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Fire Damage Mechanism of RC Structure and Assessment Method

Mechanism of Fire Damage of RC Strcuture and Assessment Method

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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.

Fig. 1: Chemical Decomposition Led to Change of Color

Table 1 Use Concrete Color to Determine Degree of Temperature and Assess Concrete Condition

Temperature, CColor changeChange in physical appearance and benchmark temperatureConcrete condition
290-590Pink to redSurface crazing at 300 C, deep cracking at 550 C, and popouts at 590 C.Concrete remain sound but its strength reduces significantly
590-950Whitish greySpalling 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 950BuffExtensive spallingConcrete 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.

Fig. 2: Spalling of Reinforced Concrete Beam Due to Exposure to Fire

Assessment Methods

1. Preliminary Investigation

1.1 Cleaning

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.

Fig. 3: Taken Core Sample

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.

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 structureTest Methods
Actual temperature reached in buildingExamination of building contents based on Table 3.
Actual temperature reached in concreteVisual examination of concrete based on Table 1, petrographic see Fig. 4, DTA, and metallurgical studies of steel.
Compressive strengthTests 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 elasticityTests on cores and Soniscope studies
Dehydration of concreteDTA, petrographic, and chemical analysis
Spalling and aggregate performanceVisual examination and petrographic analysis
CrackingVisual examination, soniscope test, and petrographic analysis
Surface hardnessDorry hardness or other tests
Abrasion resistanceLos Angeles abrasion test on concrete chips
Depth of damageVisual examination for spalling, cracking, color variation in cores, chipping, and petrographic analysis
Deformation of beamsVisual examination, straightedge and scale, and dial gages or theodolite if needed.
Gross expansionVisual examination, and Checking of dimensions and levels
Differential thermal movementVisual check of cores for loss of bond to steel, and color change in concrete next to steel.
Reinforcing steel, structural steel, or prestressing steelPhysical tests, metallurgical studies, dimensional changes, displacement, and distortion.
Load carrying capacityLoad tests on structure

Table 3 Conditions of Materials Useful for Estimating Temperature Attained Within a Structure During a Fire

MaterialExamplesConditionsTemperature, C
LeadPluming leadShape edges rounded or drops formed300-350
ZincPlumbing fixturesDrops formed400          
Aluminum and its alloysSmall machine parts, toilet fixturesDrops formed650
Molded glassGlass block; jars and bottlesSoftened or adherent700-750
Molded glassGlass block; jars and bottlesrounded750
Molded glassGlass block; jars and bottlesThoroughly flowed800
Sheet glassWindow glass, plate glassRounded800
Sheet glassWindow glass, plate glassThoroughly flowed850
Sheet glassWindow glass, plate glassSharp edges rounded or drops formed950
SilverJewelry, coinsRounded800
SilverJewelry, coinsThoroughly flowed850
SilverJewelry, coinsSharp edges rounded or drops formed950
BrassDoor knobs, locks, lumpSharp edges rounded or drops formed900-1000
BronzeWindow framesSharp edges rounded or drops formed1000
CopperElectric wireSharp edges rounded or drops formed1100
Cast ironPipes, radiatorsDrops formed1100-1200

Read More:

Fire Resistance Ratings of Concrete and Masonry Structural Elements

Explosive Spalling of Concrete Structural Elements during Fire

Behavior of Concrete in Extreme Fire

Fire Resistant Buildings Requirements

Fire Proof Concrete

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