Structural Behaviour of Polystyrene Foam Lightweight Concrete Beams Strengthened with FRP Laminates – International Journal of Concrete Structures and Materials

Cracking Loads, Failure Loads, and Crack Pattern

Table 4 presents the failure load and cracking load for shear and flexural cracks. As expected, all the beams detailed as shear deficient were failed in shear before the flexural capacity was reached. While beams in Flexural group were failed in flexure after attaining their capacity. There was no slippage of flexural reinforcement during the testing. As shown in Table 4, the failure load is higher in Shear group and its corresponding control as compared to the failure load in Flexure group and in its control. This is due to the shear span to depth ratio which is smaller in shear deficient beams. It can also be noticed from Table 4 that strengthening for shear using CFRP resulted in higher loads compared to those of GFRP laminates. In addition, increasing the width of FRP strips for GFRP laminates is more significant than that for CFRP in increasing the failure loads. This is due to the better bonding of GFRP which plays an important role when sufficient width of laminate is provided.

Response of Shear Dominant Specimens

For Group 1, as mentioned in Table 4, the failure load, first shear cracking, and flexural cracking loads for beam BGS1 having 30 mm width of GFRP strip, were higher than those of the control specimen CBS by 13.7%, 57.1%, and 90%, respectively. Increasing the strip width to 50 mm (BGS2), resulted in raising the failure load, first shear cracking, and flexural cracking loads over those of the control specimen CBS by 25.7%, 100%, and 110%, respectively. Similarly, Beam BGS3 having 100 mm width of GFRP strip, increased these loads by 37%, 136%, and 140% as compared to control.

For Group 2, as mentioned in Table 4, the failure load, first shear cracking, and flexural cracking loads for beam BCS1 having 30 mm width of CFRP strip, were higher than those of the control specimen CBS by 20%, 81.4%, and 120%, respectively. Increasing strips’ widths to 50 mm (BCS2), resulted in raising the failure load, first shear cracking, and flexural cracking loads over those of the control specimen CBS by 29%, 128.6%, and 150%, respectively. Similarly, Beam BCS3 having 100 mm width of CFRP strip, increased these loads by 50%, 171.4%, and 200%, respectively. The increase in the width of strip of GFRP and CFRP played a dominant role in improving the loading capacity. Shear causes diagonal tension perpendicular to the direction of diagonal crack and increase in the width with fixed length enhanced the tensile capacity of GFRP and CFRP. Therefore, the results are incoherent with the response and propagation of diagonal crack.

Response of Flexural Dominant Specimens

For Group 3, as mentioned in Table 4, the failure load, first shear, and first flexural cracking loads for beam BGF1 having one-layer of GFRP, were higher than those of control specimen CBF by 11.5%, 5.3%, and 0.7%, respectively. Increasing the number of GFRP layers to two (BGF2), resulted in raising the failure load, first shear cracking, and flexural cracking loads over those of the control specimen CBF by 27%, 26.3%, and 19.3%, respectively. Similarly, Beam BGF3 having three layers of GFRP, increased these loads by 50%, 63.2%, and 48%, as compared to control.

For Group 4, as mentioned in Table 4, the failure load, first shear, and first flexural cracking loads for beam BCF1 having one-layer of CFRP, were higher than those of control specimen CBF by 26.2%, 10.5%, and 6.9%, respectively. Increasing the number of CFRP layers to two (BCF2), resulted in raising the failure load, first shear cracking, and flexural cracking loads over those of the control specimen CBF by 50.5%, 36.8%, and 34.5%, respectively. A further increase of CFRP layers to three (BCF3), increased these loads by 71.5%, 105.3, and 86.2%, as compared to control.

The gain in load carrying capacity after cracking is evident from the above discussion and at the same time there is a deflection-hardening response. Thus, loading capacity along with ductility is enhanced by the strengthening of LWC beam through GFRP and CFRP.

Crack Pattern

The crack pattern was marked to provide the necessary information for defining the failure mechanism of each specimen, as shown in Fig. 9. For beams strengthened for shear, the first diagonal crack suddenly developed at the mid-depth within the shear span. Diagonal cracks were observed parallel to the compression strut, and they propagated toward the loading region and supports (see Fig. 9). For all flexural specimens, the flexural cracks initiated on the tension side in the mid span of the beam, and the cracks propagated upward with increasing load. All beams strengthened for flexure exhibited flexural failure with pealing out of bottom FRP layers in the specimens BGF1, BGF2, and BCF2, as shown in Fig. 9. This is similar to the study conducted in which pealing out of layers of CFRP in some of their NWC concrete specimens strengthened with CFRP laminates for flexure was observed (Valivonis & Skuturna, 2007). However, the loading capacity and deflection-hardening response was observed in all beams strengthened for flexural failure. This infers that the peeling out of FRP layers do not hinder in attaining the ductile response of LWC beam strengthened through FRP.

Crack patterns of all beam specimens.

Load–Deflection Response

The load–deflection curves for all the beams are shown in Fig. 10. The load–deflection was approximately linear from zero-load to crack initiation in all the beams. The large reduction in stiffness caused by excessive cracking resulted in a relatively large increase in the deflection values. Closing to the failure load, the deflection continued to increase, even when the applied load was constant.

Load–deflection curves for all beam specimens.

Fig. 10 shows that the stiffness and load carrying capacity was increased by increasing the width of FRP strips for shear strengthening or increasing the number of FRP layers for flexural strengthening. Beam specimens BGS1, BGS2 and BGS3 were strengthened via surface attachment of U-shaped GFRP laminates with widths of 30, 50, and 100 mm, respectively. Fig. 10a shows that the load carrying capacity of specimens BGS1, BGS2 and BGS3 were higher than that of CBS control specimen; however, the deflection was lesser at the same load level for beams BGS1, BGS2 and BGS3 by approximately 11%, 18% and 28%, respectively. Beam specimens BCS1, BCS2 and BCS3 were strengthened via surface attachment of U-shaped CFRP laminates with widths of 30, 50, and 100 mm, respectively. Fig. 10a shows that the load carrying capacity of specimens BGS1, BGS2 and BGS3 were higher than that of CBS control specimen; however, the deflection was lesser at the same load level for beams BCS1, BCS2 and BCS3 by approximately 18%, 35% and 40%, respectively. It can be observed that there is an improvement in stiffness as a result of increasing the FRP strip width from 30 to 100 mm regardless the type of FRP. However, the effect of increasing the width of CFRP strengthening strips on the stiffness of the studied beams is slightly higher than that for GFRP strips. To take maximum advantage of FRP strengthening, it is recommended to employ the maximum width of FRP for strengthening LWC beams for shear.

Fig. 10b shows that beam specimens BGF1, BGF2, and BGF3 were strengthened by attaching one, two and three layers of GFRP laminates to the bottom surface of each specimen. The load carrying capacity of specimens of BGF1, BGF2 and BGF3 specimens were higher than that of CBF control specimen; however, the deflection was lesser at the same load level for beams BGF1, BGF2 and BGF3 by approximately 18%, 33% and 48%, respectively. Beam specimens BCF1, BCF2 and BCF3 were strengthened by attaching one, two and three layers of CFRP laminates to the bottom surface of each specimen. The load carrying capacity of specimens of BCF1, BCF2 and BCF3 specimens were higher than that of CBF control specimen; however, the deflection was lesser at the same load level for beams BCF1, BCF2 and BCF3 by approximately 30%, 40% and 52%, respectively. Based on these results, it is inferred that there is an improvement in stiffness as a result of increasing the FRP strengthening layers. However, the effect of increasing the number of CFRP layers on the stiffness is more significant than that for GFRP layers. As a result, this strengthening technique reduces or eliminates the rate of crack formation, delays initial cracking, reduces stiffness degradation with residual deflection, and extends the fatigue life of LWC beams. CFRP is the greatest alternative for strengthening LWC beams.

Crack Width

The crack width was measured using LVDTs, as shown in Fig. 11. By comparing the crack widths of the tested beams at the same load level, it was observed that the crack width was decreased with increasing the width of strips or the number of strengthening layers.

Load–crack width curves for all beam specimens.

For Shear strengthened beams, the crack widths of beam specimens BGS1, BGS2 and BGS3 were less than that of CBS control specimen at the same load by approximately 26%, 38% and 45%, respectively. Similarly, the crack widths of BCS1, BCS2 and BCS3 specimens were less than that of CBS control specimen at the same load level by approximately 32%, 51% and 58%, respectively. For Flexural strengthened beams, the crack widths of BGF1, BGF2 and BGF3 specimens were less than that of CBF control specimen at the same load level by approximately 9%, 18% and 37%, respectively. Similarly, the crack widths of BCF1, BCF2 and BCF3 specimens were less than that of CBF control specimen at the same load level by approximately 24%, 35% and 48%, respectively.

These results also show that the crack width was decreased due to the increase in the overall beam stiffness as a result of increasing the width of FRP strips or increasing the number of FRP strengthening layers. It can also be observed that Shear strengthened beams with CFRP laminates generally had less crack width as compared to the beams strengthened using GFRP laminates. The crack width was almost zero in the elastic range of LWC beams strengthened through FRP, as shown in Fig. 11, this is important as reducing the crack width also limit the exposure of reinforcement to the deleterious substances, such as chloride and sulphates. The reduction in crack width is also depending on the steel strain which is directly proportional with the crack width. The effect on the steel strain through FRP a is discussed as under:

Steel Strain

Fig. 12 shows the strain at steel level measured through electrical strain gauges mounted on the beam longitudinal reinforcement and stirrups.

Load–steel strain curves for all specimens.

Strain at Longitudinal Reinforcement Level

Fig. 12a shows the load vs strain at longitudinal steel level for shear strengthened beams. It is evident that the strain was increased after strengthening and it depends upon the widths of strengthening strips. The strains at failure were below the strain at yielding point of steel. This shows that the shear strengthened beams were failed on the higher load, but the mode of failure was compression rather than yielding. Strengthening with CFRP has a higher effect on the steel strains compared with strengthening with GFRP. In addition, the effect of increasing the widths of CFRP strengthening strips on the longitudinal steel strains was more significant than for GFRP strips. Fig. 12b shows the load vs strain at longitudinal steel level for flexural strengthened beams. It is evident that the strain was increased after strengthening and it depends upon the number of layers of laminates. The strains at failure were higher than the strain at yielding point of steel. This shows that the flexural strengthened beams were failed on the higher load with the mode of failure was tension. Strengthening with CFRP has a higher effect on the steel strains compared with strengthening with GFRP. In addition, the effect of increasing the number of CFRP layers on the longitudinal steel strains was more significant than for GFRP layers.

Based on the preceding discussion, it is clear that FRP strengthening does not change the mode of failure; rather, the increased strain, number of fractures, and loading capacity indicate that LWC beams are exhibiting symptoms prior to failure. This is especially crucial for shear deficient LWC beams to show signs of failure before approaching the brittle failure phase.

Strain at Stirrups Level

Fig. 13 shows the steel strains in the stirrups of the tested beams at the same load level. The steel strains in the stirrups dropped when the widths of strengthening FRP strips or the number of strengthening FRP layers increased, as shown in the figure.

Load–Stirrups steel strain curves for all specimens.

Fig. 13a demonstrates that the stirrup steel strains of BGS1, BGS2, and BGS3 specimens were smaller than that of CBS control specimens at the same load level by approximately 7%, 11%, and 18%, respectively, for beams strengthened for shear. Furthermore, at the same load level, the steel strains of BCS1, BCS2, and BCS3 specimens were roughly 46%, 65%, and 71% lower than the CBS control specimen. It’s worth noting that the reduction in stirrup strain caused by CFRP strip stiffening is nearly 7 times more than that caused by GFRP strip stiffening. For GFRP laminates; however, increasing the strip width from 30 to 100 mm had a greater impact.

Fig. 13b shows that the steel strains of the stirrups of BGF1, BGF2, and BGF3 specimens were lower than those of the CBF control specimen at the same load level by about 16%, 25%, and 43%, respectively, for beams strengthened for flexure. The steel strains of the BCF1, BCF2, and BCF3 stirrups, on the other hand, were roughly 36%, 65%, and 84% lower than the CBF control specimen at the same load level. The difference between the two FRP types on the stirrup’s strains is less than the difference between the other examined parameters in the preceding sections, as can be seen from the above values. It may conclude that the number of layers is more effective for GFRP laminates as compared to CFRP. However, all beams strengthened through GFRP and CFRP demonstrate deflection-hardening, reduction in crack width, and longitudinal steel strains.