Ultra-high performance concrete (UHPC), a new cement-based material, has character of superior mechanical properties and excellent durability. Especially, it possesses compressive strength higher than 150 MPa, which is approximately 3 times as that of conventional concrete. UHPC is nearly impermeable to carbon dioxide, chlorides and sulphates. The superior durability leads to long service life with significantly reduced maintenance.
This thesis focuses on the dimensional stability of UHPC, which is one of the most significant factors of the material. Unlike in normal concretes which are mixed at a regular water/cement ratio (w/c = 0.32-0.5), UHPC usually consists of extremely low w/c (0.140.24). At the conditions of ultra-low w/c and high dose of superplasticizer, the hydration process of cement particles is different, resulting in the occurrence of high early age shrinkage. High autogenous shrinkage correlated with high binder content makes UHPC highly vulnerable to shrinkage cracking, which requires urgent resolution for its engineering application. On the other hand, at the conditions of ultra-low w/c, abundant amounts of un-hydrated cementitious materials in the matrix may affect the long-term performance of UHPC. The ongoing hydration of cement particles in UHPC has a retrieving effect on physicochemical properties of UHPC, also known as “autogenous/self-healing” products, which plays an important role in determining the microstructure of finally obtained UHPC. This project is proposed to conduct fundamental research on the dimensional stability (early age shrinkage and long-term stability) of cement with partial replacement of ultra-fine fly ash (UFA) under the UHPC condition, especially long-term stability of UHPC in different exposure conditions (the seawater, tap water or outdoors).
Results show that the addition of 30% UFA significantly improved the early-age as well as later-age compressive strengths of ordinary and high-volume fly ash concretes. The effectiveness of UFA in the blended system lies in producing high packing density and in accelerating the pozzolanic activity to produce more C–S–H gel by consuming calcium hydroxide (CH) in HVFA concretes. In the case of UHPC specimens exposed to seawater, the CH was efficiently even after 1080 days. TGA/DTG results indicated that the CH was consumed, which was accompanied by the formation of CaCO3 (calcite), due to the
carbonation effect at outdoor conditions. Whereas for the water and seawater immersion conditions, the CH was transformed into other reaction products including Mg(OH)2 and ettringite. Fiber addition improved the performance of fly ash based UHPC, because of the formation of a denser microstructure, as evidenced by the dramatic decrease in the diffusion coefficient and porosity. XRD and SEM analyses imply that the UHPC sample exposed to outdoor and seawater condition underwent less deterioration compared to fly ash (FA) containing UHPCs, more calcite was formed than the calcite formed in water condition. The surface layer of the sample immersed in seawater had some brucite and calcite formation as well as Friedel’s salt and sulfoaluminates. However, under the outdoor condition, the surface pH dropped due to the penetration of CO2 into the binder neutralizing the pore solution. As time passed, either large or small sized pores were formed in the seawater because of the expanding effect induced by ettringite, whereas for outdoor environments, the formation of calcites tended to promote the development of medium-sized pores.