shear friction milestones

8/10/2019 Shear Friction Milestones

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8th fib PhD Symposium in Kgs. Lyngby, Denmark June 20 23, 2010

Assessment of the Shear Strength betweenConcrete Layers

PEDROM. D. SANTOS(a) ANDEDUARDO N. B. S. JLIO(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 [email protected]

(b)Professor, ISISE, Dept. of Civil Engineering, University of Coimbra, Faculty of Sciencesand Technology, Rua Lus Reis Santos, Polo II, 3030-788 Coimbra, Portugal

[email protected]

AbstractConcrete-to-concrete interfaces are present in new and existing RC structures. Precastmembers with cast-in-place parts and the repair and rehabilitation of existing RC members aretypical examples. The behaviour of RC composite members is highly influenced by thesurface conditions of the interface and by the differential shrinkage and stiffness of bothconcrete parts.

Current design codes present expressions for the assessment of the longitudinal shearstrength of concrete-to-concrete interfaces. Some drawbacks can be pointed: 1) the evaluationof the surface roughness is purely qualitative; 2) the curing conditions of both concrete partsare not considered; and 3) the difference between Young modulus of both concrete parts is notaddressed either.

This paper describes a research study conducted to investigate the influence of the surfaceroughness, differential shrinkage and differential stiffness. Modifications to the current shear-friction provisions of Eurocode 2 are proposed.

1. IntroductionThe bond strength at the interface between concrete layers cast at different ages is importantto 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 thelongitudinal shear strength of concrete-to-concrete interfaces. These expressions are based onthe 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 currentlyadopted by all design codes. It is common to classify the surface asvery smooth , smooth ,rough or very rough or simply asintentionally roughened or not intentionally roughened .Typical finishing treatments of concrete surfaces are usually linked to this classification andthe values of two coefficients, friction and cohesion, are given to be adopted in the designexpressions. This approach is clearly inaccurate because it is highly influenced by thetechnician opinion and, therefore, subjected to human error.

Since no method or device is specified by design codes to help the designer in theroughness classification it is common to use the Sand Patch Test [4] or the Concrete Surface


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Profiles [5]. Although simple, the use of both methods presents major drawbacks. The first isonly 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 thedifference 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 theinterface.

For all these reasons, current design expressions need improvements to increase theiraccuracy. This research study aims to add a contribution to the development of such designexpressions. The influence of the surface roughness and differential shrinkage and stiffnesswas investigated. A new optical measuring device [6] was specifically developed tocharacterize the roughness of concrete surfaces. A fullin 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-frictionThe shear-friction theory assumes that the shear strength of a concrete-to-concrete interfacesubjected simultaneously to shear and compression forces is ensured by friction only. Asimple 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 newconcrete layers, the interface crack width increases, the steel reinforcement yields in tensionthus 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 stressat the concrete-to-concrete interface (vu). The five most significant contributions are presentedin Table 1.

Birkeland and Birkeland [7] proposed the design expression currently known as theshear-friction expression. These researchers suggested the following values for thecoefficient of friction: a) = 1.7, for monolithic concrete; b) = 1.4, for artificiallyroughened construction joints; and c) = 0.8 to 1.0, for ordinary construction joints and forconcrete-to-steel interfaces. Later, Mattock and Hawkins [8] proposed an improved designexpression, known as the modified shear-friction expression, which includes a constant dueto cohesion. The coefficient of friction is considered constant and equal to 0.8.

s

s


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Loov [9] was the first to explicitly include the concrete strength. Walravenet al. [10] proposed a non-linear function to predict the shear strength of initially cracked interfaces. Aninnovative 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 normalstresses to the interface and the longitudinal relative slip between concrete parts; and dowelaction, 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 u yv f =

Mattock and Hawkins [8] 1972( )

1.38 0.8u n y

v f = + +

Loov [9] 1978 ( )u n ycv k f f = +

Walravenet al. [10] 1987

( ) 21C

u yv C f = 0.406

1 0.822 cC f = 0.303

2 0.159 cC f =

Randl [11] 1997 ( )1 3u c n y y cv cf kf f f = + ++

In Table 2 are also presented the design expressions of three major design codes for RCstructures, 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 ( )u ctd n yv cf f = + +

Eurocode 2 [2] 2004 ( )sin cosu ctd n yv cf f = + + +

ACI 318 [3] 2008 ( )sin cosu yv f = +

In these expressions (Table 1 and Table 2), is the coefficient of friction; is thereinforcement ratio; f y is the yield strength of the reinforcement; n is the normal stress actingon the interface due to external loading;k is a constant (Loovs expression); f c is the concretecompressive strength;c is the coefficient of cohesion;k is a coefficient of efficiency relatedwith the reinforcement (Randls expression); is a coefficient for dowel action (Randlsexpression) or the angle between the shear reinforcement and the shear plane; f ctd is the tensilestrength 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 samesurface condition. This incongruence is probably the main drawback of the design expressionsand of the design codes referred to.


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neglected is the difference between the substrate and added concrete Young modules and thusthe differential stiffness between concrete parts.

In relation to roughness, it is proposed that a quantitative methodology be adopted to

avoid the subjective assessment proposed in all design codes. The authors proposed aninnovative and non-destructive method, the 2D-LRA method [6], to predict the bond strengthof concrete-to-concrete interfaces. This new method proved to be effective, since it is possibleto obtain 2D profiles of the surface texture; to compute texture parameters from these; and tocorrelate the latter with the bond strength of the concrete-to-concrete interface, both in shearand in tension, with high coefficients of correlation. Moreover, it was demonstrated that the proposed new method presents all the advantages, with even higher accuracy, and overcomesall the disadvantages of existing methods [4, 5, 12].

Based in recent research studies [13] and adopting the design expression of Eurocode 2[2], the authors propose that the coefficients of cohesion and friction be predicted using thefollowing expressions:

0.1451.062vm

d

coh

Rc

= (1)

0.0411.366vm

d

fr

R

= (2)

wherecd is the design coefficient of cohesion; d is the design coefficient of friction; Rvm isthe Mean Valley Depth [13] of the primary profile of the surface in millimetre; coh is the partial safety factor for the coefficient of cohesion; and fr is the partial safety factor for thecoefficient of friction.

The proposed expressions were obtained by adjusting a power function to theexperimental values of the coefficients of cohesion and friction, Figure 3, determined for thefive different surface conditions considered: left as-cast; wire-brushing; sand-blasting; shot- blasting and hand-scrubbing. Based in the coefficient of variation of both coefficients, theauthors propose the values of 2.6 and 1.2 for the partial safety factors of the coefficients ofcohesion and friction, respectively.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.0

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0.4

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0.8

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1.2

1.4

1.6

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Coeff. of cohesion

Power Regressionfor Coeff. of cohe-sionCoeff. of frictionPower Regressionfor Coeff. of fr iction

Mean Val ley Depth, Rvm (mm)

C o e

f f i c i e n

t o f c o

h e s i o n

C o e

f f i c i e n

t o f f r i c t i o n

Figure 3: Correlation between the Mean Valley Depth ( Rvm) and the coefficients of cohesion

and friction.

It also proved that differential shrinkage and differential stiffness can have a significantinfluence on the shear strength of the interface between concretes cast at different times [13].These effects should at least be mentioned in codes and analyzed for each specific situation.


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AcknowlegmentsThe authors acknowledge the financial support of the Portuguese Science and Technology

Foundation (FCT), PhD Grant number SFRH/BD/25510/2005. Acknowledgements areextended to the companies Maprel Empresa de Pavimentos e Materiais Pr-Esforados Lda,Sika Portugal SA, AFAssociados Projectos de Engenharia SA, Weber Cimenfix, Cimpor Cimentos de Portugal, Beto-Liz, Euro-Planning Engenharia & Gesto Lda, TrueGage andSYCODE also for their financial support.

References[1] CEB-FIP Model Code, Comit Euro-International du Bton, Secretariat Permanent, Case

Postale 88, CH-1015 Lausanne, Switzerland, 437 p., 1990.[2] Eurocode 2, Design of concrete structures - Part 1-1: General rules and rules for

buildings, European Committee for Standardization, Avenue Marnix 17, B-1000Brussels, Belgium, 225 p., 2004. (with corrigendum dated of 16 January 2008)[3] ACI 318, Building code requirements for structural concrete (ACI 318-08) and

commentary, American Concrete Institute, PO Box 9094, Farmington Hills, MI 48333-9094, USA, 471 p., 2008.

[4] ASTM E 965, Standard test method for measuring pavement macrotexture depth using avolumetric technique, American Society for Testing Materials, 100 Barr Harbor Dr.,West Conshohocken, PA 19428, USA, 3 p., 2001.

[5] ICRI (International Concrete Repair Institute), Selecting and specifying concrete surface preparation for sealers, coatings, and polymer overlays, Technical Guideline No. 03732,Des Plaines, Illinois, USA, 41 p., 1997.

[6]

P. Santos and E. Jlio, Development of a laser roughness analyser to predict in situ the bond strength of concrete-to-concrete interfaces, Magazine of Concrete Research , vol.60, no. 5, pp. 329-337, 2008.

[7] P.W. Birkeland and H.W. Birkeland, Connections in precast concrete construction, Journal of the American Concrete Institute , vol. 63, no. 3, pp. 345-368, 1966.

[8] A.H. Mattock and N.M. Hawkins, Shear transfer in reinforced concrete recentresearch, Journal of the Precast/Prestressed Concrete Institute , vol. 17, no. 2, pp. 55-75, 1972.

[9] R.E. Loov, Design of precast connections, Paper presented at a seminar organized byCompa International Pte, Ltd., Singapore, 8 p., 1978.

[10] J. Walraven, J. Frnay and A. Pruijssers, Influence of concrete strength and load history

on the shear friction capacity of concrete members, Journal of the Precast/PrestressedConcrete Institute , vol. 32, no. 1, pp. 66-84, 1987.[11] N. Randl, Investigations on transfer of forces between old and new concrete at different

joint roughness, PhD thesis, University of Innsbruck, 379 p., 1997. (in German)[12] P. Santos, E. Jlio and V.D. Silva, Correlation between concrete-to-concrete bond

strength and the roughness of the substrate surface,Construction and Building Materials , vol. 21, no. 8, pp. 1688-1695, 2007.

[13] P.M.D. Santos, Assessment of the Shear Strength between Concrete Layers, PhDthesis, University of Coimbra, 338 p., October 2009.

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