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Myths on composite rebars

    “Elektroulgamgurlushyk” Company implements an investment  project to create an integrated enterprise for production of continuous glass fiber (CGF) with the subsequent production of composite materials using CGF, especially glass-fibre reinforcement bars (GFRB) manufactured according to TDS 31938-2012, for its subsequent application in power transmission poles, foundations of high-voltage substations and civil engineering facilities.

Structurally, the company is organized on the basis of two industries.

The first one is the production of GFRB(glass-fibre reinforcement bars), consists of four lines and is located at the Bezmein enterprise of reinforced concrete products in Ashgabat city. The equivalent capacity of the enterprise is 10,000 tons of metal construction reinforcement bars.

The second production is the production of glass continuous fiber (CGF) will consist of ten furnaces with a capacity of 2000 tons per year, operating on quartz sand with the addition of the broken glass or waste of the Ashgabat glass factory. An important advantage of this enterprise is that this production can be transferred to the production of basalt fiber without additional costs.

At the current period, the company operates one line with a capacity of up to 2400 tons per year of GFRB calculated in the metal equivalent.

All necessary certificates and legal title documents have been received.

Myths on composite rebars

Before purchasing composite reinforcement, any buyer will want to know the features of its using and application, its advantages and disadvantages, and in this subject the main source of information is the Internet. But it is not always possible to understand what information is true and reliable. Let’s try to answer the most difficult questions and dispel the prevailing myths:

First Myth : The composite reinforcement is a rubber. It is considered that modulus of elasticity of composite rebar is lower than steel bar. Is it OK?

In the current period, GFRB production is based on producing of the CGF (continuous glass filbers) produced by OJSC Polotsk-Fiberglass (Belarus) and chemical resins produced by Dugolak Company (Russia). Mostly, non-accelerated, non-thixotropic orthophthalic resin (DEPOL 100) is used. This resin has good mechanical properties and increased resistance to aggressive environments.

For Customers who demand high requirements for mechanical properties and especially important requirements for exposure to aggressive environments, we use an unspecified epoxy vinyl ester resin based on Bisphenol A. (DEVINIL 910). This resin kepps its stable physical and mechanical properties of products based on this resin in conditions of high temperatures and aggressive environments due to the high density of resin crosslinking.

The linear elasticity modulus of reinforcement on such resin is up to 100,000 MPa, which allows to use reinforcement bars based on real resin even in load-bearing (supporting ) structures. For comparison, steel reinforcement A-II (A200) 150,000 MPa, reinforcement A-III (A300) 200,000 MPa. . Yield area (ratio σy / σu = 0.1). This is the best indicator for creating prestressed structures.

Modulus of elasticity:

In order to stretch the sample for a given certain distance, it is necessary to apply a certain force known as the modulus of elasticity. For composite reinforcement bars, it is 45,000 MPa, for metal – 200,000 MPa. So the reinforcement bar made from the composite is easier to “stretch” by 4 times. But, the conducted studies have proved that elastic modulus of steel materials is not constant and decreases sharply with increasing load due to the appearance of plastic deformations. Main task of reinforcement bars in concrete is work in tensile and dusruption. In concrete, the elastic modulus varies from 20,000 to 30,000 MPa, depending on the brand, but it is difficult to call it rubber. Taking into consideration all properties of the material, it is necessary to take into account the full range of its characteristics, which includes relative elongation at tearing, temporary tensile strength, yield strength, uniform elongation. The reinforced concrete structure under load behaves as follows: After a slight stretching, microcracks appear in the concrete, after which the metal reinforcement prevents its final cracking. Microcracks in a loaded structure are a common phenomenon, since even with a minimum load it is impossible to prevent their occurrence. The size of these cracks depends on the elastic modulus of reinforcement bars; if this size is small then the concrete will more sag. To prevent collapsing of whole structure , the ultimate strength enters into action. As this limit become higher , the concrete tolerates heavier loads.

Modulus of elasticity:

In order to stretch the sample for a given certain distance, it is necessary to apply a certain force known as the modulus of elasticity. For composite reinforcement bars, it is 45,000 MPa, for metal – 200,000 MPa. So the reinforcement bar made from the composite is easier to “stretch” by 4 times. But, the conducted studies have proved that elastic modulus of steel materials is not constant and decreases sharply with increasing load due to the appearance of plastic deformations. Main task of reinforcement bars in concrete is work in tensile and dusruption. In concrete, the elastic modulus varies from 20,000 to 30,000 MPa, depending on the brand, but it is difficult to call it rubber. Taking into consideration all properties of the material, it is necessary to take into account the full range of its characteristics, which includes relative elongation at tearing, temporary tensile strength, yield strength, uniform elongation. The reinforced concrete structure under load behaves as follows: After a slight stretching, microcracks appear in the concrete, after which the metal reinforcement prevents its final cracking. Microcracks in a loaded structure are a common phenomenon, since even with a minimum load it is impossible to prevent their occurrence. The size of these cracks depends on the elastic modulus of reinforcement bars; if this size is small then the concrete will more sag. To prevent collapsing of whole structure , the ultimate strength enters into action. As this limit become higher , the concrete tolerates heavier loads.

For concrete itself, the tensile strength is 8-20 times lower than the compressive strength. Marking B25 means that this class of material is able to withstand compression pressure of 25 MPa, and during tensile only 1-4 MPa. For steel, this figure is 400 MPa, and for composite reinforcement bar is 1200 MPa. This characteristic shows that the construction with composite reinforcement bars is able to withstand 3 times greater load than structure with metal. But at the same time, it sags stronger by 4 times. Size of microcracks at the same load in concrete with metal reinforcement will be smaller by 4 times.

Tensile strain:

Using of steel reinforcement is regulated by GOSTs and SNiPs due of fact that this material will corrode, lose its properties, which can lead to collapse of the structure. The composite reinforcement does not rust and does not threaten by its destruction. However, the appearance of cracks in concrete is not only a consequence of corrosion. With tensile strength, the deformation of fiberglass is up to 2.8%, and metal 25%. In the code SP52-101-2003 states that reinforced concrete structures crack at a tensile strain of 0.015%, i.e. long time before of occuring tensile strength of the reinforcement, regardless of its material (composite or steel).

If you desire to replace metal reinforcement with composite reinforcement bars in ceilings or load-bearing supporting walls, it is necessary to recalculate the design and technical documentation, which will avoid occuring of large cracks. Recalculation shall be performed for structures exposed to maximum loads. In places where the minimum load is expected, it is allowed to replace metal reinforcement with composite rebars with a smaller diameter. SNiP Code eliminates the necessity to recalculate the opening of cracks not provided by the structure.

For concrete itself, the tensile strength is 8-20 times lower than the compressive strength. Marking B25 means that this class of material is able to withstand compression pressure of 25 MPa, and during tensile only 1-4 MPa. For steel, this figure is 400 MPa, and for composite reinforcement bar is 1200 MPa. This characteristic shows that the construction with composite reinforcement bars is able to withstand 3 times greater load than structure with metal. But at the same time, it sags stronger by 4 times. Size of microcracks at the same load in concrete with metal reinforcement will be smaller by 4 times.

Tensile strain:

Using of steel reinforcement is regulated by GOSTs and SNiPs due of fact that this material will corrode, lose its properties, which can lead to collapse of the structure. The composite reinforcement does not rust and does not threaten by its destruction. However, the appearance of cracks in concrete is not only a consequence of corrosion. With tensile strength, the deformation of fiberglass is up to 2.8%, and metal 25%. In the code SP52-101-2003 states that reinforced concrete structures crack at a tensile strain of 0.015%, i.e. long time before of occuring tensile strength of the reinforcement, regardless of its material (composite or steel).

If you desire to replace metal reinforcement with composite reinforcement bars in ceilings or load-bearing supporting walls, it is necessary to recalculate the design and technical documentation, which will avoid occuring of large cracks. Recalculation shall be performed for structures exposed to maximum loads. In places where the minimum load is expected, it is allowed to replace metal reinforcement with composite rebars with a smaller diameter. SNiP Code eliminates the necessity to recalculate the opening of cracks not provided by the structure.

Therefore, structural elements that are not subject to heavy loads can be safely performed using glass-fibre reinforcement bars.

The second myth. Equal strength  or equivalent replacement? What is the difference?

Equal strength and equivalent substitutions should not be confused. If the sample is not inferior in strength to the original structure, that means equal strength replacement. In this case, strength means “tensile strength”, the maximum mechanical stress, after which the destruction of the material occurs. In GOST 1497-84, strength means “temporary fracture resistance”, evaluating as stress which corresponds to the maximum force before rupture of the sample during tests.

If we measure two samples of metal and composite material, we get the following indicators: the tensile strength of a composite with a diameter of 10 mm will be 63,000 MPa, and for steel with a diameter of 14 mm will be 60,000 MPa. This shows that this replacement is not equally strong, since the reinforcement made from composite is stronger by 5%. So with equal strength replacement, metal reinforcement with a diameter of 14 mm can be safely replaced with composite reinforcement with a diameter of 10 mm. What is an equivalent replacement? With this replacement, the physical characteristics of the samples should be identical. If fiberglass reinforcement has an elastic modulus is less 4 times than metal, it is necessary to take it more by 4 times to replace it. The ability of solid material to deform in the case of applying tensile is called the elastic modulus. This term includes several physical values. Lets calculate the diameters of the materials with an equivalent replacement. If composite material is needed 4 times more, then using the circle area formula we get that glass-fibre with a diameter of 20 mm is required to replace metal bars with a diameter of 10 mm. The obtained calculations must be taken into account before starting of construction or the preparation of the project, and clearly understand the difference between equivalent and equally strength replacement.

In structures where the deflection of the reinforcement bars has not special importance, it is advisable to use more durable composite materials. In floor slabs or load-bearing walls, it is demanded to use metal reinforcement bars with a high modulus of elasticity or to make recalculation to equal strength substitution at using glass-fibre