GLASS FIBER - Flexural stress-strain curves are used to determine values of modulus of elasticity for design purposes. Values of flexural modulus of elasticity are normally in the 1.5 to 2.9 X 106 Psi range, and will vary in accordance with water-cement ratio, sand content, cure, density, and degree of micro cracking. There is a lack of a continuous network of micro cracks at low stress level versus well develop network of micro cracks at or near flexural strength, thus giving lower value than normally associated with precast concrete panels.