Experimental Study On Ultra High Toughness Cementitious Composites With Nano-SiO₂

Ultra High Toughness Cementitious Composites (UHTCC) is one kind of random short fiber reinforced cementitious composites with excellent ductility. In order to obtain the coordinating deform between matrix and fiber, high volume fly ash are added into UHTCC. However, the low activity of fly ash decreases its hydration rate, which seriously influences the strength and durability of UHTCC especially at early ages. This study is supported by National Key Technology Research and Development Program of the Ministry of Science and Technology of China with grant No.2012BAJ13B04 and National Natural Science Foundation of China with grant No.51378462, which plans to incorporate nano-SiO2 to compensate the drawbacks of high-volume fly ash, so UHTCC added with nano-SiO2 is being studied in the followings.1. The flowability, shrinkage and thermal conductivity of UHTCC with nano-SiO2 are tested. The results show that nano-SiO2 has little influence on the flowability of UHTCC, but the shrinkage and thermal insulation of UHTCC increase firstly and then decrease when the content of nano-SiO2 increases. Furthermore, a permanent insulation formwork for concrete dam is proposed to inhibit the crack due to temperature stress.2. Hydration products, micromorphology and pore structure of UHTCC with nano-SiO2 are tested. The results reveal that the increasing content of nano-SiO2 accelerates the hydration of fly ash, reduces the content of Ca(OH)2 and improve the microstructure.3. The durability of UHTCC with nano-SiO2 is studied. The results exhibit that the anti-penetration of water is improved with the increasing content of nano-SiO2, and seawater UHTCC beams with nano-SiO2 has higher anti-erosion ability than that of seawater concrete, which can be applied into island construction.4. The compressive and flexural properties of UHTCC with nano-SiO2 are tested. The results show that the compressive and flexural strength of UHTCC increase with the increasing content of nano-SiO2, but flexural crack width becomes larger correspondingly. The strengthening mechanism of nano-SiO2 is attributed to the pore structure.5. The influence of hybrid fibers on the mechanical properties of UHTCC with nano-SiO2 are studied and the design criterion of hybrid fibers is also proposed to obtain the steady multiple-cracking behavior. The results exhibit that hybrid fibers can improve the mechanical strength of the composites and re-obtain the harmless crack. However, the deformation decrases seriously when the content of steel fiber is too large.6. Fire resistance of UHTCC with nano-SiO2 is tested. The results show that the microstructure and compressive properties are not effected until the temperature reaches 600℃. When the steel reinforced concrete column is covered with UHTCC, both residual compressive strength and anti-spalling ability increase.7. The improvement of nano-SiO2 as cast-in-place join for bridge deck is studied through three-point bending test. The results show that when the length of join is larger than minimum length, the bearing capacity of bridge deck at serviceability limit state can improve apparently.