In this project influences of the combined impact of time and temperature on the behaviour of FRP materials used to strengthen concrete structures will be studied. The long and short term behaviour of basalt, glass and carbon FRP reinforcement will be investigated.
If the FRP reinforcement is pre-stressed (in tension) before application, the stress state of the composite varies significantly with time.
It is planned:
To study the joint effects of high-temperatures, FRP tensile pre-stressing and long-term loading on the behaviour of FRP reinforced concrete.
Cylindrical specimens (height – 30 cm, diameter – 15 cm) were manufactured (Fig.1). A commercially available concrete mixture of class C25 was used. The samples were cured for 28 days. The cylinders will be winded with unidirectional fiber reinforced epoxy matrix composite (basalt, carbon and glass fibers will be used).
The mechanical properties of basalt fibers are consistent within broad temperature range. Glass and carbon fibers could pose a problem in this aspect.
The orientation pattern of winded basalt fibers must also be taken in to consideration. Increased reinforcement quality can be primarily obtained by two winding methods – by winding the fibers in a circumferential or in a spiral pattern (Fig.1).
The winding process of the circumferential pattern does not pose significant manufacturing difficulties. Epoxy-soaked fibers are placed on the rotating cylinder at 90 degree angle w. r. t. the axis of rotation. This type of pattern can only handles radial stresses within the specimen.
The spiral winding pattern allows for some longitudinal stress from specimen to be transferred to the fibers. By varying the spiral winding angle, the amount of transferred longitudinal stress may be controlled. On the other hand, the winding procedure poses some technical difficulties.
Fig.1 - Specimens preparation
At room temperatures, the effects of creep for the composites in question are insignificant. In a good approximation, the amount of creep deformation w. r. t. the initial deformation for a given period of time is not a function of the applied load, and therefore can be neglected.
Complete failure occurs after the failure of the winding due to increasing radial stresses.
Fig.2 - Pre-stressing
Effect of high temperature
Epoxy resins may be used at relatively high temperatures. More heat resistant epoxy types contain phenol base. ASTM allows for use of epoxy in temperatures up to 300°.
Concrete cylinders (height - 30 cm,diameter - 7 cm) were manufactured from a commercially available concrete mix (manufacturer - SAKRET, concrete class - C25). Some of the cylinders were strengthened by FRP composites (basalt fiber or carbon fiber textile reinforced epoxy resin). Some properties of the FRP composite constituents are listed in Table 1. The reinforcing basalt fibers were winded on to the concrete cylinders in uniform (fibers tightly spaced) and in a wide angle (spiral) pattern. The various types of specimens and their labels are listed in Table 2.
Table 1 - Relevant properties of FRP composite constituents
Table 2 - Types of specimens considered
In order to monitor the longitudinal deformations of samples, tensoresistors (name - HBM 50/120 LY41, maximum compressive deformation — 0.75%) were glued to the sides of cylinders. A total of 3 tensoristors were glued on the plain concrete specimens (type C). Due to difficulties in application of the reinforcing composites, only two tensoresitors were placed on the reinforced specimens (types CB, CC and CBS). The specimens were loaded (on servohydraulic testing system MTS 809.40) under a constant displacement loading (0.05 mm/min), until the failure of concrete or reinforcing composite. During loading the applied force, displacement at the top of the specimen and deformations of the tensoresistors were registered using data collection unit 'Spider'.
Fig.3 - Schematic depiction of the experiment and locations of the tensoresistors. a) schematic depiction of the experimental setup. Location of tensoresistors on the specimens of type b) С and types c) СС, СB, СBS are shown to the right
The applied force as a function of the deformations at the center of samples are shown in Fig.4.
Fig.4 - The applied load as a function of sample deformation for samples of type a) C, b) CB c) CC d)CBS. Red curves are the average of all curves obtained for a sample type
Fig.5 -Sample type-average of the applied load-deformation curves
The plain concrete specimens (C) failed by initiation of a macro-crack along the specimen axis. The failure occurred at applied load around 160 kN. Reinforced samples displayed an increased load caryng capacity, as well as increased resistance to crack propagation. During the loading of both samples of type CB and one sample of type CC the deformation reached the maximum allowed values for the tensoresistors(0.75%). After this point the samples were loaded until failure to obtain the failure load. Due to the singificantly increased load carrying capacity of the CB samples, sample buckling was observed. The loading of these samples was stopped after intiiation of a macroscopic crack along a cross section orthogonal to the sample axis. Critical failure was not observed. Specimens of type CBS failed due to the failure of reinforcing basalt fibers near center of the specimen. Some micro-cracks were observed.
Fig.6 - Samples after failure: а) СBS, b) CC, c) СB (left) and С
Table 3 - Maximum applied load
Significant changes in specimen failure modes and increase in load carying capacity and cracking resistance were observed for all types of reinforcement. The uniformly winded basalt fiber reinforcement and carbon textile reinforcement display the highest increase in sample integrity. Spiral winded basalt fiber reinforcement poses significantly less difficulties during manufacturing, and has potential for future study.
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