Marked shrinkage behavior when heated is typical of semicrystalline po
lymer fibers such as polypropylene (PP) and polyacrylonitrile (PAN). S
hrinkage of PP and PAN fibers may give the possibility to control the
spalling tendency of fiber concrete under the heat exposure of fire. C
ut staple fibers are normally delivered for concrete reinforcement. Mo
dern methods for continuous fibers cannot be used by the end-user for
shrinkage determination of commercial staple fiber grades. The shrinka
ge of five different commercial staple fibers specially designed for c
oncrete reinforcement was studied under a hot-stage microscope. Signif
icant differences in cumulative shrinkages of the various PP and PAN f
ibers were detected, shrinkages being 3-15% with PP fibers and 6-7% wi
th PAN fibers at a temperature of 150-170 degrees C. At about 160-165
degrees C, PP fibers melt, whereas PAN fibers continue shrinking. Hot-
stage microscopy provides a simple and a relatively accurate method fo
r estimating thermal shrinkage of staple PP and PAN fibers, the deviat
ions from measured average values remaining typically at 10-15%. (C) 1
995 John Wiley and Sons, Inc.