Fire Effect on Concrete

CONCRETE IN FIRE EXPOSURE

1- Real Life Cases

2- Types of Fires

•Cellulosic Fires: • from finishings, furniture, paper, wood.• Reaches 900C in 45min. And 1050C in 2hrs.

•Hydrocarbon Fires:• From fuel such as oil, gasoline,…• Reaches 900C in 4min. And 1100C in 20 min.

2- Types of Fires

3- Behaviour Essentials

• Yield strength of steel is unaffected till 400C.

• Yield strength is 55% of its original value at 550C (Critical Temperature).

• Stresses in steel members are around 55% of the yield strength. So if steel temp reaches 550C, failure will occur.

• Concrete starts to lose strength very early. It loses 10% of its strength at 150C. It loses 20% of its strength at 300C. The major loss is at 550C (concrete loses between 50% and 75%).

• Concrete strength is considered completely lost at 600C.

3- Behaviour Essentials

4- Failure Mechanisms of Concrete in Fire

• Strength Loss in cement Matrix:

• Loss of free water (or capillary water) at 100C.

• Loss of bound (adsorbed) water starts at 150C

• Degradation of cement paste hydrates at 300C (calcium oxide is released).

• Major strength loss between 550C – 600C

• Complete failure between 850C – 900C.

4- Failure Mechanisms of Concrete in Fire

• Spalling:

• Takes place slowly with fire propagation. Slabs and beams soffits are more susceptible to spalling.

• It is affected by the degree of saturation of concrete, and its permeability.

• Moisture content of 3% (or more) is considered critical.

• Low permeability Concrete is prone to explosive spalling. (HSC)

6- Improving The Fire Resistance of Concrete

• Increasing the concrete thickness.

• Adding sand cement render or gypsum plaster or other passive protection layers.

• Adding polypropylene fibers to the mix. Why?

5- The Influence of Aggregate Type

• The difference in thermal properties between aggregates and cement matrix is the main concern.

• Siliceous aggregate is the worst (gravel, granite).

• Dolomitic aggregate is better. Basalt is the best.

• One reason is that; their coefficients of thermal expansion are closer to that of the matrix.

• Another reason is: siliceous aggregate changes phase at 570C with considerable volume increase.


• Carbonate aggregates degrade at elevated temperature as follows:

• Calcium carbonate releases carbon dioxide at 660C.

• Magnesium carbonate releases carbon dioxide at 740C.

• Carbon dioxide is believed to blanket thermal propagation.

• Light weight aggregates, sintered and synthetic aggregates perform very good in dry fires because of the thermal expansion compatibility with cement matrix and the high bond with the paste.

7- Evaluation of Concrete Structures Exposed to Fire

• Visual Inspection: concrete colour changes to pink when temp. reaches 300C and becomes grey again when temp reaches 600C.

• Cores and strength testing is not very helpful.

• Non-destructive testing using rebound hammer is not helpful.

Assignment 2:

1- Sketch (on the same axes) the relation between Temperature and Strength for Concrete and Steel.

2- How is each of the following factors affect concrete resistance to fire exposure:

concrete strength and aggregate type?

In your answer explain the main concrete failure mechanisms.

Assignment 2:

3- An industrial building in a new city near Cairo suffered a fire. You were asked to check the structure after the fire and prepare a report. The main observations you made were: the concrete colour is pink; the coarse aggregate is dolomite; and No steel rebars are exposed.a)Estimate the concrete temperature at the end of the fire and the percent of concrete strength loss.b)If the coarse aggregate were basalt, would the concrete strength loss be the same? c)How would a high strength concrete column fail in fire?

Fire Effect on Concrete

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