Concrete as the dosage is the biggest building materials, use a very wide range of. But in the construction to the road, floor or thin-wall parts, often appearing on the surface of" powder"," exposed sand" phenomenon. Although on the surface of the concrete" dusting" does not affect the compressive strength grade, but may cause serious damage to concrete pavement or the floor of wear resistance, impermeability, appearance and long durability, adverse to project quality. Caused by concrete surface" dusting" reason is also often the construction sector and the focus of controversy between the concrete supply station. The construction sector often mixing concrete mixed with fly ash and cement manufacturers grinding cement doped with mixed material such as hydraulic poor material as to the road" dusting" arch-criminal, think this part material proportion is light, easy to enrichment in fresh concrete surface, which leads to the concrete surface hardness range decline was caused by the" dusting" main reason. Concrete supply station thinks, "dusting" in concrete surface is the main construction process vibration of excessive or improper maintenance after the construction, and concrete material itself and is mixed with the fly ash. In this paper, based on the concrete surface" dusting" case analysis, discussed the surface powder reasons, and put forward the prevention or mitigation of concrete surface powder corresponding technical measures.
One, case analysis
Guangzhou City, a street extension project, using C35the strength grade of concrete ( cement using the same manufacturers with a variety of cement ), which is part of pavement is used without fly ash ( pure cement concrete of commodity concrete, pavement ) is used with10% fly ash concrete. After the opening, the pure cement concrete pavement without" dusting" phenomenon, fly ash concrete pavement has a section does not " dusting" phenomenon, there is a show" powder" and" exposed sand" phenomenon. Quality inspection departments core pulling test results showed that, all the concrete compressive strength, flexural strength can meet the design requirements. The construction sector is fly ash floating slurry resulted in a surface layer of low concrete strength. Through field sampling and analysis, found the surface powder is not fly ash floating pulp, but the surface of the concrete in the construction and setting and hardening of cement water ratio induced by large. The analysis process is as follows:
Sample A -- without fly ash concrete pavement surface mortar ( not powder )
Sample B -- fly ash concrete pavement surface mortar ( played a part)
Sample C -- without fly ash concrete pavement layer of mortar
The sample grinding, using 0.08mm square hole screen will remove most of the sand in order to obtain the desired sample. The prepared samples for chemical composition analysis, the acid insolubles content analysis, the results as shown in Table1,2 below:

Table 1chemical analysis results can be seen : the ratio of A, C kind of the same chemical composition and acid insoluble content basically similar, A sample loss was significantly higher than that of C like B A, C; and comparing, loss on ignition, SiO2and acid insoluble content is higher, low CaO content low, indicating that B kind of calcareous material content is less, the siliceous material content more. Usually cement products chemical analysis of acid insolubles is not a clean separation of sand, cement, admixture used in concrete admixed with fly ash and curing process into the clay material. The sand is the main chemical components of SiO2, fly ash and clay material is the main chemical components of SiO2and Al2O3. Table 2results, acid insoluble matter is the major components of SiO2and Al2O3, A and B Al2O3sample sample similar content, and not greater than C Al2O3 content sample. The sample B without a large number of fly ash, visible" dusting" instead of fly ash in concrete surface enrichment.
According to the cement hydration degree and chemically combined water content, determination of chemically combined water content of CaO, CaO comparison unit with chemically combined water how many, can compare relative hydration degree. Table 1the loss on ignition ( Loss ) consists of raw materials ( cement unhydrated ) own loss and cement hydration after chemical combined water, setting the original cement loss was 3.5%, excluding acid insoluble matter results as shown in table 3. From the chemical combined water content, sample A, B hydration levels were higher than that of sample C, wherein the sample B hydration degree highest, the unit CaO with chemical combined water high amounts to 0.73, is the pure cement road surface layer concrete sample C 2.49 times, than the" dusting" pure cement road surface sample A higher than 56.53%. The surface of the concrete cement particles in the hydration degree of concrete internal particles bigger than. This paper argues that it is in the construction process of concrete bleeding, caused by surface water cement ratio is too large, the cement hydration is fully caused by. Although the cement with high water level and large water space, but the hydration products joint loose, low intensity is the surface" played " the real reason.
Similar to the pavement played phenomenon is common in large area floor, parking lot, concrete engineering, problems of this sort of multiple site analysis and sampling analysis, the results showed that the," played" is not the main cause of fly ash or other mixed materials or admixture of concrete surface is covered, but loose structure, low strength. Cause of concrete surface strength low of loose structure, there are two main reasons why:
(1) the surface of concrete water cement ratio ( W / C ) is greater than the internal concrete, surface hydration products are overlapped between not compact, large porosity, loose structure, low strength;
(2) improper maintenance construction concrete, early loss of moisture too fast, resulting in a large number of holes, the surface of the cement can not get enough water for hydration, thus the surface of concrete structure is loose, low strength. The surface of concrete water cement ratio is too large and the improper maintenance caused by surface prematurely lose too much water can both lead to concrete" dusting" phenomenon. Detection of concrete surface of cement hydration degree, can help to distinguish" dusting" reason. The surface of cement hydration degree higher mainly due to bleeding induced by. The surface of cement hydration degree low, mainly due to the improper construction maintenance. In many cases analysis, because of bleeding and lead to concrete surface powder case resides majority.
Two, the effect of concrete surface water cement ratio ( W / C ) factor
Concrete is composed of particles of different sizes, different proportion of a variety of solid and liquid composed of a composite material, in cement ( or other cementitious materials ) condensation process, than the major particle to settlement, thus creating the solid particles from the water, namely the new mixed concrete will inevitably cause bleeding, the bleeding is more severe the surface of concrete, water cement ratio ( W / C ) more. Influence of concrete bleeding of concrete mixture ratio, material composition, construction and maintenance etc..
In 1, the mix proportion of concrete with:
Concrete water cement ratio is larger, the setting and hardening of cement longer, more free water, water and cement separation time is longer, the more easily bleeding of concrete; concrete admixture dosage is excessive, or retarding component content is overmuch, can cause a large number of new mixed concrete bleeding and precipitation Shui Mi, a large number of free surface of concrete, the effect of cement setting and hardening of concrete, water retention performance, leading to severe bleeding.
In 2, the component materials of concrete:
Aggregate containing mud more, will seriously affect the initial hydration of cement, clay clay will be wrapped in cement particles, slow and hindered the cement hydration and setting of concrete, thereby increasing the concrete bleeding; fineness modulus of sand and particle composition, fineness modulus of sand is sand, the more coarse, more easily cause bleeding of concrete, especially the 0.315mm below and above 2.5mm particle content on bleeding effect, this part of the fine particles less coarse particles, more concrete, more bleeding; mineral admixture particles distribution also influences on concrete bleeding performance, if the mineral admixture of fine particle content, coarse aggregate content, the easier it is to cause the concrete bleeding. Fine grinding slag as admixture of cement mixing ratio, due to reduced the amount of slag, hydration is slower, and slag glass water retention performance is poor, will often increase the urinary content of fly ash concrete; too coarse, fine aggregate effect abate, also will make the concrete bleeding volume increases.
Cement concrete as the most important cementitious materials of concrete bleeding, and closely related to the performance of. The setting time of cement, fineness, specific surface area and distribution of particles will influence the concrete bleeding performance. Cement setting time is longer, the prepared concrete setting time and setting time is longer, the extension than the cement paste has grown exponentially in concrete, static, the setting and hardening of cement particles prior to settlement, the longer, more easily demonstrated bleeding of concrete; cement fineness thicker, specific surface area and more small, particle size distribution of fine particles ( < 5um ) content is few, the early hydration of the cement quantity is little, less hydration products is not sufficient to block the pores in concrete, the internal moisture easily bottom-up movement, more severe bleeding of concrete. Usually the shaft kiln cement plant for saving energy and reducing consumption, in the preparation of raw materials to add more fluorite mineralization agent, resulting in clinker setting time is greatly delayed, the cement grinding fineness thicker, control, small specific surface area, and therefore often cast the user using the cement is easy to cause the concrete surface" powder". In addition, some large mill ( especially with high Xiaoxuan powder machine system ) grinding cement, while the larger specific surface area, fineness finer, but due to choose powder efficiency is very high, cement particles have crushed less, fine particles ( less than 3~ 5um ) content is little, also easy to cause bleeding and concrete surface dusting phenomenon. Therefore, in the cement production process control of suitable technical parameters and performance index is also effective in ameliorating the C40 concrete surface" dusting" in one way. Different varieties of different strength grade of cement, water retention, setting time, early strength difference is bigger, should be used according to their respective characteristics, the selection of appropriate construction methods, curing condition and time, to minimize the cement varieties and label on the surface of concrete" dusting" effect.
In 3, construction and maintenance:
Construction process of vibration is not the proportion of lighter in concrete admixtures or mixed material vibration to the concrete surface, but increased on concrete bleeding, making the surface of concrete water cement ratio increases; when the cement has not yet hardened concrete surface on water conservation or surface from rain, also will cause the surface of concrete water cement ratio increasing. In addition, in concrete construction and maintenance process, the sun or the weather is very dry, surface water evaporation rate is greater than the concrete bleeding, will lead to large water surface evaporation surface, the cement can not be fully hydrated, build do not have sufficient surface strength and" dusting" phenomenon. Therefore, construction and maintenance methods should be adopted according to different climate, different strength grade of concrete and different varieties of cement and adjust in time, ensure the concrete in the construction to build up enough strength before fully wet curing and does not appear serious bleeding.
Three, conclusion
To avoid concrete appearance" dusting" phenomenon, the first concrete itself should has good water retention, preventing serious bleeding resulted in surface of concrete water cement ratio is too large. From mix proportion and composition of material selection, pay attention to control the water cement ratio should not be too large, not mixed with admixture, retarding time should be appropriate. Sand, stone aggregate should accord with national quality requirements, pay particular attention to the sand below 0.315mm particle content. Cement setting time not too long, specific surface area is too small, particle size should not be too concentrated; secondly, the construction process to prevent vibration caused by excessive concrete serious segregation and weeping; again, the construction should pay attention to timely maintenance, both to prevent concrete surface hardened before being washed by water cement ratio of concrete surface too large, but also to prevent the water in the concrete surface to establish strength before a loss, especially with fly ash or slag concrete, because of its low early strength, surface without enough water to sealing surface of large pores, if not pay attention to early adequate moist curing of concrete surface, water dispersion lost faster more, surface cement do not have access to adequate hydration, will also lead to the surface layer of low strength concrete, loose structure. Usually, in the concrete close to the final set of concrete, two surface ( or pressure ), so that the surface of the concrete structure more compact.
The original author: Wu Xiaomei Fan Yueming Jane Kang