Braided Hybrid Composites for Bridge Repair
Frank K. Ko
Goal:
The primary outcome of this research will be textile production technology for one of the largest material use industries in the projected future. The potential growth provided by the success of this project is estimated to be more than 1 million pounds of newly developed textile materials. In addition to creating an innovative manufacturing process, a graduate student will be trained in this area and technology will be transferred to the participating industries.
Abstract:
Hybrid fiber reinforced rebars are attractive for two primary reasons: they resist corrosion and they reduce weight. Both of these features result in lower lifetime cost for bridge construction, maintenance, and repair.
In this research, a continuous production methodology has been developed to produce suitable reinforcing elements for rebar applications in a continuous fashion. Issues of scale-up in production , testing and modelling are the focus of this research.
Thus far, production scaleup and process improvement has been realized. Initial testing has been performed to evaluate the materials and modelling schemes are under development and improvement.
Background
Composite rebars are under development for use in bridge repair and construction. Notably, these rebars are designed to allow graceful failure when ultimate load or extension has been reached. In traditional metallic rebars, the steel will reach a yield point and lose modulus while extending. In the current composite rebars (typically fiberglass reinforced epoxy) when the reinforcing element reaches a critical strain level the bar ruptures in two. This type of catastrophic failure is unacceptable for civil constructions because there is no opportunity to avoid critical and catastrophic failure of the entire bridge structure.
An ideal reinforcing material will both fail gracefully and resist corrosion. Thus the hybrid fiber construction is established.
The central concept behind this development is the use of a braided construction in a sheath-core combination. The core should be a high modulus, low strain to failure material whereas the sheath will be a high extension, high strength material. In this research, P-55 carbon yarns have been used for the core and Kevlar 49 has been used for the sheath.
In particular, a 24 carrier braiding machine was employed to form the structure. The braiding yarns were 3 ply 1,140 denier Kevlar 49, except one of the bobbins was loaded with a 15 ply 1,140 denier Kevlar 49. This large bundle is to create a rib in the braid for mechanical bonding between the resulting composite rebar and the concrete. The design of this rib is to be similar in concept to the current steel rebars. In a previous study, ribs were formed similarly, but braided at 20° braid angle. It was seen that the bonding between the rib and concretae was not good enough, so the direction of this project has directed that 30° braid angles be employed.
The process takes the braided fabric through a forming ring, and runs the braid through an infusion zone wherein epoxy resin is dripped onto the fell of the cloth. The wet fabric is then run through a heated chamber to cure the resin. The fabric has a 30 minute dwell time before being collected. The resin was selected to completely cure within 30 minutes. This process is schematically illustrated in Figure 1.
Figure 1. Schematic illustration of rebar formation process.
In the original process, it was observed that significant bubbling occurred during the infusion process. To address this, an additional infusion aid was developed to assist in pushing the resin into the braided fabric and simultaneously remove excess resin from the surface of the braid. The additional device was developed and inserted into the infusion zone. The parts formed in this way showed remarkably better surface appearance than before. Figure 2 is a picture of the braided rebar before the infusion assistance device, and Figure 3 shows the braided rebar formed with the device.
Figure 2. Braided rebar formed before resin infusion process.
Figure 3. Braided rebar formed with infusion assistance device.
Further, it was found that applying the infusion assistance device resulting in superior mechanical properties and more repeatable mechanical response. As shown in Figure 4, the stress-strain response of these rebars was rather repeatable and showed a high effective ductility behavior.
Figure 4. Mechanical properties of braided hybrid rebars formed with (S-1 through S-7) and without (NS-1 and NS-2) infusion assistance .
Comparing the designed structure with the current metal devices, the mechanical properties are shown in Figure 5. This data is for 5 mm rebars. As can be seen the steel rebar demonstrates a higher tensile modulus and higher yield strength in terms of stress per unit area - resistance based on the physical size of the part. The composite rebar curves shown in this plot are typical curves from the master set shown in Figure 4. Again it is clear that the infusion device has dramatically changed the mechanical response of the composite rebar.
Figure 5. Comparison of Stress-strain response of steel and composite rebar.
If the materials are compared on the basis of weight, there is a difference in evaluation. Considering 1040 steel to have a density of 7.85 g/cm3 and the composite rebar to have a density of approximately 1.53 g/cm3, it can be seen that it takes almost three times as much composite to equal the weight of the steel. Changing the stress units to force per area per density, or the weight of rebar required to resist load, Figure 6 illustrates the weight based comparison.
Figure 6. Comparison of Composite and Steel Rebars on weight basis
Materials were also produced with 20° braid angles in a previous study. These were similarly formed with P-55 cores and Kevlar 49 sheath. Extracting average mechanical properties from the two data sets and comparing, it is not surprising that the 20° braids had higher tensile strengths (see Figure 7). The lower angle braiding yarns make a higher contribution to the load bearing ability of the rebar.
Figure 7. Yield and maximum load for 20° and 30° braided composite rebars.
Figure 8 shows a comparison of the yield and maximum strains for the two different rebar systems. As can be seen, the 30° braided part had higher strain to failure and approximately equal strain to yield. This again is expected because the yielding occurs as a result of failure of the longitudinal P-55 yarns, which should be approximately independent of the braided yarn configuration - the only contribution the braided yarns make is in inducing crimp in the core. The ultimate strain is dictacted by the sheath yarns which suggest a higher strain to failure for a higher braid angle.
Figure 8. Comparison of tensile strain to yield and failure for 20° and 30° braided rebar.
Comparing tensile modulus, as shown in Figure 9, there is a remarkably higher tensile modulus for the 30° braid. This is unexpected. The higher braid angle should produce a lower tensile modulus because of the orientation of the sheath yarns. The explanation is found in the infusion assistance device. The 20° braids were formed before this device was developed and subsequently there is a poor bonding between fiber and resin and there are many bubbles and pores within the structure. This poor bonding allows reorientation of the braiding yarns during loading which results in lower tensile modulus. If the 20° braids had been made properly, they would show a high tensile modulus than the 30° braids.
Figure
9. Comparison of tensile modulus for 20° and 30°
braids.