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News > Dynamic industry > Why mass concrete will crack?

Why mass concrete will crack?

2 massive concrete measures to review and disposal of the cellular reasons Pock produce
Internal reasons concrete structure surface Honeycomb 2.1 is formed
2.2.1 Concrete excessive gas, and the poor quality of air-entraining agents. Currently a large amount of concrete pumping, pumping concrete in order to ensure pumpability, often adding the right amount of air-entraining agent in pumping concrete. Because the performance of various air-entraining agents are quite different, and therefore the state presented in concrete are not the same, some air-entraining agent in the concrete will form larger bubbles, and the surface energy is low, it is easy to form China Unicom large bubbles, if coupled vibrating unreasonable, large bubbles can not be completely discharged, will definitely give the hardened surface of the concrete structure caused by Honeycomb.
2.2.2 concrete mix properly, concrete is too thick, the bubble is difficult to discharge when vibrated. Due to improper concrete mix, such as binder side, sand ratio is too large, too small amount of water, admixture unreasonable thickening component, etc., will result in fresh concrete is too thick, the concrete in the mixing will Bubble wrap large number of people, even if vibration is reasonable, the bubble in the thick of concrete discharge is also very difficult, thus leading to the emergence of hardened concrete structure surface Honeycomb.
2.2.3 due to poor workability of concrete, resulting in segregation bleeding. In order to prevent the concrete layers, the concrete into the mold can not sufficiently vibrated after a large number of bubbles did not break out, can lead to the surface of hardened concrete structure Honeycomb appear. Some cement plants in order to increase cement fineness, improve early strength cement, and consider saving energy, often adding a number of people in the mill grinding aids, such as wood, calcium, diethylene glycol, triethanolamine, propylene glycol (l. 2 ) and other substances, some of which aids have cited since the gas, and the introduction of non-uniform bubbles and too large will cause the concrete to harden the surface structure Honeycomb.
2.2 Method to solve concrete internal negative factors
2.2.1 choose to use high-quality air-entraining agent. The quality of air-entraining agent in concrete introduction of gas should be 10 to 200 microns in diameter gown, a relatively high surface energy bubble, bubble distribution more evenly in the concrete (average distance of not more than 0.25 mm). The author has tested 11 kinds of air-entraining agent for concrete air content, compressive strength, setting time and air-entraining agent mixed gas containing losses over time, that the trip to the tannins and ene as the main raw material of entraining agent comprehensive performance than Okay.
2.2.2 reducing concrete viscosity. Appropriate adjustments concrete water-cement ratio, sand percentage component, cement and admixture of material consumption, improve the viscosity of the concrete, but also can improve the quality of concrete structures window layer.
2.2.3 control of fresh concrete workability. If the concrete segregation bleeding, and strictly control the vibration time must be timely complex mode.
2.2.4 If the cement contains entraining component mixing concrete should be added thereto defoamers. For example, adding an appropriate amount of phosphoric acid tributyl ester, silicone defoamers, polyether defoamers and surface tension less than 30 dyne / cm many additives, which can eliminate air bubbles.
External causes 2.3 concrete surface Honeycomb structure formation
2.3.1 "Technical Specification for Concrete Pumping" provides "concrete pouring layer thickness, is preferably 300-500 mm," but in the actual construction, often casting thickness are high, the bubble travel is too long, even if the vibrator time to reach the procedures required, the bubble can not be completely discharged, it will also cause Honeycomb structure to harden the surface of concrete.
2.3.2 irrational use of mold release agents is caused by hardening of the concrete structure surface Honeycomb main reason. Currently releasing agent market is chaotic, good and bad, the product broadly divided into the following categories: mineral oils, emulsified oils, water type, polymers and solvents and the like.
2.3.3 on mineral oil release agent, the different grades of oil viscosity are not the same, even if the oil is the same label, because the ambient temperature is different, not the same viscosity, low viscosity at high temperatures, low temperatures when the viscosity high. When the temperature is low, attached to the template than the sticky oil, fresh concrete structure of the surface layer of bubbles once exposed to thick oil, even if it is difficult to reasonably vibrators bubbles rise along the form is discharged, a direct result of the cellular structure of the surface concrete appear Pock. Some units fully aware of this, adding some diesel fuel in the oil, used to reduce the viscosity of the release agent, this can play a role, but still can not get satisfactory results.
2.3.4 water release agent currently more milk on the market, but there are some products use emulsifiers cited large gas will cause the surface layer to the concrete structure Honeycomb.
Ship mold agent problems 2.3.5 flora and fauna of the more fat oil, the reason is air-entraining products contain relatively large emulsifier and thickener, the surface layer of concrete structure will bring great impact. Templates also make different materials with varying surface layer of the concrete structure of the state. After contact with a variety of solid solution and will form a different contact angles, cement paste is no exception, the contact angle of a liquid on a solid adhesion of the smaller, the stronger (with a cosine law can be explained). Commonly used in everyday life, "non-stick" coating on its surface layer polytetrafluoroethylene (trade name called 特夫隆), in the production practice everyone knows other conditions under the same premise, the use of urea-formaldehyde resin pressed concrete surface layer quality bamboo or wooden template molding concrete surface quality than iron molding template has improved significantly.
2.3.6 Ambient temperature on the quality of the concrete structure of the surface layer is also affected. Because inside the bubble-containing gas, so bubbles off-plot is particularly sensitive to changes in ambient temperature, ambient temperature is high off-plot larger bubble, bubbles smaller carrying capacity, easily shattered. Ambient temperature is low bubble smaller size, large carrying capacity, not easy to form Unicom gas saturation. Even the structure of the surface layer of concrete air bubbles is small, little effect on the appearance of the concrete structure, which bring to mind summer and winter surface layer of concrete structures is better than spring and autumn.
2.3.7 spring and autumn larger temperature difference between day and night, so the bubbles adhered to the surface of the volume change of concrete structures is also great, when the strength of the concrete surface layer of cement paste strength less than the bubble, the bubble volume with the ambient temperature changes, the bubble surrounding cement paste also changes over time as the strength of the cement paste increases, when the bubble surrounding cement paste reaches a certain intensity, the volume of the bubble do not change, if at this time to catch up with the bubble diameter maximum, is bound to leave holes in the surface layer of concrete.
The method of solving concrete 2.4 adverse external factors
2.4.1 strictly in accordance with "Concrete Pumping technical regulations" in the regulations, each pouring concrete thickness should not exceed 50 cm.
2.4.2 choose to use high-quality release agent.
2.4.3 In the case of conditions should be preferred urea-formaldehyde resin pressed bamboo, wood template molding.
2.4.4 Complex mode is one of the most effective ways to eliminate the concrete structure surface Honeycomb. I have found a site in Beijing six concrete bridge stone block flat and smooth surface of the lower, higher up, the more bubbles, bubbles up to the top of a bridge stone block with the same batch of concrete, concrete and even with the same car and the same up and down the template, the results presented different states. Check the main reason is the second layer of concrete when vibrated and vibrated unconsciously the first floor, while the third layer unconsciously vibration and vibration of the second layer and the first layer, press the lower part of the approach bridge stone block equal to many times by the concrete vibrators, dimensional look smooth and clean, the higher up the relative vibration frequency decreases, so the entire pier from the bottom to the surface layer of the bubble gradually increased. Although the "Technical Specification for Concrete Pumping" clearly states: interval 20-30 minutes and then re-vibration once, in spring and autumn were particularly important concrete. According to the author at the construction site of long-term observation units such rare practical operation, which should be attached great importance to the construction manager.
2.4.5 rational use of defoamers. Defoaming including two meanings, one "suds" that prevent the generation of bubbles or foam; the second is "broken bubble", which is full of gas (or bubble) would have produced eliminated. In addition to the special requirements of defoamers foaming system, but also with anti-foaming ability, with less; added to the foaming system does not affect the basic properties of the system; chemical stability, oxidation resistance is strong; the foaming solution The solubility; no physical activity, safety and other characteristics. Further use of effects and defoamers varieties, dosage great relationship, often choose to improper or inappropriate mixing amount will not achieve the desired results.
Generally contain water reducing admixtures, superplasticizer currently used by most of anionic surfactant. When the additive added to the concrete after the concrete in the mixing water surface tension decreased to varying degrees, under a blistering ambush factors, mixing or production process will generate unnecessary bubbles in the concrete, it is in developed countries long ago discovered the problem, they are incorporated in many admixtures appropriate amount of anti-foaming agent, used to eliminate unwanted large bubbles. At present, adding anti-foaming agent in concrete admixtures product is relatively small, yet attracted admixture manufacturers (especially manufacturers of complex admixture) enough attention.

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