Bond and interfacial properties of reinforcement in selfcompacting

Transcription

1 Materials and Structures / Matériaux et Constructions, Vol. 37, August-September 2004, pp Bond and interfacial properties of reinforcement in selfcompacting concrete W. Zhu, M. Sonebi and P. J. M. Bartos Advanced Concrete and Masonry Centre, University of Paisley, PA1 2BE, Scotland, UK ABSTRACT This paper reports a study carried out to assess the impact of the use of self-compacting concrete (SCC) on bond and interfacial properties around steel reinforcement in practical concrete element. The pull-out tests were carried out to determine bond strength between reinforcing steel bar and concrete, and the depth-sensing nano-indentation technique was used to evaluate the elastic modulus and micro-strength of the interfacial transition zone (ITZ) around steel reinforcement. The bond and interfacial properties around deformed steel bars in different SCC mixes with strength grades of 35 MPa and 60 MPa (C35, C60) were examined together with those in conventional vibrated reference concrete with the same strength grades. The results showed that the maximum bond strength decreased when the diameter of the steel bar increased from 12 to 20 mm. The normalised bond strengths of the SCC mixes were found to be about 10-40% higher than those of the reference mixes for both bar diameters (12 and 20 mm). The study of the interfacial properties revealed that the elastic modulus and the micro-strength of the ITZ were lower on the bottom side of a horizontal steel bar than on the top side, particularly for the vibrated reference concrete. The difference of ITZ properties between top and bottom side of the horizontal steel bar appeared to be less pronounced for the SCC mixes than for the corresponding reference mixes. RÉSUMÉ Cet article présente une étude de l impact de l utilisation de béton autoplaçant sur l adhérence et les propriétés de l interface autour de l armature de l acier dans un élément de béton structural. Les essais d arrachement direct ont été menés afin de déterminer l adhérence entre l armature d acier et le béton. La technique de nano-échancrure a été utilisée pour évaluer le module élastique et la micro-dureté de la zone d interface autour de la barre d acier. L adhérence et les propriétés de l interface au voisinage de l armature d acier de différentes classes de béton autoplaçant 35 MPa et 60 MPa ont été examinées ensemble avec celles du même type de classe du béton de référence. Les résultats montrent que l adhérence maximale diminue lorsque le diamètre de la barre d armature augmente de 12 mm à 20 mm. L adhérence normalisée des bétons autoplaçants a été de 10 à 40% plus grande que celle des bétons de référence pour les deux diamètres de barres (12 mm and 20 mm). L étude des propriétés de l interface révèle que le module élastique et la micro-dureté de la zone l interface ont été plus petits dans la partie basse de la barre d armature d acier horizontale que ceux de la partie haute, particulièrement pour le béton de référence vibré. La différence des propriétés de l interface béton-acier entre le haut et le bas de la barre d armature horizontale se révèle moins prononcée dans les bétons autoplaçants que celle des bétons de référence. 1. INTRODUCTION Self-compacting concrete (SCC) is defined as concrete that can be placed normally by pump or skip, and flow under its own weight, maintaining its homogeneity. It will completely fill formwork of any shape, even with congested reinforcement, subject to the aggregate size. Full compaction and in-situ strength are achieved without the Editorial Note ACM Center is a RILEM Titular Member. Prof. Peter J.M. Bartos is a Member of the RILEM MAC (Management Advisory Committee) as well as the Chairman of RILEM TC 197-NCM Nanotechnology in construction materials. He also participates in RILEM TCs 183- MIB Microbial impacts on building materials - Weathering and conservation, HFC Hybrid Fibre Concrete and TRC Textile reinforced concrete. Dr. Mohammed Sonebi participates in RILEM TC CSC Casting of self-compacting concrete and Dr. Wenzong Zhu is a member of the above-mentioned RILEM TC NCM /04 RILEM 442

2 Materials and Structures / Matériaux et Constructions, Vol. 37, August-September 2004 assistance of mechanical vibration. To achieve these requirements, such concrete should have a relatively low yield value for high flowability and a moderate viscosity to resist segregation and bleeding. It must maintain its homogeneity during transport, placing and curing to achieve adequate structural performance and long-term durability. It was first introduced in the late 1980s in Japan [1, 2], the main impetus behind the development of the material being the desire to improve the quality of concrete work, reduce cost, improve the working environment and automate construction [1-3]. The effect of bleeding, segregation, and settlement on bond between any concrete and reinforcement is related to several factors, including the stability of the concrete, position of embedded bar and completion of compaction. Leakage from formwork and its roughness may also play a part in terms of loss of paste from the matrix [4]. A few investigations have been carried out in order to study the bond strength with reinforcement bars in SCC [5-8]. Khayat et al. [4] reported that SCCs exhibited insignificant variations in compressive strength and modulus of elasticity in relation to height along a 1.5 m wall. However, the topbar factor for reinforcing bars positioned approximately 140 cm from the bottom of the walls was for SCCs compared to 2.0 for control concrete. Zhu et al. [9] also found that in-situ concrete properties in practical 3-metre columns and 4-metre beams were marginally more uniform for SCC mixes than for conventional vibrated reference concrete. In another study [6], SCC containing viscositymodifying admixture was found to enhance stability and reduce the top-bar factor of fluid concrete with slump flow values of 600 to 690 mm. The improvement of cohesiveness of SCC leading to reduction in bleeding, segregation and surface settlement was considered to be able to reduce localized structural defects resulting from voidage under reinforcement, especially under top-bars in deep sections [6]. This paper reports an investigation aimed at evaluating bond strength and interfacial properties around steel reinforcement in SCC and comparing them and those in conventional vibrated concrete. The study was carried out in two parts firstly pull-out tests for bond strength; and secondly depth-sensing nano-indentation tests to directly assess properties of the interfacial transition zone (ITZ) between concrete and steel reinforcement, the area where bond is developed. Two optimised SCC mixes of strength grades, C35 and C60 (i.e. C35- SCC, C60-SCC) respectively, one C60 grade SCC mix comprising steel fibres (C60-FSCC) and two conventional vibrated reference mixes (C35-Ref, C60-Ref) were examined. Free water Portland cement 42.5 Limestone powder GGBS Fibre (RC 65/35BN) Water/cement ratio 2. EXPERIMENTAL PROGRAMME 2.1 Materials The concrete mixes used in this study were prepared with Standard 42.5N grade Portland cement, limestone powder and ground granulated blast slag (GGBS). The cement and GGBS used conformed to Standard BS 12:1996 and BS 6692:1992. The limestone powder was of high purity (99% CaCO 3 ) and high fineness (94% < 25 µm and 25% < 5 µm), with a relative density of The limestone and slag powders were used as additional filler in SCC to enhance self-compactability and segregation resistance. Continuously graded crushed granite aggregates with a nominal maximum particle size of 20 and 10 mm were used for C35 and C60 mixes, respectively. A medium grade natural sand with a fineness modulus of 2.74 was also used for all the mixes. The relative density values for the coarse aggregate and sand were 2.62 and 2.56, and their absorption rates were 0.8% and 1%, respectively. A copolymer-based superplasticizer was used which had solid content and specific gravity of 30% and 1.11, respectively. This superplasticizer was used at dosages varying from 0 to 1%, by mass of powder. 2.2 Proportions of mixes used Table 1 summarizes the mix proportions for the different SCC and reference concrete used. The SCC mixes contain high-volume replacements of limestone powder and granulated blast furnace slag to enhance fluidity and cohesiveness and to limit heat generation. Such materials are generally less reactive than cement and can reduce the problems resulting from fluidity loss of the rich concrete. The 2.5 m³ loads of SCC and reference concretes were Table 1 - Mix proportions and basic properties C35-Ref C35 C60 Fibre C35-SCC C60-Ref C60-SCC C60-FSCC Total powder content Sand (40% < 600 µm) Crushed coarse aggregate Viscocrete 2 Normal SP * * Slump flow (mm) 65 slump slump f c at 28 d (MPa) f sp at 28 d (MPa) In-situ compressive strength (MPa) *: 10 mm coarse aggregate 443

3 Zhu, Sonebi, Bartos mixed in a 6-m 3 tilting drum mixer and delivered to the laboratory by truck mixers. The mixing sequence consisted of homogenising all materials using ribbon feeding, and then introducing water. The concrete was mixed for three minutes and the SP was introduced towards the end. The mixing sequence for the SCC mixes was similar to that for the reference mixes except that the mixing time for the SCCs was 1 min longer after the addition of superplasticizer. The GGBS, the limestone powder and the steel fibres were added to the mixer together with the aggregates at the beginning. 2.3 Experimental procedures The slump and slump flow of the reference and SCC mixes, respectively, were measured. Standard 150-mm cube and Ø150x300-mm cylinder specimens were used for compressive and splitting tensile strength tests. The specimens were demoulded at one day after casting and then placed in water tank for curing until testing. The compressive strength tests were carried out at 28 days and 6 months and the splitting tensile strength (f sp ) was at 28 days Pull out testing The bond strength between reinforcing steel and the concrete was determined by pull-out tests carried out after 28 days of curing. Deformed reinforced steel bars with 12- mm and 20-mm effective diameters were used to evaluate the bond in C35, C60 and steel fibre-reinforced concrete (FSCC), in accordance with the recommendation of RILEM TC51-ALC, 78-MCA [10]. The test specimen was a prism with a cross-section of 100 x 100 mm and a length of 150 mm. Three specimens were cast per mix. Each specimen had horizontally bonded reinforcing bars of 12 or 20 mm in diameter and 1 m in length. A rigid plastic sheathing was tightly attached to the loaded end of each bar to limit the bond between the bar and concrete to the remaining portion of the bar. The anchorage length was 120 mm for all bars. The bonded length of each bar was properly cleaned to ensure an adequate bond with concrete. Average bond stresses were evaluated by pull-out test. The pull-out load is applied progressively up to bond failure and the deformation of the bar was measured using two LVDT connected to the unloaded end of the bar (Fig. 1). The test was terminated when pull-out failure occurred, the Fig. 1 - Pull-out test arrangement used to determine bond strength. reinforced steel began to yield, or the surrounding concrete cover failed in split Specimen preparation for studying interfacial properties A set of three identical full-scale beams was cast each time. For the reference mixes the concrete was cast and compacted conventionally using hand-held poker vibrators. In the case of the SCC mixes, the SCC was poured into the formwork at one end, flowed to the other end, and filled the 4-metre length without vibration. The formwork was stripped at 6 days and the beams immediately sprayed with a curing compound. Samples for the study of ITZ were extracted from the middle section of the beam by drilling 100 x 200 mm cores horizontally intersecting the steel bars. Small specimens (roughly 35 x 20 mm) with only one steel bar at the centre were then cut from the cores taken, using a diamond saw. This was then followed by procedures [11] including resin embedding, precision sectioning, grinding, polishing and ultrasonic cleaning to obtain the final disc specimens ( 40 x 15 mm) for the nano/micro-indentation testing. Due to the time and budget constraints no full-scale beams were cast for the C35-SCC mix. Therefore, specimens for the ITZ study were only prepared for C60- SCC, C60-FSCC, C35-Ref and C60-Ref mixes Micro-mechanical properties of ITZ in concrete Evaluation of micro-mechanical properties of materials on the micron to submicron scale, particularly within the ITZ of cementitious composites and concrete, poses special difficulties. One major problem is that it is not possible to isolate the material in the ITZ for mechanical testing. As a result, for a long time the evaluation of properties of ITZ relies on either indirect methods (e.g. SEM image analysis and theoretical modelling, etc.) or test of a model composite specimen that does not represent the actual material used in practice. To overcome the difficulties and limitation of existing techniques, significant progress has recently been made in the development of a revolutionary nano-technology based, depthsensing micro/nano-indentation apparatus and the associated methodology [12, 13]. The apparatus monitors load and displacement (or depth) continuously during indentation: this enables the mechanical properties to be determined even when the indentations are too small to be imaged conveniently. Such an apparatus was used in this study to examine the properties of ITZ around steel reinforcement in SCC and reference concrete. The operating principle and special features of the apparatus were described in detail elsewhere [12, 13]. A typical outcome of the nano-indentation testing is an indentation load-depth hysteresis curve as shown in Fig. 2. As a load is applied to an indenter in contact with a specimen surface, an indent/impression is produced which consists of permanent/plastic deformation and temporary/elastic deformation. Recovery of the elastic deformation occurs when unloading is started. Determination of 444

4 Materials and Structures / Matériaux et Constructions, Vol. 37, August-September 2004 Fig. 2 - A schematic diagram of an indentation load vs. displacement curve. Fig. 3 - Typical bond stress - net slip curves for C35-SCC mix. the elastic recovery by analysing the unloading data according to a model for the elastic contact problem leads to a solution for calculation of elastic modulus E and also microstrength/hardness H of the test area. Details of the theoretical background and methodology for the elastic modulus determination have been reviewed and presented elsewhere [12, 14]. Briefly, the specimen elastic modulus is determined using Equations (1) and (2): dp 2 S Er A dh (1) v 1 v 1 i (2) E E E r i where, S = dp/dh is the experimentally measured stiffness of the upper portion of the unloading data; E r is a reduced elastic modulus defined in Equation (2); A is the projected area of the elastic contact; E and v are Young's modulus and Poisson s ratio for the specimen; and E i and i are the same parameters for the indenter. For the diamond indenter used in this study, E i = 1141 GPa and = The projected area A can be derived from the plastic depth h p obtained using the unloading data and the indenter shape function, which is dependent on its geometry. For an ideally perfect 90 (corner of a cube) indenter, A = 2.6 h 2 p. In the case of imperfect indenter tip geometry, the shape function can be determined by using electron microscopy techniques or through calibration of hardness (or d = 12 mm elastic modulus) - plastic depth curve using a homogeneous material of constant hardness (or elastic modulus). A special defined parameter microstrength, H = P max /A can also be calculated, where P max is the maximum load applied. When the Vickers indenter d = 20 mm is used the micro-strength is equivalent to the Vicker microhardness. 3. TESTS RESULTS AND DISCUSSION 3.1 Bond strength A good reproducibility of bond stress development vs. net slip was obtained for all mixes and bar sizes. Typical variations in bond stress ( max ) vs. net slip for three replicate specimens of C35-SCC with 12-mm diameter steel bars are shown in Fig. 3. The individual and average bond strength values and their coefficients of variation (COV) for all the SCC and reference mixes are given in Table 2. As shown the values of COV were between 2 to 14% for all the mixes. The bond strength between embedded reinforcement bar and concrete depends on the diameter of the bar, as well as the strength of the concrete. As expected, a reduction in bond strength was observed when the diameter of the bar increased (Table 2). Bond strength is often expressed in terms of the square root compressive strength or the tensile strength of concrete. The values of bond strength are thus normalised (normalised bond strength = max /square root f c ), and the effect of variations in compressive strength eliminated. The normalised ratios of the mixes are plotted in Fig. 4. At 28 d, the actual maximum bond strength of C60-SCC Table 2 - Maximum bond strengths of reference and SCC mixes C35 C60 Bar Sample Reference SCC Reference SCC FSCC Average COV. % Average COV. %

5 Zhu, Sonebi, Bartos Fig. 4 - Variation in normalised bond strengths of all mixes ( max / f ). c mix was about 40% higher than that of the reference mix. (C60-Ref). The normalised bond strength of C60-SCC mix was 20-40% higher than that of the reference mix C60-Ref. The actual maximum bond strength of C35-SCC mix was 13.5 MPa and 10.5 MPa for 12 mm and 20 mm diameter, respectively. The normalised strengths were about 10% higher than those of the reference mix C35-Ref. The FSCC mix exhibited the highest values of normalised ratios (3.63 for 12 mm and 2.65 for 20 mm). Thus the C35-SCC and C60-SCC mixes exhibited high bond strength, and the normalised ratio values of both were higher than those of the references mixes for both diameters (12 and 20 mm). This may be due to the lower water content and particularly to the higher powder volume in SCC mixes than the reference mixes. This is considered to reduce the accumulation of bleed water under horizontally embedded reinforcement bars. In normal concrete this can increase the local W/C ratio under the bar and weaken the strength of the bond. 3.2 Properties of the interfacial transition zone A large number of indentation tests were carried out in the steel - concrete interfacial areas (varying from 0 to 80 m from the actual interface) at the top and the bottom sides (i.e. above and under) of the steel bar. A photograph of a typical test specimen is shown in Fig. 5, and a scanning electron microscope (SEM) photograph of the tested area and indents left on the specimen surface following the indentation testing is shown in Fig. 6. Since some sand particles and large voids also existed in the tested area, the test points which lie closer to the aggregate particles than to the steel bar or fall into the voids were considered invalid and discarded. A typical set of results is presented in Fig. 7 as plots of modulus of elasticity versus distance from the actual interface. As shown in Fig. 7, the elastic modulus profiles showed a trough within the ITZ, with the minimum occurring at m from the actual steel interface. The E modulus then increased when test points moved towards the bulk concrete matrix and became roughly constant at distances greater than m. A similar trend was also observed in the micro-strength profile in the ITZ. It was also noted that the results showed moderate variations even at the same distance from the interface, and that interference of neighbouring aggregate/sand particles became more frequent at distances greater than 60 m from the steel interface. The interference of steel bar was also present for Fig. 5 - Specimens for studying the ITZ. Fig. 6 - SEM image of a tested area below steel bar (scale bar = 50 µm). Fig. 7 - Profiles of E modulus in ITZ at top and bottom sides of the steel bar. test points adjacent to or less than 10 m from the interface. Therefore, the results for test points that located between 10 m and 40 m (50 m in some cases) away from the actual steel surface were taken to calculate the mean micro- 446

6 Materials and Structures / Matériaux et Constructions, Vol. 37, August-September 2004 Location of the ITZ tested Table 3 - Properties of the ITZ above and below steel bar and their standard deviations Modulus of elasticity (GPa) Micro-strength (MPa) C60-Ref. C60-SCC C35-Ref. C60-FSCC C60-Ref. C60-SCC C35-Ref. C60-FSCC Below steel bar Std. deviation Above steel bar Std. deviation CONCLUSIONS Fig. 8 - Results of ITZ properties below horizontal steel bar relative to above steel bar (%). mechanical properties of the ITZ. Table 3 presents the mean values and standard deviations of E modulus and micro-strength of the ITZ above and below the horizontal bar for the SCC and the vibrated reference mixes. The average values of the interfacial properties below horizontal steel bar relative to those above steel bar are also presented in Fig. 8. The results in Table 3 and Fig. 8 indicate that the elastic modulus and micro-strength of the ITZ were lower on the bottom side of the horizontal steel bar than on the top side. For example, the average E modulus and micro-strength of the ITZ were 20-40% lower on the bottom side of the steel bar than on the top side for the vibrated, reference concretes. The difference appeared to be less pronounced for the SCC mixes compared to the reference mixes. Particularly, the C60-FSCC mix showed considerably higher ITZ properties and very small difference between the top and bottom side of the steel bar. This may be due to the very fine limestone powder used in this mix, that may lead to an improvement of particle packing in the ITZ and stability of the fresh mix. The enhanced micro-mechanical properties of the ITZ and their uniformity for the SCC mixes were consistent with the increased bond strength with reinforcement in SCC, as shown in Fig. 4. The improvement of ITZ and bond properties in SCC is believed to be mainly due to two effects: the enhanced stability or reduced internal bleeding in the fresh mixes and the improved particle packing around steel bars owing to the fine powders used. Based on the results presented in this paper, the following conclusions can be drawn: For both diameters of reinforcement bars, the actual bond strengths of C35 and C60 SCC mixes were higher than those of references mixes. The normalised bond strengths of the SCC mixes were also about 10-40% higher than those of reference mixes. This may be due to the lower water content and particularly to the higher powder volume in SCC mixes than in the reference mixes. This is considered to reduce the accumulation of bleed water under horizontally embedded reinforcement bars. In normal concrete this can increase the local W/C ratio under the bar and weaken the strength of the bond. The depth-sensing nano/micro-indentation technique was successfully used to study the elastic modulus and micro-strength of the ITZ around steel reinforcement in practical concrete. This method enables quantitative and direct assessment of the properties of the ITZ between concrete and steel reinforcement, the area where bond is developed. The results indicated that the elastic modulus and the micro-strength of the ITZ were lower on the bottom side of a horizontal steel reinforcement than on the top side, particularly for the vibrated, reference concretes. This supported the common perception that ITZ underneath a large aggregate particles or steel bar is weaker than above it, owing to internal bleeding and settlement of particles during concrete placing. The difference of ITZ properties between top and bottom side of the horizontal steel bar appeared to be less pronounced for the SCC mixes than for the reference mixes. The enhanced micro-mechanical properties of the ITZ and their uniformity for the SCC mixes were consistent with the increased bond strength with reinforcement in such mixes. ACKNOWLEDGEMENTS The authors wish to acknowledge that the work reported here was partially funded by the European Commission through Brite-Euram Project BE /Contract BRPR- CT

7 Zhu, Sonebi, Bartos REFERENCES [1] Ozawa, K, Maekawa, K. and Okamura, H., Development of high-performance concrete, Journal of Faculty of Eng., University of Tokyo, XL1 (3) (1992) [2] Okamura, H., Ozawa, K. and Ouchi, M., Self-compacting concrete, Structural Concrete 1 (1) (2000) [3] Bartos, P.J.M. and Cechura, J., Improvement of working environment in concrete construction by the use of selfcompacting concrete, Structural Concrete 2 (3) (2001) [4] Khayat, K.H., Manai, K. and Trudel, A., In situ mechanical properties of wall elements cast using self-consolidating concrete, ACI Materials Journal 94 (6) (1997) [5] Sonebi, M. and Bartos, P.J.M., Hardened SCC and its bond with reinforcement, Proceedings of the RILEM Symposium on Self-Compacting Concrete, Ed. Skarendahl, Å. and Petersson, Ö., Stockholm (1999) [6] Khayat, K.H., Use of viscosity-modifying admixture to reduce top-bar effect of anchored bars cast with fluid concrete, ACI Materials Journal 95 (2) (1998) [7] König, G., Holschemacher, K., Dehn, F. and Weiße, D., Self-compacting concrete-time development of material properties and bond behaviour, Proceedings of the Second International Symposium on SCC, Tokyo, Ed. K. Ozawa & M. Ouchi, (2001) [8] Schiessl, A. and Zilch, K., The effects of the modified composition of SCC on shear and bond behavior, Proceedings of the Second International Symposium on SCC, Tokyo, Ed. K. Ozawa & M. Ouchi (2001) [9] Zhu, W., Gibbs, J.C. and Bartos, P.J.M., Uniformity of insitu properties of self-compacting concrete in full-scale structural elements, Cement and Concrete Composites 23 (2001) [10] RILEM TC 9-RC (1983), Bond test for reinforcement: Pullout test, (1992) [11] Zhu, W. and Bartos, P.J.M., Assessment of interfacial microstructure and bond properties in aged GRC using a novel microindentation method, Cement & Concrete Research 27 (1997) [12] Oliver, W.C. and Pharr, G.M., An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, Journal of Mater. Research 7 (1992) [13] Zhu, W., Trtik, P. and Bartos, P.J.M., Evaluation of elastic modulus at interfacial transition zone in reinforced concrete by a microindentation technique, Proceedings of the 6 th International Symposium on Brittle Matrix Composites, Eds: Brandt, AM, Li, VC and Marshall, IH, Warsaw, 2000, [14] Fischer-Cripps, A.C., Nanoindentation, Mechanical Engineering Series (Springer-Verlag New York, Inc., 2002). Paper received: January 15, 2003; Paper accepted: April 30,