Nowadays, the fire behavior of CLT panels made from solid-sawn lumber exposed to fire is well known and documented by a number of research organizations and universities. However, due to the desire to optimize how material is used in CLT, and ultimately lower manufacturing costs, CLT with thin laminations ranging from 19 to 25 mm in thickness has started to be produced in North America, which somewhat limits the applicability of some design provisions which were derived and validated from CLT made with 35-mm laminations. There is currently limited research on CLT manufactured with thin laminations, namely with respect to their fire behavior and specifically the effective charring rate.
Several charring models have been developed over the years to predict the char front of CLT elements exposed to a standard fire curve (e.g. CAN/ULC S101 and ASTM E119). Some models aimed at predicting the char front at any given time, while others were developed based on experimental time-to-failure, using an average charring rate. While these models result in similar char depths at the time-to-failure, the results from this fire test series demonstrate the non-linear impact on the charring rate from using thin laminations in the manufacturing of CLT elements. Using a “one-rate-fits-all” effective charring rate may work when trying to replicate “time-to-failure”, but such approach provides inconsistent and overly conservative predictions of char depths, thus structural fire-resistance, when there is a need to evaluate the depth of char at any time less than the time-to-failure.
In order to address the lack of consistency in the charring models of CLT with thin laminations, FPInnovations conducted a series of fire tests to further evaluate and document the impact on the charring rate from using thin laminations. The objective of this study is to evaluate the charring behavior of CLT manufactured in accordance with ANSI/APA PRG-320  with thin laminations of various thicknesses (less than 35 mm).
From the data generated in this test series, it can be observed that when the first lamination is charred through to the glue line, the general trend is that the charring rate in subsequent laminations experiences a significant increase whether it is a CLT floor or wall element. The sharp changes in the temperature profiles recorded at the glue lines suggest that such behavior is most likely attributed to the adhesive heat performance, where heat delamination (fall-off) is observed when the glue lines reaches temperatures ranging between 115 to 250ºC.
It was also found that the 1st lamination seems to char at a fairly uniform rate, whether it is a floor or a wall element and regardless of its thickness. The traditional one-dimensional charring rate of 0.65 mm/min may be used for the 1st lamination of CLT elements. However, the subsequent laminations char at a much faster rate, ranging from 0.59 to 1.07 mm/min, depending on the lamination thickness and its location within the CLT configuration. The results from this fire test series demonstrate that if no changes are made to the adhesives currently used, the impact from using thin laminations in CLT elements is not fully captured by charring models using a “one-rate-fits-all” approach. Charring models need to be adapted to explicitly account for lamination thickness. When compared to the test data, the current 2014 Canadian CLT Handbook provides reasonable, while slightly greater, charring rates based on the thickness of laminations for the 2nd and subsequent laminations.
As such, some recommendations are provided herein for future improvement of existing CLT charring models and consideration at the CSA O86 Technical Committee. It is demonstrated that the proposed changes predict char depths that closely track at any given time the actual char depths as observed during a standard fire exposure test. 301010618