A STATE-OF-THE-ART COMPOSITE REBAR FOR THE REINFORCEMENT OF CONCRETE STRUCTURES

15.08.2010

In contemporary world, along with the traditional steel rebar non-metallic composite basalt rebar is beginning to be more and more widely used. A distinctive feature of this kind of rebar is high corrosion resistance to aggressive environments, namely to chloride salts, carbon dioxide and sulfur dioxide gases, nitrogen oxide, etc. This feature enables the substantial increase in the overhaul period in comparison to ferroconcrete structures. Basalt composite rebar has a low thermal conductivity ratio, is dielectric, radiolucent, and magnet inactive. Consequently, in many cases it insures antimagnetic and dielectric characteristics of constructions.

A total of all of the above mentioned characteristics as well as reinforcement sufficient properties of durability and deformation and the ability to provide the necessary cohesion with concrete, have determined the most efficient fields of application for basalt composite rebar, which are: sea and port structures, highways, foundations/basements, engineering network constructions, supports for electric power lines, heat conserving walling structures.

Currently basalt composite rebar is most widely used on the US, Canadian, Japanese, German, and Italian markets, where proper regulations as well as design, testing and application rules exist for such type of rebar.

Recently, Ukraine has seen the emergence of basalt rebar production which is used for the reinforcement of concrete structures. It is produced by Technobasalt-Invest LLC and the production site is located in the town of Slavuta, Khmelnytsk region. The rebar is produced according to standard specifications and has the following characteristics:

- nominal diameter – 6-12 mm;
- temporary tensile strength – 800 MPa;
- modulus of elasticity – 43000 MPa;
- modulus of elongation – 2,2%;
- density 1900 kg/m³.

To study the strength regularities of concrete bendable structures reinforced by basalt composite rebar, special experimental research has been carried out with the aim to determine the mode of deformation of concrete and basalt rebar in the process of loading, the character of deformations, possible forms of disintegration, strength, hardness, and crack formation.

Beams of rectangular cross-section with longitudinal basalt composite rebar were chosen as samples. The variation factors were content (reinforcement ratio) and place of basalt composite rebar in the beam’s cross section.

Three series of identical beams were tested. The first and the second series included 6 identical beams, where basalt composite rebar was placed in the tension area. The third series included 7 identical beams, where basalt composite rebar was placed in the tension and compression areas. In the first series the reinforcement ratio was equal to 0,0059 (2Ø10), in the second to 0,0086 (2Ø12), and in the third the total reinforcement coefficient of both the tension (2Ø12) and compression (2Ø10) areas was equal to 0,0146.

The beams were of rectangular cross-section 120х220 mm, were tested as freely reclined and were charged by two concentrated forces located at every 1/3 of the span. Concrete deformation of the compression area, rebar deformation of the tension area, vertical displacement (bending) of the beam in the middle of the area, and displacement of the free end of basalt composite rebar at the tension area and at the butts were recorded.

In the process of loading the former, the load being 0,32…0,36 from the breaking load, normal cracks were formed in pure bending zone. Further loading caused vertical growth and opening-up of normal cracks. Growth of the normal cracks practically stopped with the load factor of 0,65…0,8 from the breaking load, opening-up of the cracks persisted up to break-up of the beam. Disruption on all experimental beams was of plastic nature and occurred as a result of disruption (parcelling) of concrete of compression area over the normal crack. When elements disrupted as a result of concrete disintegration in the compression area over the normal crack tension in basalt composite rebar equaled to 541…725 MPa, which corresponded to 0,50…..0,79 from the temporary resistance of the composite rebar. Herewith basalt rebar tension equaled to 541…725 MPa, which accounted to 0,50…..0,79 from the temporary tensile strength of the composite rebar. At the same time, changes in deformations of longitudinal composite rebar in the tension area of experimental beams in the process of loading were of linear nature. Concrete deformations on mid-level of the height of compression area of experimental beams on the stage prior to disruption equaled to (1,762…2,947) 10^-3 and approached the maximum concrete compression deformations.

Beam loading deflections, which corresponded to the level of normal, equaled to 2,7…4,9 mm, which accounted to (1/180…1/320) of the span. 

The width of the cracks corresponded to the level of normal (characteristic) and equaled to 0,45…0,55 mm.

In order to evaluate the possibility of using basalt composite rebar, comparative analysis was carried out with calculated values of bearing capacity (maximum moment of deflection), width of crack opening and deflection of similar beams with longitudinal steel rebar of the А500С quality according to DSTU 3760:2006 [2]. When making calculations, such indices as geometrical dimentions, longitudinal reinforcing, and concrete compressive strength were considered the same as the beams reinforced by basalt composite rebar. Yield stress of the rebar of А500С quality was taken as equal to the standard values of 500 MPa. Calculation of the bearing capacity, width of the cracks and deflections of the beams with longitudinal rebar of the А500С quality was carried out according to SNiP 2.03.01-84 [3] and to “Recommendations for application of the reinforcing bar according to DSTU 3760-98 for the design and production of ferroconcrete structures without preliminary rebar tension” [4]. Table 1 shows comparative results where mean values are given for the beams with basalt plastic rebar.

1. The patterns of resistance of the concrete elements reinforced by basalt composite rebar, such as thenatureofcrackformation, deformationanddisruption correspond to the analogous patterns for the elements reinforced by steel rebar.

2. The disintegration of beams with basalt composite rebar with single or double reinforcement with reinforcing ratios of 0,0059…0,0086 and 0,0146 occurs in the compression area of concrete, deformations in concrete being close to maximum and the tension of composite rebar equaling to 0,50…..0,79 from the temporary resistance.

3. Bearingcapacityofthebeamswithsinglebasaltcompositerebarcorresponds to the bearing capacity of the beams with steel rebar. Bearing capacity of the beams with double basalt composite rebar reinforcement is lower than bearing capacity of the beams with steel rebar, due to lower value of compression resistance of the composite rebar.

4. The width of crack opening under loading pressure on the beams with basalt composite rebar corresponds to the standard level and equals to 0,45…0,55 mm, which is higher than maximum set for steel rebar, taking into account corrosion resistance requirements. Deflections of the beams with basalt composite rebar under loading pressure corresponds to the standard level and equals to (1/180…1/320) of the span and integrally corresponds to requirements to the maximum deflections of elements with steel rebar.

The width of cracks and deflections of the beams with basalt composite rebar is higher than the width of cracks and deflections of the beams with steel rebar (see Table 1) due to lower value of modulus of elasticity of basalt rebar compared to steel rebar.

D.E., Professor of Kyiv national university

of construction and architecture,

Academician of the Academy of construction of Ukraine

Yu.А. Klimov

3. SNiP 2.03.01-84* Concrete and ferroconcrete structures.

4. Recommendations for application of the reinforcing bar according to DSTU 3760-98 for the design and production of ferroconcrete structures without preliminary rebar tension, Technical standardization committee “Rebar for ferroconcrete structures”, Kyiv, 2002.