Bond of reinforcement in eccentric pullout silica fume concrete specimens

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 31, December 1998, pp Bond of reinforcement in eccentric pullout silica fume concrete specimens Bilal S. Hamad 1, Saad M. Sabbah 2 (1) Associate Professor of Civil Engineering, (2) ME CE, Structural Engineer American University of Beirut, Beirut, Lebanon TECHNICAL REPORTS Paper received: October 30, 1997; Paper accepted: April 4, 1998 A B S T R A C T The objectives of the study were to investigate the role of specimen type and specimen size on the effect of silica fume on bond strength of reinforcing bars, and to evaluate the effect of bar size and concrete confinement on the anchorage characteristics of the reinforcing bars in silica fume concrete. Forty-eight eccentric pullout specimens were tested. The variables were the percentage replacement by weight of cement by silica, the concrete cover over the reinforcing bar, and the bar size. Failure of the specimens was governed by splitting of the concrete cover over the anchored bar. Test results indicated that as the percentage of silica fume increased, the maximum load capacity and the stiffness of the load-slip curve of the anchored bar decreased, regardless of bar size or the concrete cover used. The results showed that concrete confinement slightly improved the bond resistance in concrete containing more than 10 percent silica fume. R É S U M É Les objectifs de cette étude concernent l influence de la nature et de la taille des échantillons sur l action de la fumée de silice dans les forces de liaison entre le béton et les armatures d acier. L évaluation de l effet du diamètre des armatures et du confinement du béton sur les caractéristiques d ancrage des armatures utilisées avec du béton avec comme adjuvant la fumée de silice a été également effectuée. Quarante-huit échantillons ont été testés par le biais d une extraction excentrée, les variables étant le pourcentage de substitution du ciment par des fumées de silice (en poids), la couverture de béton et la diminution des armatures. Le mode de rupture des échantillons est dominé par l éclatement de la couche de béton recouvrant les armatures. Les résultats obtenus montrent que plus le pourcentage de la fumée de silice monte, plus la capacité maximale de charge ainsi que la courbe glissement - charge appliqué des armatures ancrées diminuent, quel que soit le diamètre des armatures ou l enrobage du béton. Un léger confinement, dans le cas d un béton à plus de 10 pour cent de fumée de silice, sert à augmenter légèrement ces forces de liaison. 1. INTRODUCTION For the past fifteen years, researchers have been interested in the effect of silica fume on the properties of concrete. Most of the research concluded that silica fume has generally a positive influence on the concrete mixture. The concrete changes from a normal concrete to a high performance concrete. These positive findings were only related to plain concrete. When it comes to reinforced concrete, little research is reported on the effect of silica fume on bond between the reinforcement and the modified concrete mix or silica fume concrete mix. In 1984, Bürge found that a percentage replacement of up to 40 percent of Portland cement by an equal mass of silica fume improved the bond strength of plain reinforcing steel embedded in high strength lightweight concrete [1]. Concrete mixtures containing more that 20 percent replacement were found to be especially effective in improving the pullout bond strength. In 1990, Gjorv, Monteiro, and Mehta studied the effect of concrete strength and percentage replacement of Portland cement by an equal mass of silica fume on the pullout strength of #6 (20 mm) Grade 60 deformed bars [2]. The specimens were designed according to ASTM C 234. Gjorv et al., concluded that up to a concrete compressive strength of 76 MPa, an increase in silica fume up to 16 percent replacement of cement improved the pullout strength. They attributed the improvement in bond to the reduction in porosity and thickness of the steel-cement transition zone. In 1994, Hwang, Lee Y., and Lee C., reported the results of beam splice tests designed to fail in bond splitting [3]. They concluded that a 10 percent replacement of cement by silica fume decreased the bond strength of tension bar splices by about 10 percent due to the loss of adhesion between concrete and steel at the ribs. In 1995, a research program was started at the American University of Beirut to investigate several aspects /98 RILEM 707

2 Materials and Structures/Matériaux et Constructions, Vol. 31, December 1998 of the bond and anchorage characteristics of reinforcing bars in high performance silica fume concrete. In 1996, Hamad and Itani reported on the first phase of the program in which 16 beams were tested in positive bending [4]. Each beam was reinforced with two 25-mm (#8) bars spliced at the center of the beam on the tension side. No transverse reinforcement was provided in the splice region. The variables were the percentage replacement by mass of cement by silica fume (5 to 20 percent), casting position (top or bottom), and the superplasticizer dosage (2 or 4 liters per 100 kg cement). The beams were designed to fail in a splitting mode. Based on the test results, Hamad et al., concluded that the replacement of 5 to 20 percent of cement by an equal mass of silica fume resulted in an average 10 percent reduction in bond strength regardless of the casting position or the superplasticizer dosage. They noted greater reduction in bond strength with higher percentage replacement silica fume when greater superplasticizer dosage was used (4 liters per 100 kg cement). Because of the lower bond strength and the brittle mode of failure, Hamad et al. suggested the use of transverse reinforcement in the splice region of high strength concrete (HSC) beams to provide ductility and to allow for the bar lugs to be utilized in the stress transfer mechanism between steel and concrete leading to higher bond strength. Recently, Machaka reported on the second phase of the AUB program designed to investigate the effect of transverse reinforcement on the bond-slip characteristics of tension lap splices in silica fume concrete [5]. Twelve beams each reinforced with two 25-mm (#8) bars spliced at the center, were tested in positive bending. The variables were the percentage replacement by mass of cement by silica fume (0, 8, or 16 percent) and the amount of transverse reinforcement in the splice region. The results indicated that the use of transverse reinforcement confining the splice region increased the bond strength of the splice and the ductility of the mode of failure. Fig. 1 Schematic drawing of the test specimen. 2. RESEARCH SIGNIFICANCE The work reported in this paper is the third phase of a multiphase research program conducted at the American University of Beirut. The first objective was to study the role of specimen type and specimen size on the effect of silica fume on bond strength reinforcing bars in high strength silica fume concrete. Previous phases of the AUB program concentrated on tests of tension lap splices in beam specimens. It was significant to find out whether the trends of results indicating reduction in bond strength due to the presence of silica fume would be valid if a different type of test specimen, an eccentric pullout specimen, were used. The second objective was to evaluate the effect of bar size and concrete confinement on the anchorage characteristics of the reinforcing bars in silica fume concrete. The variables were the percentage replacement by mass of cement by silica, the concrete cover over the reinforcing steel bar, and the bar size. 3. EXPERIMENTAL PROGRAM Six series with a total of forty-eight eccentric pullout specimens were tested. The series were identical except for the percentage replacement of Portland cement by an equal mass of silica fume (0, 4, 8, 12, 16, or 20 percent). Each series included eight specimens with four replicates to check the reliability of the test set-up and the scatter of the test results. In any series, two bar sizes were tested: 25 and 32 mm (#8 and #10), and for each bar size, two concrete covers were tested: 25 and 50 mm. The test specimen consisted of a reinforcing bar embedded 254 mm in a mm concrete block. A schematic view of the test specimen is shown in Fig. 1. The short embedment length was chosen to avoid yielding of the bar. A three-part notation system was used to identify the variables of each test specimen. The first part of the notation indicates the percentage replacement of cement by silica fume (S0, S4, S8, S12, S16, or S20). The second part indicates the bar size used in mm (B25 or B32). The third part indicates the concrete cover in mm (C25 or C50). The letters S, B, and C indicate silica fume, bar size, and concrete confinement, respectively. The variables of all test specimens are identified in Table 1. In all test specimens, the total length of the anchored reinforcing bar was controlled by providing a suitable length of the bar extending out of the concrete block. This length was required to provide room for a compression plate, a center-hole hydraulic ram, and a gripping wedge assembly. Also, at the free end, a short extension was required to measure the slip of the bar relative to the concrete block. The overall length of the bar was chosen to be 85 cm for the 25-mm (#8) bar and 90 cm for the 32-mm (#10) bar size. The steel bars were from the same heat of steel and had the same parallel (bamboo) deformation pattern. The bars met ASTM specifications and were Grade 60. Before placing the test bar in the form, a 10-mm (#3) steel cage was placed in the formwork at 50 mm below the test bar. The purpose of the cage was to provide the 708

3 Hamad, Sabbah Table 1 Test parameters and test results Specimen notation Silica fume Bar size Concrete Compressive Ultimate load Ultimate load Average Bond Average percentage cover strength P max normalized P max norm ratio bond ratio (mm) (mm) f c (MPa) (KN) (KN) (KN) Series 1 S0-B25-C S0-B25-C25 r S0-B25-C S0-B25-C50 r S0-B32-C S0-B32-C25 r S0-B32-C S0-B32-C50 r Series 2 S4-B25-C S4-B25-C25 r S4-B25-C S4-B25-C50 r S4-B32-C S4-B32-C25 r S4-B32-C S4-B32-C50 r Series 3 S8-B25-C S8-B25-C25 r S8-B25-C S8-B25-C50 r S8-B32-C S8-B32-C25 r S8-B32-C S8-B32-C50 r Series 4 S12-B25-C S12-B25-C25 r S12-B25-C S12-B25-C50 r S12-B32-C S12-B32-C25 r S12-B32-C S12-B32-C50 r Series 5 S16-B25-C S16-B25-C25 r S16-B25-C S16-B25-C50 r S16-B32-C S16-B32-C25 r S16-B32-C S16-B32-C50 r Series 6 S20-B25-C S20-B25-C25 r S20-B25-C S20-B25-C50 r S20-B32-C S20-B32-C25 r S20-B32-C S20-B32-C50 r concrete block with restraint against splitting due to the compression force applied on the block while pulling the bar during the test. The cage consisted of three 10-mm (#3) stirrups spaced at 100 mm on centers and hooped around four 200-mm long 10-mm (#3) bars at the corner (see Fig. 1). Two samples of each bar size were tested to confirm the mill test report obtained from the supplier. The average yield strength values for the 10-, 25-, and 32-mm bars (#3, #8, and # 10) were 526, 482, and 452 MPa, respectively. 709

4 Materials and Structures/Matériaux et Constructions, Vol. 31, December 1998 (a) Front view (b) Side view Fig. 2 Schematic views of the test setup. A non-air entrained concrete mix was designed to provide a nominal 28-days compressive strength of 70 MPa. Water-to-cementitious materials ratios ranged from 0.25 to ASTM Type I Portland cement was used. The silica fume used contained 96.7 percent of SiO 2. The superplasticizer used is produced by a German company and satisfies ASTM C494 Type F. A dosage of 3 liters per 100 kg cementitious material was used. The superplasticizer had 40 percent solids and had a specific mass of 1.2. All specimens of the same series were cast from the same batch of concrete. Concrete was cast in two lifts in each specimen. Care was taken in the insertion of the vibrator to avoid as much as possible the formation of air bubbles around the test bar which would hurt the bond. As the specimens were cast, concrete was also placed in mm cylinder molds. The specimens and the cylinders were cured for 28 days. Schematic front and side views of the test frame are shown in Fig. 2. The tensile load was gradually applied in 5-kN increments until bond failure occurred. At each load stage, the free-end slip was recorded. After the ultimate load was reached, the test was halted. 4. ANALYSIS OF TEST RESULTS 4.1 General load-slip behavior For almost all specimens, the free-end started to slip at a loading of 50 to 70 kn, which was in most cases about 25 percent of the ultimate load. At about 50 percent of the ultimate load, the stiffness of the load-slip curve decreased. Beyond 70 percent of the ultimate load, large free-end slip values were then measured after each load increment. After reaching the ultimate load, the load dropped rapidly, and the specimen failed with a loud brittle noise. 4.2 Mode of failure All specimens exhibited a V-notch splitting mode of failure. At a very early stage of loading, one crack formed in the cover at the loaded-end of the anchored bar. With increase in load and slip, this crack propagated along the 254-mm embedment length towards the free-end of the bar. At high level of loading (50 to 60 percent of ultimate), more cracks formed at the loaded-end and spread from the main crack in a V-pattern towards the edges of the compression plate. It was noticed that as the percentage of silica increased, the number of cracks branching from the main longitudinal crack was reduced. This is related to the increase in concrete strength of the specimen as the silica fume percentage increased which resulted in less lugs along the embedded bar participating in resisting the load. 4.3 Test results The pullout specimens used in this test program do not represent actual beam or column conditions in reinforced concrete structures. The test results are only used to indicate and study the effect of the different variables on the bond strength. Absolute values of bond stresses and free-end slips are not useful for design. The measured slips are much greater than the slip values which would be expected under very severe loading on a structure or even at regions of distress in a typical subassembly tested at failure. The load-slip data were normalized at a common concrete strength of 70 MPa. The adjustment was made by multiplying the load at each deflection by (70/f c ) 1/2, where f c is the concrete strength in MPa of the specimen under consideration at the day of testing. The test results of all forty-eight specimens are shown in Table 1. Replicates in all series gave comparable results indicating 710

5 Hamad, Sabbah the validity of the test setup and the reliability of the test data. Therefore, average values of replicates were considered in the analysis of the test results. The bond ratio is the normalized ultimate load of a specimen with silica fume divided by the normalized ultimate load of the specimen with no silica but identical otherwise (i.e. same bar size and concrete cover). Fig. 3 Load-slip curves of specimens with different percentage replacement silica fume, bar size = 25 mm, and concrete cover = 25 mm. Fig. 4 Variation of normalized ultimate load with percent replacement silica fume for various combinations pf bar size and concrete confinement. Fig. 5 Effect of percent replacement silica fume on average bond ratio Effect of silica fume Load-slip curves of pullout specimens with the same bar size and the same concrete confinement but different percent replacement silica fume indicated a reduction in load-slip stiffness (greater slip for a given load) of specimens with greater percentage replacement silica fume (see Fig. 3). In general, the peaks of load-slip curves of specimens with the same bar size and same concrete cover dropped as the silica fume percentage increased from 0 to 20 percent. The reduction in load for various combinations of bar size and concrete cover is shown in Fig. 4. Average bond ratios of series 2 to 6 with silica fume percentages of 4, 8, 12, 16, and 20, are listed in Table 1 and plotted in Fig. 5. The corresponding values are 0.93, 0.92, 0.86, 0.83, and 0.82, respectively. Each value is the average bond ratio of all specimens with a given silica fume percentage but different bar sizes and concrete covers. The reduction in bond strength is attributed to the loss of adhesion between the concrete and steel due to the presence of silica fume Effect of bar size Increasing the bar size from 25 mm (#8) to 32 mm (#10) for a given silica fume percentage and a constant concrete cover, led to an increase in load-slip stiffness (greater load for a given slip) and ultimate load. Curves shown in Fig. 6 indicate similar reduction in load-slip stiffness and peak load of 25-mm (#8) and 32-mm (#10) bar specimens due to the incorporation of silica fume while maintaining a constant concrete confinement. Variations of the average bond ratio of specimens with a given bar size (25 or 32 mm), but different concrete covers with respect to the percent replacement silica fume, are shown in Fig. 7 for each of the two bar sizes used, 25 and 32 mm. The two resulting curves are almost superimposing, which indicates that the bar diameter does not affect the relative reduction in bond strength of pullout 711

6 Materials and Structures/Matériaux et Constructions, Vol. 31, December 1998 specimens with different percent replacement silica fume. Fig. 6 Effect of bar size and silica fume on load-slip curves of specimens with 50- mm concrete cover. Fig. 7 Variation of bond ratio with percent replacement silica fume for the two bar sizes, 25 and 32 mm (#8 and #10) Effect of concrete cover Two concrete covers were used, 25 and 50 mm. The effect of concrete confinement on the load-slip behavior of specimens having the same bar size and silica fume content is shown in Fig. 8. Increasing the concrete cover for a given bar size and a given percent replacement silica fume had a positive effect on loadslip stiffness (greater load for a given slip) and on the ultimate bond strength. The reduction in load-slip stiffness and ultimate load of 25-mm (#8) or 32-mm (#10) bar specimens, due to replacement of part of the cement by silica fume, appeared to be relatively smaller when larger concrete cover was used. Variations of the average bond ratio of specimens with different bar sizes with respect to the percent replacement silica fume are shown in Fig. 9 for each of the two concrete covers tested, 25 and 50 mm. The two curves show a reduction in bond ratio with increase in silica fume. However, less reduction in bond strength was noted with higher percentages silica fume (more than 12%) when the larger confinement (50-mm) was used. This indicates the positive effect of larger concrete confinement on bond strength of high strength concrete specimens incorporating silica fume. With larger concrete confinement, the splitting mode of failure was coupled with bar pullout, where the concrete keys between the bar lugs were sheared off. In a pullout failure, the friction between the concrete and steel is much less important than in a splitting failure. In this case, the bond strength is controlled by the capacity of the concrete in direct shear. The bearing of the ribs against the concrete causes the key between the ribs to shear from the surrounding concrete. Since the bar is well confined, friction between the rib and concrete which is reduced by the use of silica fume, is not necessary to prevent sliding of the concrete key relative to the rib. 5. CONCLUSIONS Fig. 8 Effect of concrete confinement and silica fume on load-slip curves of 25- mm (#8) bar specimens. Based on the analysis of the test results, the following conclusions were made: 1. All specimens failed in a splitting mode of failure. The failure was brittle 712

7 Hamad, Sabbah 7. The results of this study of pullout specimens compared and combined with the results of the previous studies of beam splice tests conducted at the American University of Beirut [4, 5] indicate that a reduction in the load capacity of anchored bars due to the incorporation of silica fume is not affected by the size or type of the test specimen. ACKNOWLEDGMENT Fig. 9 Variation of bond ratio with percent replacement silica fume for the two concrete covers, 25 and 50 mm. The authors gratefully acknowledge the support of the University Research Board at the American University of Beirut. Also, the assistance of Mr. Hilmi Khatib, Supervisor of the materials testing laboratory at AUB is greatly appreciated. and noisy regardless of the variables used. 2. In general, as the percent silica fume increased, the number of cracks branching from the main longitudinal cracks was reduced. 3. As the silica fume content increased, there was a reduction in load-slip stiffness (greater slip for a given load) regardless of the bar size or concrete cover used. 4. The bar size had no effect on the relative reduction in the bond strength of pullout specimens with different percent replacement silica fume as compared with specimens with no silica fume. 5. Increasing the concrete cover for specimens containing high silica fume improved the load-slip stiffness and the ultimate load. 6. In general, with larger concrete cover, the splitting mode of failure of the specimen was coupled with bar pullout. REFERENCES [1] Bürge, T.A., High strength lightweight concrete with condensed silica fume, Proceedings, CANMET/ACI First International Conference on the Use of Fly Ash, Silica Fume, Slag, and Other Mineral By-Products in Concrete, Montebello,Canada, Editor: Malhotra, V.M., ACI SP-79, 2 (1983) [2] Gjorv, O.E, Monteiro, P. J. M. and Mehta, P. K., Effect condensed silica fume on the steel-concrete bond, ACI Materials Journal 87 (6) (1990) [3] Hwang, S., Lee Y. and Lee, C., Effect of silica fume on the splice strength of deformed bars in high-performance concrete, ACI Structural Journal 91 (3) (1994) [4] Hamad, B. S. and Itani, M. S., Bond strength of reinforcement in high performance concrete: Role of silica fume, casting position, and superplasticizer dosage, ACI Materials Journal 95 (5) (1998). [5] Machaka, M. F., Effect of Transverse Reinforcement on Bond Strength of Reinforcement in Silica Fume Concrete, Master Thesis, American University of Beirut, May