Design Issues - Strengthening Design of Reinforced Concrete with FRP - page 72

4.3 STRENGTHENING LIMITS DUE TO LOSS
OF COMPOSITE ACTION
These limits are assigned to protect the member from failure if the FRP system
is lost because of fire damage or vandalism. The bare section of member without
strengthening should be able to resist certain levels of the new or upgraded loads
without capacity deficiency, as described by Equation (9.1) of ACI 440.2R-08.

( ) ( )

φ R ≥ 1.1 S + 0.75 S

(4.1)

n DL LL

existing new

The dead-load factor is close to unity, since the estimate of the new or upgraded dead
load can be determined fairly accurately. The live-load factor is selected to exceed
the statistical mean of annual maximum live-load factor, given by ASCE-SEI 7-10
(2010) to be 0.5. Once the member survives the damage incident, the FRP is sup-
posed to be repaired or reinstalled.
However, in cases where the live load is sustained on the member for an extended
period of time, like the cases of library stacks and heavy storage warehouses where
live load exceeds 150 lb/ft 2 (7.2 k N/m 2 ), the live load factor of 0.75 is increased to
1.0 in Equation (4.1).

4.4 FIRE ENDURANCE
The strengthening level of externally bonded FRP is typically limited by the fire
endurance of the member with FRP. This is because FRP undergoes severe degra-
dation in bond and mechanical properties at or beyond its glass transition tempera-
ture ( T g ), which is typically equal to 140°F-180°F (60°C-82°C) (ACI 440.2R-08).
Deuring (1994) tested CFRP plated beams with and without insulation. He
showed that the unprotected FRP-strengthened beams achieved around 81 minutes
of fire endurance. Identical beams with CFRP protected by a 40-mm thick (1.57-in.
thick) calcium-silicate insulation plates endured fire for a longer 146 minutes. It is
important to note that bond between concrete and CFRP was lost within the first few
minutes of fire exposure when CFRP was not protected.
Blontrock, Taerwe, and Vandevelde (2001) tested CFRP-strengthened beams
protected with different insulation boards under full service load plus ISO 834 fire-
curve exposure (1975). They observed that the best fire endurance can be accom-
plished if U-shaped insulation boards are installed to the soffit and sides of beams.
Williams et al. (2008) tested CFRP-strengthened RC T-beams with U-shaped
vermiculite/gypsum (VG) insulation 25-38 mm thick (1-1.5 in. thick) subjected to
uniformly distributed service load and ASTM E119 standard fire curve (2002). The
strengthened RC T-beams were able to withstand the fire exposure for up to 4 hours.
The results of this test and the three-dimensional (3-D) FE analysis of the same test
(Hawileh et al. 2009) showed that the insulation controlled the temperature trans-
ferred to steel and FRP below the critical temperature values.
ACI 440.2R-08 allows the extension of the concepts established
by ACI 216R-89 suggesting limits to maintain safety against collapse due to fire.