Tóm tắt Luận án Research of improvement of tensile strength in bending and abrasion resistance of fine sand concrete for cement concrete pavement

rete pavement. In Vietnam today, the selection of concrete components that meet the requirements on tensile strength when bending is done under Decision No. 778/1998/QD-BXD. Accordingly, concrete gradation is still selected in accordance with the compressive strength based on the Bolomey-Skramtaev formula (1): Rb = A. Rx . ( + B) (1) Where: Rb, Rx - Concrete and cement intensity; X, N - Amount of cement and water used; A - Material quality coefficient; B - Equation factor. When designing components according to compressive strength, the value of Rb, Rx is the compressive strength of concrete and cement, the coefficient B is taken by ± 0.5 depending on the X/N ratio, coefficient A is determined according to the investigation table depending on the quality of materials used. According to Y.M.Bazenov, formula (1) can also be used to select concrete components according to tensile strength in bending. Then Rb, Rx is the tensile strength when bending of concrete and cement, coefficient B is taken by -0.2, coefficient A is taken from the lookup table. However, the values shown in (1) are based on cement test data using the method of plastic mortar and the use of materials in the former Soviet Union. Therefore, these coefficients are likely not suitable for the current situation in Vietnam. Besides, when designing concrete components according to tensile strength in bending, attention should be paid to mortar residuals (reasonable mortar residuals should increase by about 0.15÷0.20 compared to when designed according to compressive strength). When increasing the mortar balance calculation, the work of concrete mixture will be reduced, so it is recommended to select the appropriate initial amount of water to ensure workability. On the other hand, using fine sand in concrete, the tensile strength of bending and the abrasion resistance of concrete are reduced compared to when using coarse sand. In order to improve the tensile strength of bending and abrasion resistance of fine sand concrete equivalent to coarse sand, meeting the technical requirements for concrete pavement to grade I, the use of water-reducing 11 admixture, increasing mortar residues and adding grit combined with fine sand is really necessary. 3.1. Properties of concrete mixtures 3.1.1. Selecting concrete components for research The dissertation has used the same type of PCB40 Nghi Son cement, stone (Dmax = 20mm), superplasticizer Daltonmat-RDHP, fine sand (C1,C2,C3), coarse sand (CV), dust limestone (M) combines fine sand with the rate of replacing 40% of fine sand with crushed stone. workability of concrete, Rku, amount of cement and N/X ratio as recommended by Decision No.1951/QD-BGTVT. To ensure appropriate construction conditions, the workability in the study is not immediately after mixing but takes into account the loss of slump depending on the actual conditions and weather of the construction. The use is higher than that required for cement concrete pavement. Therefore, the amount of cement selected is 350 kg/m 3 , the rate of additives according to the manufacturer's recommendations is 1% of the weight of cement, the ratio of X/N=1.80; 2.00 and 2.30. Given an X/N ratio and sand magnitude module, the experimental gradients are designed with two different reasonable mortar residues for Rn and Rku according to Decision No.778/1998/QD- BXD and TCXD 127:1985. In particular, the reasonable mortar residual coefficient according to Rku was chosen higher than Rn from 0.15 to 0.20, Based on batches and volume of concrete mixture using fine sand and concrete using fine sand combined with crushed stone, calculated the actual concrete composition and research results are presented in Table (3.1, 3.2). 3.1.1.1. Selection of concrete components using fine sand Table 3.1. Studied components of concrete used (fine sand, rough sand) No Symbol Material quantity, kg/m3 Distribution parameters Cement Water Sand Stone PG Mdl Kd X/N 1 CP1 349 193 642 1217 3.49 1.6 1.37 1.80 2 CP2 347 193 707 1143 3.47 1.6 1.53 1.80 3 CP3 347 174 613 1291 3.47 1.6 1.23 2.00 4 CP4 345 173 685 1205 3.45 1.6 1.39 2.00 5 CP5 347 151 672 1288 3.47 1.6 1.23 2.30 6 CP6 344 149 742 1199 3.44 1.6 1.41 2.30 7 CP7 346 173 564 1332 3.46 1.2 1.16 2.00 8 CP8 344 172 647 1237 3.44 1.2 1.33 2.00 9 CP9 346 173 692 1208 3.46 1.9 1.39 2.00 10 CP10 344 172 754 1130 3.44 1.9 1.56 2.00 11 CP11 347 174 697 1212 3.47 2.5 1.38 2.00 12 CP12 345 172 759 1134 3.45 2.5 1.55 2.00 3.1.1.2 Selection of concrete components using fine sand and limestone grits Table 3.2. Components of concrete used (fine sand combined with grit, coarse sand) No Symbol Material quantity, kg/m3 Distribution parameters Cement Water M Sand Stone PG Mdl Mdlhh M/CLN Kd X/N 12 1 CPM1 348 174 280 420 1214 3.48 1.2 2.2 0.40 1.38 2.00 2 CPM2 347 173 307 460 1141 3.47 1.2 2.2 0.40 1.54 2.00 3 CPM3 349 174 282 423 1217 3.49 1.6 2.4 0.40 1.37 2.00 4 CPM4 348 174 309 463 1145 3.48 1.6 2.4 0.40 1.53 2.00 5 CPM5 349 174 283 425 1217 3.49 1.9 2.6 0.40 1.37 2.00 6 CPM6 349 174 311 466 1147 3.49 1.9 2.6 0.40 1.52 2.00 7 CP11 347 174 -- 697 1212 3.47 2.5 -- -- 1.38 2.00 8 CP12 345 172 -- 759 1134 3.45 2.5 -- -- 1.55 2.00 3.1.2. The relationship between the used amount of water and the workability of the concrete mixture 3.1.2.1 Relationship between the used amount of water and the performance of the concrete mixture using fine sand Research results show that the slump of concrete mixture tends to decrease with increasing mortar residual coefficient. The volumetric mass of a concrete mixture is less affected by the sand category but only depends on the sand magnitude module. The air bubbles content of the concrete mixture using different types of sand in the study is not much different. The sand modulus has a significant effect on the correlation between water use and the slump of the concrete mixture. When using finer sand, the ratio of surface area increases the level of water absorption, so the amount of water mixed to reach the same slump tends to increase with decreasing sand modulus. Based on the above test results, combined with the recommendations of Decision No.778/1998/QĐ-BXD, it is possible to create Table 3.6 to reference the initial preliminary water required for 1m3 of concrete using fine sand. When using polycarboxylate-based superplasticizers for concrete components to make cement concrete roads (priority for tensile strength in bending) as follows: Table 3.6. The initial amount of mixing water required for 1 m3 of concrete, liters No Slump, cm Maximum particle size of large aggregate Dmax=20mm Modulus of magnitude of sand, Mdl 1.2 1.6 1.9 1 1 ÷ 2 157 152 148 2 3 ÷ 4 163 158 154 3.1.2.2. The relationship between the used amount of water and the performance of the concrete mixture using fine sand and crushed lime stone Research results show that the slump of concrete mixture tends to decrease with increasing mortar residual coefficient. The volumetric mass of a concrete mixture is less affected by the sand category but only depends on the magnitude modulus of the fine sand mixture with crushed stone. The air content of the concrete mixture using fine sand and crushed stone in the study is not much different. The fineness modulus of fine sand mixed with crushed stone has a significant effect on the correlation between the amount of water used and the slump of the concrete 13 mixture. The amount of mixing water to achieve the same slump tends to increase gradually in the direction of decreasing the modulus of the magnitude of the fine sand mixture with crushed stone. 3.1.3. Ability to maintain the working properties of the concrete mixture 3.1.3.1. The ability to maintain the workability of concrete mixtures using fine sand The study results showed that after 60 minutes, the slump of the concrete mixture using fine sand decreased over time about 3cm, using coarse sand decreased by about 2cm. 3.1.3.2. The ability to maintain the workability of a concrete mixture using fine sand and limestone grit The results of the study showed that after 60 minutes the slump of the concrete mixture using fine sand and stone grout decreases over about 2cm equivalent to the use of coarse sand and magnitude modulus. 3.1.4. Stratification of concrete mixture 3.1.4.1 Stratification of concrete mixture using fine sand The research results show that with the same X/N ratio, the level of mortar separation tends to increase gradually with the decrease of the sand bulk module, the increase of mortar residual coefficient, the level of mortar separation for fine sand. value of (1.8÷2.8)%, coarse sand is equal to 0% and all meet technical requirements within the allowable limits according to TCVN 9340:2012. Mortar separation for concrete using fine sand is of great value when the mortar residual coefficient is high. 3.1.4.2. Stratification of the concrete mixture using fine sand combined with limestone grit Research results show that the use of grit combined with fine sand has limited the splitting of mortar of concrete mixture compared to when using fine sand alone, which means that the ability to improve the resistance abrasion of concrete to cement concrete pavement. 3.2. Properties of concrete 3.2.1. Relationship compressive strength of concrete with compressive strength of cement and X/N ratio Analysis of experiment results in the former Soviet Union recommends that the value of B coefficient be equal to -0.5 when the ratio of X/N <2.5 and equal to +0.5 when the ratio of X/N>2.5. Then, formula (1) has the form: Rb n = An . Rx n . ( + 0.5) (2) In which: Rb n , Rx n - Compressive strength of concrete and cement, MPa; An- Material quality coefficient according to compressive strength; X, N- Amount of cement and water in 1 m 3 of concrete, kg. The study also showed that the An factor depends on the proposed material quality of 0.55; 0.60; 0.65 (when X/N <2.5) and equals 0.37; 0.40; 0.43 (when X/N> 2.5), suitable for concrete with poor, medium and good quality materials. Using materials in Vietnam, the An coefficient (when X/N <2.5) has been determined to be valued at 0.50; 0.55; 0.60 and (when X/N> 2.5) has a value of 14 0.32; 0.35; 0.38 corresponds to concrete using poor, medium and good quality materials. Several other studies have suggested that the value of An coefficient (when X/N 2.5) equals 0.29; 0.32; 0.34, corresponding to concrete using poor, medium and good quality materials. Previous studies with fine sand in Vietnam were used as a basis to recommend taking the value of B=0.5, and the An coefficient corresponds to the poor, average and good material quality equal to 0.46; 0.52; 0.60 with sand of modulus of magnitude (0.7÷1.1) and equal to 0.49; 0.55; 0.62 with sand with modulus of magnitude (1.2÷2.0). 3.2.1.1. Relationship compressive strength of concrete using fine sand with compressive strength of cement and X/N ratio It can be seen that, although the studies use formula (2) as a basis for the selection of concrete components according to compressive strength (Rn), the proposed coefficients are different. Especially significant. Therefore, the study and supplement of the data to determine the calculation coefficients will have high practical significance and be mentioned in the research of the dissertation. To test the coefficients of formula (2), the thesis has conducted experiments of gradients in Table 3.1, the research results are presented in Table 3.13. Table 3.13. Relationship compressive strength of concrete using (fine sand, raw sand) and X/N ratio No Symbol Mdl Kd X/N Volumetric mass, kg/m3 Slump, cm Compressive strength, day-age, MPa 3 7 28 1 CP1 1.6 1.37 1.80 2400 17.0 16.3 29.2 33.2 2 CP2 1.6 1.53 1.80 2390 16.5 15.7 28.7 32.5 3 CP3 1.6 1.23 2.00 2420 11.0 19.3 35.6 40.5 4 CP4 1.6 1.39 2.00 2400 9.5 18.1 33.5 38.9 5 CP5 1.6 1.23 2.30 2450 8.0 33.5 45.5 50.8 6 CP6 1.6 1.41 2.30 2430 7.5 32.1 43.2 49.7 7 CP7 1.2 1.16 2.00 2410 10.0 17.3 31.2 35.1 8 CP8 1.2 1.33 2.00 2400 7.5 16.2 29.9 34.0 9 CP9 1.9 1.39 2.00 2420 12.5 21.4 39.5 44.1 10 CP10 1.9 1.56 2.00 2400 10.5 20.5 38.1 43.2 11 CP11 2.5 1.38 2.00 2430 14.5 22.8 43.1 47.7 12 CP12 2.5 1.55 2.00 2410 13.5 22.1 42.5 46.6 Based on the experimental results, CP concrete (1,3,5) were used with preferred mortar residues for Rn and formula (2), coefficient B was kept fixed by -0.5. When keeping the coefficient B constant, then for each pair (Rn - X/N ratio), one factor An can be determined. The results of determining the An coefficient for each pair of values and values for each material option at the age of 28 days are different, presented in Table (3.14, 3.15). 15 Table 3.14. An coefficient with fine sand C2 and scale (X/N = 1.80; 2.00; 2.30) No Symbol Mdl Kd X/N coefficient An 1 CP1 1.6 1.37 1.80 0.51 2 CP3 1.6 1.23 2.00 0.54 3 CP5 1.6 1.23 2.30 0.57 Research results Table 3.14 shows that for the same Mdl=1.6 and the X/N ratio varies from (1.80÷2.30), the factor An is equal to 0.51; 0.54; 0.57. Therefore, it is possible to choose an average value of 0.54 (corresponding to X/N ratio=2.00), this X/N ratio is used to study the properties of concrete and concrete mixture. concrete using sand with different modulus of magnitude, so that An coefficient can be determined for design work to select concrete components according to Rn. Table 3.15. An coefficient with different types of sand modulus and the same X/N ratio = 2.00 No Symbol Mdl Kd X/N coefficient An 1 CP7 1.2 1.16 2.00 0.47 2 CP3 1.6 1.23 2.00 0.54 3 CP9 1.9 1.39 2.00 0.59 4 CP11 2.5 1.38 2.00 0.64 Research results Table 3.15 shows that An coefficient tends to decrease with decreasing modulus of sand size. The study results also showed that the An coefficient increased as the X/N ratio increased and there was a significant change according to the sand module. These values of An factor can be referenced and used in the design of concrete component selection for concrete pavement surface. With the above An recommended coefficient, when using cement (PCB40, PC40) and superplasticizer can make ratio concrete (Rn/Rku), MPa is: 40/5.5 and 50/6.0 corresponds to the correlation of Rn/Rku ratio reaching level 2 level. 3.2.1.2. Relationship compressive strength of concrete using fine sand combined limestone grit with compressive strength of cement and X/N ratio To check the coefficients of formula (2), experiments of gradients are shown in Table 3.2, the research results are presented in Table 3.16. Table 3.16. Relationship compressive strength of concrete using (fine sand combined with grit, rough sand) and X/N ratio No Symbol Mdl Mdlhh Kd X/N Volumetric mass, kg/m3 Slump, cm Compressive strength, day-age, MPa 3 7 28 1 CPM1 1.2 2.2 1.38 2.00 2430 10.0 21.3 38.8 43.7 2 CPM2 1.2 2.2 1.54 2.00 2420 9,0 20.4 37.4 42.8 3 CPM3 1.6 2.4 1.37 2.00 2440 11.0 22.1 40.4 45.6 4 CPM4 1.6 2.4 1.53 2.00 2430 10.0 21.2 38.5 44.5 5 CPM5 1.9 2.6 1.37 2.00 2440 13.0 23.2 42.1 47.8 6 CPM6 1.9 2.6 1.52 2.00 2440 11.5 22.1 40.8 46.3 7 CP11 2.5 -- 1.38 2.00 2430 14.5 22.8 43.1 47.7 8 CP12 2.5 -- 1.55 2.00 2410 13.5 22.1 42.5 46.6 Based on the experimental results, CPM concrete (1,3,5) and CP11 were used with preferred mortar residues for Rn and formula (2) (coefficient B was kept fixed by - 16 0.5). When keeping the coefficient B constant, then for each pair (Rn - X/N ratio), one factor An can be determined. The results of determining the An coefficient for each pair of values and values for each material option at the age of 28 days are presented in Table 3.17. Table 3.17. An coefficient with fine sand of different magnitude modulus coordinate with grit, coarse sand and the same rate of X/N = 2.00 No Symbol Mdl Mdlhh Kd X/N Coefficient An 1 CPM1 1.2 2.2 1.38 2.00 0.59 2 CPM3 1.6 2.4 1.37 2.00 0.61 3 CPM5 1.9 2.6 1.37 2.00 0.64 4 CP11 2.5 -- 1.38 2.00 0.64 The research results show that the An coefficient tends to decrease when reducing the magnitude modulus of (fine sand combined with gravel, coarse sand). An coefficient (concrete using fine sand combined with grit) is higher than An factor (concrete using fine sand), this shows that when adding limestone dust mixed with fine sand, the coefficient An increase. The coefficient of An increases when the ratio of X/N increases and there is a big change when changing the magnitude of small aggregate module. These values of An can be referenced in the design of the selection of concrete components for cement concrete pavement when using fine sand and crushed stone. With the above An recommended coefficient, when using cement (PCB40, PC40) and superplasticizer can make ratio concrete (Rn/Rku), MPa is: 40/5.5 and 50/6.0 corresponds to the correlation of Rn/Rku ratio reaching level 2 level. 3.2.2. Relation tensile strength of concrete with tensile strength when bending of cement and X/N ratio According to Y.M. Bazenov, formula (1) can also be used to select concrete components according to tensile strength in bending. Then the coefficient B is taken as a value of - 0.2. and formula (1) has the form: Rb ku = Aku . Rx ku . ( - 0.2) (3) In which: Rb ku , Rx ku - Flexural tensile strength of concrete and cement, MPa; Aku - material quality coefficient according to tensile strength when bending; X, N - Amount of cement and water in 1 m 3 of concrete, kg. In which, Aku coefficient varies depending on the quality of materials used. It can be seen that, although the studies use formula (3) as a basis for the selection of concrete components according to tensile strength in bending (Rku), the proposed coefficient has significant difference. Therefore, the study and supplement of the data to identify the calculation coefficients has high practical significance and is mentioned in the research of the dissertation. 3.2.2.1. Relation tensile strength of concrete using fine sand with the tensile strength of cement and X/N ratio To test the coefficients of formula (3), the thesis has conducted experiments of gradients in Table 3.1, the results are presented in Table 3.18. 17 Table 3.18. The relationship of tensile strength when bending of concrete used (fine sand, rough sand) and X/N ratio No Symbol Mdl Kd X/N Volumetric mass, kg/m3 Slump, cm Tensile strength when bending, day-age, MPa 3 7 28 1 CP1 1.6 1.37 1.80 2400 17.0 3.35 3.93 5.52 2 CP2 1.6 1.53 1.80 2390 16.5 3.96 4.34 5.78 3 CP3 1.6 1.23 2.00 2420 11.0 4.13 5.06 6.24 4 CP4 1.6 1.39 2.00 2400 9.5 4.21 5.34 6.51 5 CP5 1.6 1.23 2.30 2450 8.0 5.36 7.31 8.20 6 CP6 1.6 1.41 2.30 2430 7.5 5.73 7.47 8.45 7 CP7 1.2 1.16 2.00 2410 10.0 3.64 4.53 5.97 8 CP8 1.2 1.33 2.00 2400 7.5 3.95 4.81 6.29 9 CP9 1.9 1.39 2.00 2420 12.5 4.43 5.53 6.76 10 CP10 1.9 1.56 2.00 2400 10.5 4.57 5.72 7.02 11 CP11 2.5 1.38 2.00 2430 14.5 4.95 5.98 7.50 12 CP12 2.5 1.55 2.00 2410 13.5 5.20 6.29 7.72 Based on the experimental results, CP concrete (2,4,6) were used with preferred mortar residues for Rku and formula (3), coefficient B was kept fixed by -0.2. By keeping the coefficient B constant, for each pair (Rku-X/N ratio), an Aku coefficient can be determined. The results of determining Aku coefficients for each pair of values and values for each material option at the age of 28 days were presented in Table (3.19, 3.20).. Table 3.19. Aku coefficient with fine sand C2 and ratio (X/N = 1.80; 2.00; 2.30) No Symbol Mdl Kd X/N Coefficient Aku 1 CP2 1.6 1.53 1.80 0.40 2 CP4 1.6 1.39 2.00 0.41 3 CP6 1.6 1.41 2.30 0.45 The research results in Table 3.19 show that with the same Mdl=1.6 and the X/N ratio varies from (1.80 ÷ 2.30), the Aku coefficient has a value of 0.40; 0.41; 0.45. Therefore, it is possible to choose an average Aku coefficient value of 0.41 (corresponding X/N ratio=2.00), this X/N ratio is used to study the properties of concrete mixtures. and concrete using sand with different modulus of magnitude, from which to determine Aku coefficient for design work of selecting concrete components according to Rku. This ratio (X/N=2.00) is consistent with the chosen X/N ratio when determining the value of An coefficient in section 3.2.1.1, and is also suitable for selection in calf components. concrete using fine sand and grit in Section 3.1.1.2. Table 3.20. Aku coefficient with different types of sand modulus of different sizes and the same X/N ratio = 2.00 No Symbol Mdl Kd X/N Coefficient Aku 1 CP8 1.2 1.33 2.00 0.39 2 CP4 1.6 1.39 2.00 0.41 3 CP10 1.9 1.56 2.00 0.44 18 4 CP12 2.5 1.55 2.00 0.48 Research results Table 3.20 shows that the Aku coefficients tend to decrease when reducing the modulus of sand size. The study results also show that Aku increase coefficient when X/N ratio increases and there is a significant change according to tissue simulating the size of sand. These values of Aku factor can be referenced and used in the design of concrete component selection for concrete pavement surface. With Aku coefficients recommended above, when using cement (PCB40,PC40) and superplasticizer can make road concrete with the ratio (Rn/Rku, MPa) is: 40/5.5 and 50/6.0 corresponds to the ratio correlation (Rn/Rku) ratio reaching level 2 level. It can be seen that when the mortar residual coefficient increases, Rku of concrete using fine sand tends to increase. Based on the above research results, recommendations and selection tables of material quality coefficients (An, Aku) can be used for reference in the practical application of calculating the selection of concrete components using fine sand for roads when using cement (PCB40, PC40) and superplasticizer, is presented as follows: - When designing and selecting concrete components using fine sand according to compressive strength, the reasonable mortar residue coefficient shall be used in the table. Use the formula: Rb n = An.Rx n .( – 0.5) (4) (with An material quality coefficient according to Table 3.15) - When designing and selecting concrete components using fine sand according to tensile strength when bending, it is recommended to use mortar residual coefficient higher than the table lookup value from 0.15 to 0.20. Use the formula: Rb ku =Aku.Rx ku .( -0.2) (5) (with Aku material quality coefficient according to Table 3.20) 3.2.2.2. Relations of tensile strength when bending of concrete using fine sand combined with limestone grit with tensile strength when bending of cement and X/N ratio To test the coefficients of formula (3), the dissertation has conducted experiments of gradients in Table 3.2, the results are presented in Table 3.21. Table 3.21. Relations of tensile strength when bending of concrete used (fine sand combined with grit, rough sand) and X/N ratio No Symbol Mdl Mdlhh Kd X/N Volumetric mass, kg/m3 Slump, cm Tensile strength when bending, day-age, MPa 3 7 28 1 CPM1 1.2 2.2 1.38 2.00 2430 10.0 4.70 5.94 7.62 2 CPM2 1.2 2.2 1.54 2.00 2420 9.0 5.05 6.31 8.01 3 CPM3 1.6 2.4 1.37 2.00 2440 11.0 4.89 6.19 7.95 4 CPM4 1.6 2.4 1.53 2.00 2430 10.0 5.21 6.57 8.35 5 CPM5 1.9 2.6 1.37 2.00 2440 13.0 5.09 6.43 8.25 6 CPM6 1.9 2.6 1.52 2.00 2440 11.5 5.42 6.84 8.68 7 CP11 2.5 -- 1.38 2.00 2430 14.5 4.95 5.98 7.50 8 CP12 2.5 -- 1.55 2.00 2410 13.5 5.20 6.29 7.72 19 Based on the experimental results, CPM concrete (2,4,6) and CP12 were used with preferred mortar residues for Rku and formula (3) (coefficient B was kept fixed by - 0.2). By keeping the coefficient B constant, for each pair (Rku - X/N ratio), an Aku coefficient can be determined. The results of determining Aku coefficients for each pair of values and values for each material option at the age of 28 days are presented in Table 3.22. Table 3.22. Aku coefficient with fine sand of different magnitude modulus combined with grit, raw sand and the same X/N ratio = 2.00 No Symbol Mdl Mdlhh Kd X/N Coefficient Aku 1 CPM2 1.2 2.2 1.54 2.00 0.50 2 CPM4 1.6 2.4 1.53 2.00 0.52 3 CPM6 1.9 2.6 1.52 2.00 0.54 4 CP12 2.5 -- 1.55 2.00 0.48 Research results show that Aku coefficients tend to decrease when reducing modulus of small aggregates (fine sand combined with grit, coarse sand). The results showed that Aku coefficient (concrete using fine sand combined with crushed stone) is higher than Aku coefficient (concrete using fine sand), this shows