Prevention of Debonding Failure of Intermediate Anchor to Eliminate Premature Shear Failure of Flexurally Strengthened Reinforced Concrete Beams

The aim of the present study is to review the performance of plate bonded flexurally strengthened reinforced concrete beams. This paper also describes the methods and materials used for flexural and shear strengthening of reinforced concrete beams and weaknesses of plate bonded systems. The plate bonding method often has some serious premature debonding failure which can be classified as plate end debonding (end peeling), tension delamination and premature shear failure due to insufficient shear reinforcement. Premature failures must be prevented in order to utilize the full flexural capacity of flexural strengthened reinforced concrete beams. Premature shear failure is one of the major concerns of the flexurally strengthened reinforced concrete (RC) beams. Hence, proper design of external shear strengthening system is required for eliminating premature shear failure of flexurally strengthened RC beams. The review focuses on the possible model and design guideline available in the literature for eliminating premature failures. The paper also discusses a probable approach to eliminate premature shear.

Flexurally strengthened RC beams usually fail by means of premature shear due to low shear as compared to flexure. Intermediate anchors in the length of shear span of those beams would successfully eliminate this problem. This paper presents the experimental studies on the effects of intermediate anchors in preventing premature shear failure of CFRP laminate flexurally strengthened RC beams. Design guidelines to optimize the intermediate anchors for eliminating premature shear failure are proposed. In the experimental programme, four RC beams were cast. One beam was tested in the un-strengthened condition to act as the control beam. The remaining beams were strengthened with CFRP laminates. Among the strengthened beams, one beam was prepared without intermediate anchors, one was intermediate anchored based on the proposed design method, and the last one was intermediate anchored using arbitrary anchor plates. Results showed that strengthened beam with having optimal intermediate anchors had higher ultimate strength as compared to that of the control beam. The optimal anchors significantly increased both the ultimate load as well as ductility of the said beams as compared to the beam without intermediate anchors. Moreover, the optimal intermediate anchors also reduced the number of cracks and crack widths in the shear span region. In conclusion, the beam with optimal intermediate anchors had identical failure load, crack widths, deflections and strain characteristics as that of arbitrarily anchored strengthened beam.

Externally bonded method would be the most potential technique for optimal shear strengthening of deficient reinforced concrete (RC) beam subjected to eliminate premature debonding failure. The main aim of this research was to eliminate debonding failure of steel plate using embedded connector systems for optimal shear strengthening of RC beam. Dimensions of steel plates were optimized based on the proposed design guideline in accordance with EC2. The strengthened beams with connectors were then compared with those of U-jacketing system. Bond strength enhancements of steel plate using connectors were experimentally investigated through pullout test of prisms. The beam specimens were fabricated and tested to investigate the effects of embedded connectors. Result showed that steel bar and adhesive embedded connectors significantly increased the bond strength of externally bonded plates. The connectors completely prevented premature debonding failure of steel plates and allowed the beams to fail by flexure with full ductility and strength, whereas U-jacket showed the premature debonding of plates. The embedded connectors optimized/reduced the dimension of the externally bonded steel plate by 60% as compared to those of arbitrarily strengthened beam. The experimental results satisfactorily verified the proposed design guideline.

To investigate the durability of reinforced concrete (RC) beams under the combined actions of transverse cracks and corrosion, corrosion tests were conducted on a total of eight RC beams with different water–cement ratios and cracking states. The effects of the transverse crack width, water–cement ratio, and the length of the wetting–drying cycle on the distribution of the free chloride concentration, the cross-sectional loss of the tensile steel bars, and the chloride diffusion coefficient are analyzed. The results show that the widths of the transverse crack and the water–cement ratio of concrete greatly affected the chloride profile and content of the RC beam specimens. Specifically, the chloride contents in all the cracked RC beams at the depth of the steel bar exceeded the threshold value of 0.15%. As the width of the cracks increased, the chloride concentration and penetration of the cracked concrete beam increased. However, the chloride concentration at the reinforcement position did not seem to be obviously affected by increasing the wetting–drying cycles from 182 days to 364 days. Moreover, the decrease of the water–cement ratio effectively inhibited the penetration of chloride ions in the RC beam specimens. In terms of the cross-sectional loss of the steel bars, the average loss of the steel bar increases with increasing crack width for the beams with 182-day cycles, while the effect of crack width on the average loss is not as noticeable for the beams with 364-day cycles. Finally, a model is proposed to predict the relationship between the crack width influence coefficient, μ, and the crack width, w, and this model shows good agreement with the experimental results.

First Name is required invalid characters Last Name is required invalid characters Email Address is required Invalid Email Address Invalid Email Address

Crack based bond strength model of externally bonded steel plate and CFRP laminate to predict debonding failure of shear strengthened RC beams

Several techniques have been developed for shear strengthening of reinforced concrete (RC) members by using fiber reinforced polymer (FRP) composites. However, debonding of FRP retrofits from concrete substrate still deemed as a challenging concern in their application which needs to be scrutinized in details. As a result, this paper reports on the results of an experimental investigation on shear strengthening of RC beams using near surface mounted (NSM) FRP reinforcing bars. The main objective of the experimentation was increasing the efficiency of shear retrofits by precluding/postponing the premature debonding failure. The experimental program was comprised of six shear deficient RC beams. The test parameters include the FRP rebar spacing, inclination angle, and groove shape. Also, an innovative modification was introduced to the conventional NSM technique and its efficiency was evaluated by experimental observation and measurement. The results testified the efficiency of glass FRP (GFRP) rebars in increasing the shear strength of the test specimens retrofitted using conventional NSM technique. However, debonding of FRP bars impeded exploiting all retrofitting advantages and induced a premature shear failure. On the contrary, application of the proposed modified NSM (MNSM) technique was not only capable of preventing the premature debonding of FRP bars, but also could replace the failure mode of specimen from the brittle shear to a ductile flexural failure which is more desirable.

This study experimentally investigated corrosion-induced deterioration in reinforced concrete (RC) structures: concrete cover cracking, steel-concrete bond loss, and mechanical degradation of corroded steel bars. Pullout and RC beam specimens were prepared, subjected to accelerated corrosion in a wet sand bath, and tested under loading. A 3D laser scan was employed to measure the surface profile of corroded steel bars and determine the corrosion effect on the distribution of residual cross section area. The crack width on the concrete surface was sampled randomly and analyzed statistically. Corrosion reduced the bond strength between steel bars and concrete, particularly in the form of corrosion-induced number and width of cracks. Both the yield and ultimate strengths depended upon the critical cross sectional area of steel bars, whereas the elongation changed with the cross section distribution over the length of the steel bars. Corrosion also changed the distribution of the cross sectional area of steel bars. The crack width on the concrete surface can be well represented by a normal distribution regardless of corrosion levels.

This research presents the results of an experimental study to look into the effects of intermediate anchors on end anchored CFRP laminate strengthened beams. Three beams of 125A�250A�2300 mm in dimensions were cast. Out of these, one beam was left un-strengthened and acts as the control beam and another two beams were strengthened with CFRP laminates. Both strengthened beams were end anchored to prevent premature end peeling. From the strengthened beams, one beam was intermediate anchored in the shear span to prevent premature shear failure. The anchorage lengths provided by the end and intermediate anchors were of 200 and 40 mm, respectively. The results showed that the intermediate anchors in shear span zone prevented premature shear failure. Result also showed that the strengthened beams with intermediate anchors had significant effects on failure loads, failure modes, strain characteristics, deflections and cracking patterns over the end anchored strengthened beam.

A load carrying capacity of the reinforced concrete (RC) member is degraded by the corrosion of reinforcing steel bars due to chloride ion ingress. A lot of researches on the effect of corrosion in the longitudinal tensile reinforcing steel bars on the load carrying behavior have been available up to now. Accurate and quantitative estimation of capacity, however, is often difficult, because of the non-uniformity of corrosion in the member. Thus, a relationship between the spatial distribution of corrosion in the reinforcement including its scatter and the flexural loading capacity of RC member with such distribution of corrosion should be clarified so that the flexural capacity of corroded RC member can be estimated accurately. On the other hand, in case of the practical RC member under the corrosive environment, it should be considered that the flexural capacity often have to be derived from not a large number of inspection data on cross sectional areas of corroded reinforcements. So, in this study, a flexural loading test was performed by using RC beam specimens with the corroded tensile reinforcements provided the distribution of sectional areas. An estimation method of the flexural capacity of corroded RC beam was also shown, considering the distribution and its scatter in sectional areas of corroded reinforcements under the limited inspection data. Furthermore, the estimation of the longitudinal distribution of the cross sectional area of corroded reinforcement was performed by the spatial interpolation using Kriging method. Test results showed the yield and maximum load capacity in the corroded RC beam decreased as the corrosion rate increased. The failure mode of rupture in the reinforcement was shown in the large corrosion. The proposed estimation method was able to lead the safe evaluation of those experimental flexural capacities, determining the appropriate longitudinal characteristic value of the cross sectional area of corroded reinforcement. The flexural capacity can be also safely calculated using the characteristic value of diameters estimated by the corrosion crack width on the surface of the concrete, while the ratio of the experimental flexural capacity to the estimated one decreased as the corrosion loss increased. The distribution of bar diameters in the corroded reinforcement was able to be roughly estimated by using Kriging method. However, it was suggested that the measurement points close to the minimum bar diameter should be included to estimate the flexural capacity on the safe side.

Earlier studies on large reinforced concrete (RC) beams found that the width of side-face cracks can be larger than the width of cracks at the extreme tension face. Accordingly, many RC design standards and codes recommend using skin reinforcement to limit the width of side-face cracks. In large reinforced masonry (RM) beams, side-face cracks and skin reinforcement are known as intermediate cracks and intermediate reinforcement, respectively. In line with the studies on large RC beams, it is intuitively presumed that the width of intermediate cracks in large RM beams can be critical for serviceability requirements. Accordingly, the Canadian standard CSA S304.1 recommends using intermediate reinforcement in large RM beams, although no research was conducted to validate this. Hence, this study was completed to understand the cracking pattern in large RM beams and the beneficial effect of intermediate reinforcement as recommended in the Canadian standard. The study found that the cracking pattern in large RM beams is different from that of large RC beams. This study also found that the recommendation of CSA S304.1 on intermediate reinforcement for large RM beams does not yield the best use.

Repairing and strengthening of damaged reinforced concrete (RC) beams using externally bonded (EB) steel plates has gained universal acceptance. The disadvantage of this method, however, is the premature debonding of the externally bonded strip, which is brittle and an undesired mode of failure. It is also known that the debonding failure of EB steel plates prevents the RC beam from reaching its full strengthening capacity. This study aims to increase the scientific understanding of the behaviour of fully damaged RC beams strengthened/retrofitted in shear by means of EB steel plates. It also concentrates on preventing or delaying premature debonding of the adhesively bonded steel plates using new embedded adhesive and steel connectors. To achieve these objectives, seven beams with a deficient shear design were loaded monotonically up to maximum load capacities and repaired using the proposed connectors. One beam was used as a reference beam; the other six damaged beams were repaired with vertical and inclined steel plates. The presence/absence of the new embedded connectors was investigated with these two types of configurations. Theoretical models were also proposed to predict the effective debonding strain and the shear capacities of beam specimens. A comparison among these beams was done to investigate the efficiency of these connectors to enhance the bond strength between the externally bonded steel plates and the concrete surface on the web sides of the repaired RC beams. It is concluded that using steel and adhesive connectors to fix EB steel plates for repairing severely shear-damaged RC beams can delay the premature debonding failure and restore the original shear capacity of the these beams. Finally, the proposed model was satisfactorily verified through the experimental investigations and with the current design guidelines.

It is an effective way to improve the flexural behavior of reinforced concrete (RC) members by externally bonded carbon fiber reinforcement polymer (CFRP) laminates on the soffit of the members. However, there is little investigation on flexural performance of RC beam flexurally strengthened by side-bonded FRP laminates. To investigate the flexural behavior of RC beams side-bonded CFRP laminates and the difference of RC beams strengthened by soffit-bonded and side-bonded CFRP laminates, a total of 8 CFRP-strengthened beams and 1 control beam were tested. The experimental results show that: 1) the first crack loads of RC beams strengthened by side-bonded CFRP laminates are much higher than that of RC beams strengthened by soffit-bonded CFRP laminates. The first crack loads of side-bonded CFRP laminates beams improved significantly; 2) Side-bonded and soffit-bonded CFRP laminates have almost the same effect on the flexural stiffness of RC beams strengthened with same quantity of CFRP laminates before tension rebar yielding. However, side-bonded CFRP laminates can affect crack width and crack pattern of the strengthened beams, and the pre-crack stage of RC beam by sidebonded CFRP laminates extended remarkably. 3) different to soffit-bonded CFRP laminates RC beams, side-bonded CFRP laminates cannot improve the first yielding and the ultimate load bearing capacity of RC beams.

Design codes have specified the minimum shear reinforcement requirement for reinforced concrete (RC) and prestressed concrete (PSC) members to prevent brittle and premature shear failure. They are, however, very different from one another, and particularly, ACI318 code allows the required minimum shear reinforcement to be reduced in PSC members, compared to that in RC members, by specifying the additional equation for PSC members whose basis is not clear. In this paper, the minimum shear reinforcement ratio for PSC members was proposed, which can provide a sufficient reserved shear strength and deformation capacity. The proposed equation was also verified by the test results of PSC specimens lightly reinforced in shear, comparing to design codes and other proposed equations from previous studies.

Hysteretic behavior of RC shear walls strengthened with CFRP strips