- Open Access

# Evaluation of Fire-Damaged Concrete: An Experimental Analysis based on Destructive and Nondestructive Methods

- Gang-Kyu Park
^{1}and - Hong Jae Yim
^{2}Email author

**11**:211

https://doi.org/10.1007/s40069-017-0211-x

© The Author(s) 2017

**Received:**20 June 2016**Accepted:**17 June 2017**Published:**12 September 2017

## Abstract

Fire damage to concrete causes contact-type defects that degrade its durability through impaired mechanical properties. Various nondestructive tests are used to evaluate defects induced by fire damage. Recently, nonlinear ultrasonic methods such as the nonlinear resonance vibration method and nonlinear modulation method have been introduced. These nonlinear methods are more sensitive to fire-induced contact-type defects than the linear ultrasonic method. This study involved an experimental analysis of the residual material properties of fire-damaged concrete, specifically, compressive strength, splitting tensile strength, and static elastic modulus. The residual material properties of 116 cylindrical concrete samples with various mix proportions and subjected to various heating temperatures were measured by a destructive method, and their nonlinearity parameters were measured by two nonlinear ultrasonic methods. Through regression analysis, correlated relationships that can facilitate the prediction of residual material properties of fire-damaged concrete using measured nonlinearity parameters were identified. In addition, the effect of fire damage on the mechanical strength of concrete was investigated by comparison with the relationships for undamaged concrete, and relationships for the evaluation of fire-damaged concrete were identified through regression analysis.

## Keywords

- nonlinear ultrasonic method
- fire-damaged concrete
- mechanical property
- correlation study

## 1 Introduction

Although concrete is popular as a nonflammable material with low thermal conductivity, thermophysical and thermochemical alterations induced in concrete at high temperatures can degrade its performance (Bazant and Kaplan 1996). As the constituent materials of concrete have varying thermal expansion coefficients, fire-damaged concrete has distributed contact-type defects between the individual materials. These defects manifest as openings and pores within the concrete, and the nondestructive assessment of these defects can be used to evaluate the extent of fire damage (Yim et al. 2014). Researchers have conducted experimental studies to identify the effect of fire damage on the material properties of concrete with various mix proportions under various fire scenarios (Chang et al. 2006; Handoo et al. 2002; Lee et al. 2008; Tufail et al. 2016). It has been reported that high temperatures degrade the material properties of concrete, including strength (Al-Nimry and Ghanem 2017; Dos Santos et al. 2002; Li and Liu 2016). To evaluate the durability and reusability of fire-damaged concrete, destructive and nondestructive tests are performed in this study with various proposed methods.

An increase in contact-type defects within fire-damaged concrete results in the degradation of mechanical properties including stiffness and strength. Among the nondestructive methods, ultrasonic measurement methods demonstrate high potential and are applicable for evaluating damaged concrete (Dilek 2007; Dilek and Leming 2007; Ham and Oh 2013). These methods can be divided into two categories depending on the measurement techniques and target defects: linear and nonlinear ultrasonic methods. Linear ultrasonic methods, which measure wave velocity, wave attenuation, and impact echo, are conventional ultrasonic methods and have been widely used to evaluate damaged concrete, particularly fire-damaged concrete (Chaix et al. 2003; Colombo and Felicetti 2007; Dilek and Leming 2007; Epasto et al. 2010; Kee and Nam 2015; Yang et al. 2009). However, these linear methods exhibit lower sensitivity to distributed defects and contact-type defects at the micro-scale (Jhang 2009; Park et al. 2015) compared to nonlinear ultrasonic methods, which are more sensitive to early-stage micro-scale defects as these defects cause nonlinear behavior of an incident wave (Zaitsev et al. 2006). To investigate this phenomenon in concrete, Chen et al. (2010) performed an experiment to characterize micro-scale defects induced by an alkali-silica reaction by using a nonlinear impact resonance acoustic spectroscopy technique. In addition, Payan et al. (2007) conducted nonlinear resonant ultrasound spectroscopy to assess the damage to concrete caused by exposure to high temperatures. Moreover, recently, defects in fire-damaged concrete with various mix proportions and under various temperature scenarios have been evaluated by various nonlinear ultrasonic methods (Park et al. 2015; Yim et al. 2014).

Park et al. (2015) measured the hysteretic nonlinearity parameter (HNP) of fire-damaged concrete with various mix proportions and under various fire scenarios by using a nonlinear resonance vibration method (Van Den Abeele et al. 2000a). The splitting tensile strengths of the samples were also evaluated, and the correlation between both the measurement results was reported. On the other hand, Yim et al. (2014) measured the nonlinearity parameters of concrete samples fabricated with mix proportions same as to those in a previously mentioned study, using a nonlinear modulation method (Van Den Abeele et al. 2000b). Moreover, measurements of residual material properties such as compressive strength, static elastic modulus, and peak strain were also performed to obtain correlations with the nonlinear parameter. Notwithstanding the numerous correlation studies of fire-damaged concrete that have been reported to date, several correlations remain to be analyzed. In this context, the present study involves an experimental analysis to evaluate the residual material properties of fire-damaged concrete based on nonlinearity parameters measured using two dissimilar nonlinear ultrasonic methods. To examine the sensitivity of the two nonlinear ultrasound methods in evaluating fire-damaged concrete of various mix proportions at various temperatures, their results are compared, and correlated relationships are proposed to assess residual mechanical strengths using measured nonlinearity parameters. In addition, the relationship between the compressive strength and tensile strength of fire-damaged concrete is proposed and compared with the corresponding relationships for undamaged concrete.

## 2 Sample Preparation: Fire-Damaged Concrete

*w/cm*) and fine-to-coarse aggregate weight ratios for normal strength concrete. Type I Portland cement was used to produce all the samples with crushed gravel as coarse aggregate (maximum size 19 mm) and fine aggregate (maximum size 4 mm). Additional admixtures or materials were not used in any of the concrete samples. According to the various mix proportions, the samples were labeled from C1 to C4, as presented in Table 1. The concrete samples were cast into a cylindrical shape with a height of 200 mm and diameter of 100 mm. Totally, 116 cylindrical concrete samples (29 samples for each mix) were then cast and cured for 28 days prior to high temperature exposure.

Mix proportions of concrete samples.

Mix | Water (kg/m | Cement (kg/m |
ratio | Fine aggregate (kg/m | Coarse aggregate (kg/m | Fine-to-coarse aggregate ratio |
---|---|---|---|---|---|---|

C1 | 160 | 320 | 0.5 | 744 | 1100 | 0.68 |

C2 | 171 | 285 | 0.6 | 744 | 1100 | 0.68 |

C3 | 160 | 320 | 0.5 | 922 | 922 | 1 |

C4 | 171 | 285 | 0.6 | 922 | 922 | 1 |

For measurements using the nonlinear modulation method and compressive strength test, 16 cylindrical samples, including four reference specimens, were used for each mix. On the other hand, for the nonlinear resonance vibration method and splitting tensile strength test, 13 cylindrical samples, including one reference specimen, were used for each mix. Moreover, each cylindrical sample was divided into five disk samples that were 25 mm in height.

Prior to exposure to high temperatures, the prepared concrete samples were placed in a drying oven at 100 °C for approximately 24 h to avoid hygrothermal spalling, i.e., explosive spalling, during the fire experiment. The concrete samples, apart from the reference samples, were then exposed to various peak temperatures (200, 400, and 600 °C). For the homogenization of temperature in the entire volume of each sample, exposure to peak temperatures was maintained for 2 h using an electric muffle furnace as it was reported that heat conduction up to the center of a sample is complete when it is subjected to 2 h of exposure (Yim et al. 2014). After exposure, all the samples were cooled in water at 20 °C for 5 min to avoid unexpected recovery phenomena during cooling, such as refilling of the fire-induced defects due to rehydration under a high-humidity condition, and were subsequently kept under an air-curing condition.

The degree of thermal damage in fire-damaged concrete is influenced by several factors including exposure temperature, exposure time, after-fire curing periods, and sample sizes. Among these factors, it has been reported that the after-fire curing period insignificantly affects the residual material properties under an air-cured condition, while fire-damaged concrete under a completely saturated curing condition appears to recover its mechanical strength (Park et al. 2015). Accordingly, for a correlation analysis, the concrete samples in this study were air-cured after heating to ignore the curing condition. Additionally, sample size influences the degree of thermal damage; however, it has been reported that varying the sample size negligibly influences the measured nonlinearity parameters (Yim et al. 2014). Therefore, this study attempts to correlate the measured HNP of thin disk samples with the measured nonlinearity parameter of the cylindrical samples.

## 3 Nonlinear Ultrasonic Methods

### 3.1 Nonlinear Resonance Vibration Measurement

*α*: obtained from the amplitude-dependent resonance frequency shift) can be used as the damage factor. The resonance frequency varies linearly with increasing input amplitude, and the degree of shift increases with damage. Therefore, the extent of fire damage can be evaluated through an analysis of the amplitude-dependent resonance frequency shift; further details are available in the study by Van Den Abeele et al. (2000a).

### 3.2 Nonlinear Modulation Measurement

*f*

_{ 0 }), instead of a low-frequency ultrasonic longitudinal wave, was generated using an impact hammer with a soft tip (086C03; PCB Piezotronics, Inc.) to create a resonance vibration mode in the sample (Donskoy et al. 2001; Warnemuende and Wu 2004). High-frequency sinusoidal signals were generated using a function generator (NI PXI-5421; National Instruments Corp.) with a sample rate of 100 MS/s, and a power amplifier (BA4825; NF Corp.) was used to amplify the generated signal. Two longitudinal narrow-band transducers (Panametrics X1019; Olympus NDT, Inc.) with a center frequency of 46.1 kHz were used for transmitting and receiving the high-frequency ultrasonic wave passing through the sample, and a tri-axial accelerometer (356A33; PCB Piezotronics, Inc.) placed opposite to the impact region measured the impact vibration (low frequency) in the three orthogonal directions. The measured high-frequency signal was digitized (NI PXI-5105; National Instruments Corp.) with a 60 MS/s sampling rate (12 bit resolution), and the dynamic vibration was measured (NI PXI-4472B; National Instruments Corp.) with a sampling rate of 102.4 kS/s (24 bit resolution).

## 4 Mechanical Strength Measurements

## 5 Results and Discussion

The average results of destructive and non-destructive tests.

Mix | Temp. | Compressive strength (MPa) | Splitting tensile strength (MPa) | Secant elastic modulus | HNP | Nonlinearity parameter (D) |
---|---|---|---|---|---|---|

C1 | Initial (20 °C) | 52.11 | 4.33 | 33.25 | 1.82 (10 | 0.046 |

200 °C | 37.17 | 3.96 | 22.89 | 5.98 (10 | 0.225 | |

400 °C | 30.37 | 2.59 | 8.169 | 1.12 (10 | 4.645 | |

600 °C | 23.96 | 1.00 | 2.911 | 2.84 (10 | 21.25 | |

C2 | Initial (20 °C) | 39.03 | 3.56 | 28.61 | 9.02 (10 | 0.022 |

200 °C | 27.08 | 3.34 | 18.74 | 4.73 (10 | 0.240 | |

400 °C | 25.67 | 2.15 | 6.651 | 1.54 (10 | 3.093 | |

600 °C | 18.29 | 0.84 | 2.154 | 2.97 (10 | 20.536 | |

C3 | Initial (20 °C) | 46.71 | 4.11 | 30.82 | 7.68 (10 | 0.030 |

200 °C | 34.78 | 3.62 | 18.88 | 4.30 (10 | 0.224 | |

400 °C | 27.68 | 2.46 | 7.190 | 1.15 (10 | 2.330 | |

600 °C | 21.81 | 1.07 | 3.002 | 2.04 (10 | 14.26 | |

C4 | Initial (20 °C) | 38.70 | 3.78 | 27.32 | 6.82 (10 | 0.021 |

200 °C | 28.42 | 2.93 | 16.33 | 3.19 (10 | 0.153 | |

400 °C | 24.35 | 2.10 | 5.964 | 9.03 (10 | 1.513 | |

600 °C | 17.95 | 0.70 | 1.976 | 2.1 (10 | 16.70 |

These relationships for all the thermal damage cases are adequately described by a negative power function. In the initial phase, the ratios of the mechanical properties exhibited a remarkable decrease until the nonlinearity parameter ratios increased to 10 for the HNP (\( \alpha \)) and 200 for the nonlinearity parameter (\( D \)) (approximately before 400 °C); this is illustrated in Fig. 8. In the following phase, relatively widespread values of the nonlinearity parameters are apparent with marginal variations in the mechanical properties. Generally, this widespread section can be attributed to the fire-damage to the concrete when subjected to a temperature of approximately 600 °C. This implies that the variations in the nonlinearity parameter ratios increase with temperature to a significantly higher degree than the variations in the strength ratios and static elastic modulus ratio. In addition, it can be observed that the compressive strength ratio, splitting tensile strength ratio, and static elastic modulus ratio converged to approximately 0.5, 0.25, and 0.1, respectively. This implies that among the three parameters, the static elastic modulus is most sensitive to elevation in temperature, followed by the splitting tensile strength and compressive strength.

It may also be feasible to correlate the measured nonlinearity parameters with the mechanical properties regardless of the mix proportions used in this study. Therefore, the residual material properties of concrete induced by fire damage can be evaluated using these criteria under the assumption of the used mix proportions. For example, the residual compressive strength of fire-damaged concrete can be estimated by measuring the HNP via the nonlinear resonance vibration method using thin concrete disks.

The regressed relationships illustrate that fire damage degrades compressive strength more than it degrades tensile strength. Furthermore, based on Eqs. (9) and (10), the residual compressive strength of fire-damaged concrete can be estimated from the measured value of splitting tensile strength, and vice versa; thus, it is advantageous to select the more efficient method based on the circumstances.

## 6 Conclusion

This study involved an experimental analysis of the nonlinearity parameters measured by two nonlinear ultrasonic methods. For the evaluation of fire-damaged concrete, the nonlinearity parameter measured by the nonlinear modulation method provided higher sensitivity than that measured by the nonlinear resonance vibration method. The measured nonlinearity parameter can be affected by experimental conditions such as the sample size, the impact type and location of impact that is generated, wave reflection, and boundary condition of the sample. Accordingly, the experimental results of this study, rather than representing general relationships, can provide a guideline on the relationship between the mechanical properties of fire-damaged concrete and nonlinearity parameters measured by the two nondestructive methods. Based on the results of these experimental methods, correlated relationships were proposed to evaluate the residual properties of fire-damaged concrete using the measured nonlinearity parameter. From these relationships, the residual compressive strength, tensile strength, and static elastic modulus after being exposed to fire can be estimated using the measured nonlinearity parameters without considering the mix proportions of the concrete. In addition, the effect of fire damage on the mechanical strength of concrete was investigated by a comparison of undamaged relationships, and relationships determined through regression analysis were proposed for fire-damaged concrete.

## Declarations

### Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1C1A1A01055474).

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

## Authors’ Affiliations

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