Fire can cause various types of damage to concrete surfaces, including cracking, crazing, chemical decomposition, microcracking, and spalling. These effects can significantly impact the strength, durability, and structural behavior of concrete.
Assessing and evaluating the damage caused by fire is crucial to determine the condition of the structure. This assessment serves as a basis for deciding whether the structure should be demolished or repaired. Since concrete is a highly fire-resistant material, demolition is less likely, and repair is usually the preferred option. The specific repair techniques are determined based on the assessment of the building’s damage.
The assessment process typically involves two phases: preliminary investigation and detailed investigation. Each phase includes specific observations and tests to determine the severity and extent of the damage. Non-destructive testing (NDT) methods can be utilized in assessing the residual durability properties, while core extraction tests are performed to evaluate the residual mechanical properties.
Fire Damage Mechanisms
1. Surface Cracking
Fine surface cracks in concrete, commonly observed during the early stages of concrete formation, result from the shrinkage of the outer layer. Various factors such as low humidity, fire, thermal incompatibility, intense sunlight, and excessive drying contribute to their occurrence. These cracks typically have a depth of 3 mm and form a grid pattern with diameters smaller than 50 mm.
2. Chemical Decomposition
The temperature increase during a fire causes water to evaporate and cement paste to dehydrate, leading to the decomposition of calcium hydroxide and calcium aluminates in concrete. This decomposition occurs after the evaporation of capillary and free water. The chemical and physical changes, as well as dehydration of the cement paste, result in a color change in the concrete, indicating the degree of exposure temperature and estimating the corresponding fire damage. Table 1 provides guidance for interpreting the color change.
Up to a temperature of 300°C, the compressive strength of concrete remains unchanged. However, this temperature serves as a threshold for the speed of strength loss in mortars. Although the concrete’s strength does not change significantly up to 300°C, it is significantly reduced by 30-40% due to internal cracks caused by thermal expansion. The strength of concrete does not recover after cooling.
Table 1 Use Concrete Color to Determine Degree of Temperature and Assess Concrete Condition
| Temperature, C | Color change | Change in physical appearance and benchmark temperature | Concrete condition |
| 0-290 | None | Unaffected | Unaffected |
| 290-590 | Pink to red | Surface crazing at 300 C, deep cracking at 550 C, and popouts at 590 C. | Concrete remain sound but its strength reduces significantly |
| 590-950 | Whitish grey | Spalling and exposing less than 25% of steel bar surface at 800 C, powdered; light, and dehydrated cement paste at 890 C. | Concrete is weak and friable |
| Greater than 950 | Buff | Extensive spalling | Concrete is weak and friable |
3. Microcracking and Spalling
Spalling in concrete occurs when small cracks develop and surface layers separate due to rapid temperature changes, such as in a fire. This process exposes the steel reinforcement, which then undergoes rapid deformation from the heat. When the temperature of a fire reaches around 600°C, the yield strength of the steel reinforcement can be reduced by half. However, if the reinforcement bars cool from a range of 450-600°C, their yield strength can be fully regained depending on the type of steel bar used in manufacturing. Spalling caused by high temperatures can result in the gradual destruction of concrete or the sudden explosive detachment of smaller or larger pieces of concrete measuring less than a few centimeters, particularly during the early stages of heating.
Assessment Methods
1. Preliminary Investigation
1.1 Cleaning
Properly cleaning smoke deposits is essential as they conceal fire-related spalling and cracks. Additionally, it enables clearer observations and more accurate identification of deflected and distorted members within the building. Several methods, including water blasting, dry ice blasting, and chemical washing, can be employed for cleaning. Dry ice blasting and chemical washing are particularly preferred due to their minimal risk of causing secondary damages to the concrete structure.
1.2 Visual Inspection
A visual inspector is responsible for documenting various structural issues, including cracks, spalling, deformations, misalignments, distortions, and the exposure of steel reinforcements. Additionally, they need to measure and record the geometry and deflection of certain suspicious structural members.
1.3 Fire Intensity
If previous preliminary investigations do not provide sufficient information to assess the severity of fire damage in a concrete structure and determine future actions, simple field tests can be conducted. One such method involves using a striking hammer and chisel in combination with visual inspection. By striking the concrete material and listening to the sound produced, valuable information can be gathered. Good and hard concrete typically produces a solid ringing sound, while weak concrete tends to create a dull thud and hollow sound. The chisel is employed to examine any softened regions on the concrete surface. These tests assist in assessing the fire damages to the concrete structure.
1.4 Field Tests
Fire intensity can be gauged by assessing the contents of a building and examining the condition of materials after the fire. By inspecting the contents of a structure and considering the melting point of certain materials, one can gain insights into the maximum temperature reached during the fire.
2. Detailed Investigation
Detailed investigations of concrete damages resulting from fire involve a comprehensive analysis that builds upon the findings and recommendations obtained from the preliminary evaluation. These investigations employ a combination of non-destructive and destructive tests to gather essential information.
During the detailed investigations, non-destructive testing techniques are employed to assess the condition of the concrete without causing further damage. These methods allow for the identification of potential issues, such as cracks, spalling, or weakened structural integrity, by utilizing technologies like ground-penetrating radar, ultrasonic testing, or infrared thermography.
In addition to non-destructive tests, destructive tests are also conducted as part of the detailed investigations. These tests involve taking samples from the fire-damaged concrete for laboratory analysis. The samples are carefully extracted and subjected to various tests, including compressive strength testing, chemical analysis, and microstructural examination. These destructive tests provide deeper insights into the concrete’s properties, such as its load-bearing capacity, chemical composition, and the extent of fire-induced damage.
By combining the results from both non-destructive and destructive tests, a thorough understanding of the concrete damages caused by the fire can be obtained. This knowledge serves as a foundation for developing appropriate repair or rehabilitation strategies to restore the structural integrity and functionality of the fire-affected concrete elements.
2.1 Non-destructive tests
Non-destructive tests, like pulse velocity, impact-echo, radar, Windsor probe, and rebound hammer, offer effective means to assess specific concrete properties, such as compressive strength. These tests allow for the evaluation of concrete without causing damage, enabling the determination of important characteristics while preserving the integrity of the material. By utilizing techniques like pulse velocity measurement, impact-echo analysis, radar scanning, Windsor probe testing, and rebound hammer assessment, engineers can accurately specify concrete properties, including compressive strength, without compromising the structural integrity of the material being examined.
2.2 Destructive Test Methods
Destructive test methods require more time and effort compared to non-destructive tests, and caution must be exercised during the sampling process. These tests serve different purposes and can be conducted either in a laboratory setting or in the field. They provide detailed information about material properties, the depth of fire damage, and the location of cracks.
One example of a destructive test is coring, which is typically performed in a laboratory. Coring is primarily used to determine the poison ratio, modulus of elasticity, and compressive strength of concrete. When taking core samples, it is important to select locations where their impact on strength would be minimal while still providing necessary data. Careful consideration is necessary to ensure accurate results.
To gather reliable information about changes in concrete due to temperature, core samples are collected from both fire-exposed and non-exposed areas. By comparing the results of these tests, valuable insights can be obtained. Core samples provide insights into various aspects such as interior cracking, bonding with reinforcing steel, and interior temperatures, which can be inferred from changes in color (as shown in Fig. 4). Table 2 outlines the test methods used to assess the condition of concrete affected by fire.
Table 2 Test Methods Used for Details of Condition of Fire Damaged Concrete
| Condition of concrete structure | Test Methods |
| Actual temperature reached in building | Examination of building contents based on Table 3. |
| Actual temperature reached in concrete | Visual examination of concrete based on Table 1, petrographic see Fig. 4, DTA, and metallurgical studies of steel. |
| Compressive strength | Tests on cores, impact hammer test, penetration resistance, and soniscope test. |
| Soundness at highly stressed areas (upper side at center of beam; beam supports; anchorages for reinforcement near support; frame corners) | Hammer and chisel, visual examination, and Soniscope test. |
| Modulus of elasticity | Tests on cores and Soniscope studies |
| Dehydration of concrete | DTA, petrographic, and chemical analysis |
| Spalling and aggregate performance | Visual examination and petrographic analysis |
| Cracking | Visual examination, soniscope test, and petrographic analysis |
| Surface hardness | Dorry hardness or other tests |
| Abrasion resistance | Los Angeles abrasion test on concrete chips |
| Depth of damage | Visual examination for spalling, cracking, color variation in cores, chipping, and petrographic analysis |
| Deformation of beams | Visual examination, straightedge and scale, and dial gages or theodolite if needed. |
| Gross expansion | Visual examination, and Checking of dimensions and levels |
| Differential thermal movement | Visual check of cores for loss of bond to steel, and color change in concrete next to steel. |
| Reinforcing steel, structural steel, or prestressing steel | Physical tests, metallurgical studies, dimensional changes, displacement, and distortion. |
| Load carrying capacity | Load tests on structure |
Table 3 Conditions of Materials Useful for Estimating Temperature Attained Within a Structure During a Fire
| Material | Examples | Conditions | Temperature, C |
| Lead | Pluming lead | Shape edges rounded or drops formed | 300-350 |
| Zinc | Plumbing fixtures | Drops formed | 400 |
| Aluminum and its alloys | Small machine parts, toilet fixtures | Drops formed | 650 |
| Molded glass | Glass block; jars and bottles | Softened or adherent | 700-750 |
| Molded glass | Glass block; jars and bottles | rounded | 750 |
| Molded glass | Glass block; jars and bottles | Thoroughly flowed | 800 |
| Sheet glass | Window glass, plate glass | Rounded | 800 |
| Sheet glass | Window glass, plate glass | Thoroughly flowed | 850 |
| Sheet glass | Window glass, plate glass | Sharp edges rounded or drops formed | 950 |
| Silver | Jewelry, coins | Rounded | 800 |
| Silver | Jewelry, coins | Thoroughly flowed | 850 |
| Silver | Jewelry, coins | Sharp edges rounded or drops formed | 950 |
| Brass | Door knobs, locks, lump | Sharp edges rounded or drops formed | 900-1000 |
| Bronze | Window frames | Sharp edges rounded or drops formed | 1000 |
| Copper | Electric wire | Sharp edges rounded or drops formed | 1100 |
| Cast iron | Pipes, radiators | Drops formed | 1100-1200 |