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- Comparison of Methods for Texture Assessment of Concrete SurfacesPublication . Santos, Pedro Miguel Duarte; Júlio, Eduardo Nuno Brito SantosThe bond strength of the interface between concrete layers cast at different times is important to ensure the monolithic behavior of reinforced concrete (RC) composite members. The roughness of the substrate surface has a significant influence in this scope. Current design codes use a qualitative approach based on visual inspection to assess roughness. This procedure is highly dependent on the designer and, therefore, can lead to inaccurate results. Previous studies conducted by the authors proved that it is possible to use a quantitative criterion to classify roughness. This paper describes an experimental study conducted to compare four roughness quantification methods: the processing of the digital image (PDI) and the two-dimensional (2D) laser roughness analyzer (2D-LRA) methods, both developed by the authors; an upgrade of these, using a three-dimensional (3D) laser scanner; and the sand patch test (SPT), a simple and widespread method. The 2D-LRA proved to be the best from the four methods considered because it gives a quantitative assessment of the roughness with adequate accuracy, is nondestructive, easy and fast to use, and is cost effective.
- Recommended improvements to current shear-friction provisions of model codePublication . Santos, Pedro M. D.; Júlio, Eduardo N. B. S.The shear-friction theory, first proposed by Birkeland and Birkeland (1966), was adopted by CEB-FIP Model Code 1990 to predict the longitudinal shear strength between parts of concrete members cast at different times. This is a relevant subject for different situations, such as the connection between precast members with cast-in-place parts and strengthening of existing RC members with a new concrete layer. Other situations, such as corbels; metallic supports subjected mainly to shear forces; regions near supports; and the connection between columns and foundations can also be designed using the shear-friction theory. It is known that surface roughness plays a significant role on the bond strength of concrete-to-concrete interfaces. Recent studies, using a laserbased device specifically developed to quantify the texture of concrete surfaces, proved that roughness can be measured and, moreover, correlated with the cohesion and friction coefficients present in the Model Code's design expression. Therefore, the current qualitative assessment of the surface roughness, merely based on a visual inspection, can be replaced by an accurate and quantitative approach. The curing conditions of the substrate concrete (old concrete) and of the added concrete layer (new concrete) are not considered either. This can have a significant influence because additional stresses can appear at the interface between both concrete layers due to differential shrinkage. Differential stiffness due to the difference between Young modulus is not addressed either. In this paper, improvements to Model Code 90's design expression for longitudinal shear strength between concretes cast at different times are recommended. Furthermore, a comparison between the design expressions proposed by the CEB-FIP Model Code 1990, the Eurocode 2 and the ACI 318 is presented.