Load Bearing Capacity and Deflection Behavior

Bearing Capacity

Sample C2B0 was able to resist an ultimate load of 44.6 kN. The deflection at this point was 5.21 mm. Delamination of the CFRP laminates occurred at this load level. As the loading reduced from 44.6 kN to 23.49 kN, there was quite some fall in the load-bearing capacity. Contrary to this, the deflection remained more or less at the same point.

At this load point, the CFRP laminates still remained with some bit of bonding. Even so, the rise in the value of the load-bearing capacity did not go beyond the original value of 44.6 kN. This may be attributed to the fact that the CFRP laminates were still affixed to the concrete surface. Therefore, the CFRP laminates still have an effect on the general bearing capacity.

Also, it was discovered that the return of the sample to its initial bearing capacity took place over a considerable amount of deflection. This may be taken as artificial yielding.

The load decreased up to 34.76 kN, after which there was a sharp fall. The corresponding deflection recorded was 29.44 mm. When the deflection got to about 70 mm, the experiment was stopped. The load at this point was 19.78 kN.

Cracks development

The control sample developed cracks at a load level of 6.3 kN, at which point the deflection was 0.7 mm. Sample 2 was a bit different from the control sample. Cracks started appearing when the deflection was 1.8 mm and the load value was 17.32 kN. But even so, the cracks appeared at similar points. The cracks first appeared at mid-span, and thereafter at quarter span. It was noticed that cracks on sample 2 progressed at a slower pace when compared to those of the control sample. When the load was increased to 30.6 kN and the recorded deflection was 12.2 mm, the original cracks grew further and secondary cracks were formed. When the deflection was between the values of 14 mm and 15 mm, which corresponds to load values of 31.2 kN and 31.75 kN respectively, there was no further development of cracks. The existing cracks just got deeper. It was also observed that the top face of the concrete was somewhat stretching.

The increase in deflection continued at a predetermined rate of 0.3 mm per minute. When the 19.4 mm mark was reached, 4 major cracks were seen on the lower face of the sample. All these were situated between the quarter spans, or at exactly the quarter spans. They were also connected to the lateral cracks, though they extended to greater depths. The lateral cracks were many numerically, but they were not as deep as the 4 major cracks.

When the deflection value read 21.6 mm, the corresponding load was 33.94 kN. Beyond these values, it was seen that the cracks showed more on the end where the CFRP laminates remained affixed to the concrete surface. When the deflection reading got to 33.4 mm, the upper face of the slab developed a crack that was roughly aligned to the face which first experienced disconnection.

The disconnection of the CFRP laminates from the concrete surface made the sample not to fail at once. The first instance of failure took place when the deflection was 5.6 mm and the load level was 44.6 kN. At these values, both laminates disconnected from the concrete surface on the same side up to the midpoint.

It can be assumed that the concrete took over after the CFRP laminates failed. Furthermore, one can claim that the CFRP laminates stiffened the concrete due to the minor deflections exhibited before disconnection took place as compared to the control sample. The disconnection of the CFRP laminates went on as the experiment progressed.

The figures below show the extent of delamination when the failure of the CFRP laminates started and when the experiment came to a close. The extent is shown in red. It can be seen from the figures that the debonding of the laminates kicked off simultaneously. At the start of the debonding, nearly a fifth of the laminates were disconnected from the concrete surface. The other four-fifths still contributed to the general bearing capacity of the samples, as discussed above. By the end of the experiment, about 65% of delamination had occurred.

Sample 3 - C2B1

Sample 3 comprised of reinforced concrete and 2 externally bonded (EB) CFRP laminates. Each CFRP laminate was 50 mm wide and 900 mm long. An extra longitudinal NSM BFRP bar was included at the center of the slab.

Bearing capacity

Compared to the sample discussed above, the deflections for this sample are somewhat smaller at similar load levels. Failure begins when the deflection is between 6.3 mm and 7.8 mm. At this point, the load reads 44 kN. After the CFRP laminates detach from the concrete, deflection goes on increasing. Likewise, the load goes on increasing, but with some bit of opposition. At a deflection value of 23.3 mm, the bearing capacity goes beyond the original failure load when the detachment of the CFRP laminates started. The moment the BFRP bar fails, the load remains somewhat constant, but thereafter decreases up to the end of the experiment, at which the deflection value is 74.2 mm and the load is 14.8 kN.

Cracks development

A crack was first seen at the quarter span mark of the sample. The next crack was seen at mid-span. After this, another crack was seen at the other quarter span. The deflection at this point was 2.4 mm, and the load was 19.62 kN. The cracks developed slowly and did not get very deep. No secondary cracks were observed for quite some time.

The first secondary crack was spotted when the deflection value was 20.3 mm and the load value was 44.28 kN. At this point, the existing cracks had grown deeper. When the deflection value hit the 27.2 mm mark, major cracks were seen on the lower face of the slab.

When the deflection value was 31.7 mm and the load was 36.36 kN, a big crack developed on the upper face of the sample at quarter span. It can be recalled that a crack similar to this was observed in the previous sample roughly at the other quarter span.

Failure mode

For this sample, debonding took place in 2 stages. This is unlike the previous sample where the laminate detached in only 1 stage. When the deflection was 6mm and the load had hit the 44 kN mark, the first laminate detached on one side. When the deflection was 7.8 mm and the load was 43.4 kN, the other laminate detached on the other side of the sample. As the load decreased from 44 kN to 38 kN and rose again to 43.4 kN, there is an evident decrease in the bearing capacity of the system.

After the second laminate detached, there was a considerable decrease in the bearing capacity. The load greatly dropped to 30 kN, at which point the deflection was 8.59 mm.

When the deflection was 28.1 mm, the NSM BFRP bar got disconnected from its slot on the lower face of the concrete slab from one edge up to the midpoint. At this point, the load-bearing capacity value was 46.46 kN.

he above figures and photos illustrate the extent of delamination when failure of the CFRP started (left) and the extent of delamination when the detachment of the NSM BFRP started. The detachment of the whole sample took place in 3 phases. The first two phases involve the detachment of the 2 CFRP laminates. The third phase involves the detachment of the BFRP bar. At first, the laminates detached to about a quarter span and then continued detaching slowly up to the end of the experiment. As for the BFRP bar, the first detachment extended from the edge up to the mid-span. When this happened, the delamination of the CFRP laminates had progressed to about 70%.

Sample 4 comprised of the following: i) reinforced concrete and ii) 2 externally bounded (EB) CFRP laminates each measuring 50 mm width by 900 mm length.

Bearing capacity

The load increased linearly in correspondence to the increase in deflection. This progressed up to a maximum load-bearing capacity of 46.94 kN, at which point the deflection was 6.3 mm. After the 2 CFRP laminates failed, there was a sudden decrease in the bearing capacity, i.e. up to a low of 28.16 kN, at which point the deflection value was 8mm. From this point onwards, the load-bearing capacity went on rising until the first NSM BFRP bar started detaching. At this point, the bearing capacity decreased a bit from 50.58 kN to 40.40 kN. The corresponding deflection values were 32 mm and 33 mm respectively. When the second BFRP bar detached, the bearing capacity also decreased from 40.90 kN to 29.62 kN. After this point, the load-bearing capacity remained constant until the deflection was approximately 42.5 mm. After this point, the load-bearing capacity went on dropping.

Cracks development

Cracks were first seen on both sides of the mid-span mark of the sample. This was at a load value of 21 kN and a deflection value of 2.1 mm. The cracks developed at a relatively slower pace. Most of the cracks developed when the load was between 36 kN and 46 kN. The corresponding deflection values were 4.8 mm and 6.3 mm respectively. The first CFRP laminate detached at a load value of 46.94 kN. Before the CFRP laminates failed, the cracks did not widen and deepen much. After the CFRP laminates detached, the cracks started progressing faster. Secondary cracks were also formed.

Failure mode

The 2 CFRP laminates detached when the deflection was at 6.3 mm and the load value at 46.94 kN. When the laminates detached up to the mid-span mark, the deflection was 15.8 mm and the load value was 41.70 kN. When the deflection increased to 32 mm, the first BFRP bar detached. The bearing capacity at this point was 50.58 kN. When the deflection value rose to 33.6 mm, the second BFRP bar detached. The load value at this point was 40.90 kN. Beyond this point, the load went on dropping.