INTERFACE SHEAR TRANSFER FOR HIGH STRENGTH CONCRETE AND HIGH STRENGTH REINFORCEMENT

INTERFACE SHEAR TRANSFER FOR HIGH STRENGTH CONCRETE AND HIGH STRENGTH REINFORCEMENT 641 Susumu KONO 1 And Hitoshi TANAKA SUMMARY The shear transfer at construction joints for members with high strength concrete and shear friction reinforcement was studied experimentall using twent-three direct shear tpe specimens. The contribution of dowel action to the total shear increased from % to about 6% as slip increased up to 4 mm. The larger the area and the ield strength of reinforcement, the smaller the stress drop after the initial peak and the larger the stress regain after the drop. The shear capacit also increased linearl with the normal force provided b reinforcement but it was found that reinforcing bars did not ield when the shear capacit was reached. The interface roughness was measured using laser digitizing devices and quantified with indices in order to correlate the surface roughness to stiffness and capacit in shear. Although those indices did not show the clear correlation with stiffness and capacit, the surface roughness is considered to affect the stress of reinforcement at the maximum shear, in addition to other factors like the area and ield strength of reinforcement and the concrete strength. A more appropriate index is under search to express surface roughness which can be correlated to the shear transfer mechanisms. With a linear approximation of the relation between the stress at the maximum shear and ield strength of reinforcement, an equation to evaluate the shear capacit was proposed, the format of which is similar to that of Mattock and Hawkins [7]. INTRODUCTION Interface shear at construction joints is transferred mainl through concrete before cracking, and through both concrete and reinforcing bars after cracking. The shear transfer through concrete is called a conrete action in this paper and consists of friction resulting from the normal compressive stress and the interlock of aggregate protrusions. The shear transfer through reinforcing bars is called a dowel action. Total shear stress transferred at cracked interfaces can be predicted b summing the stresses due to the concrete and dowel actions. The shear friction method introduced in the ACI code [1] as well as in the Japanese building code [3] is useful to account for the interface shear transfer. However, in this method, the effects of the friction at the concrete interface and the dowel action of shear-friction reinforcement are experimentall evaluated b integrating them all into the frictional coefficient, and the contribution of the dowel action to the total shear transfer cannot be directl seen from the frictional coefficient given in the codes. In addition, the contribution of the dowel action varing not onl with the mechanical properties of concrete and reinforcement but also with a relative displacement between concrete surfaces is not considered, and the conventional shear friction method is not originall developed for members with high strength concrete and reinforcement. In this stud, the interface shear transfer was examined experimentall using direct shear tests on 3 specimens. The concrete strength ranged from 3MPa to 1MPa and the ield strength of reinforcing bars from 3MPa to 1MPa. The interface roughness was measured and quantified using laser digitizing devices in order to clarif its relation with stiffness and capacit in shear. The relative sliding and opening displacements and the axial force of reinforment were measured to separate the concrete and dowel actions. Based on the experimental results, a design equation for the interface shear transfer for normal and high strength materials was proposed. 1 Dept. of Architecuture, Koto Universit, Koto, Japan Email:Kono@archi.koto-u.ac.jp Dept. of Civil Eng., Toohashi Universit of Technolog, Toohashi, Japan Email:Tanaka@jughead.tutrp.tut.ac.jp

TEST SPECIMEN DESIGNS Specimen Designs As shown in Table 1, two series of specimens were prepared. Series 1 was designed to investigate the effects of the strength and the diameter of dowel bars, and the concrete strength on the shear transfer mechanisms, whereas Series to investigate the effects of the concrete strength and the interface morpholog. There were two tpes of specimens. C tpe (meaning combination) specimens had prescribed interface finishings and dowel bars embedded so that the external shear force was resisted b both concrete and dowel actions, while D tpe (meaning dowel) specimens were prepared to isolate the dowel action b inserting double thin plates at the interface in order to eliminate the concrete action [4]. In this manner, the external shear force was resisted b the dowel action onl, in D tpe specimen. Specimen dimensions and reinforcement arrangement are shown in Fig. 1. For C tpe specimens, the lower block was cast first, a prescribed finishing made at the construction joint, and then the other half block cast. For D tpe specimens, lower and upper blocks were cast at a same time with double thin plates placed at the joint interface. The mechanical properties of concrete and steel are shown in Table. The maximum aggregate size was mm. The shear interface had four kinds of finishings as shown in Fig.. For a trowelled surface, latance was removed later with a wire brush from a trowelled surface. For a rough surface, mortar on the casting surface was completel removed before concrete hardened so that a surface of an aggregate laer was seen. For a scratched surface, a grid was drawn b scratching the surface with nails before concrete hardened. For a triangle surface, a steel mold was pressed and a triangle shape was generated. Table 1: Specimen designation and variables Specimen Variables Test Tpe of Nominal Surface τ Series Designation Tpe u Slip at cap dowel f c (MPa) Condition (MPa) τ u ε bars L1-3C C 3 Trowelled.98.6 74 L1-3D D Plate N/A N/A N/A L1-C C SD9A (A) Trowelled 3.3.1 4 L1-D D D1 *1 Plate N/A N/A N/A L1-8C C 8 Trowelled.6.43 76 L1-8D D Plate N/A N/A N/A H1-3C C 3 Trowelled 3.43.7 3 1 H1-3D D Plate N/A N/A N/A H1-C C KSS78 (A) Trowelled.9. 9 H1-D D D1 *1 Plate N/A N/A N/A H1-8C C 8 Trowelled.16.61 34 H1-8D D Plate N/A N/A N/A H16-3C C KSS78 3 Trowelled 4.49. 8 H16-C C D16 *1 (A) Trowelled.83. 9 H16-8C C 8 Trowelled 6.3. 39 HPC C Trowelled 1.. 37 HSC C (B) Scratched.86. 61 HRC C Rough 7.31 1.4 11 HTC C KSS78 Triangle 6.97 1.8 99 H1PC C D1 *1 Trowelled 4.7.86 63 H1SC C 1 Scratched 7.11.9 96 H1RC C Rough 8.36. 8 H1TC C Triangle 7.. *1: D in D1 or D16 indicates a deformed bar. *: εcap is the strain of dowel bars when the shear capacit was reached. ε * 641

4 7 7 136 7 X 7 Dowel bars M4 Long Nut Shear Plane 16 A B 4 3 4 8 4 Latance was removed later. (a) Trowelled 16 X 34 34 34 34 3 68 68 68 68 68 3 φ6 D1 Unit: mm (a) Elevation (b) X-X section Figure 1: Specimen dimensions and reinforcement arrangement Surface mortar was taken out b a wire brush (b) Rough Table : Mechanical properties of concrete and steel Nominal f c (MPa) Block location f c (MPa) f t (MPa) Ec (GPa) 3 Lower 3. 3.16.4 Upper 31.8 3. 4.4 (A)*1 Lower 4.6 4.41 33.7 Upper 4.4 4.44 9. (B)*1 Lower 49. 1.7 9. Upper 3.8 11.6 7.3 8 Lower 79. 3.31 3.6 Upper 13 3. 3. 1 Lower 13 1.7 3.4 Upper 97.6 17.7 36. Bar diameter in mm Strength f fu Es (Area in mm Tpe (MPa) (MPa) (GPa) ) 1 (71.3) SD9A 34 481 18 1 (71.3) 999 134 184 KSS78 16 (198.6) 867 18 187 *1 A and B refers to the same label for f c=mpa in Table 1. Concrete Steel Loading A steel mold was pressed to shape 1mm the surface. Figure 3: Loading mm (d) Triangle sstem Figure 3 shows the loading and measuring sstem. The center of the horizontal force staed at the same height as the shear plane so that the direct shear force acted on the shear plane without moment. The vertical loading jack was used for D tpe specimens onl. For C tpe specimens, the horizontal force was applied monotonicall and the opening and slip displacements were recorded. For D tpe specimens, the opening and slip displacements were controlled so that the specimen experienced the same displacement path with the companion C tpe specimen. In this manner, the shear resistance b the dowel action was measured b D tpe specimens and the shear resistance b the concrete action was computed b subtracting shear force of a D tpe specimen from the shear force of a companion C tpe specimen. Opening and slip displacements were measured at four corners of the specimen. Two strain gages were placed on the surface of dowel bars mm above the interface so that the bending and axial components of deformation was computed. Shear Stress Slip Relations TEST RESULTS Line are mm deep made b scratching with 1mm-diameter nails. (c) Scratched Figure 4 shows shear stress- slip relations for five representative specimens, all of which had concrete strength of MPa. For C tpe specimens, shear stress first increased without noticeable slip and then dropped suddenl with large slip. This drop was larger for specimens with lower ρ f, where ρ is the area ratio of dowel bars to the construction joint and f is the ield strength of dowel bars. The shear stress then increased graduall with 1mm 1mm 3 641

slip and the initial peak was exceeded for specimens with higher ρ f. It is considered that the initial peak emerged due to the cohesion at the concrete interface. After a distinctive interface was formed, the shear resistance was provided b the concrete and dowel actions. Shear stress for D tpe specimens increased monotonicall with increasing slip. Setting τ D as the stress transferred in a D tpe specimen and τ C as that in a companion C tpe specimen, the contribution of the dowel action to the total shear is expressed as τ D τ C. The computed results from six sets of Series 1 specimens are shown in Fig.. Up to slip equalled 4 mm, τ D τ C increased graduall from to about 6%. Comparing L1-3 with H1-3, L1- with H1-, and L1-8 with H1-8, τ D τ C for higher ρ f is larger than that for lower ρ f. However, comparing specimens with a same ρ f value, it can be seen that concrete strength did not have clear influence on τ D τ C. Some design practices do not allow an slip at construction joints but studies [8] show that the structure with interface slip less than mm performs as good as that with rigid interfaces. Figures 4 and show that slip does not necessaril cause a brittle failure and even the increase in shear capacit can be expected for larger ρ f. The interface endures large slip without much degradation in shear capacit since the dowel action becomes dominant after slip greater than mm. For these reasons, the shear capacit was defined for C tpe specimens as the maximum shear stress for slip under mm and these values are listed in Table 1. Shear Stress (MPa) 8 L1-C(ρf=1.6) L1-D(ρf=1.6) H1-C(ρf=4.9) 6 H1-D(ρf=4.9) H16-C(ρf=1.) 4 1 3 4 Slip (mm) τ D /τ C.8.6.4. L1-3(ρf=1.6) L1-(ρf=1.6) L1-8(ρf=1.6) H1-3(ρf=4.9) H1-(ρf=4.9) H1-8(ρf=4.9) 1 3 4 Slip (mm) Figure 4: Shear stress slip relations Surface Roughness Figure : τ D /τ C slip relations The surface roughness of joints was quantitativel evaluated to stud its correlation with the shear capacit, the stiffness in shear-slip relation, and the slip-opening relation. In this stud, the existing four indices were emploed to express the roughness; the fractal dimension, D, the root mean square height, H rms, the average height, H ave, and the core roughness depth, R k. The details for these indices are not explained here and readers are suggested to read references, 6, and 1 for details. To obtain D, a rectangle of 1 mm b mm were divided into a 3 b 3 grid and the height was measured at each grid point using a three dimensional laser digitizer. One value of D was computed from each rectange and 7 rectangles were used to obtain the average. Values H rms, H ave, and R k were computed from 1 measured heights along a 1 mm long straight base line. Assuming the obtained data is expressed as = f () x as shown in Fig. 6, H rms and H ave is expressed as Eqs. 1 and, respectivel. R k was evaluated from the linear representation of the material ratio curve (also referred to as the Abbott curve) which describe the increase of the material portion of the surface with increasing depth of the roughness profile. One value of H rms, H ave, and R k was obtained from a measurement along one base line and nine base lines were used to obtain the average. In Table 3, the roughness of specimens in Series are expressed using four indices. All indices have large standard deviation and a same kind of finishing with different concrete strengths (e.g. HP and H1P) did not necessaril have similar indices. The correlations between one of these four indices and either of the shear capacit, the stiffness in shear-slip relation, or the slipopening relation were studied but none of those combinations showed the clear correlation. For example, Fig. 7 shows the shear capacit R relations and the correlation is not ver clear. Since quantitative evaluation of k 4 641

the roughness is one of the important tasks for the interface shear transfer, an effort to find a more appropriate index is under wa. H rms = 1 L L ( f ( x) ) dx (1) H ave 1 = L L f ( x) dx () Figure 6: Surface roughness Table 3: Indices for surface roughness D H rms (mm) H ave (mm) R k (mm) Ave. SD Ave. SD Ave. SD Ave. SD =f(x) Base Line HN.3.1.9..7.4.16.1 HP.71. 1.39.38.99.7.46 1.1 HR.19.9.8.3.9.1 1.4.36 HT.3.1 1.66.4 1.4.1 3.76 1. H1N.3.1.14.4.1.4.33.1 H1P.34. 1.84.66 1.4.6 3.6 1.39 H1R..49 1.6.36.81..6.64 H1T.31.18.34.14..1 6..77 Ave.: Average SD: Standard deviation 1 1 Maximum Shear Stress, τu (MPa) 1 HR HP HT Maximum Shear Stress, τu (MPa) 1 H1R H1N H1P H1T HN 4 Rk 6 8 4 Rk 6 8 MPa (a) f c = MPa (b) f c = 1 Figure 7: Shear capacit R k relations 641

Shear capacit, τ u (MPa) 1 1 Cracked Rectangular ke Monolithic Plain Scratched Triangular ke Rough.3 Triangle Ke..1 3.6. ρf 7.8 τ u =1.1ρfs+.84 R=.77 4.3 Monolithic was excluded. 4 6 8.1..3.4 ρfs (MPa) ρf.3 Figure 8: Shear capacit - relations Rectangle Ke ρf.3 Plain (a) Triangle shear ke ρf..1 3.6 6..7..1 6. 1..3.1..3.4 ρf 4.4 1.6 4.9.1..3.4 ρf (b) Rectangle shear ke (c) Plain surface.3 Rough ρf.3 Others ρf.. Cracked 7. 6..3.1 4.4.6 7.6 3.6 17. 3.1.1..3.4 ρf.1 Monolithic.3 Scratched 4.4 Cracked 6.1 Monolithic 6..1..3.4 ρf (d) Rough finishing Figure 9: /f c ρf /f c relations (e) Others Shear Capacit Although shear capacit should not be the onl design criteria to check the performance of joints, it is still a important design factor. The current design for interface shear is based on Mattock and Hawkins research [7] and expressed b Eq. 3. 6 641

τ =.8ρ + 1.4.3 f ' c (MPa) (3) u f where ρ is the area ratio of dowel bars to the joint, f the ield strength of dowel bars, and f ' c the compressive strength of concrete. In the experiment, dowel bars did not generall ield when the maximum shear was reached as shown in Table 1. So it was concluded that the shear capacit should be correlated to the normal stress ρ f s where f s is the tensile stress of dowel bars at the maximum shear. Figure 8 shows the relation between τ u and ρ f s from the test results of this stud and reference 9. Concrete strength ranged from MPa to 98 MPa, ield strength of dowel bars from 39 MPa to 1334 MPa, dowel bar ratio from.4 to 1.9, area of constuction joint from 16 cm to 76 cm, and a diameter of dowel bars from 1 mm to 16 mm. Surface conditions were plain, triangle, rectangle ke, monolithic and precracked. All experiments were under monolithic loading. Excluding three monolithic specimens shown in white circle, the relations between τ u and ρ f s can be expressed b Eq. 4 from a regression analsis. Normal force τ u = 1.1ρf s +.84 = 1.1 + Cohesion (MPa) (4) Joint area The correlation coefficient was.77. In Eq. (4), τ u is expressed b a summation of the frictional term and the cohesion term as Eq. 3. Here, f s is unknown in order to predict τ u and this value should be computed from known values in order to design joints. To predict f s, the relation between ρ f s f ' c and ρf f ' c was studied as shown in Fig. 9. Nonlinear relations which are dependent on a surface finishing seem to exist between ρ f s f ' c and ρ f f ' c. However, the number of specimens for each surface finishing was not enough to deduce the mathematical relation between ρ f s f ' c and ρ f f ' c. Expressing the roughness quantitativel was also difficult as explained in Section 3.. For these reasons, a simple linear curve was used to express ρf f ' - ρf f ' c relation. s c ρf s f ' c = 3 ρf f ' c () that is, f = 3 f s (6) Equation 6 is shown in thick solid straight lines in Fig. 9. The equation does not represent the behavior for plain surface and ρ f more than 7.6 MPa for rough finishing. Substituting Eq. (6) in Eq. (3), the maximum shear strength can be predicted as Eq. (7). τ u (exp)/τ u (cal) 3 1 ρf = to MPa ρf = to 4 MPa ρf = 4 to 6 MPa ρf = 6 to 8 MPa Shear Capacit (MPa) 1 1 f'c=-3mpa f'c=3-4mpa f'c=-6mpa f'c=7-8mpa f'c=8-9mpa f'c=9-1mpa Mattock Walraven Kono f'c=8mpa f'c=mpa f'c=4mpa and up f'c=mpa f'c=mpa k=.*f'c+. for f'c<4 k=1. otherwise 4 6 8 1 f'c (MPa) 4 6 8 1 ρf (MPa) Figure 1: τ u /τ cal f c relations Figure 11: Shear capacit - ρf relations 7 641

τ.67ρf +.84 (MPa) (7) u = Prediction using Equation (7) is shown in Fig. 1. It ma be seen that the prediction for the concrete strength less than 4 MPa is uncerservative. So the reduction factor k shown in dotted line was multiplied to Equation (7). (.67ρf +.84) τ (MPa) (8) u = k where k =. f ' c +. for f ' c 4 MPa and k = 1. otherwise. Not applicable to plain surface and ρ f more than 7.6 MPa. Prediction b Eq. 8 is shown with thick solid lines in Fig. 11 and compared with Eq. 3 and Walraven's equation [11]. The proposed equation is similar to Eq. 3 and is less influenced b the concrete strength. The test results also shows similar variations. CONCLUSIONS 1. The increase in the contribution of dowel bars to the total shear was larger for high ρ f although the contribution eventuall reached about 6% at slip of 4 mm no matter what ρ f was. The concrete strength did not have much influence on the contribution of the dowel action. With larger amount of ρ f, the stress drop from the initial peak decreased and the stress regain after the drop increased.. The shear capacit was considered to be a linear function of the normal force exerted b dowel bars but the design should take into account the fact that dowel bars did not ield when the shear capacit was reached. This normal force can be predicted using ρ f s f ' c and ρ f f ' c relations which is nonlinear and dependent on the roughness of the interface. 3. For a design purpose, the relation between ρ f s f ' c and ρ f f ' c was approximated b a linear curve and the shear capacit was expressed with an equation similar to that b Mattock and Hawkins. 4. Four indices were used to express surface roughness but none of them clearl differentiated different surface finishings prepared in this stud or showed close relation to stiffness and capacit in shear. AKNOWLEDGEMENT This research was financiall supported b Prestressed Concrete Cooperative Research and the Japan Educational Ministr (Grant Number 1743). REFERENCES 1. ACI Committee 318 (1991), Building Code Requirements for Reinforced Concrete, American Concrete Institute, Detroit, MI.. Ali, M. A. and White, R.N. (1999), Enhanced Contact Model for Shear Friction of Normal and High-Strength Concrete, ACI Structural Journal, V. 96, No. 3, Ma-June, pp. 348-36. 3. Architecture Inst. of Japan (198), Standard for Structural Calculation of Prefabricated RC Structures with Shear-Walls. 4. Dulacska, H. (197), "Dowel Action of Reinforcement Crossing Cracks in Concrete," ACI Journal, Dec., pp. 74-77.. ISO/DIS 136- (1994), Characterization of Surface Having Stratified Functional Properties, Part : Height Characterization of Using the Linear Material Ratio Curve, International Organization for Standardization. 6. Katori, K., Haashi, S., Makitani, T, and Ushigaki, K. (1998), Estimate of the shear strength of construction joint b using its surface roughness and characteristic of slip displacement of joint under shear force, J. of Struct. and Constr. Eng., Architecture Inst. of Japan, No. 7, Ma, pp. 17-116. 7. Mattock, A.H. and Hawkins, N.M. (197), Shear Transfer in RC Recent Research, PCI Journal, March-April, pp. -7. 8. Obuchi, H. et al. (1997), Stud on shear transfer considering shear displacement in connenction of precast concrete, J. of Struct. and Constr. Eng., Architecture Inst. of Japan, No. 491, Jan., pp. 97-14. 9. Okamoto, H. et al. (199-199), Shear Transfer between High Strength Precast and Cast-in-situ Concrete, Summaries of Technical Papers of Annual Meeting, Architecture Institute of Japan, pp. 39-4(199) pp. 679-68(1991) pp. 773-774(199). 1. Underwood, E. E. and Banerji, K. (1986), Fractals in Fractograph, Material Science Engineering, Vol. 8, pp. 1-14. 11. Walraven, J., Frena, J., and Pruijssers, A. (1987), Influence of Concrete Strength and Load Histor on the Shear Friction Capacit of Concrete Members, PCI Journal, Jan.-Feb., pp. 66-84. 8 641