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- 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.
- Factors affecting bond between new and old concretePublication . Santos, Pedro; Julio, Eduardo Nuno Brito SantosThe bond strength of concrete-to-concrete interfaces, of reinforced concrete (RC) members with parts cast at different ages, is highly influenced by the curing conditions. Therefore, the monolithic behavior is dependent on these conditions. Current design codes only consider: a) the concrete compressive strength; b) the normal stress at the interface; c) the amount of reinforcement crossing the interface; and d) the roughness of the substrate surface. Because the curing conditions of both substrate and added concrete are ignored, the influence of the differential shrinkage is neglected. The influence of the differential stiffness due to the mismatch between the Young’s modulus of both materials is not considered either. This paper presents an experimental study conducted to assess the influence of differential shrinkage and stiffness on the bond strength of new-to-old concrete interfaces. Both parameters were shown to have a significant influence on the bond strength and failure mode of concrete-to-concrete interfaces.
- 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.
- INFLUENCE OF CONCRETE DENSITY AND STRENGTH ON THE BOND STRENGTH OF COMPOSITE MEMBERSPublication . Saldanha, Rui; Santos, Pedro; Júlio, EduardoThe present paper addresses the influence of both material density and strength in the adhesion between concrete layers cast at different ages. A Normal Oensity Concrete (NOC), a LightWeight Aggregate Concrete (LWAC) and an Ultra High Durability Concrete (UHDC) were considered. The latter is characterized by presenting enhanced durability properties. Nevertheless, the production of the UHDC presents both economical and environmental disadvantages. In order to minimize the impact of its use and, simultaneously, take advantage of its properties, it is proposed to use it only in the cover layer of structural members, being the bulk produced in ordinary concrete. An experimental study was conducted to characterize the adhesion between UHDC/NOC and UHDC/LWAC by means of the Modified Slant Shear Test (M-SST). This new bond test was developed by the authors to always ensure adhesive failures in composite specimens. Two different surface conditions were selected for the interface: i) smooth surface, as-cast against steel formwork; and ii) surface obtained by casting against a plastic wavy formwork. Experimental results are discussed and conclusions are drawn.
- A modified slant shear test designed to enforce adhesive failurePublication . Saldanha, Rui; Júlio, Eduardo; Dias-da-Costa, Daniel; Santos, PedroPresently, there are several tests available to evaluate the adhesion between concrete layers cast at different times. Among these, the Slant Shear Test (SST) has a wide spread use, mainly for being sensitive to the roughness of the interface surface, one of the most influencing parameters in this scope, but also because of the simplicity of the experimental setup. However, the major drawback of this test is the fact of being often obtained cohesive (monolithic) failures, therefore providing a lower bound of the interface strength, which is related with the concrete compressive strength. To definitely solve this issue, i.e., to enforce adhesive (interface debonding) failures in all situations, the 'Modified Slant Shear Test' (M-SST) was herein designed and developed. Both numerical and experimental tests have been conducted to validate this new method. All relevant conclusions are presented. In summary, it can be stated that the M-SST always enforces adhesive failure, therefore allowing to accurately assessing the adhesion of concrete-to-concrete interfaces.
- A state-of-the-art review on roughness quantification methods for concrete surfacesPublication . Santos, Pedro M. D.; Júlio, Eduardo N.B.S.Surface roughness plays an important role in the behavior of concrete-to-concrete interfaces. Methods like sand-blasting and hydrodemolition are usually adopted to improve the surface roughness of the concrete substrate. Before placing the new concrete layer it is common to verify the roughness level by visual inspection. However, this procedure can lead to inaccurate conclusions since it is influenced by the technician’s opinion and, therefore, it is subjective. Current design codes of concrete structures implicitly propose this type of assessment since there is not a specified method or device to verify the conditions of the substrate surface. To overcome the drawbacks of this qualitative methodology, a quantitative approach should be adopted instead. Thus, it is necessary to measure the surface roughness and, from this, to compute roughness parameters. This paper presents a review on roughness quantification methods that can be adopted for concrete surfaces.