# A Case Study on the Rehabilitation of a Fire-Damaged Structure
## 3 Material Tests
### Neutralization Test of Concrete  
To determine the degree of neutralization of the concrete by its pH level, the phenolphthalein (C\({}_{20}\)H\({}_{14}\)O\({}_{4}\)) indicator method was adopted. The key indicator of this test is that a solution of Phenolphthalein remains colorless in acids or neutral liquids, but turns bright reddish/pink in alkalis. Therefore, it is possible to infer the temperature history and neutralization depth through the change of color in the test surface of concrete, which permits the soundness and the reusability of the concrete exposed to fire to be judged.  
For the normal condition of concrete, calcium hydroxide, Ca(OH)\({}_{2}\), is created in the hydration process of cement, which is described by  
\[\mathrm{CaO}\,+\,\mathrm{H_{2}O}\,\mathrm{\rightarrow}\,\mathrm{Ca(OH)}_{2} \tag{1}\]  
Calcium hydroxide is a strongly alkaline product with a pH value ranging from 12 to 13. One of the particular characteristic of calcium hydroxide is that it decomposes into calcium oxide (CaO) and water by chemical decomposition at elevated temperatures (above 500 \({}^{\circ}\)C), such as those in a fire environment. As a result, the concrete gets neutralized, which is described by  
\[\mathrm{Ca(OH)}_{2}\,\mathrm{\rightarrow}\,\mathrm{CaO}\,+\,\mathrm{H_{2}O} \tag{2}\]  
The test was first performed on a sample piece of concrete popped out from the slab and on a part of wall where the plaster had already spalled, respectively, and then on small part of Girder #1 and Beam #2, both on the surface and inside the side cover. Both the piece of concrete and the part of the wall changed color almost instantaneously when the phenolphthalein indicator was applied as shown in Figure 9.  
During the test on the girder and the beam, 1% phenolphthalein solution was first applied to their side surfaces that were affected by the fire without treating the surfaces. As shown in Figure 10a, the  
Figure 8: Damage to Beam #2. (**a**) East side of beam seen from the north. (**b**) West side of beam seen from the north.  
color did not change in most of the region, with a purple area where the alkaline concrete reacted to the indicator. When the indicator was applied after chipping out the surface of concrete by 10 mm, a significant change in the color was immediately observed as shown in Figures 10b and 11. Therefore, the girder and beam are judged not to have experienced a maximum temperature of more than 500 \({}^{\circ}\)C and, as a result, have remained sound. We infer that the concrete in the girders and beams spalled during the fire not due to the degradation of the concrete itself, but to the sudden increase in temperature at the upper level in the room. The elevated temperature is deemed to have caused the expansion of the aggregate inside the concrete and to have weakened the so-called transition zone between the aggregate and the cement matrix.  
Figure 11: Preparation of the surface of Girder #1 (**a**) and application of phenolphthalein indicator (**b**).  
Figure 9: Neutralization tests on damaged concrete. (**a**) Tests on a sample piece of slab concrete. (**b**) Test on the surface of wall.