GREY MATTER
ASSESSMENT AND REPAIR OF FIRE DAMAGED CONCRETE STRUCTURES
H V Venkata Krishna - Technical Advisor
M S Sudarshan - Senior Director
Sunil V Sonnad- Senior Director
S Manoranjan - Principal Engineer
CIVIL-AID Technoclinic Pvt Ltd ( A Bureau Veritas Group Co.)Bangalore
ABSTRACT
The investigation of fire damaged buildings involves detailed physical observation of various structural members of fire affected regions, verification of burnt materials, duration of fire, fire extinguishing measures adopted, non destructive and semi-destructive tests on affected concrete members. The residual strength of concrete and steel after fire is arrived at from the results of relevant tests conducted. These results are compared with the theoretical methods available for assessing the residual strength of materials. Recently an industrial building in a city was damaged due to fire. After a detailed investigation of the fire affected building appropriate repair and restoration measures were recommended.
Introduction
It is recognized fact that concrete structures have ability to withstand fire considerably and are generally capable of being reinstated after fire. In addition, the structural continuity present in buildings provides reserve strength which may enable the structure to survive fires and be reinstated. In most of the cases reinstatement of building requires less expenditure when compared to demolition and rebuilding. However, in case of severe damage replacement of the building may be necessary.
Effect of High Temperature on Concrete
The spalled and discoloured or blackened concrete surfaces and exposed reinforcement are generally apparent after a significant fire. The residual strength of dense concrete after cooling vary depending on temperature attained, duration of fire, mix proportion and condition of loading during fire. The effect of increase in temperature on strength of concrete is not much up to a temperature of about 300?C. However temperature greater than 300? C can reduce the compressive strength of concrete because considerable proportion of free calcium hydroxide present in hardened concrete loses its water around 500? C leaving calcium oxide. If this calcium oxide gets wet or exposed to moist air it rehydrates to calcium hydroxide accompanied by volume expansion resulting in disruption of concrete. Portland blast furnace slag cement is found to be more resistant to action of fire in this regard than other type of cements.
The colour of concrete can change to pink or red in the temperature range of 300-600?C indicating onset of significant loss of strength due to heating. Pink or red discoloration is due to presence of ferrous, salts in aggregates, which dehydrate and oxidise in this temperature range. Concrete which has not turned pink or red is not necessarily undamaged by fire. The colour change to pink tends to be more predominant with silicious aggregate than calcarious and igneous crushed rock aggregates. Generally concrete containing light weight aggregates perform better than concrete with normal weight aggregates under fire. Basalt and granite perform better than dolerite and sand stones. The colour of concrete may change to whitish grey or purple in the region of 600? C to 900? C due to reaction between ferric oxide and lime forming light coloured calcium ferrites. However, the concrete disintegrates above 900? C. Thus the colour of concrete is an important parameter in the assessment of damage, as it gives an estimation of maximum possible temperature attained in any particular region.
The effect of temperature on compressive strength of concrete is dependent on water cement ratio. When exposed to temperature, high strength concrete tends to have lower percentage of residual strength than concrete of lower strength. In very dense concretes such as high strength concrete, self compacting concrete, spalling and blasting phenomenon is observed due to build up of vapour pressure in concrete. The modulus of elasticity of material is a measure of the ability to resist deformation. A considerable reduction in elastic modulus occurs during fire and after cooling. The elastic modulus recovers to some extent with time provided the concrete has not been heated above 500? C.
Tests for Condition of Concrete
The traditional approach for assessing the effects of fire on concrete is to chip off concrete to find the depth of pink discolouration by chiselling, coring or drilling into concrete. This gives some idea regarding the depth of concrete affected by fire. Discoloration may also occur as a result of carbonation and depth of carbonation may be found by spraying freshly broken surface with phenolphthalein.
Apart from observing the discoloration of concrete the temperature to which the concrete is exposed can also be determined by Differential Thermal Analysis on concrete samples collected from the fire affected regions. A typical concrete sample when subjected to heating in Differential Thermal Analysis shows peaks variations at different temperatures when chemical changes take place. Absence of such peaks indicates that concrete is already exposed to that temperature. Changes in the sample either exothermic or endothermic can be detected relative to the inert reference. This difference can also be captured through thermo gravimetric analysis where in the change in mass due to significant chemical reactions can be obtained. The sudden change in gradient of mass-temperature graph indicates such reactions. Based on these two tests and changes in properties of concrete, the temperature of exposure of concrete can be determined.
In addition to the above tests, non destructive tests like Rebound Hammer Test, Ultrasonic Pulse Velocity Test and Pull Out device test (Capo test) are generally useful for assessing the strength/integrity of concrete in the fire affected regions.
Test on Reinforcing Steel
| Significant loss of strength may occur when steel is exposed to high temperature, which may also contribute for any excessive residual deflection. However, recovery of yield strength after cooling is expected for temperature up to 450? C for cold worked steel and 600? C for hot rolled steel. The effect of high temperature is more critical in pre-stressing steel than on reinforcing steel. The strength of prestressing steel during heating is likely to be reduced to less than 50 percent of normal strength when the steel temperature reaches about 400? C. |
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For majority of usage it is possible to idealise failure stress of steel and temperature as shown in Figure 1. The temperature distribution within concrete members is non linear and typical variation is shown in Figure 2 and Figure 3. This is helpful in assessing temperature of exposure of steel reinforcement which is embedded in concrete. [4]
Effects of High Temperature on Reinforced Concrete Members
If the intensity of fire is insufficient to cause collapse then the concrete members are likely to have undergone a cycle which is summarised in Table 1
TABLE 1 CYCLE OF EFFECTS UPON REINFORCED STRUCTURES [5]
| STAGE |
PROBABLE EFFECTS |
|
On heating
1. Rise in surface temperature
2. Heat Transfer to interior concrete
3. Heat transfer to reinforcement (accelerated if spalling occurs)
|
|
Surface crazing
Loss of concrete strength, cracking and spalling.
Reduction of yield strength; possible buckling and /or deflection increase |
|
On cooling
4. Reinforcement cools
5. Concrete cools |
Recovery of yield strength appropriate to maximum temperature attained (Figure 1) any buckled bars remain buckled.
Cracks close up; reduction in strength until normal temperature is reached; deflection recovery incomplete for severe fire; further deformations and cracking may result as concrete absorbs moisture from the atmosphere.
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Assessment of a Fire Damaged Building
An industrial building located near a city was severely affected by fire. The building complex comprises of various buildings viz. Production Block, Stores Block and Office Block which are interconnected. The Production Block is located in between Office Building and Stores Block and was covered with M S Sheet roofing supported on structural steel roof trusses resting on reinforced concrete columns. The Stores Block portion of the building comprises of ground plus one upper floor with r c flat slab construction in most of the regions and with conventional r c slabs and a mezzanine floor in a part portion of the building. The Office Block is a conventional reinforced concrete frame with ground floor only. It was reported that fire broke out in the Stores Block region of the building resulting in damage to the structure and materials stored in the building. The fire continued for long duration causing extensive damage to the Stores Block building and also adjacent Production Block and a portion of the Office Block.
Investigation
A detailed investigation of the buildings was done involving physical observations and investigative tests on materials and reinforced concrete members along with dimensional measurements. The Rebound Hammer Test, Ultrasonic Pulse Velocity Test, Core Test and Pull Out device test (Capo Test) were conducted to know the probable residual strength of concrete. Concrete samples were collected at random from the fire affected regions for conducting the Differential Thermal Analysis to know the maximum temperature of exposed concrete. Residual strength of concrete and steel after fire were also determined as per the recommendations [3]. In addition the soundness and stability of the buildings after fire were also assessed from the theoretical calculations.
Inferences
Based on the physical observations, results of investigative tests and theoretical calculations the following inferences are drawn:
| In Stores block, appearance of cracks, severe spalling of concrete, exposure of reinforcement in r c columns, drops and flat slabs at several locations, excessive sagging of flat slab in ground and first floor portion were observed (Figure 4 and Figure 5). The colour of concrete had turned pink to grey at fire affected regions indicating the temperature of fire is of the order of 600? C. This agreed with the results of DTA test. From the results of NDT it was inferred that the residual strength of concrete in columns, drops and flat slab is in the range of 16 N/mm2 to 20 N/ mm2 in the affected regions as against the design strength of 30 N/ mm2. |
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| The colour of concrete in the adjacent portion of the building which was having slab and beam construction was light pink, indicating the temperature of concrete is 300?C-450?C. This agreed with DTA test results. Only spalling of concrete in certain locations was observed. The strength of concrete from the NDT was about 25 N/mm2 and this also agreed with theoretical residual strength calculations. No significant distress features were observed in mezzanine floor and staircase regions. |
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| In production block, from physical observations and DTA test results on concrete samples it was inferred that the maximum temperature of exposure of building was above 600?C. This was also evident from severe distortion severe damage to steel trusses [Figure 6], purlins and roofing sheets. The strength of concrete in R C columns was found to be in the range of 14 N/mm2 to 20 N/mm2 as against the required strength of 30 N/mm2. This agreed with the theoretical calculation of residual strength of concrete. Lesser strength of concrete was noticed in the upper portion of columns when compared to strength of concrete in lower portion of columns. This is due to the fact that the fire fumes predominantly travel upwards and affect the upper reaches. |
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In office block, the physical observations and DTA test results indicated that the maximum temperature of exposure of columns and beams towards the Production Block was in the range of 400?C-500?C. As per NDT results the estimated strength of concrete was in the range of 22 N/mm2 to 26N/mm2. Severe cracking/ spalling and exposure of reinforcements were observed in columns and beams as shown in Figure 7 and Figure 8.
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The temperature exposure of beams and slabs inside the buildings was in the range of 300?C-400?C. The estimated strength of concrete was in the range of 25 N/mm2 to 28N/mm2. Cracking and exposure of reinforcements were observed in beams and slabs at a few locations. The temperature effect on end row of columns and beams was not severe enough to cause any damage to columns and beams.
A few column footings were exposed at random for each building and no significant distress features were observed in concrete of exposed footings.
Recommendations
From the above discussions on the distress features of the fire affected buildings and also from the economy and long term durability of the structures, the following measures were recommended.
In Stores block region, as the temperature of fire in this building was of the order of 600?C residual strength of r c structural members was very low. It was recommended to dismantle and re build the affected portion of the building consisting of ground and first floor with flat slab construction and a portion of the building with beam and slab construction. However in the adjacent portion of the building with beam and slab construction was recommended to be retained after strengthening columns and beams.
The severely damaging the steel roof truss system and central row of supporting r c columns were recommended to be dismantled and reconstruct as per the requirement.
In office block area, one row of columns and beams towards the Production Block was severely affected by fire. However the interior portion of the building was moderately affected by fire. It was recommended to strengthen the columns, beams and slabs by appropriate strengthening measures.
Restoration Measures
After assessing the residual strength of r c columns, beams and slabs affected by fire appropriate restoration measures were worked out. The columns and beams which were affected by fire were recommended to be restored by reinforced concrete encasement as detailed in Figure 9. However, the beams and the slabs which were affected less by fire were recommended to be restored by Glass/Carbon fibre wrapping as per design. Considering the economy, Glass fibre wrapping was suggested for slabs and Carbon fibre wrapping was suggested for beams.
Conclusion
A detailed investigation of a fire affected building involving physical observations and various non destructive and semi-destructive tests on r c members was done. After assessing the residual strength of reinforced concrete members appropriate repair and restoration measures were recommended. Subsequently, restoration measures were effectively carried out and the building is functioning satisfactorily.
References
- C S Viswanatha 'Fire Affected Concrete -Recent Investigative Techniques'–Lecture Notes-2002.
- Malhotra H L‘Design of Fire Resisting Structures’-Surrey University Press,1982.
- Bhat N S and Jain J P 'Compressive Strength of Concrete subjected to elevated Temperatures, Indian Concrete Journal, Vol.73 , No.8, 1999.
- Chakraborti S C and Jain A K- 'Design of Flexural members subjected to fire'. Indian Concrete Journal Vol.73 , No.8,1999.
- Technical Report No.33. Fire Damaged Concrete Structures, Concrete Society The Concrete Society, Wexham, Slough,1990.