Prefabrication is one approach to construction in which building components are manufactured in a controlled environment, transported to a construction site, and assembled to form buildings.
Recent studies have shown that prefabricated residential construction offers a 20.5% reduction in total energy consumption, a 35.8% reduction in resource depletion, a 6.6% reduction in health impact, and a 3.5% reduction in ecosystem damage. The performance of prefabricated buildings and building materials can be further improved by employing fiber-reinforced polymer (FRP) reinforcement.
Prefabrication is an increasingly popular alternative to traditional construction processes, in which raw materials such as iron or steel, timber, and concrete are transported to a building site and fabricated on-site. In addition to being safer and more environmentally friendly, off-site manufacturing of building components enables greater process control and consistency. This ultimately leads to greater efficiency during the construction process and to safer, sturdier finished buildings.
The use of FRP in the construction industry is increasingly common due to its superior properties, including a high strength-to-weight ratio and ease of use. FRPs are useful in structural elements such as beams, columns, and slabs, as well as in non-structural or decorative elements.
FRP bars are also often used in place of steel rebar in concrete structures, as substitutes for steel plates in concrete beams, and as connectors joining precast concrete sandwich (PCS) walls. As with all construction materials, it’s important to understand how FRP's properties will affect the final structure.
Here are a few things you should know when considering FRP in prefabricated construction projects.
Concrete construction members, such as beams, are typically made of concrete reinforced with steel rebar. Concrete provides resistance to compressive loading, while the rebar resists and distributes shear and tensile loads. However, steel rebar suffers from poor corrosion resistance due to environmental exposure.
This leads to a reduced in-service lifetime and an overall decrease in critical physical properties, including yield strength, flexural strength, and modulus of elasticity. Corrosion of steel rebar ultimately reduces the structure's lifespan, increases repair/retrofitting costs, and can affect structural integrity.
Glass-reinforced FRP (or GFRP) is recognized as having higher tensile strength than steel rebar. GFRP bars also exhibit impressive corrosion resistance to environmental moisture, salt, and alkali in concrete. This combination of properties has proven particularly beneficial in marine infrastructure projects like jetty, dock, and bridge construction.
In one study, marine structures in Australia and Canada showed no corrosion after up to 8 years of exposure to extreme temperatures ranging from -35 to +35°C (-31 to 95°F) and to both natural marine chloride and deicing salts. The matrix polymer in all sampled GFRPs was intact and unaltered from its original state. FTIR and DSC analysis showed that hydrolysis had not occurred, and there was no significant change in the transition temperature of the matrix material. Similar results were observed for a selection of thermosetting resins, including epoxy, polyester, and vinylester.
A direct comparison between concrete forms reinforced with steel rebar and GFRP bars further demonstrates the value of FRP’s exceptional corrosion resistance. After submersion in water for 28 days, the GFRP-reinforced samples exhibited a tensile strength about 13% higher than that of the steel rebar. Yield strain was about 58% higher for GFRP.
A more qualitative analysis showed that GFRP-reinforced concrete exhibited better fracturing behavior than either steel-reinforced or unreinforced concrete. Although brittle fracture occurred in all materials, the GFRP reinforced concrete did not undergo complete fragmentation. In other words, although fracture lines appeared, the material remained in one piece.
FRP bars offer an alternative to steel rebar that is easy to handle and install, maintenance-free, and highly corrosion-resistant. These characteristics make FRP bars an attractive material for use in prefabrication operations. And the lightweight of FRP bars relative to steel means that prefabricated building components will be less expensive to transport to the construction site or to your customers.
Concrete beams and slabs used in prefabricated construction are frequently bonded to steel plates to improve strength and stiffness. These improvements come at a cost, though. Steel reinforcements tend to detach prematurely from the concrete form before reaching their design strength. This detachment, also known as debonding, is a peeling action caused by concentrated mechanical stress at the plate ends. This commonly occurs in beams with side plates, angle plates, and tension face plates.
FRP reinforced with both carbon fiber (CFRP) and glass fiber (GFRP) is increasingly being selected as an alternative to steel plates. FRP offers a wide range of benefits, including a high strength-to-weight ratio and corrosion resistance, as discussed above, as well as the ability to achieve a high degree of customization. The peeling mechanism in FRP-plated beams is similar to that of steel-plated beams, despite the lower elastic modulus of the reinforcing FRP plates. Despite occurring through a similar mechanism, FRP offers significant improvements in debonding behavior.
FRP-plated beams are much less likely to debond as a result of flexural peeling than steel-plated beams. FRP reinforcements also show improved shear peeling behavior. The flexibility of FRP allows shear stirrups to carry shear load effectively after web shear cracks begin to form. In contrast, the stiffer steel reinforcements could not transfer shear load to the stirrups and debonded from the concrete beams.
The unique balance of strength and flexibility offered by CFRP and GFRP reinforcements results in a significant reduction in debonding. This makes them an ideal substitute for traditional steel plates in prefabricated concrete beams and other construction members.
In a perfect world, we wouldn’t need to worry about the thermal behavior of construction materials at extremely high temperatures. But structural fires are a reality, and we need to take that into account when selecting building materials. FRP plates and bars are used as internal reinforcement in concrete members for structures such as parking garages, multi-story residential buildings, and industrial facilities, where fire resistance is a major design requirement.
In one study, GFRP and CFRP bars reinforced with vinylester resin were placed in a kiln and loaded hydraulically to simulate conditions that might be encountered in a structure fire. Both GFRP and CFRP retained a very high level of their original stiffness up to 350°C (662°F). The tensile stress and modulus of both GFRP and CFRP are comparable to those of steel up to this temperature. GFRP exhibited somewhat better behavior than CFRP at temperatures above 200°C. The tensile modulus of GFRP remained fairly consistent up to 400°C, retaining about 90% of its ambient temperature value.
Although the decrease in CFRP properties is greater, this can be explained by the fact that CFRP initially shows a higher degree of strength at ambient temperature. At elevated temperatures, the resin becomes more influential than the reinforcing fiber, so in practice, both CFRP and GFRP exhibit similar strength.
At temperatures that would be encountered in a structure fire, the failure strength of FRP bars decreases at an almost linear rate until reaching zero at about 500°C, while elastic modulus remains almost unchanged until 300-400°C. The failure of FRP reinforcements ultimately depends on the thermal decomposition temperature of the resin used to bond the reinforcing fibers together. This highlights the importance of selecting a matrix resin with an appropriate degree of fire resistance. Luckily, this property can be adjusted to some degree by incorporating flame-retardant additives into the resin before or during pultrusion.
The fire resistance of FRPs has been tested in at least one extreme real-world example: utility poles survived a full-scale forest fire with no compromise in structural integrity. Although their fire resistance properties are not as robust as steel's, FRP reinforcements retain their properties impressively even at extreme temperatures.
When balanced with other impressive properties, such as resistance to corrosion and debonding, the fire resistance profile of FRP makes it well-suited for prefabrication applications across a wide range of structures where fire resistance is a design requirement.
Prefabrication construction offers an environmentally friendly and cost-effective alternative to traditional construction techniques. Environmentally friendly FRP reinforcements are ideally suited for use in prefab structural members. If you’re considering reinforced thermoset materials like FRP for your next project, reach out to our team. We would love to help you learn more about how Tencom’s product line can help.