Using conventional reinforced concrete has been the status quo for the past 110 years. It is what the majority of our existing and new structures are composed of. But conventional reinforced concrete contains steel that even after being protected by the cementitious material will undergo deterioration from inevitable cracks. The steel will continue to corrode, either it being through chloride ingress or rusting until intervention is needed. The surrounding concrete undergoes internal stresses, more cracking ,and spalling. This process can be mitigated by using Fiber Reinforced Polymer (FRP) as reinforcement in concrete.
The dissertation has aimed to investigate the behaviour between concrete reinforced with steel and FRP in singly reinforced beams.
The study consisted of 1 concrete beam reinforced with steel (SRC) and 4 concrete beams reinforced with Carbon Fiber Reinforced Polymers (CFRC). Unfortunately, due to instrumentation malfunction during the experiment, one beam’s data was unusable. The SANS 10100-1 was used to designed the SRC beam and ACI 440-1R-06 was used for the CFRC beams. All the beams were designed to the same moment capacity to ensure the only difference between the beams is their bottom reinforcement material. The mix design of the concrete was the same for the beams and was done according to Fultons. The 4-point flexural test was used to understand what type of failure will occur between the beams. The deflection of the beams was tracked by Linear Variable Differential Transformers (LVDT) in the centre of the beam. Strain gauges were placed on the tensile side of all the reinforcement bars to track the deformation during the experiment. Crack widths were measured at 90 kN for safety purposes. The development of cracks was marked on the beams for every 10 kN increase of load applied.
5.1. Failure Mode
The FRP reinforced concrete beams and steel reinforced concrete beam failed in the same way, concrete crushing. It was evident that the FRP reinforced beams exhibited extensive cracking and significant amount of deflection before failure. This is advantageous as it provides occupants clear indication of the structural member nearing to its absolute failure. It is understood that FRPs have a brittle failure but this failure would be reached after clear signs of possible failure occurring which would give occupants time to vacate the structure.
5.2. Deflection
The maximum deflection of FRP reinforced concrete beams is almost twice the amount of the steel reinforced concrete beam. The difference in values are because of the bond mechanism between the FRPs and the surrounding concrete, also, the lower stiffness of the CFRCs in comparison to the SRC. The FRP reinforced concrete beams pass the ACI long term deflection requirement. The over – reinforced CFRPs absorb more energy/load than the SRC before failure as show in Figure 49 which is beneficial as they will show pertinent deformation before failure.
5.3. Strain and Curvature
The FRP reinforced beams consumed around 62% of its ultimate tensile strain (1.7%) at failure and the steel reinforced concrete beam had consumed 90% of its ultimate yield strain (0.5%) at failure. This indicates towards steel reinforcement being more efficient as the material is closer to yielding at failure in comparison to FRP reinforced concrete beams. The FRP reinforced beams exhibit a much larger curvature than steel reinforced beams which aids in identifying the severity of failure the structural member has undergone. Such early indicators can help engineers identify what type of intervention is needed , if necessary.
5.4. Crack widths and patterns
The crack widths and crack density are much larger in FRP reinforced concrete beams than steel reinforced concrete beams. Crack widths are a major problem in steel reinforced concrete beams since it is where chloride ingress, carbonation, and moisture enter the concrete and corrode the steel reinforcement, this is not an issue with FRP reinforcement. The significant crack width of FRP reinforced concrete beams shows a weak bond between the reinforcement and concrete which puts it at a disadvantage.
This research shows that CFRP is capable of replacing steel in reinforced concrete. The case studies provide information on projects that have used CFRP reinforcement. The research cannot ascertain that CFRP can be used with different beam dimensions. Engineering practitioners have several standards that can be followed for the design process of structural elements. Design through software is limited as the standards are not yet incorporated into structural software. The placement and procurement of the FRPs are different as they are light-weight and cannot be deformed into different shapes after manufacture so the design process has to be very precise as the reinforcement cannot be adjusted on site.
5.5. Further Research and Recommendations
Here are some recommendations that are attributed to the observations, findings, and conclusions that could improve research in this subject area:
Acquiring more FRP reinforcement to have a more statistical robust conclusion on the performance of the material.
Due to CFRP variability, using a CFRP that has a similar Young’s Modulus to steel will negate the stiffness.
Investigate the bond strength between the FRP reinforcement and different cementitious materials.
Investigate the comparison between singly under-reinforced beams.
Perform a numerical assessment of ductility.
Investigate the space between major flexural cracks to numerically demonstrate the distribution of load
Use FRP reinforcement in both the top and bottom of the beams
Use more LVDTs to investigate the experimental curvature.