Post-strengthening of reinforced concrete beams with prestressed CFRP strips Part 2: Analysis under cyclic loading Reforço de vigas de concreto armado com laminados de PRFC protendidos Parte 2: Análise sob ação de carregamento cíclico

© 2012 IBRACON Different FPR post-strengthening techniques have been developed and applied in existing structures aiming to increase their load capacity. Most of the FRP systems used nowadays consist of carbon fibers embedded in epoxy matrices (CFRP). Regardless of the advantages and the good results shown by the CFRP post-strengthen technique, experimental studies show that, in most cases, the failure of post-strengthened structures is premature. Aiming to better use the tensile strength of the carbon fiber strips used as post-strengthening material, the application of prestressed CFRP strips started to be investigated. The main purpose of this paper is to analyze the effects of the composite prestressing in the performance of the CFRP post strengthening technique. The experimental program was based on flexural tests on post-strengthened reinforced concrete beams subjected to static – part 1 and cyclic – part 2 loading. Experimental results allowed the analysis of the quality and shortcomings of post-strengthen system studied, which resulted in valuable considerations about the analyzed post-strengthened beams.


Introduction
This paper shows the results of an experimental program developed at LEME-UFRGS, in Brazil, and at EMPA, in Switzerland, regarding the application of CFRP post-strengthening on concrete structures.Continuing the results of the first part of this paper, this second part analyses the behavior of concrete beams post-strengthened with prestressed CFRP under cyclic loading.

Post-strengthened elements submitted to cyclic loading
Fatigue may be defined as a permanent and progressive damage process that induces gradual and cumulative crack growth and might, ultimately, result in the complete fracture of the elements subjected to cyclic loads, if the stress variation and the number of load cycles are large enough.This term was established by the first researchers of the theme due to its nature: a progressive damage process caused by cyclic loads, difficult to observe, that changes the ultimate capacity of the material [Menehhetti et al. [1]].
The usual fatigue failure mechanism for post-strengthened RC beams, when subjected to cyclic loads, is marked by the rupture of one of the steel rebars, followed by a stress redistribution that overloads the remaining bars.Meier U. [2] highlights that the steel rebars fail before the FRP post-strengthening, as can be seen in Figure [01] that shows the steel rebars of a post-strengthened concrete beam after a fatigue failure.However, cyclic loads can also damage the adhesive and affect the interface concrete-adhesive and adhesive-FRP, leading to premature failures.According to Ferrier et al. [3], the performance and the durability of a concrete structure post-strengthened with FRP, when subjected to cyclic loads, depends not only on the FRP fatigue behavior but also depends on the interface concrete-adhesive and adhesive-FRP.Authors point out the importance of understanding the behavior of these materials under cyclic loading, since a typical reinforced concrete highway bridge deck with a design life of 40 years may experience a minimum of 58x10 8 loading cycles of varying intensities.

Description of specimens
Aiming to analyze the behavior of concrete beams post-strengthened with prestressed CFRP strips, under cyclic loading, three  Post-strengthening of reinforced concrete beams with prestressed CFRP strips Part 2: Analysis under cyclic loading beams were tested: one of them was tested under static loading (VFC_PE_01) and two were tested under cyclic loading (VFC_ PC_01 and VFC_PC_02).A complete description about the concrete beams, the post-strengthening and the anchorage system used may be found at the first part of this paper.Table [01] describes the experimental program.
Stress levels applied at beam VFC_PC_01 were 50% and 80% of the yielding stress observed at beam VFC_PE_01, tested under static loading.Stress levels applied at beam VFC_PC_02 were more reasonable, 50% and 60% of the yielding stress observed at beam VFC_PE_01.

Test procedure
The experimental program was developed at EMPA -Switzerland.Loading was applied according to a six point bending test scheme: simple supported beam and four vertical loads, spaced by 12000mm, symmetrically applied along the 6000mm span.

Results of beam VFC_PC_01
Beam VFC_PC_01 was submitted to stress levels of 50% and 80% Aiming to produce the first cracks, beam VFC_PC_01 was first pre-loaded up to108kN.Then, cyclic loading was applied at a frequency of 4Hz.When 282.000 cycles were reached, a crack of about 2,2mm was observed, approximately at mid-span, reaching about 90% of the cross-section.After 331 300 cycles the machine automatically stopped, when the deflection limit was reached.It was not observed any sign of apparent failure at the strips.Larger displacement limits were settled and the test was restarted.However, the post-strengthening failed before the maximum load of 108kN was along the bottom of the beam.Measurements were made during pre-loading, after 30.000 cycles, and after 100.000 cycles, with the beam subjected to the maximum load (108kN).
Figures [06] and [07] also shows that the measurements made in the CFRP strips allowed the construction of a well defined curve.However, due to the crack growing between the gauge points, this behavior could not be observed in the measurements made in the concrete.Nevertheless all obtained responses followed an expected pattern.
Strains in the CFRP strips, at mid-span, during pre-loading, varied from 2,00º/ oo up to 2,50º/ oo .At the end of 30.000 cycles, strains increased to levels that varied from 2,50º/ oo up to 3,00º/ oo .From 30.000 cycles up to 100.000 cycles, it was not observed any significant variation in the strains.
Strains measured, added to the strain applied to prestress each strip (5,95º/ oo ), give for each strip a total strain of of 8,45º/ oo and 8,95º/ oo .
It is also noted that, at mid-span, where most of the cracks could be found, strains measured in the concrete and in the FRP are quite different.It happens because the FRP strip acts as a belt, blocking the concrete crack opening.Therefore, it can be observed that several points along to the beam are subjected to different strains.Such points can, eventually, be related to the occurrence of premature failures.
Strains at a distance of 1.2 m from both beam ends, out of the loading region, are not greater than 1,50°/ oo , however, these values increased about 100% from preloading up to 100.000 cycles.
Figure [08] shows the strains in the CFRP strips measured during pre-loading, after 30.000 cycles and after 100.000 cycles.Significant variations were not observed in the range from 30.000 to 100.000 cycles.However, strains increased about 0,50º/ oo from the pre-loading up to 10.000 cycles.A high concentration of cracks can be observed at the mid-span of beam VFC_PC_01, between the four loading points.Figure [10] shows the crack openings at mid-span between the two central loading points (signaled on the figure by two vertical arrows).It is noteworthy that, before 100.000cycles, crack openings did not reached 0,05mm.The highest crack opening after 100.000 cycles, named D, was 0,4mm, located 254cm from the left side of the beam.However, apparently the post-strengthening failed due to a 2,2mm crack opening, named I, after 331.300 cycles.It is important to notice that, after 100.000 cycles, this crack opening was about 0,3mm (Figure [11]).
The decision of testing beam VFC_PC_01 under a high stress variation led to the fatigue failure before 5.000.000cycles, that was considered the pattern of infinite fatigue life.Stress levels applied to the beam VFC_PC_02, however, are more consistent with the ones usually found in real structures.Results of beamVFC_PC_02 will allow a more detailed analysis of the CFRP prestressing technique used, as well as of the gradual anchorage system.

Results of beam VFC_PC_02
Beam VFC_PC_02 was submitted to stress levels of 50% and 60% Aiming to produce the first cracks, beam VFC_PC_02 was preloaded up to78kN.Then, cyclic loading was applied at a frequency of 4Hz. Figure [12(a)] shows that vertical displacements at mid-span, for beam VFC_PC_02, measured with the beam subjected to the maximum load (78kN) varied 12,70mm from preloading up to 1.000.000cycles.From this point, up to the end of the test, vertical displacements increased just 1,49mm.Data of beam VFC_PC_01 showed a large increase in the vertical displacements at mid-span after 100.000 cycles, probably due to the fatigue failure of the steel rebars.
Strains in the concrete and in the CFRP strips (Figure [12(b)]) behave similarly to the displacements at mid-span, where most of the variations occurred before 1.000.000cycles, and, after that, showed stability up to 5.000.000cycles.Beam VFV_PC_01 also showed a similar behavior between strains and vertical displacements at mid-span, however, with a significant increasing after 100.000 cycles, probably due to the fatigue failure of the steel rebars.
Figures [03] to [15] show the strains in concrete and in CFRP strips, obtained by deformeters, which gauge points were placed along the bottom of the beam.Measurements were made during preloading, after 30.000 cycles, after 100.000 cycles, after 1.000.000cycles and after 5.000.000cycles, with the beam subjected to the maximum load (78kN).
It can be noticed that the strains obtained by deformeters, placed along the bottom of the beam VFC_PC_02, varied from the pre-loading up to 1.000.000,tending to stabilize after 5 million cycles.Portions located between the loading points (1.2 m to 4.8 m from the beam end) clearly show the presence of cracks in the concrete.
From the pre-loading up to 30.000 cycles, it was not observed any significant variation in the strains along the gradual anchorage zone (1,2m from the both beam ends).Strains increased after 100.000 cycles, and, after 5.000.000cycles, the level of strains of the beginning of the test could be observed just along the first 0,6m from both beam ends.Figures [13] to [15] show that strains in the anchorage zones of beam VFC_PC_01 were higher than the ones of beam VFC_PC_02.Data of Figures [13] to [15] show that strains in the CFRP strips, between the two central loading points, placed at 2,4m and 3,6m from the beam ends, measured at pre-loading, varied from 1,00 º/ oo up to 1,50º/ oo .After 30.000 cycles strains increased up to 2,00 º/ oo and no significant variation were observed from 30.000 cycles up to 100.000 cycles.Measurements after 1.000.000and 5.000.000cycles registered a maximum strain of 2,11º/ oo .Such strain, added to the strain applied to prestress each strip (5,95º/ oo ), give for each strip a total strain of of 8,06º/ oo.Strains in the strips of beam VFC_PC_02 were smaller to the ones of beam VFC_PC_02, since, for the second beam, the maximum load and the difference between the maximum and the minimum load were smaller.Results of beam VFC_PC_02 indicate that, up to 5.000.00cycles, it was not observed any damage on the post-strengthening system, due to the application of the cyclic loading.Figure [16 (a)] shows the strains in the CFRP strips, measured from the pre-loading up to 5.000.000cycles.The most significant variations occurred up to 1.000.000cycles.Strains in the CFRP strips varied about 0,85º/ oo from the pre-loading up to 5.000.000cycles.The greatest differences regarding strains were found at 1,8m, 2,8m and 4,6m from the left side of the beam (Figure [16 (a)]), and at 2,6m from the left side of the beam (Figure [16 (b)]).
Results indicate the existence of a kind of progressive strain at the anchorage regions, which can, eventually, generate adherence problems regarding long-term fatigue.Such effect should be better investigated, however, the long time demanded to realize fatigue tests, sometimes, inhibits this initiative.Crack growing at mid-span of beam VFC_PC_01 can be observed at Figure [17], which shows the results of all crack opening measurements made, from pre-loading up to 5.000.000cycles.Figure [17] shows the results of crack openings and the respective position from the left side of the beam, at mid-span, between the two central loading points (signaled on the figure by two vertical arrows.First cracks at mid-span appeared during pre-loading, reaching less than 0,15mm.From this point up to 100.000 cycle, crack openings increased, but did not exceed 0,20mm.After 1.000.000cycles the maximum crack opening was 0,22mm, and, after 5.000.000,this value was not exceeded.
Cracking at the gradual anchorage regions appeared just after 100.000 cycles, however, the maximum cracking opening observed was 0,05mm.From 100.000 up to 5.000.000cycles, the maximum crack opening measured at these regions was 0,10mm.. Results of crack openings obtained from beams VFC_PC_01 and VFC_PC_02 cannot be compared directly, due to the difference regarding the maximum and minimum loads applied to generate the cyclic loading.
As the maximum load applied on the beam VFC_PC_01 (108kN) was higher than the one applied on the beam VFC_PC_02 (78kN), beam VFC_PC_01 showed higher values of crack openings since pre-loading.Values of crack openings obtained after 5.000.000cycles, for beam VFC_PC_02, were reached by beam VFC_PC_01 after just 282.000 cycles.

Conclusions
Results of the reinforced concrete beams tested under cyclic loading show that when these structures are post-strengthened with prestressed CFRP strips, damages that occur due to fatigue are mainly related to the level of stress at the steel rebars.Experimental results showed that the damage, which led to the rupture of the steel rebars, is related to the level of stress during loading, and, that it is not related to the type of post-strengthening.Tests showed that an increasing of 20% in the maximum stress of the steel rebars significantly reduced the fatigue life time of the post-strengthened element, decreasing about 15 times the number of cycles up to failure.These results emphasize the importance of proceeding the monitoring of structures that are usually submitted to cyclic loading, such as highway and railway bridges.In some cases, when these structures were designed to support traffic loads smaller to the ones that they are submitted nowadays, the use of post-strengthening may increase their lifetime, since the use of post-strengthening may lead to a reduction in the stress level of the steel rebars.

Figure 1 -
Figure 1 -Failure of concrete beam after fatigue loading test

Figure 4 -Figure 5 -
Figure 4 -Failure of post-strengthening strips: (a) Next to a flexural crack; (b) Distant from flexural cracks

Figure 9 -
Figure 9 -Cracks at mid-span of beam VFC_PC_01 after 100.000 cycles

Figure 11
Figure 11 -2,2mm width crack at mid-span of beam VFC_PC_01 after post-strengthening failure

Figure 13 -
Figure 13 -CFRP and concrete strains of beam VFC_PC_02, submitted to 78kN, during pre-loading Authors would like to acknowledge CNPq (Portuguese acronym of the Brazilian Ministry of Science's National Research Council) and CAPES (Portuguese acronym of the Brazilian Ministry of Education's Higher Education Human Resources Development Agency) for providing the financial support needed to develop this project.Authors would also like to express their appreciation for the technical support given by the research team at EMPA (Swiss Federal Laboratories for Materials Testing and Research), in Switzerland.

Table 1 -
Description of Experimental Program * Related to the load that corresponds to yielding of the steel rebarat beam VFC_PE_01.