Concrete surfaces can suffer various types of damage as a result of fire, including cracking and crazing, chemical decomposition, and microcracking and spalling. These changes can significantly affect the properties of the concrete, making it weaker and less durable, and altering its structural behavior.
It is crucial to assess and evaluate the damage caused by fire to determine the condition of the structure. This assessment is used to decide whether the building needs to be demolished or repaired. Given that concrete is a good fire-resistant material, demolition is less likely, and repair is often the preferred option. The assessment plan typically involves a preliminary investigation followed by a detailed investigation. Each phase of damage investigation involves specific observations and tests to determine the severity and extent of the damage.
Non-destructive testing (NDT) methods can be used to assess the residual durability properties of the concrete, while core extraction tests can be performed to evaluate its residual mechanical properties. These tests are vital in determining the appropriate repair technique to use when repairing the building.
Fire Damage Mechanisms
1. Surface Cracking
Fine surface cracks that appear in concrete during its early stages are often the result of shrinkage in the surface layer. These cracks form a network and are typically caused by a variety of environmental factors, such as low humidity, fire, thermal incompatibility, hot sun, and drying out. The cracks themselves are not very deep, with a maximum depth of 3mm, and are arranged in a grid pattern with a diameter smaller than 50mm. These cracks can impact the overall durability and aesthetic appearance of the concrete surface, and may require repair in order to maintain the structure’s integrity.
2. Chemical Decomposition
When a fire occurs, the temperature increases and causes water to evaporate from the concrete, as well as leading to the dehydration of the cement paste. This causes the decomposition of calcium hydroxide and calcium aluminates in the concrete. This process occurs after the evaporation of capillary and free water. As a result of these chemical and physical modifications, the color of the concrete changes, and this change can indicate the degree of exposure to the fire. Table 1 can be used as a guide to estimate the level of fire damage based on the color change.
The compressive strength of concrete remains unchanged up to a temperature of 300°C. However, this temperature marks a threshold for the speed of strength loss in mortars. Despite the fact that the strength of concrete does not drastically change until 300°C, it is significantly reduced by 30-40% due to the development of internal cracks caused by thermal expansion. It is important to note that the strength of the concrete cannot be recovered after cooling.
Fig. 1: Chemical Decomposition Led to Change of Color
| 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 is a phenomenon that occurs when the surface layers of concrete separate due to rapid temperature changes, such as those caused by fire. This process begins with the development of small cracks in the concrete, which can eventually lead to the exposure of the steel reinforcement within the structure. Once the steel is exposed, it can rapidly deform due to the heat.
The effects of high temperatures on steel reinforcement can be severe. For example, if the temperature of a fire reaches around 600 degrees Celsius, half of the yield strength of the steel can be lost. However, in some cases, the steel can regain its yield strength if it cools from a temperature range of 450-600 degrees Celsius, depending on the type of steel bar used in the manufacturing process.
Spalling caused by high temperatures can occur at varying rates. In some cases, the destruction can be slow and gradual, while in other cases, smaller or larger pieces of concrete may suddenly explode at early stages of heating. This type of spalling typically affects concrete that is less than a few centimeters thick. Overall, spalling caused by high temperatures can have a significant impact on the strength and stability of concrete structures.
Assessment Methods
1. Preliminary Investigation
1.1 Cleaning
Ensuring thorough cleaning of smoke deposits is of utmost importance as these residues can obscure the spalling and cracks caused by fire. Additionally, a clean building allows for clearer observations, leading to more accurate identification of deflected and distorted structural members.
Several cleaning methods can be employed, including water blasting, dry ice blasting, and chemical washing. However, dry ice blasting and chemical washing are preferred as they are less likely to cause further damage to the concrete structure during cleaning.
Dry ice blasting and chemical washing are desirable cleaning techniques due to their low potential for causing secondary damages to the concrete structure. Proper cleaning is crucial for identifying and addressing any structural damage caused by fire, and using these methods ensures the best chance of achieving a thorough clean without causing additional harm
1.2 Visual Inspection
A visual inspector has been tasked with documenting various types of structural damage. This includes cracks, spalling, deformations, misalignments, distortions, and exposure of steel reinforcements. The inspector must ensure that these issues are properly recorded for future reference.
In addition to documenting damage, the inspector is also required to measure and document the geometry and deflection of any structural members that appear suspicious. This is an important task, as it can help identify potential problems before they become more severe. By accurately measuring and documenting any issues, the inspector can assist in ensuring the safety and longevity of the structure.
1.3 Fire Intensity
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To estimate the intensity of a fire, one approach is to examine the contents of the affected building and the condition of other materials after the fire. By inspecting the types of materials present and considering their melting points, it is possible to determine the likely maximum temperature that was reached during the fire. This information can provide valuable insights into the nature and severity of the fire, which can help investigators understand its cause and potential impact. However, it is important to note that other factors such as ventilation, fuel load, and fire suppression efforts can also influence fire intensity and should be taken into account when analyzing fire damage.
1.4 Field Tests
In cases where the initial stages of the preliminary investigation fail to yield sufficient information to determine the extent of fire damage and plan for subsequent actions, basic field tests may be employed to assess the damage inflicted on a concrete structure. One such method involves using a hammer and chisel in conjunction with a visual examination.
To begin with, the hammer is used to strike the concrete material and produce a sound. This sound is then analyzed to gauge the condition of the concrete. Hard, sturdy concrete generally produces a clear and resonant ring, while weak or damaged concrete tends to produce a dull, hollow sound. This method of taking soundings can provide valuable insight into the overall health of a concrete structure.
Additionally, a chisel can be utilized to investigate any regions on the surface of the concrete that may have become softened as a result of the fire. By carefully examining these areas, it is possible to determine the degree of damage that has been sustained and plan any necessary repairs or restoration efforts accordingly. By combining these simple field tests with a thorough visual inspection, it may be possible to obtain a comprehensive understanding of the fire damage inflicted upon a concrete structure.
2. Detailed Investigation
When conducting an investigation of concrete damage caused by fire, a thorough examination is required following the initial assessment’s discoveries and recommendations. This examination involves a combination of non-destructive and destructive testing methods to obtain comprehensive results.
The purpose of the detailed investigation is to gain a more in-depth understanding of the extent and nature of the damage that has occurred in the concrete. It is important to have a clear understanding of the damage so that appropriate repair and maintenance strategies can be implemented to ensure the structural integrity of the concrete.
To achieve this, non-destructive testing techniques are employed, which allow for the assessment of the internal conditions of the concrete without causing any damage. This is achieved through methods such as ground-penetrating radar, ultrasonic testing, and thermography.
In addition to non-destructive testing, destructive testing methods are also employed in the detailed investigation. This involves removing samples of the concrete for laboratory analysis to determine its physical and chemical properties. The samples may also be subjected to various tests, such as compressive strength testing, to determine the strength of the concrete and the extent of damage caused by the fire.
Overall, the detailed investigation of concrete damage due to fire is a crucial step in the repair and maintenance of the structure. By utilizing both non-destructive and destructive testing methods, a thorough understanding of the damage can be gained, and appropriate repair and maintenance strategies can be developed to ensure the longevity and safety of the structure.
2.1 Non-destructive tests
Several non-destructive tests are available for assessing concrete properties without causing any damage. These tests include pulse velocity, impact-echo, radar, windsor probe, and rebound hammer. These techniques are commonly used to determine concrete’s compressive strength and other related properties.
By utilizing these non-destructive methods, it is possible to specify the strength of concrete in a structure without causing any harm to the material. Pulse velocity, for instance, determines the time required for a sound wave to travel through concrete and uses this information to assess the material’s strength. Impact-echo, on the other hand, relies on analyzing the frequencies of the sound waves produced by an impact on the surface of the concrete to estimate its properties.
Similarly, radar and windsor probe techniques employ different methods to measure concrete strength, while the rebound hammer measures the energy returned by a spring-loaded mass striking the surface of the concrete. All of these methods offer non-destructive options for determining the compressive strength of concrete, and are widely used in the construction industry for quality assurance and control purposes.
2.2 Destructive Test Methods
Destructive test methods require more time and effort compared to non-destructive tests, and caution is necessary during the sampling process. These tests can be conducted either in a laboratory or in the field, and provide detailed information regarding properties of materials, depth of fire, and location of cracks. For example, coring, which is performed in the lab, is primarily used to determine the poison ratio, modulus of elasticity, and compressive strength of concrete. Careful selection of core sample locations is crucial, as they should minimally affect the strength of the structure while providing necessary data.
Fig. 3: Taken Core Sample
Core samples are collected from two different areas: those that were exposed to fire and those that were not. By comparing the results of tests conducted on both sets of samples, the most reliable information on the changes in concrete caused by the temperatures can be obtained. Additionally, core samples can provide valuable information about cracking inside a member, the bond with reinforcing steel, and interior temperatures, which can be determined by changes in color as shown in Figure 4. Table 2 lists the test methods that are used to assess the condition of concrete after exposure to fire.
Fig. 4: Petrographic Examinations of Core Sample of Concrete to Determine Extent, Type and Severity of damage
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 |