Abstract: along with the fast development of architectural industry, science and technology has improved continuously, works on the concrete in a variety of increasingly high performance requirements, construction requires not only the working performance of concrete, strength index of high, good durability, but also for the concrete structure has smooth as a mirror of the appearance, especially the fair-faced concrete structure requirements of the more prominent, therefore gives us a new topic, namely how to ensure that the surface of the concrete structure without voids, smooth as a mirror.
Key words: concrete structure of honeycomb
One, honeycomb pitting surface of concrete internal reasons for the formation of
1. Concrete air content is too large, poor quality and air entraining agent. The pump concrete dosage is bigger, in order to ensure the pump concrete pumpability, often in pumping concrete by the amount of air entraining agent. As a result of all sorts of air entraining agent properties are quite different, so in the concrete in the state are not the same, some air entraining agent in concrete will form a large bubble, and the surface energy is low, easy formation of UniCom of big bubble, if coupled with the vibration of unreasonable, large bubbles can not be completely discharged, will give the hardening the surface of concrete structure surface resulting in honeycomb.
2. Improper concrete mix, concrete is too thick, vibration when the bubble is difficult to discharge. Due to improper concrete mix, such as cement, sand rate slightly slants big, water is too small, there are unreasonable admixture thickening composition, will lead to the new mixed concrete is too thick, in the concrete mixing will wrap large bubbles, even if vibration is reasonable, bubbles in thick concrete discharge is also very difficult, therefore result in hardening the surface of concrete structure surface appearance of honeycomb.
3. Because of poor workability of concrete, segregation and weeping. In order to prevent the concrete, concrete can fully vibrator, a lot of bubbles not line, can also result in hardening the surface of concrete structure surface appearance of honeycomb. There are some cement plants in order to increase the fineness of cement, improve the early strength of cement, but also consider saving electric energy, often in the flour mix grinding agent, such as wood calcium, two glycol, propylene glycol, triethanolamine ( L. 2) and other substances, because some of them grinding agent and air entraining, introduced the bubbles are not uniform and slants big, also can give the hardening the surface of concrete structure surface resulting in honeycomb.
In two, the concrete internal disadvantage factor method
1. Choose to use the high-quality air-entraining agent. The quality of the air entraining agent in concrete is introduced in the gas and diameter in10 -200 microns, relatively high surface energy bubbles, bubbles in the concrete more evenly distributed ( average distance not greater than 0.25 mm ). The author has tested 11 types of air entraining agent on concrete air content, compressive strength, setting time and mixing air entraining agent by air loss, think to tannic acid and the brigade was as the main raw material of air entraining agent with better comprehensive performance.
2. Concrete to reduce viscosity. Appropriate adjustments to the concrete water cement ratio, sand ratio, the amount of cementing material and the admixture of the components, to improve the concrete consistency, but also can improve the concrete structure window layer quality.
3. Control the workability of the fresh concrete. If the concrete segregation and weeping, strictly control the vibration time, vibration rehabilitation must be timely.
4if the cement containing gas leading component, in the mixed concrete should be joining the defoaming agent. For example, adding proper amount of three small lipid phosphate, organic silicon defoaming agent, polyether defoamer and a surface tension less than 30 dynes / cm many additives, can eliminate the bubbles.
In three, the surface of the concrete structure formation of honeycomb external causes
In" concrete pumping technical regulations" provisions" in the concrete pouring of the layer thickness, should be 300- 500mm", but in actual construction, often casting thickness are high, due to the gas bubbles travel is too long, even if the vibration time to code requirements, the bubbles are not fully discharged, it will give caused by hardening the surface of concrete structure surface honeycomb.
The irrational use of release agent is caused to harden honeycomb pitting surface of concrete is the main reason. The current release agent market confusion, uneven, some good and some bad, products are broadly divided into the following categories : mineral oil, emulsified oil, water type, polymer type and solvent type.
Mineral oil release agent, different grade engine oil viscosity are not the same, even with the label of the oil, due to the different environment temperature, viscosity is not the same, the high temperature of low viscosity, low viscosity and high temperature. When the temperature is low, attached to the template on which the oil is viscous, fresh concrete surface layer of bubbles upon exposure to viscous oil, even reasonable vibrating bubble is also very difficult to rise along the template directly discharged, leading to the concrete structure surface of honeycomb. Some units are fully aware of this point, in adding part of diesel oil, used to reduce the viscosity of release agent, it can play a certain effect, but still not satisfied.
Emulsion type release agent present in the market more, but there are some products using the emulsifier air entraining is bigger, will also give the concrete surface layer caused by honeycomb.
Fat of animal and vegetable oils for the release agent problems more, its reason is the air-containing products of more emulsifier and thickener, surface concrete structure will be greatly affected. Templates of different materials will make the concrete structure surface layer have different state. Solution and solid contact will form different contact angle, cement slurry is not exceptional also, contact angle smaller liquid on solid adhesion is stronger ( using the cosine theorem can be explained ). In the daily life of the common" non-stick cookware" its surface layer is coated with polytetrafluoroethylene ( commodity name of turf long ), in the production practice we all know when other conditions are same, the use of urea-formaldehyde resin pressed bamboo or wooden template molded concrete surface quality than using iron template molding the concrete surface quality is improved obviously.
Environmental temperature on concrete structure surface layer quality also have effect. Because of containing gas inside the bubble, the bubble break plot changes in environmental temperature is particularly sensitive to the ambient temperature is high, larger bubbles bubble break plot, bearing capacity of small, easy to burst. Low temperature environment when the bubble size smaller, larger bearing capacity, easy to form a Unicom gas saturation. Even though the concrete structure surface with bubbles, bubbles are small, little effect on concrete structure appearance, which makes people to think that winter, summer concrete surface layer, the fall of Yu Chun.
Spring, autumn larger temperature difference between day and night, so attached to the concrete structure on the surface of the bubble volume change is very big also, when the concrete surface layer of cement mortar strength strength is smaller than the bubble, bubble volume varies with the ambient temperature changes, the bubble around the cement slurry will also change, with the passage of time the strength of the cement paste increased, when the bubbles around the cement paste achieves certain intensity, and not with the bubble volume changes, if this time is to catch up with the bubble diameter was greatest when bound to the concrete surface layer, leaving holes.
In four, the concrete method of adverse external factors
1 strictly according to" technical specification for pumping construction of concrete" in the provisions, each layer of concrete thickness of not greater than 50 cm.
2. Choose to use the high-quality release agent.
3. In the case of conditions should be preferably used urea-formaldehyde resin pressed bamboo, wood template molding.
4. The complex is to eliminate vibration of honeycomb surface concrete structure is one of the most effective ways. The author ever was in Beijing some site found6concrete abutment surface is smooth, the more the bubble, bubble up, an abutment with a batch of concrete, even with the same car concrete, and upper and lower template are the same, the results show different state. Check the reason is mainly the vibrator second layer concrete when not consciously and vibration of the first layer, the third layer vibration does not consciously and vibrating the second layer and the first layer, according to the practice of the lower part of the concrete abutment is repeatedly by vibration, so the appearance of smooth, the relative vibration frequency gradually reduced, so the whole pier surface layer from bottom to top bubble gradually increased. Although in" concrete pumping technical regulations" clearly stipulates: interval of 20- 30 points to complex vibration time, spring, autumn is especially important for concrete construction. According to the long-term observation in construction site, the actual operation of the unit so very rare, should cause height to take seriously in construction management.
5. The rational use of defoaming agent. Defoaming includes two aspects, one is the" bubble", i.e., to prevent air or bubbles; two is the" bubble breaking", is to have been produced by gas saturation ( or foam ) to remove. Defoaming agent in addition to foaming system for special requirements, but also have the defoaming ability, less dosage; added to the foaming system does not affect the system's basic properties; chemical stability, oxidation resistance is strong; the foaming solution solubility; no physiological activity, high security characteristics. In addition the use effect and defoaming agent varieties, quantity has the very big relations, often choose the improper or with improper never reached the expected effect.
General additive contained in the water reducing agent, currently used water-reducing agent for the majority of anionic surface active agent. When added to concrete admixture, in concrete mixing water surface tension decrease to varying degrees, ambush the blistering factor, in the concrete mixing or production process will produce unnecessary bubble, developed countries have long been aware of this problem, they in many additives are mixed into the right amount defoaming agent, used to eliminate harmful bubbles. Currently in the domestic concrete admixture adding defoamer products is relatively small, has not caused the admixture manufacturers ( especially compound admixture manufacturers ) is enough take seriously
Author: Zhang Deshen