Determination method of aggregate gradation of concreteDate: 2019-06-27 Views:
Particle gradation is an important performance index of aggregate. Good gradation aggregate has reasonable content of each grain size, which is not much or less, and has low voidage. Under the same workability of concrete, the amount of water consumption and cementitious material can be reduced. The traditional idea is that aggregate only plays the role of lazy filling, and the ability to improve the performance and economy of concrete is not well understood. With the use of fine aggregate such as machined sand instead of river sand, the types of concrete aggregate become complex. It is time-consuming and laborious to draw up the mixing proportion of each aggregate first, and then to carry out the mix proportion design by experimental adjustment. Aggregate two-factor theory junction According to the ACI screening suggestion, not only the aggregate gradation can be optimized, but also the workability of concrete mixture can be predicted according to the gradation situation, which provides convenience for concrete work.
(1) suggestions on screening residue of aggregate by ACI 302.1R
ACI 302.1R provides corresponding suggestions for the screen residue of concrete aggregate at all levels: the recommended range of screen residue on the maximum pore size screen and 0.15mm screen is 0% ≤ 4% and 1.5% ≤ 5% ≤ 0.6 mm, respectively, and the recommended screen balance on 0.3mm screen is 8% ≤ 15%; For aggregates with a maximum particle size larger than 25mm, the screen margin for the remaining sieves should be in the range of 8% ≤ 18%, and for aggregates with a maximum particle size not larger than 25mm, such as 25mm or 20mm, the screen margin for the remaining sieves should be controlled in the range of 8% ≤ 22%. Normally, The screen allowance on 1~2 non-adjacent sieves can be allowed to exceed this range, but the sum of screen allowance of two adjacent sieves shall not be less than 5%and that of three adjacent sieves shall not be less than 8%.
(2) Roughness factor-workability factor theory
Roughness Factor--The working factor theory is that Professor Shilstone of the United States uses the roughness factor(CF)and the workability factor(WF)to reflect the relationship between the aggregate gradation and the workability of the concrete. The CFWF diagram(Fig. 1)is summarized by Shilstone on the basis of a large number of tests, and the CFWF diagram is divided into five areas, which represent aggregates with different grading characteristics.
the coarse particles of the concrete aggregate particles in the region 1 are more coarse, and the mixture is easy to be isolated for the discontinuous gradation; the aggregate gradation corresponding to the region 2 is excellent, and the concrete aggregate particle has good workability and economy for concrete which requires the maximum particle size of the aggregate to be 20-40 mm; the aggregate corresponding to the region 3 is suitable for the concrete with the maximum particle size of the aggregate not more than 20 mm; the aggregate and the fine particles corresponding to the region 4 are more, so that the viscosity of the concrete mixture can be easily caused, not only can the pumpability and the workability be influenced, but also the long-term performance of the concrete is also unfavorable, and the problems of shrinkage, cracking and the like are easily generated; The aggregate fine particles corresponding to the area 5 are too small and the concrete mix appears to be dry The poor packing of the coarse aggregate can cause a decrease in the workability of the concrete.
The aggregate particles are divided into three parts according to the particle size. Q, I and W. Q are the particles with a particle size of more than 9.5mm in the aggregate, which mainly play the role of the skeleton and the filling function, and can reduce the generation and development of the shrinkage and the crack of the concrete. I is a particle with a particle size of more than 2.36 mm and less than 9.5mm in the aggregate, as the intermediate particle, is the main filling material of the void in the concrete system, and its existence is favorable for the fluidity of the concrete. W is a particle with a particle size of less than 2.36 mm in the aggregate, can play a lubricating function as fine particles, and is favorable for improving the workability of the concrete. The calculation formula for WF and CF is as follows :
In the formula, CF is a roughness factor.
WF is a working factor.
Q is the percentage by mass of particles with particle size greater than 9.5mm in the aggregate;
The mass percentage of particles whose particle size is smaller than 9.5mm and larger than 2.36mm in I aggregate;
The particle size of W is less than the mass percentage of 2.36mm particles.
B is adhesive quality, kg.
According to the falling point of (CF,WF), the gradation of concrete aggregate can be analyzed intuitively and comprehensively according to the above formula (CF,WF). It is necessary to adjust the composition of concrete aggregate properly according to the falling point of (CF,WF). The range of CF is between 0 and 100. if it is 100, the lack of particles between 2.36~9.5mm in aggregate gradation can be regarded as intermittent gradation, which is thicker as a whole. If it is 0, there are no particles more than 9.5mm in the aggregate, which is fine as a whole. When CF becomes smaller, the content of fine particles in aggregate is too much, and the total ratio of aggregate to aggregate is too much. The surface area is greatly increased, and in the case of not increasing the amount of the cement slurry, the cement paste is relatively small, thereby affecting the workability of the concrete. When the CF is increased, the content of the large particles in the aggregate is increased, and the content of the intermediate particles is reduced, the specific surface area of the aggregate is reduced, the interface thickness of the cement slurry body and the coarse aggregate is increased, and the workability of the concrete mixture is improved. The concrete working performance is affected by the WF when the value of CF is appropriate, the aggregate is not graded, and the segregation of the concrete is caused. The WF not only is related to the aggregate gradation, but also is affected by the amount of the rubber material. When the dosage of the rubber material is small, the viscosity of the slurry is low, It is necessary to increase the rate of sand properly to avoid segregation and the occurrence of water bleeding. In that case of the same amount of gum material, the workability of the concrete can be improve by adjusting the gradation of the aggregate, appropriately lowering the CF, and increasing the WF, so that the concrete satisfying the workability and the strength requirement can be prepared at a lower amount of the gum material. The CF-WF double-factor theory only takes into account the gradation of the aggregate, and the aggregate grain shape is not considered, and when the aggregate grain shape is not good, it is likely that it is not applicable.