1 Introduction
According to the principle of concrete design, self-compacting concrete (SVB) can be divided into three types.
The powder content of self-compacting concrete (SVB) in the code is obviously higher than that of ordinary vibratory compacted concrete. Traditional concrete strives to achieve higher aggregate volume and smaller particle void volume, which does not exist for self-compacting concrete (SVB). In other words, the coarse aggregate "floats" in the cemented mortar composed of powder (cement ≤ 0. 1.25mm aggregate concrete admixture), mixing water and self-compacting concrete (SVB) plasticizer. The excellent function of self-compacting concrete (SVB) mainly depends on its two characteristics: (1) has high enough fluidity and air permeability, which can achieve the best combination of reinforcement and concrete under the condition of high-strength reinforcement and minimize the harm caused by defects (such as honeycomb); (2) Self-compacting concrete (SVB) has good bonding ability in maintaining structural stability and preventing segregation. In general separation, two phenomena may occur:
(1) settlement: large amount of aggregate settlement and mud overflow;
(2) Separation: During the flowing process, the large aggregates will no longer flow together.
It is very important and absolutely necessary to choose the best mixing ratio of powder particles, the ratio of powder particles to water and the amount of plasticizer in order to obtain concrete with sufficient cohesiveness and satisfactory fluidity.
The amount of water is determined by meeting the water demand of powder particles and wetting the surface of particles. Obviously, the water demand of concrete can be changed by replacing the composition of powder particles or by other substitutes (such as fly ash instead of cement), which will change the composition of concrete and the performance of fresh concrete. The addition of extra water will affect the fluidity and cohesiveness of concrete, which may lead to the improvement of fluidity and reduce the cohesiveness and structural instability of concrete. The change of 3 l/M3 water addition is enough to cause concrete settlement, separation, bubbles and holes or low fluidity. When the influence of water on fluidity and cohesiveness may lead to segregation, the most important concrete fluidity can be adjusted by using the content of plasticizer.
At present, almost the only new generation plasticizer is used to produce self-compacting concrete, which is called PCE plasticizer. We must realize the mutual influence (mutual adaptability) between cement and plasticizer, and also pay attention to the influence of temperature on concrete fluidity. In addition, adding a small amount of plasticizer into the mixer will lead to additional liquefaction effect, which is called "storage effect" and may lead to concrete segregation in the near future.
Adding fine materials (limestone powder or fly ash) can improve the fluidity of concrete. Other things being equal, the content of high fine components will reduce the fluidity of concrete. In addition to the influence of concrete admixtures on the rheological properties of self-compacting concrete (SVB), there are cases that have nothing to do with technical practicability, that is, the influence of admixtures on the visual appearance art of concrete surface. According to the characteristics of limestone powder and quartz powder, it will bring brighter concrete surface than concrete mixed with fly ash.
The sensitive relationship between concrete components may lead to a relatively small change in the mix proportion of concrete, which will lead to a significant change in the performance of dense concrete (SVB).
2 requirements for fresh concrete
2. 1 liquidity
The fluidity of concrete is judged by the fluidity measured without blocking ring. Generally speaking, the fluidity (sm) of self-compacting concrete should be between 700 mm and 800 mm. The fluidity of concrete can also be judged by the fluidity (smb) measured by the retaining ring (through the gap between the steel columns of the retaining ring). It is particularly important to verify whether the large aggregate in cement mortar can flow through obstacles (such as steel columns) or whether the large aggregate passing through obstacles is separated. This effect will occur when the spacing between steel columns is large in aggregate diameter. Therefore, the number and spacing of columns blocking the circular ring during inspection depend on the maximum particle size of aggregate. The diameter of the retaining ring reinforcement column is 18mm. The diameter of the retaining ring is 30cm. The use of discontinuous graded aggregate further improves the adhesion resistance.
The composition of self-compacting concrete (SVB) is considered to be useful, if the maximum aggregate smoothly passes through the gap between steel columns, when there is no height difference between the inside and outside of the retaining ring, and the value of the maximum smb is less than the value of sm.
2.2 Funnel outflow time
The viscosity of self-compacting concrete (SVB) is determined by funnel outflow time (tTr). The determination of outflow time needs to determine the outflow time of SVB with a V-shaped funnel under the condition of continuous fluid injection (see Figure 2). Usually, the outflow time (tTr) of self-compacting concrete (SVB) is between 5 ~ 5 ~ 20s.
2.3 Settlement trend
In order to test the settlement trend of self-compacting concrete (SVB), a cylinder with a height of 500mm and a diameter of 150mm was filled with concrete. At the height of one third of the cylinder, the concrete sample is divided into three parts. After washing the cement mortar, according to the quantity difference of fresh coarse aggregate, it can be judged whether there is a settlement trend of concrete. Self-compacting concrete (SVB) with coarse aggregate content less than 20% of the average coarse aggregate content can be regarded as settlement stability.
The research shows that liquidity and funnel outflow time are undoubtedly two very important indicators, which can be used as practical application standards. These indicators are obtained through initial testing and are clear and guaranteed in a specific application window. The concrete with these indexes neither settles nor stagnates, and has satisfactory fluidity, air exhaust and settlement stability. Under the same concrete composition, temperature will lead to different application windows.
In order to detect fluidity and viscosity simply and quickly, and suitable for use in construction site, VDZ invented a combined test method, which can accurately measure two performance indexes in one test. The equipment consists of a cone with an outlet and a fluidity test plate.
3 production and transportation
The production of self-compacting concrete (SVB) requires high-quality technical equipment and well-trained personnel. Due to the high sensitivity to water content fluctuation, the following production requirements are generated:
(1) The metering accuracy of mixing equipment should be as high as possible;
(2) Before production, the moisture of aggregate in the silo should be deducted;
(3) The water content of sand should be determined continuously; The water content of coarse aggregate should also be measured continuously;
(4) According to DIN EN1008: 2002-10, the remaining water can only be used as DAfStb under certain conditions, otherwise it will bring adverse effects to concrete with high solid particle content;
(5) Mixers and transport mixers must be completely emptied to absorb the water for washing the machine.
Different transportation time will affect the consistency of concrete, and in the case of storage effect mentioned above (depending on plasticizer), the intensity of this effect is different. In addition, there are weather conditions, such as solar radiation, which causes the temperature of fresh concrete to rise and changes the consistency of concrete. For the changes of concrete consistency and real-time concrete composition that may be caused by transportation, weather and other reasons, efforts should be made to strengthen the regular information exchange between the ready-mixed concrete station and the construction site on the performance of fresh concrete.
In order to directly correct the deviation of the expected performance of fresh concrete before pouring, additional plasticizer dosage can be added according to DAfStb at the construction site. If there is a dosage specification for concrete production, in order to adjust the required fluidity or viscosity at the existing temperature and consistency of fresh concrete, the dosage of plasticizer must be specified in the specification. This description was obtained from the initial test.
Because of the sensitivity of self-compacting concrete (SVB) to fluctuation in the process of production, transportation and pouring, the concrete of each transport mixer must be checked and accepted in the construction site according to regulations. In the acceptance, as a simple test method to evaluate the construction performance of self-compacting concrete, check the fluidity of retaining ring concrete and the outflow time of funnel.
Through the analysis of two important indexes, fluidity and outflow time, it can be shown to the operator whether the self-compacting concrete (SVB) that passed the initial test is within the predetermined application window. Because the cone combined determination method with outlet is simple to use, it is applied more and more in practice.
Accurate supply planning is very important. To do this, as soon as the front car is emptied, the rear car will start feeding. In addition, we should pay attention to whether the car behind can arrive in time and whether the fresh concrete can be tested.
The reality puts forward high requirements for the logistics efficiency of ready-mixed concrete enterprises. If the specific boundary conditions of the construction site (such as different temperature ranges) have been clarified through preliminary tests, this aspect can be satisfactorily realized.
4 concrete pouring
Before pouring, check whether there is residual water in the formwork. A small amount of residual water may lead to the segregation of self-compacting concrete (SVB), because the water content of self-compacting concrete (SVB) is no longer allowed to be relaxed, and it is impossible to increase it.
Self-compacting concrete (SVB) needs a definite pouring process to obtain a satisfactory concrete and exhaust performance.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd