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Freeze-Thaw Resistance
Experience with GFRC in natural freeze-thaw environments has been good. In order to study the mechanism of behavior a series of laboratory studies have been performed.
ASTM C666, "Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing," Procedure A, gives the most severe exposure of standard unrestrained freeze-thaw tests. In this test, specimens were subjected to alternating cycles of freezing in water at 0 F for approximately 2 hours and thawing in water at 40 F for approximately 1 1/2 hours. GFRC specimens were subjected to freeze-thaw cycles after 0, 8, and 26 weeks of accelerated ageing. Unreinforced mortar specimens were subjected to freeze-thaw cycles after 0 and 26 weeks of accelerated ageing. For each of these accelerated ageing periods, six specimens were tested in flexure after 0, 100, 200, and 300 cycles of freezing and thawing. [Note: accelerated ageing was achieved by immersing specimens in lime-saturated water at 122 F to accelerate composite ageing.]
 Fig. 18. Flexural yield strength versus freeze-thaw cycles. |
Flexural yield strength versus freeze-thaw cycles are plotted with solid lines in Fig. 18 for the GFRC specimens and by dashed lines for the companion unreinforced mortar specimens. All curves in Fig. 18 represent matrix cracking strength. Numbers next to each curve indicate the number of weeks in accelerated ageing conditions prior to exposure to freezing and thawing.
As shown in the figure, presence of the glass fibers effectively preserved the cement matrix against significant freeze-thaw deterioration. Without fibers, mortar specimens were observed to completely deteriorate before reaching 200 freeze-thaw cycles. In addition, the effect of accelerated ageing prior to freeze-thaw exposure had very little effect on the resulting freeze-thaw resistance of the GFRC specimens as indicated by the relatively flat slope of the line for each accelerated ageing period.
Flexural ultimate strength versus freeze-thaw cycles are plotted in Fig. 19 for the GFRC specimens. Numbers next to each curve indicate the number of weeks in accelerated ageing conditions prior to freeze-thaw exposure. As shown in the figure, regardless of the number of weeks in accelerated ageing prior to freeze-thaw exposure, the flexural ultimate strength decreases to approximately 1500 psi after 100 cycles and approximately 1000 psi after 300 cycles.
 Fig. 19. Flexural ultimate strength versus freeze-thaw cycles. |
After 300 cycles the GFRC showed slight flaking and fiber prominence on the form side. There was severe flaking of the ‘trowel’ face and delamination cracks along the edges. The flakes, about 5/32 to 13/64 in. across and 3/64 in. thick, generally remained attached to the main body of the specimen by the glass fibers.
Freeze-thaw tests have been made on test samples cut from sprayed boards containing 5 percent AR glass fiber by weight and 20 percent sand by total weight. The tests were based on the British Standard Test for Asbestos and Asbestos Cement Building Products, BS 4624; 1970 (50 freeze-thaw cycles). These involved samples that were artificially aged by soaking in 122 F water for 90 days then subjected to 50 cycles of 16 hrs at – 4 F in air and 8 hours at 68 F in air, followed by soaking in water for 48 hours.
There was no visible change in the appearance of the samples after the tests, and the mechanical properties of flexural ultimate and yield strengths, modulus of elasticity, and impact strength were not affected.
In practice, the form side of GFRC would usually be the exterior of a component with the more vulnerable trowel face protected from the weather. GFRC is of low permeability and the trowel face would be unlikely to become saturated with water and susceptible to the action of freeze-thaw. Where an integral composite concrete face mix is used, the aggregates in the finish must be compatible with the anticipated weathering exposure as determined by tests or proven performance.
ASTM C666, Procedure A (freezing and thawing immersed in water), provides a very severe freeze-thaw condition and most materials show some degradation. GFRC is no exception but it still compares favorably with older established construction materials and the conclusion is that freeze-thaw conditions do not pose a significant problem in the use of GFRC.