Investigation on the Flexural Behavior of Corroded Concrete Beams Repaired by CFRP Sheet Under Different Corrosion Levels

2.1. Introduction of Specimens

In this paper, a longitudinal beam of high-pile wharf, which is a type of easily corroded member in marine environment, is selected as an object for investigation.

Firstly, a series of RC beams was constructed and corroded by accelerated corrosion techniques. Then they were classified into several corrosion levels according to their damage state, and repaired by different methods with or without CFRP sheets respectively. Finally, static load tests were carried out to investigate their mechanical behavior.

In Chinese code Technical Specification for Detection and Assessment of Harbour and Marine Structures (JTJ 302-2006) [17], the corrosion levels of specimens include A, B, C and D, in terms of steel corrosion, crack development, concrete peeling and so on. According to the study below, beams belong to Level C in [17] by different strengthening ways may exhibit obviously different behaviors. In this paper, based on [17], a more sophisticated classification for corrosion levels is presented, in which the Level C in [17] was divided into level C1 and C2. Thus there are five corrosion levels, namely A, B, C1, C2 and D, are defined in this paper, according to the crack pattern, the crack width and the percentage of loss of sectional area η. The classification criterion is shown in Table 1.

Since it is hard to measure the actual corrosion status of the steel bars before static load test, the corrosion level of specimens is preliminary classified by the width of cracks induced by corrosion. What’s more, half-cell potential measurement and destructive test would be carried out for further validation of steel corrosion status after the static load test.

The program consisted of twenty 150mm×250mm×1400mm reinforced concrete beams that were reinforced with HPB300 steel bars. The details of the beams are shown in Fig. (1). The mechanical properties of concrete and steel are shown in Tables 2 and 3, respectively. Considering the corrosion levels, strengthening methods, and layers of CFRP, the number of specimens under different strengthening schemes is shown in Table 4.

2.2. Accelerated Corrosion Process

As shown in Fig. (2), an accelerated corrosion instrument was adopted to simulate corrosion by electrochemical methods, supplemented by wet-dry cycles environment. The accelerated corrosion instrument can provide DC power for 18 specimens at the same time and each specimen would not affect each other. In addition, the current intensity is constant during the process of corrosion and can be adjusted continuously within a certain scope. The accelerated corrosion test devices are shown in Figs. (2 and 3).

During the accelerated corrosion process, the salt content of water in the tank was kept at around 3% and the wet-dry cycle took 3 days, which consisted of wetting the beams for 1 day followed by 2 days of drying. The electric current flowed through the tensile reinforcement was about 800 mA, which corresponds to an approximate current density of 7.5μA/mm². The time of corrosion depended on the designed corrosion level of each specimen.

Although theoretical formulae have been proposed to judge the corroded status of the specimens, the test results still have variation due to the difference of material, current density, and the time of test. After the corrosion process, the cracks were the most visible phenomenon, therefore, at the end of the corrosion, the crack width was observed (shown in Fig. 4) and was selected as the main criteria to define the corrosion level of the specimens. In addition, the measured value of half-cell potential and the sectional loss rate of rebars were also adopted to detect the corrosion level of rebar for reference.

To measure the section loss rate, the corroded rebars were taken out from the broken beams after the static load test, and cleaned by dilute hydrochloric acid. Then the Vernier caliper was used to measure the cross sections with greater area loss, except those with obvious plastic necking deformation. Then the section shapes were drawn by AUTOCAD and their area could be calculated from it.

Fig. (5) demonstrates the maximum crack width and the corresponding loss rate of the sectional area η in the specimens with different corrosion level, which shows a relatively obvious linear relationship. Therefore, it is possible to judge the corrosion levels by crack width.

Fig. (6) shows the relationship between the corrosion time and the half-cell potential. According to ASTMC 876-80, measured potentials more positive than -200 mV, represent greater than 90% probability of no active corrosion; potentials between -200 and -350 mV, active corrosion is uncertain; and potentials more negative than -350 mV, there is greater than 90% probability of active corrosion. Fig. (6) indicates that the potentials in each beam quickly increased to a value negative than -350 mV in the first few days.

According to Table 1, two of the twenty specimens without cracks were classified as Level A, and the remaining eighteen corroded beams were classified into the other levels, four of which were Level B, five were C1, six were Level C2, and three were Level D. The details are shown in Table 4.

2.3. CFRP Repairing

Generally, the surfaces of corroded beams were chiseled and cleaned before bonding CFRP. However, for the on-sites corroded RC beams, it is a hard work to remove and rebuild the concrete cover before bonding the CFRP sheets. For beams with lower corrosion levels and small crack width, it is necessary to investigate that whether the CFRP sheets could be bonded directly to the surface of the concrete beam or not. Two strengthening methods were studied in this paper.

The composite Cymax L200-C, manufactured in Taiwan was used as the CFRP materials in this test, and its mechanical properties are listed in Table 5. The wet surface bonding binder with the adhesion strength no less than 2.5 MPa was adopted as the epoxy resin adhesive.

2.3.1. Strengthening Method by Bonding CFRP Directly (DM)

CFRP sheets were bonded directly on the bottom of the test beams, where usually the flexural and tensile region. The CFRP sheet was 1180mm long along the beam and the same width as the beam. In order to prevent the debonding failure, except Level B, two 80mm wide U-shaped strips spaced every 80mm were patched at each end of longitudinal CFRP. Fig. (7) shows the details of DM.

2.3.2. Strengthening Method by Bonding CFRP After Cutting or Chiseling Off Concrete (RM1 or RM2)

Strengthening method of bonding CFRP after cutting off concrete was suitable for the beams of level C1, C2, D with large crack width.

Before patching the CFRP, lossened concrete or the protection cover around the corroded rebars should be cut or chiseled away firstly, so that the rust of corroded rebar could be cleaned. Secondly, the protection cover was repaired according to the original size of the specimen with the epoxy polymer mortar. Finally, the specimen is strengthened by bonding CFRP sheets.

In this paper, two chiseling schemes were adopted in the process of dealing with the concrete. The first scheme (RM1) is suitable for the specimens with lower corrosion level. In this scheme, a V-shape groove was cut along the longitudinal cracks on the bottom of the beams, and then the CFRP sheets were bonded after repairing the grooves with polymer mortar. The process of RM1 is shown in Fig. (8). The second scheme (RM2) is suitable for the seriously corroded specimens in which the protection cover had become loosen, and there was severe slip between steel bars and concrete. Therefore, the loosened protection cover should be removed, then a form was established and new protection cover was built by the polymer mortar. Finally, the CFRP sheets were bonded. The process of RM2 is shown in Fig. (9).

Table 4 shows that among the twenty specimens, five were unstrengthened, eleven were repaired by DM, three were repaired by RM1, and one was strengthened by RM2.

2.4. Static Load Test

According to the corroded levels and the CFRP strengthening method, the specimens were named as S_m-n. S denotes the corrosion level; m is the different strengthening methods corresponding to 0~3, while 0 represents unstrengthening, 1 represents DM, 2 represents RM1, and 3 represents RM2; n is the number of CFRP sheets layers corresponding to 0~3, 0 represents unstrengthening. For example, C1_2-1 means the specimen is C1 corrosion level, the strengthening method is RM1 and the CFRP layer is 1. The number of specimens is shown in Table 6.

A series of static load tests had been carried out on these corroded specimens to investigate the flexural behavior. The test device and the loading position are shown in Figs. (10 and 11).

The strain of rebar, concrete, CFRP and the mid-span deflection of the beams were recorded, and the development of cracks and the failure modes of specimens were observed. The loading position and the layout of strain gauges are shown in Fig. (11). The concrete strain gauges were placed on both sides of the beams. The deflection of the beam was collected by linear variable differential transducer (LVDT). Both load and displacement control regimes were adopted during the tests.

3.1. The Experiment Phenomena and Failure Mode

The experimental results of specimens are shown in Table 6. Part of the experimental failure modes of the specimens are shown in Fig. (12).

Typical flexural failure occurred in the unstrengthened specimens, A_0-0, B_0-0, C1_0-0, C2_0-0 and D_0-0, and the cracks mainly appeared at the mid-span, most of which were vertical flexural cracks. The loading process was similar to the common RC members, and can be divided into three stages. No cracks appeared in the first stage, and in the second stage the cracks appeared and developed stably, and in the third stage the steel bars yielded and finally the concrete crashed. It can be seen from Table 6 that from corrosion Level A to D, as the steel area was gradually decreased due to corrosion, the flexural bearing capacity of the unstrengthened beams gradually decreased and the failure mode changed from the under-reinforced failure to scarce-reinforced failure.

For those specimens strengthened by CFRP, the test results is somewhat different to those unstrengthened ones. Part of the phenomenon of the experiment is summarized as followed,

1. For specimen A_1-1, whose corrosion level is A and one layer of CFRP was bonded, the cracks occurred later than A_0-0 but developed rapidly. The concrete was finally crushed at the loading point and then the beam broke downward from the crush point suddenly followed by the fracture of CFRP U-shaped strips. It had the similarly failure as the over-reinforced beam.
2. For specimens with Level B or C1 corrosion level and strengthened by DM, or with Level C1 or C2 corrosion level strengthened by RM1 or RM2, the failure modes were similar to under-reinforced failure of normal RC member. Next, the tensile concrete cracked and the strains of steel bars and carbon fiber increased rapidly, in which the steel bars played a more important role than the CFRP. Nevertheless, after the yielding of the steel bars, the strain of the CFRP keep on increasing. Finally, the CFRP fractured or deboned. For example, the CFRP and the concrete cover in B_1-1 specimen debonded from the beam when the strain of CFRP increased to 7400με. The test results implies that the bearing capacity of the CFRP strengthening corroded RC beams was effectively enhannced, the crack width was restrained, and the flexural stiffness was improved.
3. For the specimens with Level C2 corrosion and strengthened by DM or with Level D corrosion and strengthened by DM or RM2, the steel bars broke without obvious yielding for the sake of severely corrosion, and the specimens failed before the CFRP exerted its full strength. The cracks developed downward from the loading point and the U-wraps broke followed by the CFRP debonded. For these specimens with high corrosion level, the CFRP sheet couldn’t effectively prevent the cracks from developing. It may be because that there was a severe slip between the tensile steel bars and the concrete around results in lower bonding stress.

The test results indicates that the failure mode of the unstrengthened specimen with lower corrosion level A was under-reinforced failure, yet it might be over-reinforced failure when strengthened by CFRP. Therefore, if Level A members need to be strengthened, we should make sure that it would not be over-reinforced. As for level B or Level C1 specimens, although the steel bars lost some sectional area in corrosion, directly bonding CFRP sheet (DM) can still effectively prevent the cracks from developing and the failure mode of the beam can still be the under-reinforced failure. For Level C2 specimens, RM1 or RM2 is better than DM to strengthen the specimens. However, for Level D specimens, due to the excessive loss of steel bars area, the strengthening cannot effectively restrain the development of original cracks, and the scared-reinforced failure is likely to occur.

In summary, it is suggested that Level A, B and C1 corroded members can be strengthened by directly bonding CFRP. While the other level members should be strengthened by bonding CFRP after cutting and chiseling the concrete cover. In any case, the over-reinforced failure should be avoided.

3.2. Parametric Analysis

Following is the analysis on the ultimate flexural capacity of RC members strengthened by CFRP according to the experimental results. The influence of different strengthening methods and the CFRP layers is to be discussed.

(1) Influence of number of CFRP sheets layers on the ultimate flexural bearing capacity

Fig. (13) illustrates the comparison of bearing capacity between unstrengthened beams and beams strengthened by DM with different corrosion levels. In Fig. (13), 0-0 represents the unstrengthened beams, while 1-1, 1-2 and 1-3 represents the specimens strengthened by DM with 1 to 3 CFRP layers, respectively. Fig. (13) indicates that except level D, all the CFRP strengthening specimens have higher bearing capacity than the unstrengthened ones, and with the increase of the CFRP layers, the bearing capacity increases. Furthermore, generally with the increase of the corrosion levels, the bearing capacity of the specimens with the same CFRP layers decreases.

Fig. (14) shows the increased range of bearing capacity with different CFRP layers. It demonstrates that the bearing capacity increased significantly when strengthened by one layer of CFRP, The ultimate flexural capacity of members with one layer of CFRP raised by 30% to 50% than those unstrengthened ones. However, the trend slows down with the increase in the number of layers. Fig. (14) also shows that the increased range of bearing capacity declined with the growth of corrosion levels. This can be ascribed to the worse reinforcing situation under serious corrosion.

(2) Influence of different strengthening methods on bearing capacity

Fig. (15) presents the comparison of the increase of bearing capacity between the members with different corrosion levels under DM and RM1. As can be seen from the figure, for specimens with obvious cracks, it’s better to use RM to increase the bearing capacity than DM.

(3) Influence of different corrosion levels on stiffness

Figs. (16 and 17) show the load-middle span deflection curves of some specimens. As can be seen from Fig. (16), the final mid-span deflection of the unstrengthened beam A_0-0 is 22mm, and the deflection of the strengthened beams is from 8mm to 15mm. Generally, the slope of the deflection curves of strengthened beams is greater than that of unstrengthened ones. The bearing capacity of the strengthened ones is higher than that of unstrengthened ones, yet the ductility of the strengthened ones are lower than that of the unstrengthened ones. This indicates that strengthened by CFRP is capable of improving the deformation performance and the integral stiffness of the beams obviously yet decreasing the ductility.

Except A_1-1, under the same loading, larger deflection of the specimen can be achieved with the increase of the corrosion level. That means the mid-span deflection of the specimens was greater and their integral stiffness was weaker with the growth of the corrosion levels. It can be concluded that the cracking caused by corrosion has a great impact on the integral stiffness of strengthened beams. The more severe the corrosion, the greater impact on stiffness is and the worse reinforcing effect is.

Fig. (17) demonstrates the load-deflection curves of C2 under two different strengthening methods (DM and RM1), it can be seen that the initial stiffness is close yet the bearing capacity is different, therefore for the specimens with serious cracks conditions, RM1 or RM2 is recommended for repair.