Essay describing the properties and technical characteristics of the FRP system for structural reinforcement.
. 1 - Introduction.
The structural design and production of concrete building elements is always linked to certain values of the stresses. However, during the lifetime of the structure, various circumstances may cause the initial hypothesis of the project becomes invalid:
-Modification of the structure.
-Aging of building materials.
-Corrosion of reinforcement.
-Earthquakes and fires.
-Changing in normative of load capacity, etc..
In these cases, and as a requirement for the development of adequate reinforcement project, rehabilitation or repair, it is necessary to evaluate the actions acting realistically.
Valid techniques for reinforcing concrete construction elements are basically the following:
-Applying reinforcements in slots made in the concrete.
-Installation of additional brackets or braces.
-Gluing steel facings.
-An alternative to these types of reinforcement is the use of composite materials (FRP)
2. SELECTION SYSTEM FRP
The fibers used in composite materials are embedded in a matrix and applied as reinforcement to an existing building element. For flexural reinforcement may be used in laminated composite materials prefabricated. As these prefabricated laminates can not be used for landfill or for shear reinforcements are provided by manual lamination fabric (sheets) "in situ", consisting of different fiber types.
Different types of fibers and composite materials:
|tissue S&P C||Uni- directional||
|tissue S&P G||Bi- directional||
|Band S&P CFK||Uni- directional||
Both tissue type unidirectional S & P C and prefabricated laminates S & P CFK type fibers are straight and are stretched (although the manual application of the tissue causes a slight ripple). Consequently, these products are suitable for absorbing tensile forces with minimal elongation. Both the fabrics and laminates are preferably used to increase the rigidity of the construction element.
The fibers are used in the production of carbon fibers (C) having a high modulus of elasticity. The basic parameter for comparison between different tissues (type C) and laminate (CFK type) is the modulus of elasticity of the fibers.
In bi-directional tissue S & P fibers are positioned wavy due to the manufacturing process (weaving). For this reason, the tensile absorption occurs with a greater deformation. The bi-directional fabrics are ideal to increase ductility by confinement of a construction element (an important field of application is the earthquake protection of concrete structures).
To ensure transmission of the load from the S & P FRP system to the support material, it is further recommended to treat the support by sand blasting or pricking surface.
3.- MECHANICAL PROPERTIES OF LONG-TERM
The creep behavior against (maintained stress cracking) of the CFRP is very superior to the composite materials. This has been proven in a large number of trials. In a trial of this style made with CFRP bars in saline solution was recorded on the bars to keep stress at a level of 70% of its tensile strength in the short term, after 10,000 hours.
This extrapolated for a 50-year period, behavior corresponds to a level of sustained tensile load under 79% of the short-term resistance.
In usual cases of CFRP reinforcement tensile stress of continuous operation under load usually reaches maximum values of 20% of the value of short-term resistance. At these levels of stress can not be considered a significant loss of strength as a result of the maintenance of the load.
FRP composites comprise carbon fibers C (CRP) are highly resistant to fatigue. In trials conducted in Japan with maximum voltages up to 87.5% of the tensile strength in the short term and amplitudes of up to 1000 MPa, more than 4,106 charge cycles were reached.
For CRP bars embedded in concrete, after 4105 cycles of load fluctuation range 0.05 - 0.5fc and a frequency of 0.5 Hz there was no failure by fatigue and subsequent tensile tests not there was some reduction in tensile strength of the bars.
Creep and relaxation.
The creep and relaxation express the viscoelastic behavior of the material. In the case of composite materials, it is possible to distinguish between flow due to axial force on the blades and interlaminar creep. In usual cases of reinforcement CFK both are very small creep under permanent loads that occur.
Carbon fibers, by themselves, do not have significant influence or power loss as a result of relaxation under the prevailing charge levels maintained in state service. In contrast, the epoxy resin matrix is a viscoelastic material can be seen that although the matrix resin used is linear elastic up to a value of tensile strain εM = ± 20% (which is well above the values they will be present in practice).
As CFRP rods, relaxation tests conducted over a period of 3,000 hours with an initial voltage level of 70% of the tensile strength gave lower losses of 2% relaxation. These losses can be extrapolated (using a logarithmic profile for relaxation curve) at 50 years relaxation losses of 2 to 3%.
All this can be justified considering that there is no significant creep or relaxation in CRP laminates using epoxy glue commonly used as structural reinforcement.
4. PHYSICAL AND CHEMICAL PROPERTIES OF MATERIAL
The realization of a large number of trials shows that CRP has a very good resistance to chemical attack of relevance in building applications. For example , in tests under load bar CRP maintained under permanent tension levels 5 to 75 % of the tensile strength in the short term , at temperatures between 21-80 ° C and subjected to liquid of pH 10-13.5 (simulating existing concrete pores ) over a period of three months, no reduction in grade interlaminar strength was observed.
Trials in prestressed concrete beams with rods CRP tensioned to 40% of its tensile strength in the short term and immersed in alkaline solutions for 81 days at pH = 12.5-13 no lost resistance was obtained . Trials parallel bars isolated CRP that were exposed to the same solutions at 60 ° C also showed no influence on the mechanical properties .
The thermal expansion coefficient for the two types of laminates S & P C in the longitudinal and transverse directions is given in the following table.
|Tipo de lámina||aT; longit. (10-6K-1)||aT; transv. (10-6K-1)|
|S&P 150/2000||± 0||30|
|S&P 200/2000||± 0||20|
The coefficient of thermal expansion in the transverse direction is not relevant for sizing . The difference between the coefficient of expansion of the S & P CFK laminate in the longitudinal direction and the concrete ( aT = 10.10 -6K- 1) could have a negative impact on the high adhesion between the laminate and the concrete temperature fluctuations .
Two series of beams subjected to static bending test at EMPA , following 100 cycles of freeze / thaw between -25 º C and 20 º C by comparing the results with those obtained in beams with an identical design that had not been exposed to these cycles of ice / thaw. The beams of the series suffered previous cracking due to initial load , while the beams of the other set had no cracks .
During cycles of ice beams were saturated with water , which meant that it was possible to study the potential loss of strength in the composite caused by the icy water, and the potential impact of thermal incompatibility of CFRP and concrete. The results showed no reduction of the bending load capacity compared to the reference beams .
Another reinforced concrete beam CFRP sheets was cooled at -60 ° C without the plate were to detach pandease concrete or as a result of any induced stress fiber understanding .
In other trials not damage the adhesive bond between CRP laminate ( fiber T700 ) and aluminum support material was observed until a temperature of -133 ° C. The thermal incompatibility problem is more relevant in CFRP composites of aluminum and CFRP and concrete due to high coefficient of thermal expansion of aluminum ( aT = 23.4 · 10-6K- 1) .
Although this trial could not assess the impact on the bond strength , the test showed that low temperature conditions expected in our buildings , buckling is not expected in the fibers in the laminate or as a result of tensions produced by compressive forces .
According to present knowledge , the different behavior with respect to thermal expansion of the laminates of CRP and concrete in any way worsen the capacity of the construction elements of reinforced CFRP laminate .
Anyway, in order to prevent potential lack of knowledge or any uncertainty is recommended (according to the guidelines contained in sizing agreed rules on the use of laminated CFRP reinforcement ) that the value for dimensioning resistance bond breakage is reduced by 10 % when very high temperature fluctuations are expected. This reduction is based on rough estimates.
Fire behavior .
The carbon fibers have high heat resistance . The glass transition point of the resin matrix is TGM = 100-130 ° C. However, the capacity of the composite system is determined by the adhesive which has a glass transition point of TGK = ± 47 ° C. This temperature will be reached after a few minutes a fire type .
If the reinforcement is dimensioned such that upon failure of the laminate , the coefficient of residual security is greater than 1 , then the time of fire resistance will not simply by the fire resistance of the concrete element , that is, by the coating existing armor . This can be increased with adequate protection.
If instead the boost level is such that the coefficient of residual safety in the event of failure of the laminate is less than 1, then you must project a sufficient fire protection to ensure that over time the adhesive will not reach desired temperature TGK . This must be considered in each particular case .