Determination of the thermal history of fire

damaged concrete by Thermo-gravimetric Analysis

MA Alqassimac

, N Nic Daeida, 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

References 1 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), 516-

522.

3 Poon, C. S., Azhar, S., Anson, M., & Wong, Y. L. (2001). Comparison of the strength and durability perfor-

mance 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), 1009-1018.

Findings 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.

Abstract

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).

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.

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.

Background

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.

Methods

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).

Results

Figure 1. TG curves for Mix A Figure 2. TG curves for Mix B

Table 1. Mix proportions of the concrete specimens

*Achieved slump class S4. a SF contains 50% water by unit weight.

Figure 3. TG curves for Mix C Figure 4. Onset Temperature of the Peak of the Dihydroxylation Reaction vs

Previous Temperature Treatment

Mix Cementitious Materials (kg/m

3) Water

(mL)

Aggregate (kg/m3) SP*

(mL) PC GGBS SF fine coarse

A 380 – – 152 927 1080 2.66

B 380 – – 190 880 1022 2.00

C 175 190 30a 137 915 1075 2.66

Figure 5. The apparatus used for the

thermo-gravimetric study

Table 2. Onset Temperature of the Peak of the Dihydroxylation Reaction vs

Previous Temperature Treatment

Pre-heat

Temperature

(°C)

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