Strengthening of hollow core precast slabs using FRP composite materials procedure, testing and rating FLORUŢ SORIN-CODRUŢ*, NAGY-GYÖRGY TAMÁS*, STOIAN VALERIU*, DIACONU DAN* * Department of Civil Engineering Politehnica University of Timisoara 2 nd T. Lalescu, Timisoara, 300223 ROMANIA codrut.florut@ct.upt.ro, tamas.nagygyorgy@ct.upt.ro, valeriu.stoian@ct.upt.ro, dan.diaconu@ct.upt.ro Abstract: - This paper presents some aspects regarding tests on two hollow core precast slabs. The behavior of the slabs ware analyzed before and after flexural strengthening with externally bonded fiber reinforced polymers (FRP). The testing and strengthening procedure are described in detail, the latter being rated in regard with several parameters. The aim of this research was to determine whether the solution used for the flexural strengthening of the hollow core slabs with FRP represents a viable solution or not. Key-Words: - strengthening, prestressed concrete, hollow core slab, FRP composite materials 1 Introduction In the case of precast slabs, one of the most common systems for multi storey industrial buildings consists of hollow core slabs. There are two kinds of manufacturing technologies for hollow core slabs available in Romania. Both involve prestressing but the design is quite different. The first technology is the classic one used in Romania before 1989. The slab is reinforced using strands at the inferior side but it also has passive reinforcement at the superior side, anchorage reinforcement and shear reinforcement in the ribs. The other technology which is new in Romania involves pretensioning with extruding technology in large longitudinal forms (one manufacturer has a 170 m long stand). When the concrete is sufficiently hardened, the panels and strands are cut to the appropriate length which may vary to whatever dimension the costumer wants. This process excludes the use of any shear reinforcement and the strands are anchored only by bond. The first element tested by the authors was manufactured using the classic technology while the second element was constructed using the latter technology of the two previously mentioned. 2 Test setups The first experimental element was named S1 and the second was named S3. After being strengthened with carbon fiber reinforced polymers (CFRP), the two elements were named RS1 respectively RS3. The element S1 was a 990 by 5000 mm hollow core slab with a height of the cross section of 260 mm. It was cast using C25/30 concrete and reinforced using eight prestressed strands TBP 9. The self weight of the element given by the producer was 3.766 kn/m 2 (a total of 18.64 kn). The slab was subjected to two linear distributed loads, one independent 100 tons hydraulic jack being used to induce the two linear distributed loads. Three displacement transducers were used to measure deflection at the middle of the span and on the direction of the two loads. The element S3 was a 1200 by 7000 mm hollow core slab with a height of the cross section of 200 mm. It was cast using C50/60 concrete and reinforced with seven prestressed strands Fp-100/1770-R2. The self weight of the element given by the producer was 2.40 kn/m 2 (a total of 20.16 kn). The slab was subjected to four linear distributed loads simulating (as much as it was possible) a uniform distributed load. Two independent 50 tons hydraulic jacks were used to induce the four linear distributed loads. The weight of the testing equipment (2 jacks, 6 steel profiles) was approximated to 6.40 kn (1.60 kn on each of the four loading lines). Five displacement transducers were used to measure deflection at the middle of the span and in the vicinity of the four linear distributed forces. Both of the slabs were simply supported on the short edges. The slab S1 rested on steel beams while the slab S3 rested on concrete blocks through mortar beds. The effective length resulted 5000 mm for the S1 slab and 6680 mm for the S3 slab. The testing equipment and the test setups can be observed in Fig. 1 and Fig. 2. The distances at which the forces and transducers were placed can be observed in Fig. 3 and Fig. 4. ISSN: 1790-2769 496 ISBN: 978-960-474-080-2
Fig. 1. Test setup for slab S1 and RS1 Fig. 2. Test setup for slab S3 and RS3 1000 850 150 Position of forces - 1 2 Measuring points - M1; M2; M3 1950 1100 1950 1 2 M1 M2 M3 1950 1 550 550 2 1950 5000 Fig. 3. Position of forces and transducers for slab S1 and RS1 1100 Position of forces - 1 2 3 4 Measuring points - M1; M2; M3; M4; M5 1600 800 800 1600 1100 1 2 3 4 200 1200 1150 M5 M4 M3 M2 M1 1250 1 1300 2 950 950 3 1300 4 1250 7000 Fig. 4. Position of forces and transducers for slab S3 and RS3 ISSN: 1790-2769 497 ISBN: 978-960-474-080-2
3. Testing of the unstrengthened elements 3.1 Testing of S1 hollow core slab It was decided to subject the slab to an equivalent distributed load of 50 kn/m 2, the slab being loaded close to its ultimate bearing capacity. The proposed value of the load was reached during one single cycle. Up to this level of load, a series of cracks have opened. In the central zone of the slab the cracks appeared due to flexure while in the sections close to the point of application of the loads, the cracks appeared due to shear. When the proposed value of the load was reached, the slab was unloaded. After the cracks were measured and marked, the slab was unloaded. After full unload, the residual deformation was measured and had a value of 6.12 mm, far smaller that the initial camber of 50 mm. The main cracks can be observed in Fig. 5. Fig. 5. Orientation of main cracks 3.2 Testing of S3 hollow core slab According to EC 2 this element had a service load capacity of 12 kn/m 2 (including the self weight of the hollow core slab). It was decided to subject the unstrengthened slab to an equivalent distributed load of 18 kn/m 2. This corresponds to a load higher with 50 percents than the service load capacity given by EC 2. The desired load level was reached through three cycles. In the first two cycles the maximum equivalent distributed load had a value of 13.2 kn/m 2 (10 percents above service load). In the third cycle the slab was loaded up to 18 kn/m 2. During the third cycle first cracks appeared. The first six pairs of cracks (six cracks on each side of the slab) appeared at a load value between 15 kn/m 2 and 17 kn/m 2 and were numbered 1 to 6 respectively 1 to 6. After marking these cracks the loading continued, and at 17.70 kn/m 2 another two pairs of cracks appeared (7 and 8, respectively 7 and 8 ). As it can be seen in Fig. 10 these were all bending cracks. It is important to mention that the symmetry regarding the order of appearing of cracks, crack width and propagation was almost perfect. After inspecting the slab and determining that the cracks have closed and there is only a slight deflection, (0.23 mm, far smaller that the initial camber of 16 mm) the slab was strengthened. 4. Strengthening procedure for the hollow core slabs RS1 and RS3 For both of the slabs CFRP unidirectional fabric with a width of 600 mm was used. The fabric was placed on the bottom side of the element which was the tensioned one. The properties of the fabric given by the manufacturer were as follows: elastic tensile modulus E f =231000 N/mm 2, specific strain ε f =0.017 and fabric thickness t f =0.12 mm. The strengthening procedure involved several steps, starting from preparing the surface to applying the fabric. The preparation of the surface began with its grinding. After grinding, it was cleaned using an air compressor. In the same time the fabric was cut and cleaned with a powerful degreaser. Inside the marked area epoxy resin was applied. The CFRP fabric was then laid over the resin layer using a laser guided beam to ensure its accurate disposal. A supplementary layer of resin was applied over the fabric in order to create better bond between the composite material and the surface of the concrete slab. The CFRP was laid for both of the slabs in such way so 300 mm would be left unstrengthened at the both ends of the elements. Since at the end of the element, the bending moment has a quite low value, the CFRP is properly anchored. The strengthened elements were named RS1 respectively RS3 and were retested. The strengthening scheme is presented in Fig. 6. 175 175 640 CFRP Composite Fig. 6. Lay-up of the CFRP on the buttom side 5. Testing of the strengthened elements 300 5.1 Testing of RS1 hollow core slab After the CFRP composite was left to cure one week, the strengthened element was tested up to failure. The failure of the element was sudden and brittle, being produced by shear efforts. One oblique crack extended on the entire height of the cross section has opened, running from the inferior side of the slab up to the point of the application of one of the two loads. The crack has progressively propagated, creating a shear plane that caused an important change in the failure mechanism, the yielding of the prestressed strands and shear failure ISSN: 1790-2769 498 ISBN: 978-960-474-080-2
of the CFRP. This behavior was somehow predictable since the two loads were placed at short distances from the middle of the slab and quite close one to another, creating important shear effects along with then flexural ones. The path of the two main shear cracks along with several flexural cracks is presented in Fig. 7. Details of the failure of the RS1 slab are presented in Fig 8., and Fig. 9. Fig. 7. Path of the shear cracks that produced failure 5.2 Testing of RS3 hollow core slab The CFRP composite was left to cure one week in order to reach its optimum characteristics. It was decided to test the strengthened element up to failure but due to some technical drawbacks the maximum load reached was 25.50 kn/m 2. Five loading cycles were performed. In the first two cycles the maximum equivalent distributed load had a value of 13.20 kn/m 2. Then followed two identical cycles in which 21.00 kn/m 2 was reached. During the first of this two cycles the cracks numbered 1 to 8 (respectively 1 to 8 ) widened and another ten cracks appeared. These cracks were numbered from 9 to 19 (respectively 9 to 19 ). During the fifth and final cycle the load reached the maximum value of 25.50 kn/m 2 and another three pairs of cracks numbered 20, 21 and 22 (respectively 20 21 and 22 ) were observed. Final crack distribution can be observed in Fig. 10. Fig. 8. The deformed slab prior to failure Fig. 9. Details of the failure for RS1 Axis of Symmetry Fig. 10. Final crack distribution for slab RS3 ISSN: 1790-2769 499 ISBN: 978-960-474-080-2
6 Conclusions based on the experimental results A comparison between the behavior of the unstrengthened and the strengthened element (S1 respectively RS1) can be made by analyzing the loaddeflection curves. Two series of diagrams are presented in Fig. 11 and Fig. 12, the load-deflection curve and the equivalent distributed load-deflection curve. In the case of S1 and RS1 the equivalent uniformly distributed load is somehow not an accurate expression of the load applied in tests, since the two loads were quite close to each other and close to the mid span of the slabs. The deflection in all of the curves presented in this article corresponds to the deflection at the mid span of each slab. curves it was reasonably close to its limits since the diagram has an almost horizontal shape right before unloading of the slab. Two important reductions of the load can be observed in both of the diagrams corresponding to RS1. They correspond to an important widening of the two main shear cracks close to the failure point of the slab. The experimental tests have proven that the failure of the slab was caused by shear efforts, and that the superior flexural capacity gained by strengthening of the slab was not harnessed at all. Nonetheless the CFRP had an important influence in keeping the cracks from premature opening and further widening. Regarding ultimate load capacity of S3 and RS3 no conclusion can be drawn since the element wasn t tested up to failure. Load [kn] 300 270 240 210 180 Load [kn] 210 180 150 150 120 90 60 30 S1 RS1 120 90 60 S3 RS3 0 30 Fig. 11. Load-deflection curve for S1 and RS1 Fig. 13. Load-deflection curve for S3 and RS3 Load [kn/m2] 70 60 50 Load [KN/m2] 28 24 20 40 30 16 20 10 S1 RS1 12 8 S3 RS3 0 Fig. 12. Load-deflection curve for S1 and RS1 In terms of ultimate load capacity it can be stated that the strengthened slab (RS1) has withstand a load higher with approximately 11 percent then the unstrengthened one (S1). However, the S1 slab was not tested quite up to its failure but as it can be seen in the load-deflection 4 Fig. 14. Equivalent load-deflection curve for S3 and RS3 The maximum equivalent load reached during the test can be compared with the value of ultimate load capacity calculated according to EC 2. But this comparison gives no answers either, since it is clear that between test ISSN: 1790-2769 500 ISBN: 978-960-474-080-2
results and the value calculated according to EC2 is no agreement at all. During the test of the unstrengthened slab a value of equivalent distributed load of 18.4 kn/m 2 was reached without failure. The ultimate load capacity calculated according to EC 2 was 16 kn/m 2. The load-deflection curves for the S3 and RS3 tests are presented in Fig. 13. In the same time a larger amount of information can be drawn from the equivalent distributed load-displacement curve presented in Fig. 14. The fact that all the control values are given by the manufacturer in form of distributed load, is an indicator that these diagrams are also very important. For both the S3 slab for the RS3 slab only the last cycle is presented. Based on the considerations previously stated it can be concluded that the proposed strengthening solution is not quite satisfactory, since it doesn t put foreword the very important mechanical capabilities of the CFRP. However, if the slab would to be subjected to uniformly distributed loads (similar to the loads considered for the design) the CFRP could prove its potential. Prefabricate din Beton, Cluj Napoca, 2005, (in Romanian). [7] ACI 440.2R-01, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, American Concrete Institute. [8] European Standard pren 1992-1-1, 2003, Eurocode 2: Design of concrete structures Part 1: General rules and rules for buildings. [9] fib TG 9.3, 2001 Technical Report in the Design and Use of Externally Bonded FRP Reinforcement for Reinforced Concrete Structures, Federation Internationale du Beton. 7 Acknowledgments The research work was granted to some extent by the Ministry of Education and Research through the CEEX and CNCSIS programs of the National University Research Council of Romania. References: [1] Dan D., Stoian V., Nagy T. C. Daescu, D. Pavlou Numerical Analysis and Experimental Studies on Welded Joint for Buildings, 3-rd International conference Applied and Theoretical Mechanics, Tenerife, Spain 2007, pp 106-112. [2] Floruţ S.C., Nagy-György T., Stoian V., Diaconu D., Strengthening of a hollow core precast slab using FRP composite materials - testing and rating, Proceedings of the International Conference CONSTRUCTIONS 2008, 09-10 May, Cluj-Napoca, Romania, pp 69-74. [3] Nagy-György T., Composite Materials: for masonry and concrete structural elements strengthening, Ed. Politehnica, 2007, (in Romanian). [4] Marco di Prisco, Marco G. L. Lamperti, Toward a standardized procedure for hollow-core slab testing, Proceedings of the fib Symposium, Hungary, 23-25 May 2005, vol I, pp. 558-563. [5] Stoian V., Nagy-György T., Dan D., Gergely J., Dăescu C., Composite Materials for Constructions, Ed. Politehnica, 2004 (in Romanian). [6] Stoian V., Dan D., Nagy-György T., Dăescu C, Diaconu D., Sas G., Performance study of a hollow core slab strengthened with CFRP fabric, Structuri ISSN: 1790-2769 501 ISBN: 978-960-474-080-2