3.1 The basis for this test as an indicator of the long term behavior of fiber reinforced composites is that elevated temperature and moisture content accelerate the formation of the products of hydration of the cement in the matrix, particularly calcium hydroxide.

3.1.1 It is known that the interaction of these products of hydration, particularly calcium hydroxide, with the fibers can have a major effect on the long-term properties of the composites.

3.1.2 The principal mechanism that occurs is that the calcium hydroxide forms within the bundles of filaments that form the glass fiber strand. This gradually bonds the filaments together, which reduces filament pull-out. This causes a reduction in the strain capacity of the composite, thereby reducing the strength of the composite and changing the composite from a ductile material to an increasingly brittle material.

3.1.3 Accelerating the formation of the hydration products accelerates their interaction with the fibers, hence accelerating the aging of the composite.

3.2 The data developed from this test are used by researchers and manufacturers to evaluate the long-term performance of different mixtures, reinforcements, mixture components, and to provide data for the development of design procedures for products made from these composites.

3.2.1 The recorded data also provides screening information as new matrices and reinforcements are developed.

3.3 The usefulness of the test as an indicator of long term composite performance is valid if the only reactions that are accelerated are those that occur more slowly under natural weathering, hot water can induce effects in some composites, for instance those containing acrylic polymer, that may not occur in natural weathering. This test method may not be a valid indicator of long-term performance of such composites.