# A Case Study on the Rehabilitation of a Fire-Damaged Structure
## 3 Material Tests
### Strength Tests  
Two types of strength tests were performed on the concrete cylinders and rebar coupons. The purpose of the tests is to evaluate the remaining capacity of the structural element and, consequently, to determine the degree of retrofit required to recover the original structural capacity.  
For the strength test of the concrete cylinders, two or three cylinders with diameters of 100 mm were sampled from Beams #1 and #2, and Girder #2 indicated in Figure 12, respectively. Examples of the cored specimens are shown in Figure 13. The compressive strength tests of these specimens were conducted per ASTM (American Society for Testing and Materials) C42 [11]. The results are summarized in Table 1. Excluding the test results from three non-confirmative specimens represented by the shaded region, which are 1-1, 3-2 and 3-3, the average compressive strength was 30.7 MPa. This is equivalent to the design compressive strength (\(f_{c}\) ), 30 MPa. According to BS EN 13791 [12; 13; 14], the strength of cored specimens is lower than that of molded specimens due to the micro-cracks developed by the impact from drilling during the sampling process. The minimum characteristic _in-situ_ compressive strength required by BS EN 13791 is calculated by multiplying a factor of 0.85 and the design strength.  
Figure 12: Locations of sampled concrete core and rebar coupons.  
Figure 13: (**a**) Cored specimens and (**b**) specimen 3-1 after the test.  
ACI 318-05 [15; 16] also provides a guideline for evaluating the original strength based on the core test in Section 5.6.5.4: Concrete in an area represented by core tests shall be considered structurally adequate if the average of three cores is equal to at least 85% of \(f_{C}\)' and if no single core is less than 75% of \(f_{C}\)'. Since the average compressive strength of the core specimens is greater than 85% of the design strength, _i.e._, 25.5 MPa, and none of the test results are below 75% of the design strength, _i.e._, 22.5 MPa, the damaged girders and beams are judged to have remained sound in terms of concrete strength.  
Three rebar coupons 600 mm in length were sampled from each span of slab and also from one of the damaged girders, Girder #1. Their yield strength, tensile strength, and elongation were obtained as Table 2 according to ASTM E8-09 [17]. As shown in the table, the yield and tensile strengths of the longitudinal rebar in Girder #1 are much higher than the designed yield strength of 400 MPa. The yield strength of the stirrup is slightly below the design strength but almost reached the level that would not degrade the shear strength of the girder. In the case of the reinforcing bars obtained from the slabs, the measured strengths are on the level of 60%-70% of the original strength, except for the slab #2 where the fire was first extinguished. The specimen \(\phi\)14-2 showed sufficient strength levels, but the degree of elongation was relatively smaller than other rebar coupons.  
The results of the tests discussed in this section show that the compressive strength of concrete cores and the yield strength of longitudinal rebar were above the design strength, and the yield strength of the stirrups was slightly below the design strength. Only the residual strength of reinforcing bars obtained from the slab generally degraded from their original capacity.