Avaliação de propriedades do concreto projetado por via seca quando se varia a direção de projeção e o uso de sílica ativa

Abstract

The shotcrete has as one of its main uses, the recovery of structures, considering the application due to the non-use of formworks and the productivity achieved. In the application process there are some factors that significantly interfere in the quality of the final result, among them are the reflection coefficient, which is the measure of the amount of projected material that is not adhered to the substrate. The addition of fumed silica to the mixture is shown to be an alternative for reducing the reflection coefficient, and it may also bring some other improvements. In this study, the objective is primarily to examine the behavior of the reflection coefficient under the influence of different levels of fumed silica addition (5%, 10% and 15%), in three different projection situations (horizontal, vertical and inclined), using traces of (fck) 30 MPa and 45 MPa resistance. The properties evaluated are divided into characteristics in the fresh state (consistency measured with a Proctor needle, moisture of the mixture before and after spraying and reflection coefficient) and in the hardened state (compressive strength at 7, 28 and 540 days, modulus, elasticity, immersion absorption, void ratio, dry density, capillary absorption and tensile strength in diametral compression). For this purpose, an apparatus was built in a metallic structure, intended to provide the aforementioned projection situations. After molding the concrete slabs, the extractions were performed, originating the concrete specimens for the properties evaluation. All tests followed current Brazilian standards. To evaluate the results, an analysis of variance (ANOVA) and the Tukey's test were used to compare the means. As a result, it was found an improvement in the properties of shotcrete with the addition of silica, notably the reduction of the reflection coefficient, the absorption by immersion and the void ratio combined with an increase in the compressive strength and in the tensile strength by diametrical compression. It was also evidenced that, as for the projection position, the horizontal position of the shotcrete presented the best results compared to the vertical one.