Can GRP rebar be used instead of steel? Understanding the issue

Steel reinforcement has almost doubled in price and many have a reasonable question: can it be replaced with a fiberglass analogue? Let's look at the main characteristics of both types and find out the exact answer to this question.

Subject matter and some theory

For unstressed structures, reinforcement bars А240, А400, А500, А500С are used.

• A240 is a smooth reinforcement that is used as an auxiliary one. Smooth reinforcement is required as clamps for lattice girders and mesh anchors from work reinforcement.
• A400, A500, A500S are working reinforcement that perceives tensile forces from the dead weight of structures and external loads.

If we talk about the A500 reinforcement, then alloys with a carbon content of more than 0.6% are used for its production. This figure indicates that the flexibility and weldability of this reinforcement is limited.

A500C contains half as much carbon, which means it is more plastic, bends well and welds well. For this reason, A500C reinforcement is mainly used in low-rise construction. The prefix "C" means that the fittings are welded. The profile of this reinforcement is made in a crescent shape to improve adhesion to concrete; the longitudinal ribs do not intersect with the lateral ones. It is also called periodic section reinforcement.

Composite reinforcement consists of rods made of glass, basalt, carbon or synthetic fibers impregnated with a polymer binder. Samples made of carbon or synthetic fibers can be disregarded due to the very high cost - it is not used in private construction. Let's dwell on the most affordable and widespread option - fiberglass reinforcement.

Fiberglass reinforcement is divided into several types:

• fittings with a periodic (classical) profile;
• sandy;
• with a periodic profile and sandy coating.

A sand coat is applied to enhance adhesion to concrete.

For comparison, let's take A500 fiberglass and steel reinforcement with the same cross-section of 12 mm.

Comparison options

Let's make a reservation right away: we will not consider purely marketing parameters such as durability, corrosion resistance, environmental friendliness, electrical conductivity, thermal conductivity, fire resistance and other indirect characteristics. Let's consider the main, one might say, fundamental parameters - these are strength, stiffness and elongation.

All supporting structures are calculated according to two groups of limit states. First: the calculation of the structure for strength and stability. Second: deflection, deformation and crack opening. Simply put, the calculation is reduced to the determination of two values: 1 - resistance to external loads or strength; 2 - resistance to deformation or elasticity. That is, a monolithic structure is considered reliable if two parameters are met simultaneously.

Strength

For A500 reinforcement, the tensile strength is 500 MPa. For fiberglass reinforcement, this figure is 1200 MPa. Judging by these numbers, fiberglass reinforcement is twice as strong as steel. Composite manufacturers like to talk about it. But there is absolutely no need for such an ultimate strength, because it will work in a concrete structure by only 20-30%. This can be compared to the body of a car - if you increase the thickness of the steel by three times, from one millimeter to three, will it make sense? Yes, the body's strength will triple, but does that make sense? In a concrete structure, everything is the same.

Rigidity or elasticity

But the next indicator - rigidity, will directly indicate the difference between the two materials. Stiffness is the ability of a material to resist shape change while maintaining volume. Stiffness of metal fittings - 200,000 MPa; for fiberglass - 55,000 MPa. This means that metal is almost 4 times harder than fiberglass. But this is a real problem for the composite, since for reinforcement the main indicator is elasticity or stiffness. That is, the reinforcement must be more elastic than concrete. And with such values, the stiffness of fiberglass reinforcement approaches the stiffness of the concrete itself. In simple words - plastic reinforcement practically does not add rigidity to the monolithic structure. And at this point a reasonable question arises - what is it for at all?

Relative extension

Now about the relative lengthening. For metal fittings, this figure is 0.25%; for fiberglass - 2%. This means that a monolithic structure reinforced with plastic is capable of deforming by 2%. Concrete simply cannot withstand such deformation. As an example, consider a monolithic beam 3 meters long. If you give it a maximum load from above, then the lower row of reinforcement will come into operation. With a relative elongation of 0.25%, the expansion of the cracks will be 7.5 mm and the beam will bend by a small amount. With a relative elongation of 2%, the crack expansion will be 60 millimeters. At the same time, the beam will bend even more and huge cracks will appear in it. Concrete is unable to maintain stability against such deformations.

Conclusion

As it was written above, if the reinforcement does not pass at least one of two parameters (strength and stiffness), then it is absolutely impossible to use it! Steel reinforcement passes both in strength and stiffness; fiberglass - only in terms of strength.

Fiberglass reinforcement can be used as flexible ties for facing masonry, when reinforcing screeds, blind areas and garden paths. This is where the scope of its application ends.

Source: YouTube channel Project LLC ( https://youtu.be/mVu9NcbrbEM).

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