Mechanical Characteristics Investigation of Ultra High Performance Concrete Using Design of Experiment and Response Surface Methodology

Attention to the mechanical properties of concrete for higher strength and ductility and also the increase in its durability has resulted in the innovation for several types of concrete. Ultra high performance concrete (UHPC) is one of the latest concrete with the unique properties such as high compressive strength, exhibiting tensile and flexural strength with increase in energy absorption (toughness), high durability, improved resistance against freezing- thawing and various chemical attacks. UHPC represents the highest development of high performance concrete in different curing conditions. One of the main disadvantages of UHPC is huge amounts of binder content used for producing UHPC. The purpose of this study was to improve the mechanical properties of UHPC relative to using local materials in two different phases: The purpose of phase one was to find the models of 7, 14 and 28-day compressive strength, 28-day splitting tensile strength, modulus of rupture, and flexural toughness of Ultra High Performance Concrete, as well as, study on the interaction and correlation of five variables including silica fume (SF), cement, steel fibers, superplasticizer (SP), and w/c ratio. The models are valid for mixes made with 1.0 part sand, 0.15-0.30 part silica fume amount, 0.70-1.30 part cement amount, 0.10- 0.20 part steel fiber, 0.04- 0.08 part superplasticizer (all values by sand weight) and 0.18- 0.32 water cementitious material ratio. In phase two, the effect of quartz powder (Qp), quartz sand (Qs), and different water curing temperatures on UHPC performance was investigated, the correlation between these variables and mechanical properties were found. The offered models are valid for the variables between: quartz powder 0 to 20% of cement substitution, quartz sand 0 to 50% of aggregate substitution, and water curing temperature 25 to 95 ºC. The experiments were designed by central composition with α=1 (face centered). The response surface methodology was analyzed between the variables and responses. The correlation of variables and mathematical models in terms of coded variables were established by ANOVA. Keywords: Ultra high performance concrete, strength, durability, silica fume, steel fiber, quality sand, modelling.