🕑 Reading time: 1 minute
Fire cause concrete surface cracking and crazing, chemical decomposition, and microcracking and spalling. These impacts can significantly change the properties of structural concrete, reducing strength and durability, and altering structural behavior.
Assessment and evaluation of damages caused by fire is substantially significant to specify the condition of the structure. The assessment of damages is used as a base to decide whether the structure need to be demolished, which is less likely since concrete is good fire resistance material, or repaired, and specifying repair technique when repair of the building is selected.
The assessment plan involves preliminary investigation followed by detailed investigation. In each phase of damage investigation, certain observations and tests are conducted to specify the severity and extent of the damage. For instance, non-destructive testing (NDT) methods can be employed to assess the residual durability properties, whereas core extraction tests can be performed to evaluate the residual mechanical properties.
Fire Damage Mechanisms
1. Surface Cracking
network of fine surface cracks in concrete which usually occurs in early ages of concrete due to shrinkage of the surface layer. They are caused by low humidity, fire, thermal incompatibility, hot sun, drying out. The depth of these cracks is 3 mm and their diameter of grids is smaller than 50mm.
2. Chemical Decomposition
The increase of temperature during fire leads to the evaporation of water and cement paste dehydration which decomposes both calcium hydroxide and calcium aluminates in concrete. The decomposition process would take place after capillary and free water are evaporated.
The chemical and physical modification and dehydration of cement paste because of fire lead to change in concrete color based on the temperature degree of the fire. This color change can be used as a sign of exposure temperature and consequently the corresponding fire damage of concrete can be estimated, Table 1 can be used as a guidance.
The compressive strength of concrete would not change up to 300?C, but this is threshold temperature for speed of strength loss in mortars. Despite the fact that concrete strength is not drastically change till 300?C, it is reduced significantly 30-40% due to internal cracks caused by thermal expansion. The strength of concrete would not be recovered 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|
|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 starts with the development of small cracks and then separation of surface layers of concrete because of rapid change in temperature, such as fire, and leads to exposition of steel reinforcement and its rapid deformation due to heat.
Half of yield strength of steel reinforced would be lost when temperature of fire is about 600 C. if the reinforcement bars cools from a temperature range of 450-600 C, its yield strength can be regained completely based on the steel bar rebar manufacturing type.
Spalling caused by high temperature can be full destruction at slow rates or sudden exploding of smaller or larger pieces of concrete with thickness less than few centimeters at early ages of heating.
1. Preliminary Investigation
It is considerably crucial to adequately clean smoke deposits since such debris cover the spalling and cracks due to fire. Moreover, cleaning of the building would facilitate clearer observations and more accurate identification of the deflected and distorted members.
Various methods such as water blasting, dry ice blasting, and chemical washing can be used for cleaning purpose. Dry ice blasting and chemical washing is desired since secondary damages to concrete structure due to these cleaning techniques are highly unlikely.
1.2 Visual Inspection
Visual inspector needs to record cracks, spalling, deformations, misalignments, distortions and exposure of steel reinforcements. And the geometry and deflection of some suspicious structural member should be measured and documented.
1.3 Fire Intensity
Fire intensity can be estimated by observing the building contents and the post-fire condition of the other materials. Inspection of building content and knowing melting point of some materials can be used as a guidance to determine maximum fire temperature.
1.4 Field Tests
If the previous phases of preliminary investigation would not reveal enough information used to determine the severity of fire and to decide future activities, simple field tests such as striking hammer and chisel in combination with visual inspection is considered to for the assessment of fire damages of concrete structure.
Striking hammer to the concrete material and taking the sounding is one of the common methods. Good and hard concrete tend to be solid and ring whereas weak concrete tends to be dull thud and hollow. Chisel is used to inspect the softened regions on the surface of the concrete
2. Detailed Investigation
Detailed investigation of concrete damages due to fire is carried out based on the findings and recommendations of preliminary evaluation. In detailed investigations, both non-destructive and destructive tests are involved.
2.1 Non-destructive tests
Non-destructive tests such as pulse velocity, impact-echo, radar, windsor probe, and rebound hammer can be used to specify certain concrete properties such as compressive strength
2.2 Destructive Test Methods
Destructive Test Methods need more time and effort to be carried out compare with non-destructive tests, and caution is necessary during the sampling process. various types of destructive tests are available for different purposes.
They can be conducted either in laboratory or in the field, and would produce detailed information regarding properties of materials, depth of fire, and location of cracks.
For instance, coring, which is tested in the lab, is mainly used to determine the poison ratio, modulus of elasticity and compressive strength of the concrete. Core samples should be taken carefully from locations where their effect on strength would be minimal, but provide necessary data at the same time.
Core samples are taken from areas not exposed to fire and those exposed to fire. The results of both tests are compared to obtain the most reliable information on changes in concrete caused by the temperatures.
Moreover, core samples can be used to achieve information about cracking in the interior of a member, the bond to reinforcing steel, and interior temperatures which is disclosed by changes in color, Fig. 4. Table 2 presents test methods which are used to determine the condition of concrete suffered from 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
|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||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|