[1]
Radial continuous distribution model of particle size of coarse aggregate In order to accurately describe the relationship between the value of SD and segregation degree, ,and in Eq.1 were essential. Remarking the coarse aggregate area on vertical section as ,and from top to bottom. However, the thickness of coarse aggregate corresponding to the area (,and )was difficulty to get because it was not a constant value. Therefore, a constant value must be determined to replace with the thickness. We called the constant value as equivalence cumulative thickness of coarse aggregate. This constant value must meet two requirements. One is and the other is that total volume from the constant value and area should equal to the true volume of the coarse aggregate in the specimen. Therefore, based on the assumption that fresh concrete is uniformity [[] Lihan Li; Xurong Ma. Influence Research of Gradation Segregation on Performance of Asphalt Mixture[J]. Journal of Tongji University(Natural Science,2006,35(12):1622-1626.(in Chinese) ,[] Xiuning Bao, Xiaoning Zhang. Segregability evaluation of granular pavement materials [J].Journal of South China University of Technology: Natural science edition, 2010,38(3): 31-36. (in Chinese) ] (components of concrete distributed uniformity in the volume space regardless the particle size). The model of radial size continuous distribution of coarse aggregate was established in order to calculate radial equivalent total thickness of coarse aggregate. Then, the particle size less than a particular size can be added based on the model. So, the distribution probability of that size can be calculated. Now, set distribution function of coarse aggregate particle size as.Then, probability density function can be calculated as by derivation. Thus, particle size of coarse aggregate on any range [a,b] conform to continuous distribution. The distribution probability was as shown in Eq.2.
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[2]
Presume the average particle size on any range [a, b] of coarse aggregate.can be obtained by Eq.3.
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[3]
After obtaining radial average particle size of coarse aggregate, can be used to represent the equivalent total thickness of the radial particle size. In which, the indexrepresent the particle size that converted all the radial particle size of coarse aggregate into average. Then, Eq.4 can be used to calculate the value aggregate volume in every layer. In which, the indexwas on behalf of the number of layer.
DOI: 10.5353/th_b3124075
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[5]
Where:is equivalence cumulative thickness of coarse aggregate in vertical section in group ;is the average area of coarse aggregate that on vertical section in group;is theoretical volume of coarse aggregate in single specimen. Calculation of SD. The area of the coarse aggregate on vertical section in cylindrical specimen can be determined according to the method that area of the coarse aggregate in chapter 3.2.1. Probability theory shows that will close to the theoretical area of the coarse aggregate on vertical section as well as enough specimens. Now, was used to instead of the theoretical area. Area of coarse aggregate on vertical section in top layer in groupwas denoted by. Meanwhile, middle layer denoted by, lower layer denoted by.Thus, Eq.1 can be developed to Eq.6 to calculation the SD of specimen . According to Eq.6, the values of SD were shown in Table 2 column ⑥.
DOI: 10.14359/2252
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[6]
Where: SD is the value of segregation degree belong to group, dimensionless; is the average area of coarse aggregate that on vertical section in 90 cylindrical specimens. Application of SD From Eq.7, when the coarse aggregate in concrete specimen is uniform or each layer contains coarse aggregate equally, SD value is equal to zero. On the contrary, the distinction of coarse aggregate content in each layer is great, in other word, the segregation degree is significant, and SD value is also greater. Segregation directly caused the decline in the strength of concrete. Fig.2 shows the standard compressive strength of concrete specimens with same mix proportion. The compressive strength increased imposed by early vibration. At the same time, the uniformity of the concrete declined. But particles of each component in concrete became even denser with the discharge of air bubbles [[] Zhi Fu. Research on the Vibrated Structure Viscosity of Freshly Mixed Concrete[J].Journal of Highway and Transportation Research Development,1996, 13(3): 1-9. (in Chinese) ]. When the vibration time coming 100 s, the strength of the specimen began a downward trend. The distribution of coarse aggregate emergence large was different from each other in Fig.1 (b) (c). Fig. 2 Compressive strength of segregation specimen The reference comparison table used to evaluate concrete segregation degree in library or on the spot was proposed in Table 4 according to laboratory data, such as SD, compressive strength and coarse aggregate distribution situation in concrete specimens. Table 4 Comparison between the SD value and degree of segregation Segregation degree
Uniform
Mild segregation
Moderate segregation
Severe segregation
SD
<0.050
0.050 ～ 0.075
0.075 ～ 0.100
≥ 0.100 Because coarse aggregate gradation was different, the SD of the calculated result was also different. So, Table 4 was more suitable for the evaluation of concrete made with coarse aggregate that maximum diameter of 35mm minimum diameter of 5 mm. The value of SD still has broad application prospects on the spot. Difference between sample of the concrete pavement and segregation specimen mentioned in this article was only in size. A similar approach can be carried out to deal with the sample. Then, segregation degree can be determined according to SD values reference in Table 4. Conclusions 1, the model that size of coarse aggregate against vertical section (the direction against vertical section of specimen) in concrete conforms to continuous distribution was established. The concept of equivalence cumulative thickness of coarse aggregate in a certain size range in concrete was proposed based on the model. 2, the index that segregation degree value(SD) used to evaluate the degree of concrete segregation was put forward based on the assumption that coarse aggregate in concrete is uniform and the model that size of coarse aggregate against vertical section in concrete conforms to continuous distribution. 3, the reference standard used to evaluate concrete segregation degree in library or on the spot was proposed according to laboratory data, such as SD, compressive strength and coarse aggregate distribution situation in concrete specimens. 4, the indexesand in the model that size of coarse aggregate against vertical section conforms to continuous distribution need further investigation. References
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