Examining the behavior of concrete masonry units under fire and post‐fire conditions

Masonry is an inert construction material with favorable thermal and mechanical properties. While masonry is widely used in buildings, the fire performance of this material has not received much attention over the years. This continues to hinder the understanding of the fire behavior of masonry. To bridge this knowledge gap, this study presents the results of an experimental campaign carried out on concrete masonry blocks (CMUs) to investigate fire‐induced degradation of the compressive strength of CMUs under elevated temperatures and post‐fire conditions. In this campaign, steady‐state tests were conducted; wherein standard‐sized CMUs are exposed to a heating scenario ranging from 25 to 800°C followed by cooling to ambient temperature. In addition, these tests were also complimented with a thermogravimetric (TGA) analysis to arrive at a comprehensive understanding of the degradation of the strength property of masonry. Results from the tests clearly show that the degradation in CMUs is lower than that typically observed in normal strength concrete. Furthermore, our findings also infer that masonry is capable of retaining a larger percentage of strength when tested under post‐fire conditions as opposed to being under heating.

not provide guidance for standard testing methods for masonry. This has forced researchers to use self-derived testing methods or simply modify/extend existing techniques available for concrete testing under fire conditions. 8,18 A deep dive into the available material testing protocols used in the past substantiate significant differences in procedures, specimen sizes, equipment setup, heating/cooling rate, etc 8 General observations from these results indicate that: (a) there is an implied agreement that the properties of masonry would follow a similar trend to that of concrete material, and (b) regardless of the origin and composition of masonry, it is also common to assume that temperature degradations in masonry are expected to follow that of the Eurocode 6 model. Perhaps one of the most notable studies was conducted by Eurocode 6. 24 This guiding document provides general guidance on the degradation of mechanical properties of masonry as a function of temperature rise. For a more comprehensive treatment of the test methods, interested readers are encouraged to review a newly published review paper by the authors 8 that summarizes various testing methods and practices (please refer to Table 1, which summarizes some of the key aspects noted in our earlier review).
On the material front, the hydrated cement phases undergo several thermal reactions, which include dehydration, dihydroxylation, decarbonation oxidation, decomposition, phase transition, and melting when subjected to heat. These reactions are generally associated with weight change, and temperatures at which these processes occur are typical for a hydrate phase. 32 Thermogravimetric analysis is used to measure the continuous mass loss due to the transition and decomposition of hydrated cement phases as a function of increments in the temperature (typically from ambient to 1000 C). The transition/ decomposition temperature signatures for each phase in anhydrous and hydrated cement phases have been studied and mapped by Collier. 33 Hence, TGA provides an effective measure of phase evolution/decomposition of the hydrated phases under elevated temperatures, such as in the case of fire, which can be correlated to the change in mechanical properties of the material. The differentiation of thermogravimetric data allows a better resolution and identification of weight loss. 34 Derivate thermogravimetry (DTG) plots the rate of material weight changes upon heating is plotted against temperature and used to simplify reading the weight versus temperature thermogram peaks that occur close together. 34 These general observations are merely accurate and adopted to overcome incomplete knowledge of the behavior of masonry under elevated temperatures. To have a better understanding of temperature-dependent material properties and behavior, this paper presents an experimental program designed to obtain the compressive strength of CMUs at different temperatures in the range of 25-800 C in hot state and residual state conditions. These results were then compared with collected data from Aditya and Naser. 8 Thermogravimetric analysis was also carried out on CMUs to understand chemical degradation reactions in concrete the CMU matrix microstructure under elevated temperatures.

| DESCRIPTION OF EXPERIMENTAL PROGRAM
This section describes the experimental campaign carried out herein. Our testing program mirrors that often adopted by other researchers. 19,20,22,23,[25][26][27][34][35][36]    painted, heavy-gauge steel case (see Figure 2). For the furnace, the inputs for temperature, heating rate, and retention time are programmable. Temperatures for ambient temperatures inside the furnace and samples were recorded using a thermocouple at an interval of 5 min.

| Universal testing and compressive testing equipment
Two loading testing equipment were used in this program. The first is a universal testing machine (UTM) (SHIMADZU AUTOGRRAPH T A B L E 1 Summary of recent and notable works on the front of masonry under fire conditions

Study Type of testing Findings
Harmathy 25 Tested 47 specimens made from 17.5% hydrated Portland cement and 82.5% expanded shale, brown clay brick, and insulating fire brick. In addition, 35 specimens were tested once, and 12 were subjected to repeated heating.
• The tested specimens showed 6%-19% increase in strength during the first fire test than in the repeated tests. • Moisture content decreases with an increase in the duration of fire testing.
• Derived a new material model for masonry.
• Noted that the compressive strength was reduced by 28% from 200 to 400 C, as well as 18% and 65% at 600 and 800 C, respectively. • When compared to concrete, the concrete masonry blocks achieved improved performance.
Bosnjak et al 28 Tested the residual response of solid clay brick and calcium silicate bricks in the temperature range of 20 to 1100 C.
• The clay bricks showed low sensitivity to the rise in temperature until 500 C. • The compressive strength of calcium silicate brick increased to 300 C and then dropped at 700 C.
The National Bureau of Standards 29 (Report 117 and Report 120) Conducted over 60 fire tests on large-scale masonry walls.
• The fire resistance of the tested walls varied between 69 min to about 7 h, depending upon thickness, moisture content, type of aggregate, and loading.
Lawrence and Gnanakrishnan 30 Carried out 146 large-scale masonry load-bearing walls and another 30 on non-load-bearing walls with various masonry types and thicknesses.
• Noted the development of high thermal gradients due to the differential expansion of the hot and cold faces of the tested walls. • Also noted a discrepancy between tests of identical specimens and emphasized the need for duplicated tests.
Keelson 31 Tested four masonry walls of 2.8 m in width and 3.2 m in height with three thicknesses (100 mm, 150 mm, and 200 mm).

F I G U R E 1 Dimensions of Standard Half-sized CMU
Precision Universal Tester AG-IS 250 kN) with a maximum loading capacity of 250 kN (see Figure 3A). Its features contain a high rigidity frame, multiprocessors, high-speed sampling and accuracy, an intelligent controller equipped with a progressive user interface, and software that supports the creation of test conditions. This equipment is capable of recording the stress-strain (Force-displacement) behavior of tested units. All the deformation and load measurements were taken using the data logging systems in-built in the machine. This equipment was proven useful to record stress-strain curves for CMU specimens tested at 800 C.
Since the load capacity of available UTM was limited, a second piece of equipment was used to test units exposed to hot state tem-  Figure 3B). Unfortunately, this equipment was not capable of recording stress-strain curves of tested CMUs. were installed inside both vertical faces of the blocks. As the vertical faces of the blocks were of different thicknesses, two thermocouples were used (see Figure 4). As one can see, both thermocouples converge, indicating that uniformity of temperature rise is attained. In total, the uniformity test was repeated three times to establish confidence-a sample of thermocouple readings is shown in Figure 4.

| Unstressed residual testing
Once the uniformity test was completed, the fire tests were started.
As mentioned earlier, two series of CMU units were tested: under residual and heating conditions. In the first series, CMUs were  At the end of the retention time, each unit was then directly tested under the compression to measure its compressive strength (and stress-strain curves for specimens exposed to 800 C). The exposed area of the testing machines was covered with a 2 h firerated glass fiber thermal blanket to prevent any damaging the equipment due to radiation from the heated specimens. Careful extraction of the specimens from the furnace was carried out using a newly designed tool with fire safety gears (rated to 1200 C)-see Figure 5.
Then, the extracted units were loaded until failure. A total of three units for each targeted temperature were tested, and their average strength value was calculated.

| Details on TGA equipment
The TGA samples were prepared by taking a sample from the center and the inner portion of the CMU block. The samples were crushed using an agate mortar and pestle until all the contents passed 150-micron sieve (#100 sieve). The samples were collected and placed in an airtight vial to prevent carbonation and subsequently loaded into the auto-sampler of the testing device and tested. The samples were analyzed using a TA instrument Q5000 in a platinum crucible up to 1000 C. For each specimen, approximately 50 mg of sample was loaded in the crucible and heated at a temperature ramp of 10 C/min up to the desired peak temperature, while the sample was simultaneously purged with nitrogen gas at 30 ml/min.

| RESULTS OF FIRE TESTS
This section presents findings from our experimental campaign.  In all tests, full documentation of failure modes was recorded.
The experimental study infers that all the CMUs (whether tested under hot and residual states) failed due to shear splitting. Diagonal cracks throughout the load-bearing membrane of the units were F I G U R E 7 Failure mode of tested CMUs under residual state conditions observed. Shear splitting due to diagonal rupture of the blocks progressing to corners was observed in blocks at ambient and elevated temperatures. Figure 7 shows all the failure patterns observed during testing under both residual and hot state conditions.
The blocks tested at 600 and 800 C showed shear cracks that were more self-evident as compared with cracks in blocks tested under lower temperatures. It is also worth mentioning that blocks prepared for residual testing at 800 C showed surface scarring after the resting period. This can also be the justification for a sudden drop in compressive strength above 600 C.

| Hot state
In this section, the fire-induced degradation in compressive strength of the CMUs tested under unstressed hot conditions is presented. Abrams. 42 This slight increase is possibly due to the presence of lightweight aggregate. 43 The same effect is not apparent in the residual conditions. This is likely due to moisture re-absorption from the atmosphere while storing the post-heated specimens, as noted by Abrams,43 or the possible increase of structural damage due to cracks upon cooling, as noted by Malhotra. 44 At the present moment, we speculate that the reason for this From 600 to 800 C, the compressive strength of the tested units drops by 70% due to the severe decomposition of constituent materials. 1 Figure 8B shows a comparison between the reduction factors as obtained from testing under the hot state and the residual state as per the above discussion. A comparative analysis of our test results pertaining to the residual strength and while hot (bot which was unstressed) notes that there is an additional loss of strength observed for the residual specimens (i.e., those heated and cooled down to ambient conditions and then tested after 2 weeks). This analysis agrees with that reported by Abrams 43 and Malhotra, 44 who, individually, reported a difference of about 20% between the two states.
In addition, a sample of three stress-strain curves was obtained for specimens tested in this series. These stress-strain curves are plotted in Figure 9. As one can see, these curves resemble that which could be obtained at ambient conditions but with a slight and smoother ascending portion. Unfortunately, the stress-strain curves for other temperatures were not recorded due to technical difficulties and mechanical failure in the used equipment. We hope our future tests to be able to record and compare such curves at different temperatures to gain further insights into the effect of temperature rise on the stress-strain response of CMUs. Figure 10A,B shows thermogravimetric analysis (TGA) data and differential thermal analysis (DTA) data, respectively. The TGA data analysis from Figure 10A shows a mass loss of 1.44%, 1.98%, and 3.18% at temperatures of 200, 400, and 600 C, respectively, compared with mass at ambient temperature. The mass loss up to 600 C is typically due to the loss of unbound and bound water from the hydrate phases. However, there is a significant mass loss of about 19.54% between 600 and 800 C.

| Thermal analysis results
Typically, this mass loss can be attributed to decarbonation of the cementitious system. As seen in DTA shown in Figure 10B, the main decomposition peak for the sample is between 600 and 800 C, and there is a significant decrease of about 40% in compressive strength between these temperatures. As the exact composition of the cementitious mortar used in the CMU is not known, it can be concluded that the loss is strength is due to an increase in porosity, which can be attributed to the decarbonation of a calcite (CaCO 3 ) rich phase. However, complimentary porosity using mercury intrusion porosimeter (MIP)/scanning electron microscopy (SEM) and XRD analysis can provide more insight into the evolution of microstructure due to the heating/fire.

| CONCLUSIONS
This paper presents findings from thermo-mechanical tests conducted on CMUs with an aim to develop temperature-dependent material models for masonry suitable for during and post-heating conditions.
Despite re-occurring challenges foreseen during this research, given the complexity of fire testing and its new nature to our institution, we believe that the outcome of our tests presents a glimpse into the fire response of CMUs, and we hope to be able to carry out an extensive research program on walls and other masonry members in the near future. We also invite interested researchers to continue and extend this research direction. The following list of inferences can also be drawn from this study: • Available testing methods in current standards lack guidance toward the testing of masonry materials at elevated temperatures.
As such, there is an urgent need to develop standardized testing methods to evaluate the mechanical and thermal properties of masonry at elevated temperatures.
• The examined CMUs were shown to retain 60%-80% of their room temperature strength post-exposure to 600 C.
• Results from our tests indicate that the temperature-induced degradation in CMUs is lower than that typically observed in normal strength concrete. Thus, we can infer that CMUs are capable of retaining a larger percentage of strength when tested under hot conditions as opposed to post-cooling, that is, in the residual state.

ACKNOWLEDGMENT
The authors would like to acknowledge and thank the support pro-

DATA AVAILABILITY STATEMENT
Some or all data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.
ORCID M. Z. Naser https://orcid.org/0000-0003-1350-3654 ENDNOTE 1 Please note that this particular model was arrived at from the presented tests and has also been recently used in a detailed statistical analysis in a companion paper. 45 That particular paper developed a new model from a comprehensive review (as well as the model developed from our tests).
To avoid building a new model that is only applicable to our tests (vs. the much rich dataset of masonry noted above), and to limit the amount of similarity between the two papers.