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Shear Friction Milestones

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this technical paper is more important for those who are looking for forecasting of shear friction theory about from 1950 to date Mast, mattock, paulay and park s contribution on shear friction has been included
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  8 th  fib PhD Symposium in Kgs. Lyngby, Denmark June 20 – 23, 2010 Assessment of the Shear Strength between Concrete Layers P EDRO M.   D.   S ANTOS (a)  AND E DUARDO  N.   B.   S.   J ÚLIO (b)   (a) Adjunct Professor, ISISE, Dept. of Civil Engineering, Polytechnic Institute of Leiria, School of Technology and Management, Campus 2 – Morro do Lena – Alto do Vieiro, 2411-901 Leiria, Portugal  pedro.santos@estg.ipleiria.pt (b) Professor, ISISE, Dept. of Civil Engineering, University of Coimbra, Faculty of Sciences and Technology, Rua Luís Reis Santos, Polo II, 3030-788 Coimbra, Portugal ejulio@dec.uc.pt   Abstract Concrete-to-concrete interfaces are present in new and existing RC structures. Precast members with cast-in-place parts and the repair and rehabilitation of existing RC members are typical examples. The behaviour of RC composite members is highly influenced by the surface conditions of the interface and by the differential shrinkage and stiffness of both concrete parts. Current design codes present expressions for the assessment of the longitudinal shear strength of concrete-to-concrete interfaces. Some drawbacks can be pointed: 1) the evaluation of the surface roughness is purely qualitative; 2) the curing conditions of both concrete parts are not considered; and 3) the difference between Young modulus of both concrete parts is not addressed either. This paper describes a research study conducted to investigate the influence of the surface roughness, differential shrinkage and differential stiffness. Modifications to the current shear-friction provisions of Eurocode 2 are proposed. 1.   Introduction The bond strength at the interface between concrete layers cast at different ages is important to ensure the monolithic behaviour of RC composite members. Precast beams with cast-in- place slabs and repair and strengthening of existing concrete structural members, such as  bridge decks, by adding a new concrete layer are typical examples of RC composite members. Current design codes [1, 2, 3] present design expressions for the assessment of the longitudinal shear strength of concrete-to-concrete interfaces. These expressions are based on the  shear-friction theory  and the shear strength is evaluated considering basically four  parameters: a) compressive strength of the weakest concrete; b) normal stress at the interface; c) shear reinforcement crossing the interface; and d) roughness of the substrate surface. A qualitative evaluation of the surface roughness, based on a visual inspection, is currently adopted by all design codes. It is common to classify the surface as very smooth ,  smooth , rough  or very rough  or simply as intentionally roughened   or not intentionally roughened  . Typical finishing treatments of concrete surfaces are usually linked to this classification and the values of two coefficients, friction and cohesion, are given to be adopted in the design expressions. This approach is clearly inaccurate because it is highly influenced by the technician opinion and, therefore, subjected to human error. Since no method or device is specified by design codes to help the designer in the roughness classification it is common to use the Sand Patch Test [4] or the Concrete Surface  8 th  fib PhD Symposium in Kgs. Lyngby, Denmark June 20 – 23, 2010 Profiles [5]. Although simple, the use of both methods presents major drawbacks. The first is only applicable to top horizontal surfaces while the second is purely qualitative. Design codes do not take into account the curing conditions of both concrete parts. Therefore, the differential shrinkage is neglected. The differential stiffness, due to the difference between Young modulus of both concrete layers, is not addressed either. However,  both parameters have a significant influence because they can create additional stresses at the interface. For all these reasons, current design expressions need improvements to increase their accuracy. This research study aims to add a contribution to the development of such design expressions. The influence of the surface roughness and differential shrinkage and stiffness was investigated. A new optical measuring device [6] was specifically developed to characterize the roughness of concrete surfaces. A full in situ  non-destructive methodology is  proposed for the assessment of the bond strength of concrete-to-concrete interfaces. Modifications to the current shear-friction provisions of Eurocode 2 [2] are proposed. 2.   Shear-friction The shear-friction theory assumes that the shear strength of a concrete-to-concrete interface subjected simultaneously to shear and compression forces is ensured by friction only. A simple “saw-tooth model” is usually adopted to exemplify the basic principles of this theory, Figure 1. This design philosophy assumes that, due to relative slippage between old and new concrete layers, the interface crack width increases, the steel reinforcement yields in tension thus compressing the interface and the shear forces are transmitted by friction. Figure 1: Shear-friction.   Several design expressions were proposed to predict the ultimate longitudinal shear stress at the concrete-to-concrete interface ( v u ). The five most significant contributions are presented in Table 1. Birkeland and Birkeland [7] proposed the design expression currently known as the “shear-friction expression”. These researchers suggested the following values for the coefficient of friction: a)  µ  = 1.7, for monolithic concrete; b)  µ  = 1.4, for artificially roughened construction joints; and c)  µ  = 0.8 to 1.0, for ordinary construction joints and for concrete-to-steel interfaces. Later, Mattock and Hawkins [8] proposed an improved design expression, known as the “modified shear-friction expression”, which includes a constant due to cohesion. The coefficient of friction is considered constant and equal to 0.8. τ σ σ τ σ   s σ   s  8 th  fib PhD Symposium in Kgs. Lyngby, Denmark June 20 – 23, 2010 Loov [9] was the first to explicitly include the concrete strength. Walraven et al.  [10]  proposed a non-linear function to predict the shear strength of initially cracked interfaces. An innovative “sphere model” was developed to analyse the interaction between the aggregates, the binding paste and the interface zone. Randl [11] proposed the first design expression that explicitly includes the contribution of: cohesion, related with the interlocking between aggregates; friction, related with normal stresses to the interface and the longitudinal relative slip between concrete parts; and dowel action, related with the deformation of the shear reinforcement crossing the interface. Table 1: Shear-friction milestones.   Researcher(s) Year Design expression Birkeland and Birkeland [7] 1966 uy vf   µρ  =  Mattock and Hawkins [8] 1972 ( ) 1.380.8 uny vf  σρ  = + +  Loov [9] 1978 ( ) uny c vkff  σρ  = +  Walraven et al.  [10] 1987 ( ) 2 1 C uy vCf   ρ  =   0.4061 0.822 c Cf  =   0.3032 0.159 c Cf  =  Randl [11] 1997 ( ) 13 ucnyyc vcfkfff   µσραρ  = + + +   In Table 2 are also presented the design expressions of three major design codes for RC structures, which are mainly derived from the first two expressions presented in Table 1 [7, 8]. Table 2: Shear-friction provisions of design codes.   Design Code Year Design expression CEB-FIP Model Code 1990 [1] 1990 ( ) uctdny vcff   µσρ  = + +  Eurocode 2 [2] 2004 ( ) sincos uctdny vcff   µσρµαα  = + + +  ACI 318 [3] 2008 ( ) sincos uy vf   ρµαα  = +   In these expressions (Table 1 and Table 2),  µ  is the coefficient of friction;  ρ  is the reinforcement ratio;  f   y  is the yield strength of the reinforcement; σ  n  is the normal stress acting on the interface due to external loading; k   is a constant (Loov’s expression);  f  c  is the concrete compressive strength; c  is the coefficient of cohesion; k   is a coefficient of efficiency related with the reinforcement (Randl’s expression); α  is a coefficient for dowel action (Randl’s expression) or the angle between the shear reinforcement and the shear plane;  f  ctd   is the tensile strength of the weakest concrete. Besides the format of the design expressions, the main difference between codes is the roughness classification and the proposed coefficients of friction and cohesion for the same surface condition. This incongruence is probably the main drawback of the design expressions and of the design codes referred to.  8 th  fib 3.   Ex A largroughnconcretA Pdiffereand coaTwshear aand the 150mmFivand thconsideinterfac blastinused in as “rakisurface Twcured radiatioadded cof diffeFig 4.   Co The revon the codes classificoncretconside based, CoconditihD Sympo perime  experimess, differee-to-concreortland cet aggregaterse limesto bond testd in tensioshear plan with the in different added cored to serve surface (SAB); athe precasng”, was eroughness curing coutside, then, rain and oncrete laential shrire 2: Surfa nclusio iew of the  bond strenhis paramation. 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