Blast Load Performance of Reinforced Concrete Beams and Columns Constructed with SCC and Fibers

Transcription

1 Blast Load Performance of Reinforced Concrete Beams and Columns Constructed with SCC and Fibers Steve Castonguay 1, Corey Guertin-Normoyle 2, Hassan Aoude 3 1 Graduate student, Dept. of Civil Engineering, University of Ottawa 2 Graduate student, Dept. of Civil Engineering, University of Ottawa 3 Associate professor, Dept. of Civil Engineering, University of Ottawa Abstract: This paper presents the results of a research program investigating the potential of using Self-Consolidating Concrete (SCC) and fibers to improve the blast performance of reinforced concrete structural components. As part of the research program a series of fiber reinforced concrete (FRC) beams and columns constructed with SCC and fibers are tested under simulated blast loading using a high-capacity shock-tube. Test parameters considered include transverse reinforcement detailing, fiber content and fiber type. Under dynamic loading, the combined use of SCC and fibers is found to improve the blast performance of the beams by increasing shear capacity, reducing midspan displacements, increasing damage tolerance. The provision of fibers is also found to improve the blast performance of columns by reducing displacements and eliminating secondary blast fragments. Keywords: Blast; FRC; SCC; Fibers; Beams Introduction The addition of fibers to concrete results in several enhancements in performance, including: improved tensile resistance, toughness, ductility and damage tolerance. These improvements result from the ability of the randomly orientated fibers to arrest and redistribute cracking. Research over the past few decades has shown that improved mechanical properties of fiber reinforced concrete (FRC) can enhance the performance of reinforced concrete structural components [1]. In beams, the enhanced diagonal tension capacity of FRC results in increased shear resistance, and if added in sufficient quantity, the use of fibers can also allow for partial or full replacement of required shear reinforcement [2]. In columns, the provision of fibers can improve behaviour by enhancing confinement of core concrete and delaying cover spalling [3]. In shear-critical members subjected to stress reversals, the use of K.H. Khayat, SCC th International RILEM Symposium on Self-Compacting Concrete, ISBN: RILEM

2 436 Steve Castonguay, Corey Guertin-Normoyle, Hassan Aoude FRC can result in enhanced ductility and seismic performance [4]. The use of FRC can also potentially allow for relaxation of required transverse reinforcement detailing, resulting in improved constructability of structural components designed for seismic resistance. The enhanced toughness and damage tolerance of fiber reinforced concrete also makes this category of material well-suited for use in the blast resistant design of structures [5]. The structural use of FRC in extreme load applications such as blast requires higher quantities of fibers which can in turn lead to problems with concrete workability. The combined use of self-consolidating concrete (SCC) and fibers has been proposed as an innovative solution to this problem [6], and can thus allow for higher fiber quantities required for such applications. This paper presents the results of a research program investigating the potential of using SCC and fibers to improve the blast performance of reinforced concrete structural components. As part of the research program a series of FRC beams and columns constructed with SCC and fibers are tested under simulated blast loading using a high-capacity shock-tube at the University of Ottawa. Test parameters considered in this study include the effect of concrete type, fiber content, fiber type and transverse reinforcement detailing. The test results are compared in terms of the effect of fibers on flexural/shear blast resistance, member midspan displacements and post-blast damage. Literature Review Table I summarizes some of the published research that has focused on the impact and blast behaviour of FRC. Several researchers have studied the impact behaviour of FRC using instrumented drop-weight and Split-Hopkinson pressure bar (SHPB) tests [7-9]. This research has shown that the compressive, tensile and flexural strength of FRC increase under dynamic loading, with the behaviour influenced by strain-rate, fiber content and fiber properties. It is noted that some of the conclusions from the literature are conflicting, with some researchers showing that the ductility and toughness normally present in FRC under static loading is absent or reduced at high-strain loading. Published data on the blast behaviour of FRC structural components is scarce; however, some limited research has been conducted on slab panels and beams [10-11]. Magnusson and Hallgren [11] tested a series of FRC beams constructed with high-strength concrete containing short and long hooked-end steel fibers. The beams were tested under quasi-static and air-blast loading. Dynamic testing was conducted using a shock-tube. The tests demonstrated that the load capacity of the beams increased when compared with static load results. The failure mode of some of the concrete beams changed from flexure to shear under dynamic loading, while the inclusion of fibers in companion beams ensured ductile flexural failure under dynamic loads. Nonetheless, the tests indicated that the positive influence of fiber

3 Blast Load Performance of Reinforced Concrete Beams and Columns 437 reinforcement is reduced under dynamic loading when compared to static loading. The longer fibers were also found to be less effective under dynamic loading when compared to the shorter fibers used in the same study. Table I. Some of the previous research on impact and blast behavior of FRC. Authors Naaman & Gopalaratnam [7] Banthia et al. [8] Lok and Zhao [9] Lok and Xiao [10] Magnusson & Hallgren [11] Type of testing Impact tests with drop-weight setups Impact testing with SHPB setup Blast testing on panels using live explosives Blast testing on beams using air-blast loading Experimental Program This study was undertaken to investigate the performance enhancements that can be gained from the combined use of SCC and fibers in beams and columns exposed to blast loads. This section of the paper summarizes the details of the test program. Material parameters All beams and columns in this test program were built using self-consolidating concrete (SCC). Each series included a set of control specimens built with plain SCC, and a companion set of FRC specimens built with SCC and steel or macrosynthetic fibers. The SCC used in all specimens consisted of a pre-packaged mix with a specified strength of 40 MPa. The mix contained a maximum aggregate size of 10 mm with a sand-to-aggregate ratio of approximately 0.45 and a water-cement ratio of approximately An air-entraining admixture, a super-plasticizer and a viscosity-modifying admixture (VMA) were incorporated into the blend in the form of dry powder. Two types of fibers were considered in this study (see Figure 1). The ZP steel fibers had a length of 30 mm, aspect ratio of 55, tensile strength of 1100 MPa, and were hooked at the ends. The SF macro-synthetic fibers had a length of 50 mm, cross-section of 0.37 mm x 1.1 mm, tensile strength of 625 MPa and partially fibrillates when mixed in concrete. Three fiber contents of 0.5%, 0.75% and 1% (40, 60 and 80 kg/m 3 ) were considered for the steel fibers, while a constant fiber content of 0.75% (6.75 kg/m 3 ) was used for the macro-synthetic fibers (the limiting fiber contents were chosen based on a series of trial batches). Figure 2 shows typical slump flow values of the SCC mixes when fibers were added; while the mixes were not fully self-consolidating (shown by the reduced slump flow diameters), all mixtures were workable and only required minor vibration during placement. Three types of steel reinforcement were considered in this study. Longitudinal reinforcement consisted of 10M (A b = 100 mm 2, f y = 483 MPa) and 15M (A b = 200

4 438 Steve Castonguay, Corey Guertin-Normoyle, Hassan Aoude mm 2, f y = 460 MPa)) bars for the columns and beams, respectively. The transverse ties/stirrups in the columns/beams were made using 6.3 mm diameter steel wire (f y = 604 MPa). (a) Steel fibers (ZP) (b) Macro-synthetic fibers (SF) Figure 1. Fibers considered in this study. Mix ID SCC 0% SCC 0.5%ZP SCC 1.%ZP SCC 0.75%SF Slump 625 mm 585 mm 525 mm 510 mm Figure 2. Typical slump flow results for the fiber-reinforced SCC mixtures. Specimen details A total of 4 columns and 4 beams were tested in this research program. Details of the beam and column specimens can be found in Table II. Typical reinforcing cages are shown in Figure 3. The columns had a total height of 2450 mm with partiallyfixed supports and were tested under uniformly distributed blast loading over a span of 1980 mm. The columns were 150 x 150 mm in cross-section and were reinforced 4-10M longitudinal bars and 6.3mm diameter ties having a centre-to-centre spacing of s = 75 mm. The series included one control column built with SCC and three companion specimens built SCC and fibers. Two fiber contents of 0.5% and 1% where used in the case of the specimens with steel fibers, while the column built with macro-synthetic fibers had a fiber content of 0.75% by volume of concrete. The specimen nomenclature indicates the concrete type, fiber content and fiber type; for example column SCC-0.5%ZP was built with SCC and 0.5% of ZP steel fibers. The beams in the test program had cross-sectional dimensions of 125 x 250 mm, length of 2438 and were tested under four-point blast loading over a simply-supported span of 2232 mm (shear span, a/d of approximately 3.7). The longitudinal reinforcement

5 Blast Load Performance of Reinforced Concrete Beams and Columns 439 in all beams consisted of two 15 M bars and the clear cover was kept at 35 mm. The series included two control beams built with SCC and two FRC beams built with SCC and fibers. The control set included one beam without shear reinforcement and one specimen reinforced with 6.3 mm diameter stirrups spaced at s = 100 mm in the shear spans (specimens SCC-0%-0 and SCC-0%-100, respectively). Specimen SCC- 1%ZP-0 was cast with 1% of ZP fibers and did not contain transverse reinforcement, while specimen SCC-0.75%SF-100 contained 0.75% of SF fibers and 6.3 mm diameter stirrups at s = 100 mm. Table II. Specimen test matrix. Series Specimen ID Fiber type Fiber content Transverse reinf. (% by volume) spacing, s (mm) SCC-0% Columns SCC-0.5%ZP ZP SCC-1%ZP ZP SCC-0.75%SF SF SCC-0% Beams SCC-0.% SCC-1%ZP-0 ZP SCC-0.75%SF-100 SF (a) Typical column steel reinforcing cage (b) Typical beam steel reinforcing cage (specimen with stirrups) Figure 3. Reinforcing cages for column and beam specimens. Instrumentation and test setup The University of Ottawa shock tube can safely simulate blast-induced shock waves without the need for live explosives. The shockwaves are generated by a compression chamber that rapidly releases compressed air into an expansion chamber, where it travels along its length until it interacts with a test specimen. Figure 4 illustrates the shock-tube setup for the column and beam tests. Due to the non-planar nature of the specimens tested in this study a load transfer device (LTD) is used to redirect the shockwaves generated at the shock-tube opening onto the

6 440 Steve Castonguay, Corey Guertin-Normoyle, Hassan Aoude specimens. In the case of the columns the LTD transfers the shockwave pressure as a uniformly distributed load along the tension face of the specimens. In the case of the beams, the LTD transfers the shockwave pressure as a series of two point loads, resulting in four-point blast loading. Partially-fixed support conditions were considered for column specimens while beams were tested under simply supported conditions. The columns were tested under combined transverse blast loads and an axial load of 300 KN which was applied to the specimens at the start of testing. The beams and columns in this study were subjected to gradually increasing blast pressures until failure. Typical pressure-time histories for the shockwaves used in the testing the columns and beams are shown in Figure 5. In the case of the columns, Blasts 1, 2 and 3 resulted in average reflected impulse of 125, 400 and 750 kpa-ms, respectively. For the beam series, Blasts 1 to 4 resulted in average reflected impulse values of 230, 350, 475 and 560 kpa-ms, respectively. (a) column setup (b) beam setup Figure 4. Shock-tube setup load for column and beam specimens Reflected Pressure (kpa) "Blast 1" "Blast 2" "Blast 3" Reflected Pressure (kpa) Blast 4 Blast 3 Blast 2 Blast 1 "Blast 1" "Blast 2" "Blast 3" "Blast 4" Time (ms) Time (ms) (a) Blasts 1-3 in column series (b) Blasts 1-4 in beam series Figure 5. Typical blast wave pressure-time histories for columns and beams.

7 Blast Load Performance of Reinforced Concrete Beams and Columns 441 Results Results - Column series This section summarizes the results from the column testing program. As discussed previously, each column was subjected to a series of shockwaves that aimed at testing the columns under elastic, yield, and ultimate loading conditions (denoted as Blasts 1, 2 and 3, respectively). Table III summarizes the maximum (D max) and residual (D res) displacements of the columns as recorded by the midspan LVDTs at Blasts 2 and 3. Figure 6a shows the displacement time histories for the various columns after Blast 3. Examination of the results demonstrates that the addition of steel fibers in SCC columns reduces maximum and residual displacements, and consequently improves blast performance. For example, while the control specimen (SCC-0%) had maximum and residual displacements of 126 and 108 mm at Blast 3, it can be seen that column SCC-0.5%ZP had reduced maximum and residual displacements of 94 and 61 mm (see Figure 6a). The increased fiber content in column SCC-1%ZP reduced the maximum and residual displacements by 30% and 56% when compared to the control specimen (D max = 88 mm vs. 126 mm; D res = 48 mm vs. 108 mm). The results also show improved column performance when SCC is combined with macro-synthetic fibers, particularly in terms of the effect on residual displacements. When compared to the specimen without fibers reinforcement (SCC-0%) at Blast 3, the residual displacements are decreased by a factor of 28% for specimen SCC-0.75%SF (D res = 77.9 mm vs. 108 mm) which contained 0.75% of SF fibers (see Figure 6a). Figure 7 shows the damaged state of columns SCC-0%, SCC-0.5%ZP, SCC-1%ZP and SCC-0.75%SF at the end of testing and demonstrates the improved damage tolerance afforded by the use of fiberreinforced SCC. Results - beam series This section summarizes the results from the beam testing program. The beam specimens were subjected to incremental shockwaves referred to as Blast 1, 2, 3 and 4. Table III presents the maximum (D max) and residual (D res) displacements of the beams as recorded by LVDTs at blasts 2, 3 and 4. Figure 6b shows the midspan displacement time history after Blast 4. Examining the beam failure blasts depicted in Figure 8, it is observed that by implementing transverse shear reinforcement (SCC-0%-100) a brittle shear failure can be avoided and altered to a more ductile flexural mode of failure when compared to the control specimen (SCC-0%-0). Correspondingly, the steel fiber reinforced beam (SCC-1%ZP-0) yields a similar ductile flexural mode of failure, underlining the fact that an adequate steel fiber content can effectively replace transverse steel reinforcement. The hybrid combination of transverse reinforcement and macro-synthetic fibers (SCC- 0.75%SF-100) resulted in a flexure dominant response, with limited evidence of shear cracks near support regions. Figure 6b illustrates that the addition of steel fibers

8 442 Steve Castonguay, Corey Guertin-Normoyle, Hassan Aoude (SCC-1%ZP) reduced the maximum and the residual displacements by factors of 22% and 51% when compared to the control specimen (SCC-0%-100) at Blast 4 (D max = 66 mm vs. 84 mm and D res = 27 mm vs. 55 mm, for the FRC and control specimen, respectively). Comparing the results of the synthetic fiber reinforced specimen (SCC-0.75SF-100) to the control specimen (SCC-0%-100), the performance enhancement is more evident when comparing the residual displacements, where the provision of synthetic fibers results in a displacement reduction of 33% (D res = 37 vs. D res = 55 mm, respectively). The reduction in residual displacements can be explained by the improved damage tolerance of the column with synthetic fibres when compared to the control SCC specimen (see Figure 8 which shows reduced crushing and spalling in beam SCC-0.75%SF-100 at the end of testing). The steel fiber reinforced concrete specimen (SCC-1%-ZP) shows a similar improvement in damage tolerance. Table III. Test results for column and beam tests. Specimen Blast 2 Blast 3 Blast 4 Dmax Dres Dmax Dres Dmax Dres (mm) (mm) (mm) (mm) (mm) (mm) SCC-0% SCC-0.5%ZP SCC-1%ZP SCC-0.75%SF SCC-0% SCC-0.% SCC-1%ZP SCC-0.75%SF Mid-Span Displacement Time History - Blast Mid-span Displacement Time History-Blast 4 Displacement (mm) SCC-0% SCC-0.5%ZP 0 SCC-0.75%SF SCC-1%ZP Time (msec) Time (msec) (a) Columns: Blast 3 (b) Beams: Blast 4 Figure 6. Displacement-time histories for columns and beams. Displacement (mm) SCC-0%-100 SCC-1.00%ZP-0 SCC-0.75%SF-100

9 Blast Load Performance of Reinforced Concrete Beams and Columns 443 a) SCC-0% b) SCC-1%ZP c) SCC-0.5%ZP d) SCC-0.75%SF Figure 7. Column damage at midspan at the end of Blast 3. a) SCC-0%-0 Blast 2 b) SCC-0%-100 Blast 4 c) SCC-1%ZP-0 Blast 4 d) SCC-.75%SF-100 Blast 4 Figure 8. Beam failure blasts and damage at the end of testing. Conclusions The addition of fibers to conventional concrete can cause problems in workability, particularly at the higher fiber contents required for structural applications. The combined use of SCC and fibers is an innovative solution to this problem. This paper presented a summary of research that has been conducted on the structural use of

10 444 Steve Castonguay, Corey Guertin-Normoyle, Hassan Aoude fiber-reinforced SCC in blast applications. Results were reported for beams and columns tested under simulated blast loading. The results demonstrate that the combined use of SCC and fibers results in important blast performance enhancements in beams and columns. In the case of columns the provision of steel and macro-synthetic fibers reduced column midspan displacements at equivalent blasts and resulted in improved damage tolerance. The results also showed that the provision of steel fibers in the shear-deficient beams can prevent brittle shear under blast loading. In flexural-dominant beams, the addition of steel and macro-synthetic fibers was found to improve flexural response by reducing midspan displacements and improving damage tolerance under equivalent blast loading. Acknowledgments The authors gratefully acknowledge the donation of materials from KING Prepackaged materials (SCC), Bekaert (steel fibers) and Euclid (synthetic fibers). References [1] Li, V.C. (2001), Journal of App. Poly. Sci., vol. 83, n. 3, p [2] Parra-Montesinos, G. J., (2006), Conc. Int., vol. 28, n. 12, p [3] Ganesan, N. and Ramana Murthy, J. V. (1990), ACI Mat. Journal., vol. 87, n. 3, p [4] Parra-Montesinos, G. J., and Chompreda, P. (2006), ASCE Journal of Struct. Eng., vol. 133, n. 3, p [5] Banthia, N. (2008). In: Resilience of Cities to Terrorist and other Threats, NATO Science for Peace and Security Series C: Environmental Security, pp , Pasman, H. and Kirillov, I. Springer Netherlands, Moscow, [6] Khayat K.H. and Roussel Y. (2000) Materials and Structures, vol. 33, n. 6, p [7] Naaman, A.E. and Gopalaratnam, V.S. (1983), Int. J. Cem. Compos. Lightweight Conc., vol. 5, n 4, p [8] Banthia, N., Mindess, S., and Trottier, J.-F. (1996). ACI Mat. J., vol. 93, n. 5, p [9] Lok, T.S., and Zhao, P.J. (2004). J. of Mat. in Civ. Eng., vol. 16, n. 1, p [10] Lok, T.S., and Xiao, J.R. (1999). Proc. of the Ins.t of Civ. Eng.: Strct. and Bldg., vol. 134 n. 4, p [11] Magnusson, J., Hallgren, H. (2010). Mag. Concrete Res., vol. 62, n. 2, p