Flexural behaviour of strengthened reinforced concrete beams

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 30, April 1997, pp SCIENTIFIC REPORTS Flexural behaviour of strengthened reinforced concrete beams Regina Helena F. Souza 1 and Julio Appleton 2 (1) Universidade Federal Fluminense, Brazil (2) Universidade Técnica de Lisboa, Portugal A B S T R A C T A large experimental research programme has investigated the flexural strength of simply supported reinforced concrete beams. The beams were first damaged so that they could be strengthened by means of jackets (cast-inplace shotcrete or pre-packed special mortar plus additional new reinforcement). This paper analyses the flexural strength of these beams. The behaviour in service and ultimate state as well as the bond characteristics are studied. R É S U M É Au cours d un vaste programme expérimental, on a étudié la résistance en flexion de poutres en béton armé à appui simple. Ces poutres ont d abord été endommagées, pour ensuite être renforcées par du béton projeté ou par du mortier spécial préconditionné, avec l adjonction de nouvelles armatures. Cet article discute de la résistance en flexion de ces poutres. Le comportement en service et à l état limite ultime, ainsi que les caractéristiques d adhérence, sont étudiés. 1. INTRODUCTION Although a great number of structures have recently been strengthened or repaired, the responsibility for these repairs depends on the experience of the experts and designers involved in the work. There is little experimental research work on this subject. Theoretical methods for redimensioning crosssections after intervention are not yet well-developed. The aim of this study is to provide a scientific contribution in this area. The cast-in-place concrete or mortar technology required is very simple and allows not only great efficiency but also relatively low costs. However, a few points must be carefully considered: adequate treatment of the base concrete surface, critical selection and correct application of the repair materials, and effective curing. In this way, bond improvement and shrinkage control can be achieved. Providing good bond is one of the main requirements for successful repair. Bond failure between new and old concrete is sometimes caused by shrinkage. While frequently referred to as bonding agents, some commercial materials result in poor or even detrimental effects on bond strength values. Commercially available repair systems vary greatly in physical and chemical properties [1 5]. It is fundamental that the characteristics of both the repair material and the original concrete be compatible, in particular as concerns modulus of elasticity, creep strains and coefficients of expansion. According to Warner [4] Portland cement concrete or similar cemen- titious compositions are frequently the best choices for the repair material. The choice of the material will depend on the conditions the structural element will be exposed to before and after its application, the repair dimensions, the material s compatibility with the old concrete and its bond and durability properties [1-5]. Shotcrete is widely used in repair for three main reasons: 1) its excellent bonding properties without any adhesive agent; 2) its improvement of strength, density and durability characteristics due to high compaction and low water/cement ratio; 3) the possibility of spraying it on every kind of surface (vertical, inclined or over-head) with a minimum of formwork [2, 3]. When a smooth surface is required, the excess material should be sliced off with a sharp-edged cutting screed while the concrete is still fresh, taking care not to debond the concrete by overworking. If a very smooth finish is desirable, a special thin layer can be sprayed, followed by the application of a steel trowel [2, 6]. The present work, based on a previously developed experimental programme, compares the behaviour of beams strengthened by means of cast-in-place shotcrete or a special repair mortar plus additional re-bars. 2. EXPERIMENTAL PROGRAMME Six T-beams were cast from the same batch. Four beams (FC1, FC2, FP1, FP2) were first damaged in a preliminary test. After being strengthened, these beams were then tested until failure and renamed: FC1R, /97 RILEM 154

2 Souza, Appleton FC2R (with mortar jacket) and FP1R, FP2R (with shotcrete jacket). The remaining two were reference beams: DF to be compared with the damaged beams and RF to be compared with the strengthened ones. The concrete used in the models was prepared taking into account the required size of the reduced model. Therefore, the maximum aggregate size was limited to 9.5 mm. In order to determine the concrete strength, some test specimens were prepared: 200 mm cubes for compression, 150 x 150 x 550 mm prisms for flexural-tension and 150 x 300 mm cylinders for the modulus of elasticity. Their storage conditions were the same as for the beams. Three were tested at the age of 28 days and the other three at the age of the beam test. A commercial cement-repair mortar, modified by polymers with acrylic fibres, was used for recasting FC1R and FC2R. Its compressive and tensile strength and its modulus of elasticity were obtained by testing 40 mm cubes and 40 x 40 x 160 mm prisms, respectively. For beams FP1R and FP2R, a dry mix shotcrete was used. Cores of 50 mm diameter and 100 mm height were extracted from test panels, to determine its strength. To make them comparable, these test results were corrected by conversion factors as recommended by REBAP [7] and Model Code/90 [8]. The respective strength values obtained are listed in Table 1. Table 1 Material Properties Age, fcm fctm Ec Beams days MPa MPa GPa *1 *2 *3 Base FC1;FC2;FP1;FP Concrete FC1R;FC2R;FP1R FP2R;RF DF Mortar FC1R;FC2R Shotcrete FP1R;FP2R *1 Values refer to cylinders of 150 x 300mm. *2 Values refer to axial tensile strength. *3 Values refer to cylinders of 150 x 300mm for the base concrete, to prisms of 40 x 40 x 160mm for the mortar and to cores of 50 x 100 mm for the shotcrete. For the longitudinal reinforcement, 6 and 8 mm diameter high-bond steel bars were used and for the stirrups, 4.5 mm diameter plain steel bars, specially produced for these tests. The original beams were reinforced with a low geometric percentage of longitudinal reinforcement to simulate a design fault. Fig. 1 presents the formwork and reinforcement details of the beams tested before strengthening and the reference beam DF. The additional reinforcement, which was from the same source as the original reinforcement, is shown in Fig.1 Formwork and reinforcement of beams FC1, FC2, FP1, FP2, DF. 155

3 Materials and Structures/Matériaux et Constructions, Vol. 30, April 1997 Fig.2 Strengthening reinforcement of the beams. Fig. 2. The monolithic reference beam RF was cast with the same size and reinforcement as the repaired beams. Because it was not necessary to include new stirrups, the transverse reinforcement used was basically constructive. It was also verified that the new longitudinal bars need not be anchored into the supports. 3. LABORATORY TESTS Tests were carried out on a loading frame where the beams were subjected to increasing loads controlled by a hydraulic jack and a load cell. Photo 1 shows DF beam at failure. Two displacement gauges were located at the bearings and one at the midspan in order to measure the deflections. The reinforcement strains, both near the supports and at the midspan, were verified by strain gauges. The mean strains in concrete and steel at midspan, as well as the crack width, were estimated with a Demec gauge. At the end of each loading and after recording all device measurements, the location and extent of cracking were observed. Photos 2 and 3 show the crack pattern of the strengthened beams and RF. During all the tests, a cycle of loading and unloading was applied on the service load to characterize a loading history (see Figs. 3 and 4). Drawings of these cycles were omitted in the other graphics to make the presentation clearer. The first beam tests were performed only in order to produce damage; the loading was interrupted at a load level near the beams flexural capacity. After strengthening, the tests were performed and the load was continued up to failure. Photo 2 Crack pattern of the beams FC1R, FC2R, RF. Photo 1 DF beam at failure. Photo 3 Crack pattern of the beams FP1R, FP2R, RF. 156

4 Souza, Appleton Fig.3 Eight cycles of loading and unloading applied to beams before strengthening. application of mortar after thorough mixing; formwork was used only on the bottom of the beam; cure by successive wettings; formwork removal the next day. A cement-based mortar modified by polymers, presented as two pre-packed components, was used. Photo 4 illustrates the repair mortar when hand-applied. b) Shotcrete Jacket: dust removal from concrete and steel by rinsing with water and sand blasting; careful application of shotcrete to ensure the desirable dimensions; reworking the surface to make it smoother; application of a curing membrane. A dry shotcrete process was used. Photo 5 shows the FP1 beam after water and sand blasting. Fig.4 Five cycles of loading and unloading applied to beams after strengthening. Photo 5 Surface treatment with water and sand blasting. 4. TEST RESULTS AND DISCUSSION First, it is interesting to note some aspects observed during the execution of the repair: a) the surface treated with water and sand blasting was cleaner than that treated with the wire brush; b) the application of the repair mortar used was easy; c) the final aspect of the reworked shotcrete surface was good. Photo 4 Mortar application. At the end of the first set of tests, cracking was welldeveloped and the mean crack width was about 0.20 mm. In order to proceed with the repair work, the beams were totally unloaded. All the concrete cover around the web was removed with a chisel and a hammer, and the additional reinforcement was applied. It was decided not to treat the cracks. The following work was performed: a) Special Mortar Jacket: dust removal from concrete and steel by means of a wire brush; careful wetting of the surface with water; Fig.5 Load-midspan deflection relationship of beams before strengthening. 157

5 Materials and Structures/Matériaux et Constructions, Vol. 30, April 1997 The homogeneity in the results of the beams tested before strengthening can be verified in Fig. 5, which confirms the uniformity of the models and the adjustment of the tests. Fig. 6 shows the comparison between the Demec gauges with the strain gauges measured at midspan. A very good correlation was observed in all the beams tested, either after or before the strengthening. The strain gauges give the direct value of the local steel strain. The Demec gauges give the mean value in the concrete fiber at the steel position. All the beams strengthened presented nearly the same stiffness, as shown in Fig. 7. It was also observed that the beams tested before strengthening (Fig. 5) and RF (Fig. 7) showed a typical stiffness loss after cracking. However, this was not so clearly observed for the strengthened beams (Fig. 7). This fact became more evident due to the lack of crack treatment. Both the original longitudinal reinforcement and the added reinforcement behaved at midspan like the first and second reinforcement layers of a normal reinforced concrete beam (see Fig. 8). The response of the repaired beams is summarized in Table 2. As expected, the experimental cracking moment value (Mr) of the monolithic reference beam RF (which was not damaged) was greater than the value of the strengthened beams. Table 2 also shows that in spite of the great tensile strength of the mortar used (Table 1), the beams repaired with this mortar cracked first. This fact seems to indicate that the bond quality of the interface is more relevant to the cracking behaviour than the tensile strength of the repair material. The tests regarding the beams repaired with mortar (FC1R and FC2R) presented a premature cracking bond, as illustrated in Photo 2. As it may be seen in Table 2, the beams jacketed with shotcrete had a crack width close to the monolithic reference beam. The beams strengthened by means of mortar jackets presented a greater cracking width. The damage caused by the first tests resulted in the longitudinal reinforcement reaching the steel yield stress. They also imposed residual deflections and curvatures. The experimental deflections and the mean width crack values presented in Table 2, which are measured at a bending moment M 12 knm, include the residual deformation values. The premature cracking, the greater deflection and the mean crack width verified in the strengthened beams could be partially due to the lack of crack treatment. Table 2 Experimental test values Type of beam Special mortar jacket Shotcrete jacket Reference Mr, exper (knm) a (mm) w m (mm) M u (knm) Fig.6 Bending moment-longitudinal steel strains relationship. Comparison between Demec gauge values and strain gauge values at midspan, for beam FC1R. Fig. 7 Bending moment-midspan curvature relationship of beams after strengthening. Fig. 8 Bending moment-longitudinal steel strains relationship. Comparison between original and added longitudinal reinforcement at midspan, for beam FP2R. As concerns the strengthened beams, although the deflections and the crack width values at midspan were greater than those of the monolithic reference beam, these values are considered acceptable according to REBAP [7], ModelCode/90 [8] and Eurocode [9] under the frequent combination of actions and a rather aggressive environment. 158

6 Souza, Appleton All the beams failed by bending, with large deflections at midspan and high steel deformations. The ultimate experimental bending moments are also presented in Table 2. This method of strengthening did not reduce the ultimate strength capacity of the beams compared with the reference beam. It is worth mentioning that the beam s strengthening behaviour can be expressed by the determination of two empirical correction factors: one related to the ultimate load capacity (γ n,m ) and the other related to the stiffness at State II (γ n,k ). Table 3 presents these correction factors for the strengthened beams studied. γ nk, ( Mk ) = Mk ( ), strengthened, reference γ nm, M = M u, strengthened u, reference Table 3 Empirical correction factors in State II and at Failure Type of beam Special mortar Shotcrete γ n,k γ n,m In accordance with CEB [2, 10], the γ n values of such repaired elements can be taken as: γ n,k = 0.7 and γ n,m = 0.8. In case of treated cracks and if there is no bond degradation, γ n factors can be taken equal to unity [2]. 5. CONCLUSIONS Considering the results of this experimental study, it is possible to say that: 1) Cast-in-place mortar or shotcrete combined with additional new reinforcement is a simple and efficient strengthening technique. 2) Flexural redesign should comply with the same rules as for new structures, according to national or international codes, in order to guarantee safety at ultimate state and an acceptable behaviour under service conditions. 3) The first cracking and the service behaviour are specially influenced by the efficiency of the interface. Therefore, it is recommended to treat the cracks whenever the steel reinforcement reaches the steel yield stress. 4) The choice of the repair material should preferably be guided by the quality of the interface bond. The value of its tensile and compression strength need only be slightly higher than the concrete base values. The most important aspect to consider is the compatibilities between the materials involved. 5) While the beams repaired with mortar presented extensive bond cracking, the beams repaired with shotcrete presented better cracking behaviour. This shotcrete, which offers high compaction and low water/cement ratio, is a Portland cement-based concrete, providing better compatibility with the substrate concrete. ACKNOWLEDGEMENTS This research was conducted at the Instituto Superior Técnico (Lisbon, Portugal). Support provided by INIC, JNICT, CMEST, LNEC and CNPq is gratefully acknowledged. The authors also thank Betão Liz, STAP, Abrantina, Trefilaria Portuguesa, Heliaço, Fragisantos, Jetbeton and Sital. REFERENCES [1] Cànovas, M.F., Patologia y Terapeutica del Hormigón Armado, 2 a Edición (Editorial Dossat S.A., Madrid, 1984) 620 pp. [2] Comité Euro-International du Béton, Assessment of Concrete Structures and Design Procedures for Upgrading, Bulletin d Information n 162 (Lausanne, 1983) 288 pp. [3] Souza, R. H.F., Análise do Comportamento de Vigas de Betão Armado Reforçadas à Flexão e ao Esforço Transverso, Tese de Doutoramento (Instituto Superior Técnico, Lisboa, 1990) 320 pp. [4] Warner, J., Selecting repair materials, Concrete Construction Magazine (October, 1984) [5] Clímaco, J.C.T., Repair of Structural Concrete Involving the Addition of New Concrete, Engineering Doctoral Thesis (Polytechnic of Central London, London, 1990) 239 pp. [6] ACI Committee 506, Recommended Practice for Shotcreting, ACI Manual of Concrete Practice, Part 4 (American Concrete Institute, USA, 1984) 15 pp. [7] Regulamento de Estruturas de Betão Armado e Pré-Esforçado (Ed. Imprensa Nacional, Lisboa, 1985). [8] Comité Euro-International du Béton CEB/FIP Model Code 1990 (Thomas Telford, 1993) 457 pp. [9] Eurocode N 2, Design of Concrete Structures, Part 1: General Rules and Rules for Buildings (1988). [10] Comité Euro-International du Béton, GTG-21, Redesign of Concrete Structures, 0 Draft (1988) 367 pp. 159