1. No category

Determination of the thermal history of fire
damaged concrete by Thermo-gravimetric Analysis
ac
a
MA Alqassim , N Nic Daeid , MR Jones
b
a
Centre for Anatomy and Human Identification, University of Dundee, Scotland, UK
b
Concrete Technology Unit, University of Dundee, Scotland, UK
c
General Department of Forensic Evidence and Criminology, Dubai Police GHQ, UAE
Abstract
Findings

In certain fires in concrete structures, the core temperature in the concrete may easily
reach 1000 °C. It is known that concrete exhibits a sequence of changes in its
mineralogical composition when exposed to a continuous heat flux. Thermo-gravimetric
(TG) techniques are often utilized to assess these thermal gradients, and the information
gathered can be helpful for fire investigators to determine the thermal history of an
incident and to gain a better understanding of the reactions taking place. While previous
works to date have only researched Portland cement (PC) mixes, this study looks at
modern cementitious materials containing silica fume (SF) and slag (GGBS), as well as
other mix constituents relevant to the United Arab Emirates building standards, including
Gabbro aggregate and dune sand. Also, for the purpose of this study, two water/cement
ratios were chosen (0.4 and 0.5).

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The major weight loss in all mixtures happens owing to the decarbonation of CaCO3
(650°C-800°C). The second peak occurs in the temperature range of 40-120 °C due to loss of
water.
The highest total weight loss was in Mix C (19%), which could be attributed to the variations in
the properties of the cementitious materials used .
The third peak happens between 400-500 °C and was owing to the dihydroxylation of the
portlandite. Ca(OH)2 reforms after fire exposure [4]. The onset temperatures of the reformed
portlandite, obtained by Differential TG, is shown in Figure 4 and Table 2.
The weight loss due to the dihydroxylation of the portlandite is very little for samples
pre-heated to 600 °C and 900 °C amongst all groups. This peak is also less visible in Mix C.
A PERKIN ELMER TGA 7 system was used for the thermos-gravimetric analyses, and
specimens were collected from pre-heated 75 mm cubes. The cubes were subjected to
incremental temperature rises to: 150, 300, 600 and 900 °C for five hours continuously in
an electric furnace, before being left to naturally cool down to room temperature for
another 24 hours. Subsequently, they were tested for residual strength, and powders were
also drilled from the cores then ground to fine particles.
Results
Experimental characterisation results show that the main % weight loss for all the
specimens was in the temperature range of 650-800 °C. This corresponds to the
decarbonation of calcium carbonate. A second peak occurs in the temperature range of 40
-120 °C and is due to the loss of evaporable water. Both of these reactions could be used
as tracers for the thermal history of an incident and could provide useful information to fire
investigators. The third peak is due to the dihydroxylation of the portlandite, which reforms
during the cooling down of concrete. Albeit this reaction is reversible the thermal history
can still be determined by comparing the onset temperature of reformed components,
which are usually dissimilar to the initial ones.
Figure 1. TG curves for Mix A
Figure 2. TG curves for Mix B
Figure 3. TG curves for Mix C
Figure 4. Onset Temperature of the Peak
of the Dihydroxylation Reaction vs
Previous Temperature Treatment
Background
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Absence of CaCO3 in the cement paste formulation following a fire is a tracer that the
temperature had exceeded 800 °C during the incident [1].
In temperatures between 180-300 °C, loss of bound water from the decomposition of
the calcium silicate hydrate (CSH) happens [2].
The portlandite (Ca(OH)2) decomposes between 400-500 °C. Albeit this reaction is
reversible, the thermal history can still be determined by comparing the onset
temperatures of reformed components.
Concrete made with GGBS and SF is known to show better fire resistance [3].
Increasing the water content in concrete mixtures may change their behaviour during
fire conditions.
TG Analysis is a method used to determine the changes in physical and chemical
properties as a function of increasing temperature.
Table 2. Onset Temperature of the Peak
of the Dihydroxylation Reaction vs
Previous Temperature Treatment
Methods

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Three concrete mixtures were prepared in the Civil Engineering Laboratories at the
University of Dundee (see Table 1).
The concrete mix design was proportioned to have a 28-day compressive strength of
40 MPa. The water/cement ratios ranged from 0.4 to 0.5.
75-mm cubes were heated in a CARBOLITE OAF chamber furnace up to various
predetermined temperature regimes (150°C, 300°C, 600°C, 900°C).
Each sample had been kept at the steady-state heat peak for 5 continuous hours and
was then left to naturally cool down to the ambient temperature.
The powders for the TG test were collected from the inner cores of the cube-crushed
specimens. The amount of material to be subjected to TGA was ground until a grain
size of 80 μm was obtained.
The temperature of the TG furnace was programmed to rise at a constant heating rate
of 10°C/min up to 900°C under an air flow of 30 ml/min (Figures 1 to 3).
Mix
A
B
C
Cementitious Materials (kg/m3)
PC
GGBS
SF
380
–
–
380
–
–
175
190
30a
*Achieved slump class S4.
a
SF contains 50% water by unit weight.
Water
(mL)
152
190
137
Aggregate (kg/m3)
fine
coarse
927
1080
880
1022
915
1075
SP*
(mL)
2.66
2.00
2.66
Mix A
Mix B
Mix C
100PC
0.4 w/c
100PC
0.5 w/c
46PC50GGBS4SF
0.4 w/c
25
482.9
464.4
469.6
150
482.4
471.7
473.5
300
467.5
467.0
471.7
600
448.4
437.8
473.4
900
389.1
398.1
398.8
Figure 5. The apparatus used for the
thermo-gravimetric study
References
1
Table 1. Mix proportions of the concrete specimens
Pre-heat
Temperature
(°C)
Alarcon-Ruiz, L., Platret, G., Massieu, E., & Ehrlacher, A. (2005). The use of thermal analysis in assessing
the effect of temperature on a cement paste. Cement and Concrete research, 35(3), 609-613.
2
Arioz, O. (2007). Effects of elevated temperatures on properties of concrete. Fire Safety Journal, 42(8), 516522.
3
Poon, C. S., Azhar, S., Anson, M., & Wong, Y. L. (2001). Comparison of the strength and durability performance of normal-and high-strength pozzolanic concretes at elevated temperatures. Cement and Concrete
Research, 31(9), 1291-1300.
4
Handoo, S. K., Agarwal, S., & Agarwal, S. K. (2002). Physicochemical, mineralogical, and morphological
characteristics of concrete exposed to elevated temperatures. Cement and Concrete Research, 32(7), 10091018.