Enhancing the matrix-fiber bond in ultra-high-performance fiber-reinforced concrete using a high performance plasticizer. Impact on the flowability, physical and mechanical properties

One of the main problems in the use of ultra-high-performance fiber-reinforced concrete (UHPFRC) with steel fibers is low flowability. The addition of 2,3,4,5,6-Pentahydroxycaproic acid sodium salt (SPS) in small amounts (0.05, 0.1 and 0.15 %wt of the cement content) can significantly increase the flowability and setting time. The effects on porosity, matrix chemical composition and fiber-matrix interaction were examined, as well as its influence on strength. Improving the flowability, pores larger than 50 μm show a significant decrease, while those smaller than 0.1 μm show a significant increase. According to the results of thermo-gravimetric analysis, the production of ettringite with finer grains and staggered symbiosis improves the pore structure of the UHPFRC matrix. An improvement of the adhesion of the steel fibers to the matrix was determined by stereomicroscope images, showing evidence of a relationship between SPS dosage and the expanded concrete area surrounding the fiber. The flexural and compressive strength increased by 10 % and 8 %, respectively, when the plasticizer/cement ratio was 0.15 %. • The workability and flowability have been improved with the addition of SPS. • Stereomicroscopic provided evidence of a improved fiber-matrix bonding. • SPS reduced pores larger than 10 µm and augmenting those smaller than 0.1 µm. • SPS was shown to stop the propagation of microcracks and postpone the development of macrocracks. • Mechanical properties have been improved by the better fiber-matrix adhesion.