PSI - Issue 68

Rami Hawileh et al. / Procedia Structural Integrity 68 (2025) 259–265 R. Hawileh et al./ Structural Integrity Procedia 00 (2025) 000–000

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3. Results and discussion 3.1. Residual compressive strength

The performance of the tested samples was analyzed in terms of compressive strength values, mass loss percentages, strength reduction, and extent of spalling. Fig. 1 shows the concrete samples from each mix after being exposed to a temperature of 600°C. Severe spalling can be noted in the concrete samples containing only steel fibres. In contrast, concrete samples from the HSC-ST+PP mix remained intact after being exposed to high temperatures. Fig. 2 illustrates a comparative analysis of the compressive strength of concrete samples subjected to increasing temperatures. The results demonstrate a continuous decline in compressive strength across all samples as the temperature rises. This reduction in strength can be attributed to the formation of micro-cracks, which arise from the shrinkage of the cement paste due to moisture evaporation and the thermal expansion of aggregates. These micro cracks compromise the bond strength between the cement paste and the aggregates, thereby diminishing the overall structural integrity of the concrete. Up to 400°C, all samples exhibit enhanced resistance to strength reduction, primarily due to the reinforcing effect of steel fibers on particle bonding. The inclusion of steel fibres helps to bridge micro-cracks and redistribute stress, thereby enhancing the tensile strength and toughness of the concrete. The steel fibers act as a secondary reinforcement, which delays the onset of significant cracking and helps maintain the structural integrity of the concrete under elevated temperatures. However, a notable strength reduction begins to manifest at 600°C. At this elevated temperature, the increasing effect of vapor pressure buildup within the HSC-ST samples becomes significant. This buildup of vapor pressure, caused by the evaporation of residual moisture trapped within the concrete matrix, leads to the formation of visible cracks and the explosive spalling concrete cover. The increased vapor pressure exacerbates the development of internal stresses, which results in a substantial reduction in compressive strength. Therefore, the HSC-ST samples exhibit a more severe reduction in compressive strength compared to the HSC-ST+PP samples. Specifically, HSC-ST experiences a reduction of 70% in its original compressive strength, while HSC-ST+PP only loses 15%. The presence of polypropylene fibres in HSC-ST+PP helps to mitigate the effects of thermal degradation. These fibres melt at high temperatures, creating additional voids within the concrete matrix that act as pressure relief channels, thereby reducing the internal vapor pressure and subsequent cracking.

Table 3: Test Matrix

Exposure temperature (°C)

Designation

Sample type

Mix

Quantity

C-ST-25 C-ST-200 C-ST-400 C-ST-600

Cylinder Compression Cylinder Compression Cylinder Compression Cylinder Compression Cylinder Compression Cylinder Compression Cylinder Compression Cylinder Compression

HSC-ST HSC-ST HSC-ST HSC-ST

25

3 3 3 3 3 3 3 3

200 400 600 200 400 600 25

C-ST+PP-25 C-ST+PP-200 C-ST+PP-400 C-ST+PP-600

HSC-ST+PP HSC-ST+PP HSC-ST+PP HSC-ST+PP

3.2. Strength Reduction Factor The strength reduction factor is a key method used by design standards to account for the effects of elevated temperatures on the compressive strength of concrete. These factors enable a comparison of the performance of high strength concrete (HSC) and normal-strength concrete (NSC), as illustrated in Fig. 3. Upon analysing the results, a clear trend is observed in the behavior of both HSC-ST and HSC-ST+PP. Initially, both materials follow similar patterns, with only a marginal reduction in strength up to 400°C. However, at 600°C, the effects of explosive spalling become significant, leading to a sharp increase in the reduction factor for HSC-ST, resulting in a strength loss of 15% lower than that of NSC. This reinforces concerns regarding the use of HSC reinforced with steel fibers in structural