a certain volume. Free water in capillary is the main internal cause of concrete freezing damage. Because water freezes and expands when it meets cold, it destroys the internal structure of concrete. However, it should be pointed out that, in general, the freezing of water in capillary holes will not cause serious damage to the internal structure of concrete. Because there are some gel pores formed by cement hydration and non-capillary pores formed by other reasons in concrete, these pores are often mixed with air. Therefore, when the water in the pores freezes and squeezes into the gel pores, the expansion pressure decreases to prevent the internal structure of concrete from being destroyed. But when the concrete is saturated, the situation is completely different. At this time, when the water in the capillaries freezes, the water in the gel pores is in a supercooled state. Because the freezing point of water in concrete pores decreases with the decrease of pore size. The temperature of ice core in gel pores is lower than-17. 8. Water in supercooled state in gel pores penetrates into the interface of ice in capillary pores because its vapor pressure is higher than that of ice at the same temperature. Then an osmotic pressure is generated in the capillary. In addition, the permeability of gel pores will inevitably further expand the amount of ice in the pores. It can be seen that when saturated concrete freezes, the pore wall of its capillary bears both expansion pressure and osmotic pressure. When these two pressures exceed the tensile strength of concrete, concrete will crack. After repeated freeze-thaw cycles, the damage of concrete will continue to expand and gradually accumulate. After a certain freeze-thaw cycle, the cracks in concrete will penetrate each other, and their strength will gradually decrease or even be completely lost, which will destroy the concrete structure from the outside to the inside. This hypothesis about concrete freezing damage is more accurate than the hypothesis that pore water freezes and the volume expands by 9% after concrete freezing, causing internal structure damage. From the above, it can be seen that the problem of freeze-thaw damage does not exist in cement soil with medium strength or above, which is generally not in direct contact with water. Generally speaking, when concrete absorbs a lot of water, after freeze-thaw cycle, internal cracking and surface spalling often occur, resulting in the decrease of elastic modulus and weight. The speed and extent of this decline determine the frost resistance of concrete.