case study on a fire exposed precast slab

1. A case study on fire damage
assessment of a two-story
building with precast
pretensioned hollow core slabs
Journal Name:Engineering Sciences
Author: Mazen Ali Musmar
Year of Publication:2019
https://doi.org/10.1016/j.jksues.2019.09.003
Presented By: Paramathmeka S
P.S.G college of technology

2. Abstract
 The study is about a structural assessment of a two-story building operated as a textile factory.
 pre-tensioned precast hollow core slab units of 1.2 m width, 9.0 m length and 0.265 m thickness
each – used as floor slabs
 The slab units are assembled side by side
 The remaining structural elements of the building are of reinforced concrete.
 The work involved assessment of the damage and the assessment of the severity of fire attack
 A two stage structural analysis was performed to determine the fire endurance exposure time
 the fire endurance exposure time was estimated to be one hour
 The study encompasses proposing appropriate repair methods or to decide whether the
demolition of elements or the whole structure is more appropriate
 it was concluded that the demolition of the structure is more convenient.

3. Introduction
 Concrete has a high capacity to resist fire.
 Elevated temperatures results in restrained thermal dilation owed to kinematic restraints.
 Thermal load results in micro-structural alterations that modify the concrete behavior
 when the concrete temperature rises, pore water inside the concrete thixotropic structure
evaporates. This evaporation leads to pressure increase inside the concrete - (Nechnech et al)
 High pressure may lead to cracking and concrete spalling – (Hertz et al)
 Prestress concrete slabs are exposed to fire Substantial losses in effective strands prestressing
that lead to tangible degradation of moment strength would take place after one hour –
(Woznaik et al)

5.  two story concrete building with the first and second floor slabs made up of three simply
supported pre-tensioned precast hollow core slab units to cover the 27 m span in one direction,
and assembled side by side in the other direction to cover the whole floor slab area
 slab unit is 9 m long, 1.2 m wide, and 0.265 m deep. It is a simple span pre-tensioned precast
hollow core slab, supported on two interior reinforced concrete beams along the interior space of
the building in addition to two exterior beams at the edges.
 The pre-tensioning consists of 6 straight strands 1/2 in diameter, located near the slab bottom
surface.

6. Design parameters
 Concrete mix included siliceous gravel that is more sensitive to high temperatures than
calcareous aggregate.

7. Field investigation
Field investigation include
A detailed building inspection was carried out to spot the spalling concrete, and to track the
extension of cracks and scale off in concrete within the two stories
 Fire severity assessment
 Nondestructive evaluation of concrete strength
 Laboratory analysis of concrete cores

8. Fire severity assessment
 first floor was pinkish in several locations, reddish in other locations the concrete surface in the
second floor had a black color all over
 In both floors, there were discrepancies in the level of bottom surfaces for adjacent slab units
that make up the floor slabs, owing to the relative severity of the absorbed heat by each slab
unit.

9. Nondestructive evaluation of concrete strength
 Rebound number according to ASTM C805 (2018)
 Ultrasonic Pulse Velocity, ASTM C597 (2016)

10. Laboratory analysis of concrete cores
 Concrete cores were extracted in order to determine the compressive strength of various
structural elements,
 Such cores were chosen where the concrete surface contained the outermost layer of discolored
concrete.

11. Test Results

12. Structural analysis

13.  when the slab is uniformly loaded, the applied moment (M) will be as presented, with the
largest moment at midspan of:
 the nominal flexural strength of pre-tensioned concrete panel slabs at ambient temperature,
when prestressing straight strands exist in the tension side is:

14.  As the material strength decreases with elevated temperatures, the retained moment
capacity becomes
 Flexural failure occurs when Mn◦ is reduced to M◦
 Fire endurance is influenced by the applied load and the strength temperature
characteristics of the strands.
 The fire exposure time before the critical strands temperature is reached depends on the
amount of concrete cover.

15. Finite element analysis
 Finite element analysis was performed using Ansys (2013) software.
 It comprises a heat transfer thermal analysis to predict the isotherms within the cross section of the slab
unit and a mechanical analysis.
 Coupling is made by implementing findings from the heat transfer analysis, to the structural analysis.
 The structural analysis is composed of two stages;
1. the initial stage involves the performance of the slab unit under fire attack to predict the structural end
point.
2. The second stage involves the post fire condition of the fire damaged slab unit.
 Initially a finite element analysis was carried out to simulate the performance of the slab units during the
fire attack and up to failure in order to be able to determine the structural end point
 In the second stage, a finite element structural analysis was carried out for the fire damaged slab unit up
to the structural end point that was determined in the initial stage

16. Discussion of results
 The field investigation indicated - first floor was pinkish, reddish, second floor - black color
 Nondestructive testing indicated
1. Rebound hammer testing, the average residual concrete strength ranged from 58% of concrete
design strength for the first floor slab to 62.8% for the second floor slabs. The average residual
concrete strength ranged from 64.2% of concrete design strength for the first floor slab columns
to 74.8% for the second floor slabs columns
2. According to the Ultra sonic velocity testing, the average residual concrete strength ranged from
62.8% of concrete design strength for the first floor slab to 72.4% for the second floor slabs. The
average residual concrete strength ranged from 71.4% of concrete design strength for the first
floor slab columns to 86.5% for the second floor slab columns.
3. According to core strength, the residual concrete strength ranged from 62.8% of concrete design
strength for the first floor slab to 71.4% for the second floor slabs
 Heat transfer analysis indicated - in the case of the first floor slabs the average temperature of the
strands was 550 C, whereas the average temperature for concrete at the compression zone was
800 C. In the case of the second floor slabs the average temperature of the strands was 520 C,
whereas the average temperature for concrete at the compression zone was 21 C.

17. Conclusion
 The findings of field investigation and finite element analysis indicated that the
rehabilitation of the building requires in principle
1. replacing the pre-tensioned slab units in the first and second floors,
2. applying column jacketing in addition to repairing many damaged structural elements.
 It was concluded that such an alternative is impractical, costly and risky.
 Demolition of the structure is more convenient.

18. References

19. Thank You