Thermal Behavior of Adhesively Bonded Timber-Concrete Composite Slabs Subjected to Standard Fire Exposure

Fire tests were performed for the first time on adhesively bonded timber-concrete
composite slabs. The two medium-scale (1.8 x 1.25 m) slabs were produced by gluing an 80-mm thick three-layer cross-laminated timber (CLT) board to a 50 mm thick
prefabricated reinforced concrete (RC) slab with epoxy and polyurethane (PUR) adhesives, respectively. The behavior of the composite slabs under elevated temperature
was monitored by (1) observing the burning behavior of the used CLT, for example,
charring and delamination and (2) measuring the temperature development at different locations of the CLT slabs, in the adhesive bond between concrete and timber
boards, and in RC slabs. It was found that employing a one-dimensional charring
model for pure softwood, as prescribed by Eurocode 5-1-2, underestimated the charring depth of CLT due to the delamination effects. Measurements revealed that the
average charring rates in the middle layer of CLT panels were approximately
0.65 mm/min, suggesting that the presence of concrete does not significantly affect
the thermal behavior of the CLT panel. Delamination within the CLT was observed
when its adhesive temperature was around 230°C. It was followed by the free-fall of
delaminated wood plies, which progressed slowly and lasted until the end of the test.
At 90 min into the test, the temperatures of epoxy at the nine locations ranged
between 55°C and 130°C, while that of PUR between 60°C and 100°C. The adhesive
between concrete and CLT could lose stiffness significantly along the rising of temperature after surpassing of glass transition temperature (58°C for epoxy and 23°C
for PUR in this study). The results indicated a high risk of weakening the composite
action between the concrete slab and timber board. The measured temperatures of
steel rebar were lower than 50°C. However, the concrete temperature reached about
120°C and the concrete cracked due to the distinct thermal expansions between
concrete and timber and the rigid constraint of adhesive bond.