Serviceability of members reinforced with fibrereinforced polymers - Concrete Structures: Stresses and Deformations - page 459

coe cient in radial direction of FRP bars can cause hoop tensile stress in the

concrete adjacent to the bar, and produce radial hair cracks when the tem-
perature rises. Once cracking occurs, the tensile stresses are relieved; thus, the
radial hair cracks do not extend to the surface of concrete member when the
cover is not overly small. It should be noted that the thermal expansion
coe bre and

the resin matrix. The values mentioned above are only approximate. Again,
the manufacturer commonly provides information on thermal properties.
The compressive strength of FRP bars is relatively low and their contribu-
tion to ultimate strength as compression reinforcement in concrete sections is
often not considered. Also, because the modulus of elasticity of FRP is
relatively low, particularly in compression, the contribution of FRP bars
situated in the compression zone to the

cients of FRP vary with the method of production, the type of fi

fl exural rigidity of cracked members

is ignored.
Carbon FRP has tensile strength that exceeds the tensile strength of steel
used for prestressing. To avoid very wide cracks the high strength of FRP
cannot be fully used in non-prestressed concrete members. When used for
prestressing, information about relaxation of carbon FRP is required; this
data should take into account the temperature and the ratio of the initial
tensile stress to the tensile strength. At 20 degree Celsius and initial stress 70
to 80 percent of the tensile strength, the relaxation of FRP in 0.5 million
hours (57 yrs) is approximately 15 percent, regardless of the type of bre. The

manufacturers of FRP for prestressing should provide relaxation data.
Certain FRP products are vulnerable to rupture when they are subjected to
sustained tensile stress. This phenomenon, referred to as creep rupture,
occurs in a shorter time when the ratio of the sustained stress to the tensile
strength is larger. To control width of cracks in non-prestressed members, the
permissible strain in FRP in service should be relatively low compared to the
tensile strength. The permissible strain in service proposed in Section 14.3 for
non-prestressed FRP is below the strain that can produce creep rupture.
However, when FRP is used for prestressing, the ratio of stress at transfer to
the tensile strength should be small compared to the permissible ratio for
prestressing steel.
The basic assumptions in analysis of stresses, strains and displacements of
steel-reinforced concrete structures in service are also adopted when FRP is
employed. Thus, concrete and reinforcement are assumed to have linear
stress-strain relationships. Sections that are plane before deformation remain
plane after deformation. Concrete in tension in a cracked section is ignored;
the tension sti

fi

ect is accounted for empirically by interpolation
between the uncracked state and the state of full cracking. The analysis pro-
cedures and equations presented in the remainder of the topic for structures
reinforced or prestressed with steel can be applied with FRP, using the
appropriate characteristic material properties. However, because of some of
the di

ff ening e ff

ff erences of properties of FRP and steel, particularly in the moduli of