Has the following characteristics:
(1) If the concrete is unconstrained (no ribs or less ribs), the alkali aggregate reaction crack is net-like (turtle-back pattern), the crack network is close to the hexagon, and the crack is split from the net node for three minutes. The angle is about 120°, and small cracks can be developed between the larger hexagons.
(2) When the alkali aggregate reaction is restrained by steel bars or external forces, the expansion force will be perpendicular to the direction of the binding force, and the expansion cracks will be parallel to the direction of the binding force, and the ribs will appear.
(3) The alkali-aggregate reaction of large-volume and unreinforced concrete has a depth of up to several tens of centimeters and a width of up to about 25px.
(4) The depth of the alkali aggregate expansion crack of reinforced concrete generally does not exceed the thickness of the protective layer, and the width is mostly (0.3 to 0.5) mm.
(5) Alkali aggregate reaction cracks are usually accompanied by exudates, mostly translucent milky white or yellowish brown, sometimes brown or black.
(6) Similar to the network crack caused by shrinkage, but the shrinkage crack occurs earlier, mostly within a few days after construction. The alkali aggregate reaction cracks appear later, many years after construction or even 10 to 20 years later.
Alkali aggregate reaction crack
The causes of the alkali aggregate reaction cracks are as follows:
(1) The alkaline material formed by the reaction of active silicon and cement in the aggregate, or the alkaline material contained in the admixture, or the alkaline material brought in by the outside (such as maintenance water, groundwater, alkali in the adjacent structure) A slow expansion chemical reaction occurs in the solution, which may gradually lead to cracking of the concrete.
(2) The result of the alkali-silicon reaction will form an expansive gel that will absorb moisture from other parts of the structure, which will result in local expansion while also causing tensile stresses, ultimately resulting in structural Completely destroyed.
(3) In some cases, carbonate rocks participate in chemical reactions with alkalis, with the result that they cause harmful swelling and cause cracking. These harmful alkali-carbon reactions are mainly related to the dolomitic limestone of cohesive soils, which have very fine textures (cryptocrystalline). The affected concrete exhibits a network of cracks. This reaction differs from the alkali-silicon reaction in that there is no accumulation of gels in the crack.
Precaution:
(1) Control the alkali content of cement.
(2) Control the alkali content of concrete.
(3) Control the use of alkali active aggregates.
(4) can inhibit alkali-aggregate reaction admixture, such as powder coal ash, slag, silica fume and so on.
(5) The use of air entraining agent to alleviate the expansion pressure caused by the alkali aggregate reaction.
(6) Isolation of water and air sources.
Japanese practice:
The alkali aggregate reaction conditions are formed during the preparation of concrete, that is, only enough alkali and reactive aggregates are formed in the prepared concrete, and the concrete gradually reacts after the concrete is poured. The amount of the reaction product is swelled and the internal stress is sufficient to make the concrete When cracking, the project began to crack. Such cracks and damage to the engineering develop with the development of the alkali-aggregate reaction, which can cause the project to collapse. Some people have tried to control the development of alkali-aggregate reaction by blocking the source of water. For example, the overpass of the Songyuan section of the expressway from Dagu to Kobe in Japan, large-area alkali aggregate reaction cracking in piers and beams, Japan has injected all cracks. Epoxy resin, after injection, the entire beam and pier surface are completely covered with epoxy resin coating, in an attempt to control the progress of the alkali aggregate reaction by preventing moisture and humid air from entering. The result is only one year, and many cracks. . Therefore, all countries in the world take measures when preparing concrete, so that concrete engineering does not have the conditions of alkali aggregate reaction.
Detailed measures:
1. Controlling the alkali content of cement Since the United States proposed in 1941 that the cement content is less than 0.6% oxygenated sodium equivalent (ie Na2O + 0.658 K2O), since the safety limit of alkali aggregate reaction is prevented, although some areas of aggregates are in cement. When the content is less than 0.4%, the alkali aggregate reaction may still cause damage to the project, but in general, the cement content is less than 0.6%. As a safety limit for preventing alkali aggregate reaction, it has been accepted by most countries in the world. More than a dozen countries have included this safety margin in national standards or norms. In many countries, such as New Zealand, the United Kingdom, Japan and other domestic cement plants produce cement with less than 0.6% alkali. Canadian Railroad Bureau provides that, if the defendant does not use active aggregate, concrete railway projects Always use low-alkali cement alkali content of less than 0.6%.
2. Controlling the alkali content in concrete Since the source of alkali in concrete is not only from cement, but also from mixed materials, admixtures, water, and sometimes even aggregates (such as sea sand), it controls the total alkali of various raw materials of concrete. The amount is more important than simply controlling the alkali content of cement. In this regard, South Africa has stipulated that the total alkali content per cubic meter of concrete should not exceed 2.1 kg. The United Kingdom has proposed that the total alkali content (Na2O equivalent) of all raw materials per cubic meter of concrete should not exceed 3 kg, which has been accepted by many countries.
3. Selection of aggregates If the alkali content of concrete is less than 3kg/m3, the activity test of aggregates may not be carried out. If the alkali content of cement is high or the total alkali content of concrete is higher than 3kg/m3, the activity detection of aggregates should be carried out. If it is tested as active aggregate, it can not be used, or after being mixed with non-active aggregates in a certain ratio, it can be mixed according to the ratio specified in the test when there is no damage to the project through the test.
4. Mixing mixed materials with some active mixed materials can alleviate and inhibit the alkali aggregate reaction of concrete. According to national test data, S-containing 10% silica fume can effectively inhibit alkali aggregate reaction. It is reported that since 1979, Iceland has been mixing 5-7.5% silica fume during cement production to prevent alkali aggregate reaction. Engineering damage. In addition, the fly ash is also very effective, and the alkali content of the fly ash is different. After the test, even if the fly ash with high alkali content replaces 30% of the cement, the alkali aggregate reaction can be effectively inhibited. In addition, the commonly used inhibitory mixture material is blast furnace slag, but the dosage must be greater than 50% to effectively inhibit the damage of the alkali aggregate reaction to the project. The recommended dosages of blast furnace slag in Damei, Yingde and Germany are all above 50.
5. Sources of Isolation of Water and Wet Air If the source of water and air can be effectively isolated in the area where the alkali-aggregate reaction occurs in concrete works, the effect of alleviating the alkali-aggregate reaction on engineering damage can be obtained.
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