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Session: CLT Adhesive Tests in Support of Mass Timber Buildings

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Cross laminated timber is manufactured by laying up kiln dried sawn boards or structural composite lumber in layers that are perpendicular to each other and glued. Each billet of solid wood can be from 4 to 18 inches thick and up to 11 feet wide by 65 feet in length. Exact sizes vary by manufacturer. The resulting panel of wood is dimensionally stable in plane because there is parallel to grain lumber resisting shrinkage in both panel directions.

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American Wood Council

Engineered and Traditional Wood Products

Here’s a close-up photo of some 7-ply CLT. Of course, we’re looking at the edges here – so you can clearly see the alternating orientation of each successive layer. It is hard to get an idea of scale from this photo, but each lamination is often about 1-3/8”-thick – so 7-ply CLT like this can easily be close to 10 inches thick! And, really, the main limit to the length of CLT panels is transportability. They can be manufactured to just about any length that would be required for a tall mass timber building.

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The history of Cross Laminated Timber (CLT) is discussed in this slide. The first patent for CLT was obtained in France in 1985. The first constructed projects occurred in Switzerland and Germany in 1993. in 1995, development of new press technology made manufacturing easier and more precise. Multi-story applications came in 1998 with a residential building in Styria, Austria. Throughout the decade, starting in the year 2000, Europe expanded the use of CLT and Mass timber significantly. For example, due to regulatory regulations to promote green and sustainable construction, England has seen an explosion in the use of mass timber construction with over 500 buildings to date. The US and Canada have lagged behind the world in recognizing the benefits of Mass Timber construction. In 2016 the first tall wood building, known as the Brock Commons Residence Hall, was started at the University of British Columbia in Vancouver, Canada. At 18 stories, this was the tallest structure in North America utilizing mass timber products. The US building code, known as the International Building Code (IBC) adopted provisions recognizing CLT as a heavy timber product.

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• So what is CLT? Like GLT, it utilizes conventionally sawn lumber pressed together through an adhesive bond. Unlike GLT, it alternates the direction of the adjacent layers similar to plywood. This results in a large panel configuration suitable for floors and walls. In the United States, GLT is traditionally used in multi-ply, single width beams and columns.

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The components of CLT include laminations, layers and adhesives.

As it pertains to CLT, a lamination is a single piece of sawn lumber or structural composite lumber.

A layer is defined as an arrangement of laminations of the same thickness, grade and species combination, laid out essentially parallel to each other in one plane.

So, CLT is made up of multiple layers – at least three. Adjacent layers are orthogonal to each other, and bonded together with a structural adhesive.

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Now that Sam has reviewed with us the basics of CLT and how it’s made, I’m going to talk a little about some of the fire testing that has been done on CLT structures.

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Several series of large-scale fire tests have been performed in order to guide and inform code development for tall mass timber buildings. The objective of this code development activity is to ensure safe construction practices and safe designs for tall mass timber buildings. Many of these fire tests have been what are referred to as “compartment fire tests”.

A compartment fire test is one that is performed within a compartment that has at least one opening. These tests simulate a natural fire within an enclosed (or partially-enclosed) area, such as a room or apartment. As such, the fuel type used for compartment fire tests often consists of real furnishings and other contents that would be typical of the simulated occupancy. Other fuel types are sometimes used – such as wood cribs or gas – either in addition to, or as an alternative to, the real furnishings.

The fire growth phases of a typical compartment fire may be characterized by the time-temperature diagram shown on the upper-left corner of this slide. At first, they are usually slower to develop than a standard ASTM E119 fire exposure. However, once they reach flashover, growth typically exceeds that of a standard E119 exposure. Flashover, by the way, is the point at which all exposed combustible surfaces within the compartment suddenly become involved in the fire.

This begins what is called the fully-developed phase of the fire, in which temperatures are often in the range of 1800-2200°F (1000-1200°C). Once the combustible contents within the compartment have

been mostly consumed, the fully-developed phase ends and the

temperature starts to drop. This marks the beginning of the decay

phase. Barring any fire re-growth resulting from the introduction or

involvement of new fuel, the fire will eventually extinguish due to lack

of fuel.

By comparison, the standard ASTM E119 fire exposure grows fairly

rapidly at first, but then levels off to a certain extent. As such, the

standard E119 exposure does not reach the same level as the typical peak

temperature in a compartment fire until more than 4 hours into the test. Of

course, by that time, most natural compartment fires would have cooled because

they had already consumed the available fuel.

One of the biggest fire test series that was performed as part of the code development process was a set of 5 full-scale building tests that were performed by the US Forest Products Lab at the ATF Fire Research Lab in Beltsville, MD. The test plan for this series was developed by the Fire Work Group of the ICC Tall Wood Building Ad-Hoc Committee (TWB) and ratified by the full TWB.

The purpose of this test series was to perform tests of realistic fire scenarios applicable to tall wood construction to evaluate occupant and firefighter tenability for egress and suppression efforts, and provide data necessary to guide further development of relevant code and standard provisions.

The results of this test series are publically available at www.awc.org/tallwood.

Just to give some of the specifics of the mass timber test building that was constructed for the ATF Full-Scale building tests:

• The floor plan simulated a multi-story condo configuration. The building was constructed with 2 stories because that was deemed sufficient to simulate multiple stories for the purpose of data collection.

• The interior dimensions of the simulated apartment/ condo on each level was 30’x 30’ (900 ft2 on each level). The building also included a corridor and stair shaft to the exterior of the apartments on each level, such that the overall footprint of the building was about 37’x 46’. The general floorplan for the first level is shown in this figure. The floorplan for the 2nd level was the same.

• The TWB Fire WG, which Sam just mentioned, specified a target fuel load of 550 MJ/m2. This value was established as corresponding to the mean fuel load plus 1 standard deviation, based on a survey of typical Group R occupancies.

Five fire tests were performed in the test structure. The first three represented unlikely scenarios in which the automatic fire sprinklers, which are mandated by code, completely fail to activate and the fire service is unable to respond at all throughout the duration of the fire.

In ATF Test 1, all of the mass timber surfaces within the test structure were protected with 2 layers of 5/8” Type X GWB. Since the mass timber surfaces did not contribute to the fire, it served as a baseline to which the subsequent tests could be compared.

In ATF Test 2, 30% of the CLT ceiling area was exposed in both the living room and the bedroom.

In Test 3, the CLT walls were exposed in both the living room and the bedroom.

Test 4 had a code-compliant automatic fire-sprinkler system.

In Test 5, the sprinklers were manually charged after a 20-minute delay to simulate manual charging at the FDC by the fire service. In both Tests 4 and 5, all mass timber surfaces were exposed in both the living room and the bedroom.

The right column of this table shows the duration of each fire test. Test 1, in which all mass timber surfaces were protected, went for 3 hours. By that time, the

furnishings and other contents were almost completely consumed. Tests 2 and 3 were allowed to continue until 4 hours, just so that it could be verified that no fire re-growth would take place.

A good portion of this fuel load was achieved through a combination of typical residential furnishings, combustible floor coverings, cabinetry, and other combustible contents.

Even with all of these furnishings and contents, it was still necessary to add additional fuel load in order to achieve the TWB-specified fuel load. This is because the specified fuel load was one whole standard deviation above average for Group R occupancies. Thus, in order to make up the difference, wood cribs were also added within the test structure for each test.

Here’s a compilation of photos showing the interior of the bedroom and bathroom, courtesy of the U.S. Forest Products Lab and ATF.

The photo on the right and the one on the bottom-center show some of the combustible fuel load that was present within the bedroom in each test. The rest of the photos were taken prior to installation of the furnishings and contents.

…And these photos show the interior of the kitchen and living room – again, courtesy of FPL and ATF.

The top-center photo shows the cabinetry which was installed in the kitchen. The bottom-right and bottom-center photos show some of the combustible fuel load that was present within the living room in each test. The rest of these photos were taken prior to installation of the furnishings and contents.

Note the exposed portions of CLT on the ceiling, visible in three of these photos. These were taken prior to Test #2 which was performed on the 2nd level.

We don’t have time in this session to delve into the details of all five tests performed in this series, so let’s just hone-in on one of them…Test 2:

In Test 2, 30% of the CLT ceiling areas in the living room and the bedroom was left exposed. This amounted to 20% of the overall ceiling area throughout the apartment. The photos at the bottom of this slide show the exposed ceiling areas.

In both Tests 2 and 3, a load was applied to the floor/ceiling assembly using large barrels filled with 130 gallons of water each.

{The load resulted in maximum moments equivalent to the moment that would be induced by a 20 psf uniform live load.}

Let’s watch an accelerated video of ATF/FPL Test 2…

Flashover occurred in the living room at around 12 minutes, and in the bedroom at around 171/2 minutes.

Towards the end of the decay phase, there was no significant flaming on the exposed CLT areas on the ceiling.

You can also see the condition of the exposed CLT areas at the end of the test. The exposed CLT surfaces had self-extinguished by the end of the 4-hour test.

Here are some photos from key stages during the fire growth and decay phase of Test 2.

The photos on the bottom right show the condition of the exposed areas at the end of the test. They had self-extinguished by the end of the 4-hour test.

The term “flashover” is generally defined as the point at which all exposed combustible surfaces within a given compartment (e.g., furnishings, contents, etc.) suddenly become involved in the fire. For the purpose of this test series, this condition was considered to have been reached when ceiling gas temperatures measured six feet above the compartment floor reached 600˚ C.

This photo shows the condition of the CLT following Test 2, after removal of the gypsum wallboard. Note that the areas protected with 2 layers of 5/8” Type X GWB were uncharred, with the exception of minor localized surface charring in limited spots.

The exposed CLT surfaces self-extinguished during the decay phase.

In this photo, you can see not only the post-fire condition of the CLT, but also that of the glulam columns and beams.

Again, the areas protected with gypsum wallboard were uncharred, with the exception of minor localized surface charring in limited spots – for example: at the lower corners of the glulam beam.

Here’s a view of this same CLT ceiling panel, taken from outside the building as the workers were hoisting it with a crane.

Again, you can clearly see that the areas of CLT ceiling protected with GWB did not char.

On those surfaces of the ceiling that were left exposed (which are the black areas visible on this ceiling panel), the char depth only amounted to between 1” and 1¼”.

The scale may not be readily apparent in this photo, but that panel being hoisted by the crane in this picture is very large – with dimensions of 8’ wide by 37’ long.

Here’s a photo of a sample cross-section that was cut out of the exposed CLT ceiling following Test 2. It clearly shows the protective layer of char on the first lamination…

…And this photo shows another sample cross-section, which was also taken from the exposed CLT ceiling following Test 2. On this sample, the char has been scraped off, in order to more clearly show the depth of the char. As you can see, the char was not quite 1 lamination thick in this location – about 1¼” deep.

The reason a portion of the lower lamination was not charred is because this cross-section was cut such that it spanned a portion of the protected area on the ceiling. This shows the transition of the char depth from an exposed area to an area that is protected.

the Research Institute of Sweden (RISE) performed modeling predictions for this ATF full-scale building test series.

This graph shows how closely the modeling predictions were to the actual measurements for heat release rate in Test 2. The model slightly under-predicted the peak HRR, but it followed fairly closely for the remainder of the test.

Although this graph only shows the first 2 hours (120 minutes), the test was actually carried out for 4 hours. As you can see, the fire was well on it’s way to self-extinguishment even at the 2-hour mark – and there was no fire re-growth observed throughout the test.

Although we only have time to discuss one of the ATF tests, I did want to give a summary of results and conclusions from all five of the tests in this series. This table compares the times at which some of the key events occurred in each test.

As you can see, for each test, flashover in the living room was fairly consistent at around 12 or 13 minutes after ignition. And the time-to-flashover in the bedroom was even more consistent – at around 17 minutes, despite the differences in exposed mass timber areas.

The time that it took for flames to breach the 20-minute-rated door into the hallway was about 27 and 30 minutes for Tests 1 and 2, respectively.

In Test 3, flaming on the back-side of the 20-minute-rated door occurred early because the door was not installed correctly. But in any event, the flames did not spread beyond the door within the corridor. That was evident in the first three videos that we saw.

I also wanted to mention some of the other important findings from the ATF tests:

• Analysis of the ATF test results has shown that exposed CLT did not have a significant impact on the outcome of the event. The fires eventually burned-out, or were suppressed by the sprinklers – leaving the structure intact.

• Penetration firestops compliant with ASTM E814 prevented the spread of fire beyond the compartment of origin.

• Tenable conditions were maintained in the exit corridor and stairs for more-than-sufficient time to allow safe egress of occupants.

• And also, conditions tenable for fire fighters were maintained in both the corridor and the exit stair shaft throughout the entire test duration.

In 2017, the Fire Protection Research Foundation, which is part of the National Fire Protection Association, commissioned a series of six compartment fire tests. I spoke a little about this test series in a webinar last week, but I’m going to delve a little deeper into it here, because is it of great importance to the topic of this webinar. The tests were done for NFPA’s Property Insurance Research Group (PIRG). The purpose of the test series was to collect data on the contribution of CLT elements to compartment fires, for the purpose of insurance modeling. Testing was performed by the National Research Council of Canada at the National Institute of Standards and Technology fire lab in Gaithersburg, MD.

All 6 of the test compartments were constructed of CLT, and all had the same overall dimensions – 15’ wide by 30’ deep. The opening, which was 6.5’-tall x 6’-wide on four of the compartments, and 6.5’-tall x 12’-wide on the other two, was situated on one of the 15’ end walls. In two of the tests, all of the CLT surfaces were protected. These were considered the baseline tests, because the CLT did not become involved in the fire, and therefore did not contribute to the fuel load.

In the other four tests, various CLT surfaces were exposed. Fire performance parameters, such as HRR, temperatures and heat flux, were measured in each test. The contribution of the exposed CLT was determined by subtracting the values of these parameters, as measured in the corresponding baseline test.

All six of these tests represented unlikely scenarios in which the automatic fire sprinklers, which are mandated by code, completely fail to activate and the fire service is unable to respond at all throughout the duration of the fire.

Among the findings of this test series were, that the CLT did not significantly contribute to the fuel load in compartments which had gypsum wallboard protection on all surfaces.

And, for the tests in which certain CLT surfaces were left exposed (or unprotected), the CLT contribution naturally increased with increasing exposed surface area.

As I noted in last week’s webinar, the degree to which the CLT contributed was modeled by the Research Institute of Sweden (RISE) in advance of the tests. The actual test results, in terms of HRR and temperature, were very close to the model predictions. So the contribution of exposed CLT in a compartment fire can certainly be modeled accurately.

In addition to the findings I just mentioned, fire re-growth was observed in three of the six FPRF tests. By the term fire re-growth, I’m referring to a behavior in which the fire, after having cooled significantly during the decay phase, starts to intensify again.

In each of these cases, the observed fire re-growth coincided with the failure of an adhesive bond line between the CLT layers. Based on data from thermocouples that were embedded within the CLT at depths corresponding to the bond lines, it was apparent that the failures were occurring before the char front reached the bond lines. This behavior is known as heat-delamination.

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Before I go to the next slide, I wanted to point out that the photos on this slide were both of FPRF Test 1-4. The one on the left was taken about 2 hours into the test, which was well into the decay phase. As you can see, the fire was not very intense that that stage, because most of the combustible contents had been consumed, and the

exposed CLT ceiling was insulated with a layer of char.

One might expect that the fire would slowly continue to cool from this point, and eventually self-extinguish, due to insufficient remaining fuel. But if you look at the photo on the right, you can see that this is not what happened in this particular test. The photo on the right was actually taken ½-hour after the one on the left – and yet the fire had re-grown to the point of a 2nd flashover.

So let’s focus on FPRF Test 1-4 in particular (I’ll just refer to it from here on-out as FPRF Test 4). Remember that this is the test shown in the pictures on the previous slide:

In FPRF Test 4, the entire CLT ceiling was exposed. This compartment had an opening size of 6.5’ high-by-6’ wide, which was the smaller of the two opening sizes tested in this series. The comparative baseline for Test 4 was Test 1-1. Test 1-1 was the same as Test 4 in every way, except that it did not have an exposed CLT ceiling. Thus, there was no contribution or involvement of mass timber elements in Test 1-1. By subtracting the measured parameters of Test 1-1 from those of Test 4, it was possible to directly determine the effects of the exposed CLT ceiling in this configuration.

For example, the graph on this slide shows the measured heat-release-rate of the baseline test (red curve), the measured HRR of Test 4 (light blue curve), and the difference between these two (magenta curve). As I mentioned, the difference in heat-release-rate, shown by the magenta line, indicates the contribution of the exposed CLT ceiling

over the course of the 2-hour 40-minute test.

You can see that there are two distinctive humps, or peaks, on this magenta curve. The first one started at around 50 minutes and ended at around 80 minutes. The second hump started at around 150 minutes, and continued for the next 10 minutes until the end of the test. The start of these humps corresponded with heat-delaminations that occurred in the 1st and 2nd bond lines, respectively.

The heat-delamination in the 1st bond line occurred around 50 minutes. This introduced new fuel, in the form of un-charred wood, near what would have been the beginning of the decay phase, thereby extending the fully-developed phase by about ½ hour. The heat-delamination in the 2nd bond line occurred at around 150 minutes, introducing new fuel, resulting in fire re-growth that lead to a 2nd flashover.

Now, the video we just saw was FPRF Test 4, but fire re-growth behavior was also observed in other FPRF compartment tests in which there were exposed areas of CLT, either on the ceiling, or on the walls.

In all of these cases, it appeared that the fire re-growth was caused by heat-delamination of an adhesive bond line within the CLT.

Of course, the implication of this observed behavior is that, in rare instances such as the scenarios represented by this test series, a fire may not necessarily burn out on its own. In other words, it may not self-extinguish, even after all of the furniture and other contents are consumed.

One example of another FPRF test in which fire re-growth occurred is shown in this photo….This was taken about 4 hours into Test 3, which had an exposed CLT wall. You can see some flaming on the exposed CLT wall in this photo. Earlier in the decay phase of this test, there was an extended period without flaming on the wall, and very low heat-release-rate. Here again, this fire re-growth was a result of a heat-

delamination that occurred in one of the bond lines. However, it should be noted that in this test, the fire re-growth was not nearly as intense, did not result in a 2nd flashover, and the compartment eventually cooled down again on its own.

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In the next segment, we’re going to learn about a couple of different adhesive qualification tests that are used, and how they were developed.

But first, I wanted to speak briefly about part of the impetus behind the development of these adhesive qualification tests. Specifically, I’m referring to the direction provided by the ICC Ad-Hoc Committee on Tall Wood Buildings (TWB).

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The ICC-TWB Ad-Hoc Committee took the position that heat-delamination which leads to fire re-growth is unacceptable performance for CLT used in tall mass timber buildings. The importance of this position was confirmed and underscored by the findings of the FPRF test series that we just discussed.

Inherent to this position is the idea that CLT members should perform like solid wood. In other words, they should exhibit steady, easily-modeled char development when exposed to fire, with an unheated core that stays insulated by the char layer throughout the exposure. Furthermore, the CLT should not exhibit a behavior that results in significant fire re-growth.

Direction from the TWB: There need to be CLT adhesive qualification test protocols that are capable of identifying heat-delaminating adhesives which can result in fire re-growth.

Secondly, these adhesive qualification test protocols and associated acceptance criteria need to be required by the code. For example, by means of reference within the CLT product standard, PRG 320.

Based on the direction provided by the ICC-TWB Ad-Hoc Committee, in the fall of 2017, AWC commissioned Southwest Research Institute to develop a test protocol which would be capable of distinguishing between adhesives that can lead to fire re-growth, versus those that do not. Specifically, there was interest in replicating the severe exposure conditions of the FPRF compartment tests, to determine whether or not the same fire re-growth behavior would have been observed if the FPRF tests had been performed using CLT products made with other adhesives.

In order to determine this, it was necessary to replicate many aspects of the FPRF compartment tests. For example, a large scale compartment test was used, rather than a small- or intermediate-scale compartment, and the aspect ratio of the compartment footprint was about the same (about a 2:1 depth-to-width ratio). Also, the compartments were constructed to have the same ventilation ratio as that of the FPRF tests having the small opening (about 0.03 m0.5).

In this test procedure, the CLT ceiling is left exposed, similar to the

configuration that was tested in FPRF Test 4. The CLT ceiling is loaded to the same uniform load, and the span of the ceiling was about 15’ – which is the same span as that used in the FPRF tests.

As for the exposure, the gas burners within the test compartment were calibrated to replicate the exposure conditions measured in one of the FPRF baseline tests. Specifically, it replicated the exposure in FPRF Test 1-1, which was the baseline test with the small (6.5’ high-by-6’ wide) opening. This exposure replication was based on the measured heat flux near the ceiling in the FPRF test.

The graph on the left shows temperatures measured on one of the thermocouple trees in FPRF Test 1-1, with respect to time. The graph on the right shows temperatures measured in the large-scale compartment test calibration performed at Southwest. Note that these time-temperature curves are similar. They’re not exactly the same; but, again, that’s because the replication was based on heat flux to the ceiling, rather than temperature.

The discontinuities, or apparent “steps” in the graph on the right are a result of the fact that the gas flow rate followed a step function, rather than a continuous curve. But the heat flux to the ceiling at any given time – and more importantly, the cumulative heat flux to the ceiling

throughout the test – were very close to the measured values in FPRF Test 1-1.

So, the calibration test confirmed that it was possible to replicate the conditions of FPRF baseline Test 1-1. However, it was still necessary to validate the test protocol by testing the same CLT that was used in the FPRF test series. As I noted previously, this test protocol involves an exposed CLT ceiling, which is the same condition that was tested in FPRF Test 4. So, it follows that the test protocol would be considered valid if a test performed on the same CLT results in the same fire growth behavior as that observed in FPRF Test 4.

These are some of the photos of the validation test performed at Southwest. The photo on the left was taken at about 10½ minutes –just prior to flashover. You can still somewhat make out the exposed CLT ceiling, because at this point, it had not yet ignited And the photo on the right was taken just after flashover – at about 14 minutes after the start of the test.

Here are two photos of the fully-developed stage of the validation test. The one on the left was taken near the beginning of the fully-developed phase- at about 15 minutes. If you’re wondering what was causing that little bit of flaming on the floor near the opening, this was actually just the gypsum wallboard facing paper burning…the lab placed gypsum wallboard on the concrete floor to protect it from the fire.

The photo on the right was taken at about 1 hour, which was about 2/3

of the way through the fully-developed phase. It was right around this time that the first bond line started to undergo heat-delamination. This corresponded closely with similar performance observed in FPRF Test 4 at this stage.

On this slide, the photo on the left gives you a visual of the conditions within the test compartment during the decay phase. This photo was taken at about 3 hours into the test, but it is generally representative of the conditions throughout much of the hour or so preceding this as well. That is, with the exception of a few episodes of less intense fire re-growth. You will see these on the video I’m going to show next. The reason I chose to show a photo from the 3-hour mark is because this was just before the fire re-growth started.

The photo on the right was taken about 12 minutes after the one on the left – at about 3:10. You can see that there was, indeed, significant fire re-growth, leading to a 2nd flashover – and this fire re-growth was caused by heat-delamination of the 2nd bond line of the exposed CLT ceiling. Here again, this corresponded well with the performance that was observed in FPRF Test 4.

Here are the measured time-temperature curves from the four different compartment tests we’ve talked about so far. The two graphs on the left are from the FPRF tests, while the two on the right are from the tests performed at Southwest Research Institute. The time-temperature curve of the validation test we just saw in the video is shown in the lower-right. The curve of FPRF Test 4, which we saw in the first video, is shown in the lower-left.

The graphs at the top are from the tests performed without any exposed CLT, while the ones on the bottom are from the compartments having exposed CLT ceilings. As you can see, there are similar differences between the graphs on the top, versus the respective graphs on the bottom. This indication of how the exposed CLT ceiling affected the temperatures within the compartments was consistent between the two test series.

Now let’s just look at the two tests in which there was an exposed CLT ceiling:

A comparison of the temperatures at the ceiling reveals that during the decay phase, just prior to the fire re-growth, the temperatures were around 450°C. This was the case for both the FPRF test and the validation test performed at Southwest.

The measured temperatures from the two tests were also consistent with each other at the end of each test, during the fire re-growth stage. In both tests, the measured temperatures were right around 800°C at this stage.

This provides conclusive validation of the adhesive qualification test protocol….The objective of functionally replicating the exposure conditions measured in the FPRF compartment tests, within a more repeatable and slightly smaller-scale test protocol had been achieved.

As I mentioned earlier, one of the reasons for developing this test protocol was to determine whether or not the same fire re-growth behavior would have been observed if the FPRF tests had been performed using CLT products made with other adhesives.

To that end, CLT products made with other adhesives were also subjected to the same exposure under this test protocol. For example, the time-temperature graph from a test performed on CLT made with melamine-formaldehyde resin is shown in the lower-left. And then, in the lower-right is a time-temperature curve from a test performed on CLT made with an improved formulation of polyurethane resin.

Inspection of both of these graphs at the bottom of the slide reveals that no fire re-growth occurred during either of these tests. This is obviously very different performance than that which was observed with the CLT made from a heat-delaminating polyurethane resin. In fact, these graphs confirm performance that is very much in line with performance that would be expected from solid wood.

In addition to the large-scale compartment adhesive qualification tests that we’ve been discussing, there is also an additional, small-scaleadhesive qualification test which is used for CLT adhesives in North America. This test is described in Section A.2 of standard CSA O177, as referenced and modified by the CLT product standard PRG-320, which we’ll talk about shortly.

In this test, a specimen of CLT made with the adhesive being qualified is exposed to a Bunsen-burner flame having a temperature of 800-900˚C (1470-1650˚F). The exposed face is the edge (i.e., cross-section) of the CLT, such that the edges of the glue lines are directly exposed to the flame. The duration of the exposure is specified as 10 minutes – 5 minutes in two different orientations.

This photo, which was provided by FPInnovations, shows the small-scale adhesive qualification test in progress, as it is performed on a CLT sample.

Following the test, the samples are cut in half, such that the cross-section of the specimen is visible. The CLT samples are then inspected for delamination using optical comparator to measure the five inner bond lines for the total length of delamination.

Under the Acceptance Criteria specified within the CSA O177 standard, the sum of the delamination of 5 exposed bond lines on each specimen shall not exceed 3 mm (i.e., a little less than 1/8 of an inch). As such, this small-scale test distinguishes between adhesives that undergo heat-delamination, and those that do not exhibit heat-delamination.

The photo on the left of this slide shows what the CLT specimens look like before the test, while the photo on the right shows some of the cut test specimens after the test. These cut specimens are the ones that are inspected for delamination. Thanks to FPInnovations for providing both of these photos.

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In this last segment of today’s presentation, we’re going to learn about the applicable code requirements for CLT in general, and specifically, for adhesives used in CLT.

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Let’s start with the International Building Code (IBC). I’m sure most of you are already familiar with the IBC, as it is the model building code which sets the regulatory basis for construction throughout the US.

Chapter 23 of the IBC requires that CLT be manufactured and identified in accordance with ANSI/APA PRG 320. PRG 320 is the American National Standard for Performance-Rated Cross-Laminated Timber. Specifically, this requirement is given in Section 2303.1.4 of the IBC, which is shown here.

As a product standard, PRG 320 specifies a wide range of requirements for CLT used in building construction. The provisions of PRG 320 include requirements governing:• dimensions and tolerances• Specifications and performance requirements for each of the

components, such as the laminations and adhesives,• Qualification and marking requirements for the CLT, and • Quality assurance requirements.

In other words, this code-referenced product standard is all-encompassing, and covers every aspect of importance for CLT used in buildings in North America.

But since the topic of this webinar specifically pertains to CLT adhesives, I’m just going to briefly mention the PRG 320 provisions governing this particular component. These provisions are contained within Section 6.3 of PRG 320.

Section 6.3.1 requires compliance with ANSI 405, which specifies various types of adhesive tests, including the small-scale CSA O177 test that I described a little earlier.

So, Section 6.3.1 mandates compliance with the small-scale CSA O177 qualification test for adhesive heat-delamination. But then, in addition to this small-scale test requirement, Section 6.3.3 of PRG 320 requires that the adhesive be qualified in accordance with a new mandatory annex – Annex B – which is now part of the 2018 version of PRG-320.

The test protocol described in Annex B is a large-scale compartment test that is essentially the same test I described earlier, which was developed at Southwest Research Institute. As you’ll recall, this large-scale compartment test measures fire re-growth - or, more

importantly, lack thereof, as the case should be.

The text shown within the box here on this slide is an excerpt from Section 6.3.3 of PRG 320-2018. One of the notes, which you can see directly below Section 6.3.3, clearly states the intent of this provision. That is, to identify and exclude the use of adhesives that exhibit behavior resulting in fire re-growth during the cooling phase of a fire.

Here’s the scoping language of the large-scale compartment test in Annex B. As you can see, it is a mandatory part of this code-referenced standard. The provisions of Section B1.3 are the same as the pass/fail criteria, which we’ll discuss in a minute or so…

Here’s the calibration time-temperature curve specified in PRG-320 for the large-scale compartment adhesive qualification test. This curve should look familiar to you, since it is essentially the average of the measured time-temperature curves from the calibration test we talked about earlier. The laboratory performing the adhesive qualification calibrates their test apparatus by adjusting the fuel-flow-rate to achieve this specified time-temperature curve.

As we discussed earlier, this is not exactly the same as the measured time-temperature curve from FPRF Test 1-1. Rather, it represents the time-temperature curve necessary within the large-scale adhesive qualification test compartment, in order to replicate the heat flux to the ceiling in FPRF Test 1-1.

Once the test apparatus is calibrated, an 8’-wide by 16’-long CLT specimen, made with the adhesive being qualified, is placed in the test compartment, such that it forms an exposed ceiling within the compartment.

The exposed CLT floor/ceiling assembly is then loaded, and subjected to the prescribed exposure, which replicates the relatively severe exposure of the FPRF Test. This exposure is carried out for a period of 240 minutes (4 hours).

Here, you can see another picture of the large-scale adhesive qualification test in progress, and an excerpt from the PRG 320 provisions describing the test method.

The acceptance criteria for determining whether or not the adhesive qualifies for use in CLT under the IBC are given in Section B12 of PRG 320. They require that:

1. The exposed CLT assembly must sustain the applied load throughout the entire 4-hour test, and

2. The temperatures measured by the five thermocouples at the ceiling must all remain below 510˚C throughout the time period from 150 minutes, to the end of the test.

Recall that significant fire re-growth was observed in the validation test performed during the development of this large-scale test protocol. During the fire re-growth in that particular test, measured temperatures far exceeded the threshold limit now specified in PRG 320 – by several hundred degrees. Therefore, the adhesive used for the CLT that was tested in the FPRF test series would clearly fail these new requirements, and would not be permitted by the code for tall mass timber buildings. ________(Pause)________

In summary, these new provisions provide the mandatory, code-prescribed, regulatory means of identifying and excluding adhesives that allow heat-delamination leading to fire re-growth. They establish the means for ensuring that CLT used in buildings will perform like solid wood in the event of a fire. In other words, when exposed to fire, the members will undergo steady, easily-modeled char development, maintain an unheated core insulated by the char layer,…and, most importantly, the will not exhibit any behavior leading to fire re-growth.

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