Performance of Glass Fiber-Reinforced Polymer Reinforcing Bars in Tropical Environments-Part II: Microstructural Tests

In the first part of this study, the structural scale tests on the synergistic effects of moisture, temperature, alkalinity, and stress level on the performance and durability of glass fiber-reinforced polymer (GFRP) reinforcing bars in concrete have been discussed. In this part, investigations on microstructural studies, carried out to find out the nature, quantum, and mechanism of deterioration in the conditioned reinforcing bars, are reported. Micrographic investigations were carried out using a scanning electron microscope (SEM) to visualize the changes in the microstructure. The other tests that have been carried out are energy-dispersive x-ray analysis (EDX) and inductively coupled plasma mass spectrometry (ICP-MS) to determine the chemical changes in the composite.

Keywords: bars; fibers; hot weather; polymers; testing.

INTRODUCTION

In the first part of the paper, the structural scale test of glass fiber-reinforced polymer (GFRP) reinforcing bars in concrete in a tropical environment was discussed. The structural scale tests revealed that, although the strength of the beams went up during environmental conditioning, the reinforcing bars were found to have lost up to 65% of their strength. Another interesting observation was that, although the strength of the reinforcing bars was drastically affected, the environmental conditioning did not significantly affect the stiffness of the reinforcing bars. In this part, the microstructural tests to investigate the reasons for such behavior are reported.

At present, no standard durability test method exists, and this makes the results obtained by different researchers difficult to compare. As a result, researchers report degradation of GFRP reinforcing bars varying from 4.9 to 100% depending on the parameters selected for durability tests, namely, alkalinity, temperature, stress, and duration of the tests.1-11 The investigators, however, agree on the susceptibility of reinforcing bars to alkali attack and this mechanism has been highlighted in all durability tests. The chemical composition of E-glass is 54.3% SiO^sub 2^; 15.2% Al^sub 2^O^sub 3^ and Fe^sub 2^O^sub 3^; 17.3% CaO; 0.6% Na^sub 2^O/K^sub 2^O; and 8 to 10% B^sub 2^O^sub 3^.12,13 In an alkaline environment, glass is attacked by hydroxide ions causing hydrolysis (Eq. (1)).14,15

Si-O-Si + OH- [arrow right] -Si-OH(solid) + Si-O-(solution) (1)

As concrete creates a highly alkaline environment base, hydrolysis of the matrix can be expected to some extent. The models built to predict degradation are based on the diffusion theory. For stressed reinforcing bars in a high-alkaline environment, they do not yield satisfactory results.11 To estimate and visualize the damages in the GFRP, the tests carried out by various researchers are tensile tests and scanning electron microscope (SEM) tests.1-11 The SEM micrographs show degradation of glass fibers.5,11

In the present investigation, the SEM images have been quantified by energy-dispersive x-ray analysis (EDX), an accessory to SEM, which allows simultaneous nondestructive elemental analysis of the sample at a selected spot of approximately 2 microns wide. An EDX test was performed to find changes in chemical compositions at selected points in reinforcing bars. Though x-ray diffraction (XRD) and x-ray fluorescence (XRF) are preferred tests by metallurgists, these did not yield clarity for the nonmetallic substances like glass and polymer. ICP-MS is a very powerful tool for trace (parts per billion to parts per million) and ultra-trace (parts per quadrillion to parts per billion) elemental analysis. This test was performed to identify the chemical changes in the conditioned reinforcing bars.

RESEARCH SIGNIFICANCE

GFRPs are predicted to have great potential use as reinforcement in concrete. In practice, they have started to get a toehold in tropical regions. The use of GFRP reinforcing bars in construction is hampered by the lack of long-term durability and structural performance data. In this paper, concrete-GFRP beams have been subjected simultaneously to stress and an alkaline environment, as well as temperature and humidity conditions of the tropics. Structural scale behavior such as load-deflection and durability has been studied in Part 1. In Part 2, microstructural scale tests that assess the nature, quantum, and mechanism of degradation are reported.

Alkalinity

First, the alkalinity of the concrete was measured. Three beams were cast and cured in water for 28 days. The beams were allowed to dry for 1 month. Then parts of beams were crushed to be very fine. The sample passing 300 micron sieve and retained on 150 micron sieve was taken for alkalinity testing. The powder was diluted in water in 1:1, 1:5, and 1:10 ratios. The pH values were 12.25, 12.182, and 12.05, respectively. The actual pH value at the hydrates level was expected to be higher than these values.13 It may be noted that in the concrete mixture, normal portland cement has been used without any other cementitious materials. As a result, the pH of concrete is rather high and the glass may be susceptible to alkali attack. The mixture proportion used here is commonly used in this part of the world. To study the conditions of the exposed reinforcing bars, the SEM and EDX tests were employed.