Development of hybrid fibre reinforced self-compacting concrete as per Nan Su criteria

This paper studies the development of hybrid fibre reinforced self-compacting concrete as per Nan Su criteria. Results predicted that various packing factors adopted in the study are 1.12, 1.14, 1.16 and 1.18. Fine aggregate /Total aggregate ratios (s/a ratio) adopted in the study are 0.5, 0.53 and 0.57. The optimum combinations of packing factor and s/a ratio are found to be 1.12 & 0.53 and 1.14 &0.57 for M30 grade SCC mixes because these optimum PF and s/a ratio combination gives comparatively better particle packing density in SCC mixes. Better particle packing density enhances the microstructure of SCC mix subsequently more strength and durability can be achieved. As PF increases powder content decreases and aggregate content increases requiring more paste to make the SCC mix workable. Less value PF will have high particle packing density yielding more strength due to improved microstructure of SCC mixes. At PF & s/a combinations of 1.12 & 0.53 and 1.14 & 0.57, the workability of SCC mixes is superior because of high paste volume and less aggregate content. Compressive, split-tensile and flexural of M30 grade SCC mixes made with optimum combinations of packing factor and s/a ratios are found to be high.


Introduction
The European Federation of National Associations Representing Concrete (EFNARC) has published specifications and guidelines for the use of SCC that include a wide range of themes, including material selection and mixture design, as well as the importance of testing procedures. Nan Su suggested following important guidelines: 1. The volume ratio of aggregate after lubrication and compaction in SCC is about 59-68%. 2. Since PF value is closely related with compressive strength, by adjusting PF from 1.18 to 1.10 with decrement of 0.02, the SCC thus obtained could satisfy the compressive strength of range 20 -100 MPa. 3. Reduction in PF value would decrease the content of aggregates and increase the volume of paste, thus, enhancing the passing ability through reinforcement and segregation resistance of SCC. 4. s/a ratio is the volume ratio of fine aggregates to total aggregates, which ranges from 50% to 57%.

Step-wise procedure of Nan Su mixes design method
The procedures of the proposed mix design method can be summarized in the following steps: Step 1: calculation of coarse and fine aggregate contents Step 2: calculation of cement content Step 3: calculation of mixing water content required by cement Step 4: calculation of SCM quantity Step 5: calculation of mixing water content needed in SCC Step 6: calculation of SP dosage Step 7: Adjustment of mixing water content needed in SCC Step 8: Trial mixes and tests on SCC properties Step 9: Adjustment of mix proportion 3 Glass and steel fibre dosage in SCC mixtures at their optimal levels Table 1 shows the recommended glass and steel fibre dosages for SCC mixes in the M30 grade. For the best PF and s/a ratio combinations, Table 6 shows the compressive strengths of PSCC, SFRSCC, GFRSCC, and HFRSCC blends. At various ages of curing, Table 7 shows compressive strengths of M30 grade plain and reinforced SCC mixes produced with optimal combinations of PF and s/a ratios.

Split-tensile strength tests on plain and fiber-reinforced SCC mixtures
At various phases of curing, Table 8 shows splittensile strengths of M30 grade plain and reinforced SCC mixes produced with optimal combinations of PF and s/a ratios. 7 Plain and fibre reinforced SCC mixtures were tested for flexural strength. Table 9 shows the flexural strengths of plain and reinforced M30 grade SCC mixes produced with the best PF and s/a ratios at various curing ages.
8 Plain and fibre reinforced SCC mixtures were evaluated nondestructively. Table 10 gives the criteria for evaluating the concrete quality based on Rebound hammer test and ultrasonic pulse velocity (USPV) test.

Conclusions
Based on the results reported in this research work and key findings during the experimental investigations, the following conclusions are drawn: 1. 1.12, 1.14, 1.16, and 1.18 were some of the packing variables used in the study. The study used fine aggregate/total aggregate ratios (s/a ratios) of 0.5, 0.53, and 0.57. The optimal packing factor and s/a ratio combinations for M30 grade SCC mixes were determined to be 1.12 & 0.53 and 1.14 &0.57 because these optimum PF and s/a ratio combinations offer substantially greater particle packing density in SCC mixes. Better particle packing density enhances the microstructure of SCC mix subsequently more strength and durability can be achieved. 2. The PF & s/a combinations 1.12 & 0.53 and 1.14 & 0.57 were found to be the most effective, resulting in the highest compressive strengths, which can be related to the high particle packing densities achieved in SCC mixtures. These optimum combinations of packing factors and s/a ratio is further used in the development of the fibre reinforced SCC mixes of grade M30. 3. As PF increases powder content decreases and aggregate content increases requiring more paste to make the SCC mix workable. Less value PF will have high particle packing density yielding more strength due to improved microstructure of SCC mixes. 10. Addition of fibre reduces workability in SCC mixes. Workability is reduced drastically in SFRSCC when compared to GFRSCC. In HFRSCC mixes, due to addition of steel and glass fibres workability is affected which can be improved using fly ash and super plasticizers. 11. The strengths of the M30 grade PSCC mixes are found to be increased by the addition of fibres. The percentage increase of strength is more significant in M30 grade PSCC mixes made with PF=1.14 and s/a=0.57 12. The increase of compressive strength of SFRSCC ( steel fibre reinforced self-compacting concrete) and GFRSCC ( glass fibre reinforced self-compacting concrete)) for M30 grade concrete at 28 days in comparison with PSCC (plain self-compacting concrete) was found to be 10-12%. 13. Because steel has a greater modulus of elasticity than glass fibres, the compressive strength of SFRSCC mixes prepared with the best combinations of PF and s/a ratios was found to be higher than that of GFRSCC mixes. 14. The compressive strengths of HFRSCC were found to be significantly increased due to the combined action of glass and steel fibres, with a 16 percent improvement in compressive strength for M30 grade above PSCC produced with optimal combinations of PF and s/a ratios, respectively. 15. When compared to equivalent HFRSCC and GFRSCC mixes, the inclusion of fibres enhanced split tensile strength, which was found to be highest in M30 SFRSCC mixes. 16. Fibre addition increased flexural strength, which was found to be highest in M30 grade HFRSCC mixtures. As a result, it has been determined that the hybridization of glass and steel fibres is beneficial in enhancing the strength properties of FRSCC. Under flexural loading, GFRSCC blends outperformed SFRSCC mixtures. 17. Non-destructive testing of M30 grade PSCC, SFRSCC, GFRSCC, and HFRSCC mixes made with the best PF and s/a ratios revealed improved rebound numbers and ultrasonic pulse velocity values for HFRSCC mixes, indicating that HFRSCC mixes have better concrete quality than SFRSCC and GFRSCC mixes due to fibre hybridization enhancing the confining effect partly due to the presence of hi-tech fibres.