Comparison Of Concrete Beams Reinforced With Steel And Glass Fibre-Reinforced Polymer (GFRP)

Literature Review

Discuss about the Comparison Of Concrete Beams Reinforced With Steel And Glass Fibre-Reinforced Polymer (Gfrp).

Concrete is the commonest material in the construction industry and is characterized by high compressive strength, durability and stiffness. However, plain concrete is weak in tension because of low tensile strength and is brittle due to low fracture strain[i]. The concrete is usually reinforced so as to overcome these limitations. Reinforcement plays a major role in improving structural stability and soundness of buildings. This is because concrete has good compressive strength and poor tensile strength hence reinforcement increases the concrete’s tensile strength. Therefore reinforced concrete has the capacity to adequately resist both compressive and tensile stresses, which helps in preventing unacceptable structural failure. This report presents the comparison between concrete beams that are reinforced with steel and glass fibre-reinforced polymer (GFRP). The report contains the following sections: literature review, advantages and disadvantages of steel reinforced concrete beams and GFRP reinforced concrete beams, future research recommendations, conclusion, etc. The findings of this report can be used to make a decision on whether to reinforce concrete beams using steel reinforcement or GFRP reinforcement, depending on the construction project requirements, goals, objectives and constraints.

Beams are structural elements that are subjected to bending only. As a result of this, one section of the beam (top or bottom) is subjected to compression while the other section is subjected to tension, depending on where the load is applied. Therefore the compression section of the beam must be designed such that it resists crushing and buckling, whereas the tension section must be designed to resist tension adequately. Reinforced concrete beams are able to resist both compressive and tensile forces. The concrete (which has high compressive strength) resists compressive stresses and strain while the reinforcement (which has high tensile strength) resists tensile stresses and strain. Therefore reinforced concrete is a composite material comprising of concrete and reinforcing material. The main benefits of reinforced concrete are: high compressive strength, ability to withstand high tensile stress, rational weather and fire resistance, durability, flexibility, economical, low maintenance costs, minimal deflection, and less skilled labor requirements. Some of the drawbacks of reinforced concrete are: low tensile strength in comparison with compressive strength, its final quality is affected by production and casting processes, higher cost of forms needed to cast the concrete, and can loss strength when cracks starts developing due to shrinkage.  

Steel and GFRP Reinforcements

Steel has been used as a reinforcing material for concrete beams over a very long period of time. In steel reinforced concrete, steel bars (also known as rebars) made from twisted filaments with folds are anchored firmly in concrete with no risk of sliding. When the concrete cures, it hardens with the steel rebars forming a strong composite material. One of the reasons why steel is widely used as a concrete reinforcing material is because its rate of expansion in heat and contraction in cold (thermal expansion coefficient) is almost the same as that of concrete[ii]. This prevents concrete from cracking when it is hardening or subjected to expansion and contraction due to fluctuating environmental conditions. The steel has also been proven to be effective in making high-strength concrete[iii]. Besides the traditional steel bars, steel fibres[iv] are also used nowadays as concrete reinforcing materials[v]. The steel fibres have a wide range of structural applications in construction industry[vi]. Steel fibre reinforced concrete beams have been found to support higher ultimate load and they remain stable even after reaching their yield stiffness[vii].      

Use of GFRP as a rehabilitating or reinforcing material for structural components of buildings began few decades back[viii]. This material has several advantages over the traditional steel rebars but other factors such as high cost have hindered its widespread application in construction industry. GFRP are available in different forms, including: woven fabric, cranette, hewn strand mats, ropes, wool, etc. Some of them contain epoxy resin coatings to provide protection against cement’s alkali attack. The GFRP performs the same function as the steel rebars – increasing tensile strength of the reinforced concrete[ix]. Since both steel rebars and GFRP performs the same function in reinforced concrete beams, it is vital to investigate the comparison between these concrete reinforcing materials. 

Background

Use of steel and other metals as concrete reinforcing materials can be traced back to the 15^th century. In 1800s, many French and German engineers promoted use of reinforced concrete and came up with several innovations such as twisting the steel rebars so as to improve their bonding with concrete. It was until 1910 when the first steel reinforcing bar specifications were issued. Since then, a lot of developments have followed in relation to use of steel rebars. On the other hand, glass fibre reinforced polymer (GFRP) was first discovered in mid 1930s and has since become a reliable reinforcing material for a wide range of civil engineering projects[x]. Use of GFRP as a reinforcement material was first discovered in Russia in 1975 where the material was used for reinforcing a timber bridge. In 1980s, several researchers, scientists and engineers carried out studies to establish the feasibility of GFRP replacing steel in strengthening and repairing bridges. In 1990s, GFRP was widely used in Japan for construction of train support structures. In 1996, design guidelines for use of GFRP in reinforced concrete were introduced by the Japanese. Since then, GFRP has become an alternative reinforcing material to steel. The material is now being used across the world to reinforce concrete, timber, steel and masonry structures[xi]. The wide application of GFRP in construction industry is largely because of its benefits over steel reinforcement. Today, many buildings across the world are constructed using reinforced concrete, which makes them stronger and able to endure different weather and time ravages.

Desired Properties of Concrete Reinforcing Materials

The properties of a good concrete reinforcing material are: high resistance to tensile stress and strain, greater relative strength, good bond or compatibility with concrete, durability, thermal compatibility, resistance to chemical and weather attack, fire resistance, cost effective, sustainability and environmental friendliness. Steel and GFRP reinforcing materials have unique properties, which make them suitable for different construction applications.

GFRP reinforcing materials presents versatile design options because of their exceptional fabrication flexibility, structural efficiency, high durability and low costs of production and erection. GFRP is a type of plastic composite comprising of glass fibres that mechanically enhance stiffness and strength of plastics[xii]. The fibre gets additional protection from the resin as a result of bonding between these materials[xiii]. Some of the fundamental attributes of GFRP that make it a suitable reinforcing material for concrete beams are as follows:

High strength: GFRP is about three times stronger than steel for an equal amount of weight. Strength to weight ratio of the former is very high, which makes it a suitable material for reinforcing concrete beams.

Lightweight: GFRP weighs much less than steel reinforcement. This helps in reducing its shipping costs, enabling easier and faster installation, and reducing the need for structural framing.

Resistance to external damages: GFRP resists a wide range of environmental conditions, including chemicals, salt water, acid rain, corrosion, etc. This makes it suitable for projects in almost any environment, both indoors and outdoors.

Versatility: GFRP can be molded in virtually any form or shape, including very complex shapes. This makes it suitable for making any form of concrete beam.

Low maintenance: GFRP does not lose its physical or mechanical properties for a very long time – up to 30 years. If used as intended, the structure made from GFRP can last for decades with very minimal or no maintenance needs.

Durability: GFRP is very strong and structures built from it can withstand different expected and unexpected loadings, such as hurricanes, earthquakes, etc.  

Insulation: GFRP has good heat resistance and high electrical insulation. This helps in protecting buildings or other structures against excessive heat.

Cracking: GFRP prevents concrete beams from cracking caused by thermal or mechanical stress that may accumulate over time.

Fatigue: GFRP has greater resistance to fatigue making it a suitable reinforcing material to protect concrete beams against fatigue failure mode.

Radio signals: GFRP does not cause interference to the radio signals, like it is the case for steel rebars.

Attributes of GFRP as a Concrete Beam Reinforcing Material

Low elastic modulus: the elastic modulus of GFRP is almost a fifth that of steel. This reduces the capability of GFRP to withstand varied loadings. 

Low flexural and tensile strength: this affects the ability of GFRP to resist loads even if their dimensions are increased. 

Creep: the creep property of GFRP changes the beams endurance performance that affects the reliability of the beam in the long run[xiv].

High cost: the cost of GFRP is relatively high compared with that of steel. This is one of the major factors that have significantly contributed to slow adoption of GFRP as a reinforcing material for concrete beams.

Some of the attributes of steel reinforcement include:

High ductility and tensile strength: steel is more ductile and has high tensile strength that makes it able to bear greater loads. The high ductility and tensile strength of steel also increases the overall strength of reinforced concrete beams[xv]. 

Cheaper: steel is abundantly available in many parts of the world and this has continued to reduce its cost.

Reduced cracking: steel resists cracking strongly in different ways. One of these ways is through the strong bonding between steel reinforcement and concrete. This bonding reduces cracking of concrete when it is hardening.

Low maintenance cost: steel has low maintenance needs, which translates into low maintenance costs. The fact that the steel is engraved inside the concrete makes it less vulnerable to deterioration and damages.

Durability: the high flexural strength, impact resistance, fatigue resistance and high abrasion of steel creates durable concrete beams. 

Impact and fatigue resistance: this is another great property of steel reinforcement that makes it undergo minimal deflection (if any) and attain very high yield strength.

Strength deterioration: strength of steel reinf0rced concrete decreases significantly when the beam is exposed to extreme temperatures[xvi].

Corrosion: alkalinity of cement causes corrosion of steel reinforcement. If this happens, the structural soundness of the entire beam declines.

Spalling: steel can also react aggressively with some aggregates in the concrete causing spalling of the beam. This has costly effects on the structural member.

In the future, there is need to conduct studies that will establish the impacts of amount of reinforcing material used on the strength, stability and durability of reinforced concrete beams. More studies should also be carried out to determine the impacts of steel and GFRP reinforcing materials on the environment and the effect of type of loading on the performance of steel reinforced or GFRP reinforced concrete beams[xvii]. Researchers should also focus on establishing the qualitative and quantitative differences between steel reinforced and GFRP reinforced concrete beams. Last but not least, researchers should establish the feasibility of combining steel and GFRP reinforcing materials and if this can improve the properties and performance of reinforced concrete beams[xviii].

Conclusion

Reinforcement plays a key role in preventing development of cracks in concrete beams. These cracks form when concrete shrinks as it hardens. Plain concrete has low modulus of rupture, is brittle in nature and its capacity to resist strain is low. As a result of this, plain concrete has high compressive strength and low tensile strength. In order to improve flexural strength of concrete beams and make them able to resist both compressive and tensile stresses, reinforcement is usually added to the plain concrete. Steel and GFRP are some of the materials that are used to reinforce concrete beams. These materials have different attributes and therefore their effects on mechanical and physical properties of concrete beams also differ. It is important to carry out an experimental study to determine the effect that steel and GFRP reinforcements have on concrete beams. This comparison will help in selecting the most suitable reinforcement between the two for a concrete beam depending on the project requirements and constraints.

References

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