Steel fibre reinforced concrete toppings exposed to shrinkage and temperature deformations

Abstract: The work presented in the thesis primarily focuses on problems that are characteristic for thin layers of concrete exposed to imposed loads. More specific, the work aims at investigating, or rather finding ways to evaluate the efficiency of steel fibres as regards the limitation of crack widths in concrete under restrained conditions. To fulfil the ambitions a test method was developed in which concrete toppings were placed on the upper face of a bottom slab that constituted a stiff foundation. The toppings were then exposed to temperature loads that successively resulted in both the formation of vertical cracks in the concrete as well as horizontal cracks along the interface to the substructure. A total number of eight tests divided into two series were then performed using the proposed technique of testing. At each such occasion two toppings were tested simultaneously, one Plain Concrete (PC) and one Steel Fibre Reinforced Concrete (SFRC) specimen. In this way a comparative evaluation of the influence of steel fibres was facilitated. Results showed that end hooked steel fibres, in amounts of 30 to 60 kg/m3, in most cases reduced the maximum widths of appearing cracks substantially although the reinforcement ratios were not sufficient to get completely crack free structures. It is also clear from the experiments that the effect to some extent was influenced by the properties of the interface in the sense that the fibre contribution seemed to increase as the bond quality became poorer. A series of restrained shrinkage tests were also performed primarily for the sake of verification. Within the frames of this study a total of eight half- scale toppings, four with a depth of 6 cm and four with 12 cm depth, were cast on the surface of an old concrete floor. Both plain and steel fibre reinforced concrete specimens were included in the series. Also, for comparison reasons some conventional steel bar reinforced toppings were produced as well. After an initial curing period of three days the specimens were exposed to one-sided drying that successively resulted in the development of visible cracks. From a crack limiting point of view it was concluded that steel fibres were at least as effective as the steel bar mesh, although it was clear that none of the alternatives were adequate for the purpose of achieving crack-free structures. However, the main reason as to why the effect of reinforcement was not as pronounced as anticipated was believed to be that the bond proved to be insufficient. In particular, considering that the ends of the toppings were fastened to the floor by means of expanding bolts this resulted in a too severe load situation. Regarding the influence of the depth it was shown that cracks appeared at a considerably earlier stage for the thin toppings. At the end of the measuring period the cracks were also considerably wider. This was explained as being a result of the considerably faster rate of desiccation experienced for a thin layer. A simple linear elastic Finite Element analysis was also conducted to verify the effect of the sectional depth in addition to the influence of the concrete creep on the development of stresses in the concrete. Results from this study showed that stresses are substantially reduced due to creep effects. It was further shown that the progress of tensile stresses in the concrete is somewhat slower for a thicker section, mainly due to the slower shrinkage strain development.