Advancements in FRP Concrete

Abstract

Fiber Reinforced Polymer is fast gaining popularity in lieu of steel bars. The construction industry is now moving from steel-reinforced concrete to FRP concrete. One advantage of FRP is improved structural strength and high resistance against corrosion. However, FRP concrete is not as ductile as Steel-reinforced concrete. To amend this, it may be worthwhile to have a hybrid reinforcement system, which is predicted to improve the ductility of FRP concrete. This research seeks to assess the structural performance of hybrid concrete beams that are reinforced with steel and Basalt FRP rods. To reach a meaningful conclusion, experiments were set up using concrete beams reinforced with BFRP and others reinforced with both steel and BFRP. The ratio of BFRP was varied in different experimental setups for proper comparison. A lesser degree of deformation was exhibited in the setup where 2 BFRPs and 1 Steel bar were used when compared to a concrete beam fully reinforced with steel. It was also noticed that the load capacity of the concrete beam with 2BFRPs and 1 steel beam was higher than that of the normal steel-reinforced concrete beam. The hybrid beam recorded a load capacity two times that of the ordinary concrete beam. The only drawback of the hybrid beam was high deformability, but this can be corrected by using 2 instead of 1 steel bar for the hybrid beam. In the event of failure, the hybrid beam exhibits a sudden increase in its load capacity, which makes the failure process to proceed at a much slower pace than what is seen in ordinary concrete beams.

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Description of test beams

Different concrete beams were used for this study. The beams under study were reinforced with 2 BFRPs and 1 Steel bar. For control purposes, the ordinary steel-reinforced concrete beams were used. The dimensions for the beams under test were 2440 mm long by 200 mm deep by 125 mm wide. The links for shear reinforcement were of diameter 6mm. Concrete of class 35 was used. The BFRPs and Steel bars used both had a diameter of 6 mm.

In total, 72 high yield steel bars were used for the shear links, each with a length of 450 mm. These were bent into rectangles of 150 mm by 75 mm. This ensured a concrete cover of about 25 mm on all sides. For the longitudinal reinforcement bars, both steel bars and BFRP bars were used, each having a length of 2390 mm. In total, 14 steel bars and 6 BFRP bars were used. Different combinations of steel bars and BRFP bars were used for the longitudinal reinforcement. For the transverse reinforcement, the shear links were spaced at 100 mm c/c. Other combinations had the following configurations:

i) 2 steel bars for tension reinforcement and 3 steel bars for compression reinforcement (denoted 3S Beam)

ii) 2 steel bar for tension reinforcement and 3 BFRP bars for compression reinforcement (denoted 3B Beam)

iii) 2 steel bars for tension reinforcement and a combination of 2 steel bars and 1 BFRP for compression reinforcement (denoted 2S and 1B beam)

iv) 2 steel bars for tension reinforcement and a combination of 1 steel bar and 2 BFRP bars for compression reinforcement (denoted 1S and 2B Beam)

For the above 4 configurations, the links were spaced at 300 mm c/c.

To evaluate the yield behavior at various points along the beam, additional reinforcement bars were added as follows to the bottom side of the beam.

i) At the center of the beam

ii) 100 mm from the end

iii) At quarter points along the beam

The strain gauge used was of great precision so that proper comparison would be made regarding the deflection of the beams.

Experimental setup for the test

Beams of dimensions 150 mm by 200 mm by 2440 mm were used. A static load was then applied at the rate of 0.5mm per minute. The deflection of the beam was measured at the following points:

i) At the edges - denoted LVDT 1 and LVDT 7

ii) At the supports - denoted LVDT 2 and LVDT 6

iii) At quarter span - denoted LVDT 3 and LVDT 5

iv) At the center - denoted LVDT4

For proper measurement of the deflections, strain gauges were fixed at 9 locations along the beam. A number of them were installed on the upper side of the concrete beam, while some were installed on the lower side of the concrete beam. The strain gauges were well spread along the beam, making sure that the end spans, mid-spans, and quarter spans are considered. The strain gauge measurements were then analyzed by the use of computer software.

Results and Data Analysis

Compressive strength of concrete

It was seen that the compressive strength of concrete for the different systems was almost similar.

Ultimate load capacity

The values of tensile strength for the different systems differ considerably. Similarly, the ultimate load capacity for the two systems is significantly different. Below is a bar chart showing this finding.

Load definition at midspan

It was found out that the mid-span deflection for the steel-reinforced beam was greater than that of the BFRP reinforced beam. It is also a fact that high yield steel bars are more ductile than BFRP rods. In regards to the deflection recorded at the destructive phase (where span/100 =20 mm), it is correct to say that BFRP Reinforced beams, as well as hybrid beams with 25% steel, have a higher load-carrying capacity than steel-reinforced beams.

Inclusion of the steel bars in the hybrid reinforced beams added some degree of ductility.

Load deflection at quarter-span

The figure clearly shows that the hybrid beams have a higher load-bearing capacity than the BFRP reinforced beams. Also, hybrid beams with BFRP rods fixed at the center have a higher load-bearing capacity. The beam deflected by only 14 mm. Beyond the 14 mm quarter span deflection, beams reinforced with BFRP alone exhibit the domination property on the load, and this value corresponds to the degree of deflection. In cases where the beam deflects more than 40mm, it becomes very hard to evaluate the characteristics of the beam. The first beam is expected to exhibit the same behavior as the fourth beam.

Displacement of the neutral axis from the bottom surface

The different beams in the experiment exhibit different modes of movement of the neutral axis. This can be attributed to the differences in their midspan deflections. Below is an illustration showing the difference in the movement of the neutral axis.

Strain for reinforcement bars at mid-span

The different beams in the experimental setup make use of different types of reinforcement bars, different amounts of reinforcement bars, as well as different amounts of concrete. Such differences in the beams lead to differences in the value of strain.

Conclusion

High yield steel bars have for long been used by engineers for reinforcement of concrete structural elements such as slabs, beams, and columns. This practice has of late been associated with high costs due to inflation of material prices. One alternative that his been pinpointed is the manmade fiber-reinforced polymer. The adoption of BFRPs is seen to be more beneficial as compared to the use of steel-reinforced concrete. However, one downside with the FRP is its brittle nature and the inability to fail gradually. The following conclusions can be drawn from the experimental results and analysis:

As long as the load does not exceed 7 km, the level of deformability remains the same for all the beams.

When loading goes beyond 14 km, the steel-reinforced beams are more effective.

In regards to deflection, beams with a higher percentage of high yield steel bars exhibit less deflection than those with a smaller amount of reinforcement.

When the loading is between 7kN and 14kN, the beams with a bigger amount of steel reinforcement exhibit less deformation.

Pure BFRP beams exhibit a deflection (14.7 mm) that is almost four times that of the steel-reinforced beam (4.5 mm).

The pure BFRP beams exhibit the largest ultimate load (41 kN)

The concrete beams reinforced with 2 BFRP and 1 Steel show a small degree of deformation

When the reinforcement is broken at quarter-span for the beam with 1 BFRP and 2 steel bars, the bearing capacity reduces to 9 kN.

The beams reinforced purely with BFRP fail as a result of concrete crushing.

The hybrid beams exhibit gradual destruction, as opposed to beams with FRP alone.

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Chapter 4 - Data and Results

Bearing Capacity, crack analysis, failure mode

Sample 1 (Control) - c0b0

For control sample 1, only high yield steel bars were used for reinforcement. No FRP was added to the beam.

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Bearing capacity

The load levels went up in a nearly linear fashion up to 27.42 kN. At this load level, the deflection was recorded as 8.5 mm. Beyond the 27.42 kN mark, load levels exhibited very small increments, and thus the graph formed a plateau-like ending. This can be attributed to the internal yielding of steel. The peak value of the load achieved was 34.56 kN. The deflection at this point was 36.3 mm. Beyond this mark, the load levels varied unpredictably, after which they started decreasing. This is actually the failure phase of the beam. Yielding of the steel took place over quite a large range of deflections - from 8.5 mm to 36.6 mm. Beams strengthened with FRP should ideally display such a ductile mode of failure.

Cracks development

The very initial crack was observed at the center of the beam. The deflection at this point was 0.7 mm. The load at this point was 6.3 kN. At the quarter position along the length of the beam, the first crack was seen when the load hit the 8 kN mark and the deflection recorded was 1.4 mm. The cracks went on widening until the load reached the 18.6 kN mark, beyond which the crack fully split. As more load was added, fresh cracks started appearing when the load was about 21 kN, and they widened until the load reached 25 kN. At the same time, the cracks at the middle section along the length of the beam continued growing bigger and bigger until the front and rear faces of the beam were connected with a huge crack. This occurred precisely at a load of 27.42 kN. As the load increased, the cracks at both mid and quarter span continued deepening and widening. Upon reaching the 31 kN load mark, secondary cracks started showing on the beam. The deflection at mid-span at this load level had gone up to 15.1 mm. As the deflection increased to about 16.1 mm to 16.5 mm, no deepening and widening of cracks were witnessed. The number of cracks was defined and the beam showed no sign of failure. The load was increased until the deflection recorded hit the 70 mm mark. The beam was by then showing signs of concrete crushing on the top surface. However, it still did not collapse.

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