Study on Pore Pressure Spalling in Hybrid Fibre – Reinforced High Strength Concrete at Elevated Temperatures

Mugume Rodgers Bangi Candidate for the Degree of Master of Engineering Supervisor: Assoc. Prof Takashi HORIGUCHI

Division of Built Environment

INTRODUCTION

Concrete generally should shows good performance when exposed to high temperatures compared to other building and construction materials. This is because it incombustible when compared to other materials such as wood and plastics and has good insulation properties compared to steel because of its low thermal diffusivity which decreases with increasing temperature.

However, fire accidents associated with infrastructures and various studies have shown that concrete has a high occurrence of thermal instability in form of spalling which leads to breaking off of layers or pieces of concrete from the thermally exposed surface. This greatly compromises the structural integrity of the concrete structures [1, 2] since it exposes the core of the concrete element and the reinforcing steel leading to a reduction in load – bearing cross – sectional area. In particular High Strength Concrete (HSC) is more susceptible to spalling compared to Normal Strength Concrete (NSC) due to its low water cement-ratio which produces a dense and almost impervious microstructure, which keeps the moisture vapour from escaping in a high temperature environment resulting in build-up of pore pressure in the cement paste and hence spalling. This is of great concern to engineers since high strength concrete has been increasingly utilized in construction of many civil engineering structures world wide such as bridges, high – rise buildings and tunnels because of its superior performance compared to normal strength concrete due to its low permeability and improved durability.

This experimental investigation aims at improving the performance of high strength concrete during and after exposure to high

temperature by mitigating explosive spalling and developing a better understanding of the mechanisms of pressure development and relief inside concrete as well as improvement of its residual mechanical properties after heat exposure. Hybrid – fibre reinforced high strength concrete has been studied which contains PP fibres or PVA fibres for explosive spalling mitigation and steel fibres for maintaining the residual mechanical properties after heat exposure. EXPERIMENTAL DETAILS Materials and mix proportions

Concrete specimens were prepared using OPC (Ordinary Portland Cement) and crushed stone with the maximum nominal size of 13 mm. Some parameters of the mix proportion were kept constant for all series: W/C of 30 %, water content of 170 kg/m3 and sand to aggregate ratio (s/a) of 50%. Addition of polypropylene (PP) fibres, polyvinyl alcohol (PVA) fibres, and a combination of polypropylene or PVA with steel fibres was the main differentiation of the series. Two types of steel fibres i.e. S13 and S30 were used in this experimental study. A polycarboxylate ether hypeplasticizer was used at a dosage of 0.9 % of cement content to achieve the desired workability. Concrete mix proportions of all series cast are shown in Table 1. Experimental Procedures

All specimens tested during pore pressure measurement experiment were 175 mm in diameter and 100 mm in height. Thermal load was applied on one face of the concrete specimen by means of a computer-controlled

Table 1 Mixture proportions

Series W/C (%)

s/a (%)

Fiber vol. (%) W C S G SPAE*1

(%×c) PP (PVA) (S30) (S13) (kg/m3)

Plain

30 50

- -

-

170 567

796 781

0.9

PP

0.1

795 780

HY1 0.3 790 776

HY2 0.5 788 773

HY3 0.2 0.1

790 776

HY4 0.4 788 773

SPAE*1: Super plasticizer and air entraining agent radiant heater placed 10 mm above it. The heater of power 500 watts exposes the whole surface of the specimen and generates maximum temperature of up to 600⁰C. Ceramic fibre was used to heat-insulate the lateral faces of the specimens to ensure quasi-unidirectional thermal load upon it.

Three heating patterns were applied in the experiment. The first pattern was a slow heating rate (5°C/min) and the second pattern was a moderate heating rate (10°C/min). The third pattern, a fast heating rate was conducted by thermally shocking the specimen after the heating device has reached the designated maximum temperature of 800°C. The specimen was exposed to the maximum temperature of 800°C lasting for a few hours. The three heating patterns and ISO 834 pattern are shown in figure 1.

All specimens were instrumented with pressure gauges that allow pore pressure measurements. The gauges were made of a disk of porous sintered metal (Ø 12 mm×4mm) encapsulated into a metal cup that was brazed to a metal tube with inner diameter of 1.5 mm. The free end of the tube then stuck out at the rear face of the specimen. Three gauges were placed with in the central zone of the specimen at 10, 30 and 50 mm respectively, from the heated face. Thermocouples were attached on the sides of the gauges which were used to measure the temperature inside the heated specimens. The set up of the experimental test is shown in Figure 2.

Heating Curve

0

200

400

600

800

1000

1200

0 50 100 150 200Time (min.)

Tem

pera

ture

(℃

)ISO834-1 MiddleSlow Fast

Figure 1. Heating patterns for pore Pressure

measurement test and ISO 834

Figure 2. Experimental test set-up

RESULTS AND DISCUSSIONS

Explosive Spalling

During heating of all concrete specimens for all heating rates, explosive spalling only occurred in plain concrete exposed to a slow heating rate. Maximum pore pressure of 4.009 MPa was observed at depth of 10 mm at a temperature of 349.3°C. Spalling led to the breaking–off of pieces of concrete from the heated surface which resulted in moisture vapour rapidly escaping to the atmosphere. Build – up of pore pressure

Figure 3 (a) and 3 (b) shows the evolution of pore pressure measured during slow and fast heating respectively for the different concrete series.

slow (30 mm)

0.0

0.5

1.0

1.5

2.0

2.5

0 50 100 150 200 250 300

time (min)

pres

sure

(M

Pa)

plain

pp

HY1HY2

HY3

HY4

(a) Slow heating rate

Fast (30 mm)

0

0.5

1

1.5

2

2.5

3

0 50 100 150 200 250time (min)

pres

sure

(MPa

)

plain

pp

HY1

HY2

HY3

HY4

(b) Fast heating rate

Figure 3. Build – up of pressure with time

It can be observed that the maximum pressures measured in plain concrete for all heating rates are much higher than those of PP and HY concrete series. Since all the other series of concrete contain PP fibres except Plain concrete, it clearly shows the effectiveness of PP fibres in mitigating the build – up of pore pressure in concrete. Effect of Heating Rate

In PP concrete series as shown in figure 4, it was observed that pore pressure near the surface of concrete (10 mm depth) is nearly the same for both slow and fast heating rates at 0.3 and 0.35 MPa respectively. However, it was observed that in deeper regions of concrete (50 mm depth), a fast heating rate leads to a much higher pore pressure of 2 MPa

PP (SLOW)

0

0.5

1

1.5

2

2.5

0 200 400 600

Temperature (⁰ C)

Incr

ease

of P

ress

ure

(MPa

) 10mm

30mm

50mm

SVP

(a) PP under slow heating

PP (FAST)

0.0

0.5

1.0

1.5

2.0

2.5

0 200 400 600 800Temperature (⁰ C)

Incr

ease

of P

ress

ure

(MPa

) 10 mm

30 mm

50 mm

SVP

(b) PP under fast heating

Figure 4. Rise of Pressure with temperature

which is more than twice that of a slow heating rate at 0.9 MPa. This clearly shows that a fast heating rate leads to higher pore pressure in the deeper regions of concrete compared to a slow heating rate.

Furthermore, it was observed that 0.1 % (0.9 kg/m3) dosage of only PP fibres is not sufficient in pore pressure reduction for a fast heating rate since a high pore pressure of 2 MPa was measured in PP concrete series.

Role of Steel Fibres in Pressure Reduction Comparing PP and Hybrid concrete series under a fast heating rate as shown in figure 5, it was observed that in deeper regions (50 mm depth) the pore pressure in Hybrid concrete series with a maximum pore pressure of 1.0 MPa reduces to at least half( ½) that in PP concrete at 2 MPa. This clearly shows that addition of steel fibres plays some role in pore pressure reduction in deeper regions of concrete under a fast heating rate.

Fast (50 mm)

00.5

11.5

22.5

33.5

44.5

0 50 100 150 200 250time (min)

pres

sure

(M

Pa)

plain

pp

HY1

HY2

HY3

HY4

Figure 5. Build – up of pressure with time

under fast heating

Study on Pressure Measurement Technique Effect of sintered metal

There is significant effect of using a pressure gauge comprising of a cup with sintered metal since it measures a higher amount of pore pressure for all tests done.

It was further observed that pressure gauges made of only a tube generally measured higher pressures at an earlier time than the other types. This is probably because the pressure transfer from the measuring point to the transducer is shorter for a pressure

gauge made of only tube compared to other gauges.

Effect of silicon oil

Higher pore pressures were measured for specimens filled with silicon oil compared to specimens with empty gauges for all different types of pressure gauges used. Furthermore, it was observed that pore pressure measurements for gauges without silicon oil resulted in unstable pore pressure results with the pressure rise being very different for each different type of pressure gauges.

CONCLUSIONS

The experimental results showed that PP fibres are very effective in mitigating the build-up of pore pressure inside heated concrete. Steel fibres were found to contribute to pore pressure reduction in the deeper regions of concrete during exposure to a fast heating rate.

Pore pressure development inside concrete is highly influenced by the severity of fire with pore pressures increasing with increasing heating rate specifically for deeper regions of concrete.

Pressure gauge comprising of a sintered metal measures a higher amount of pore pressures compared to other two gauges tested.

Addition of silicon oil in pressure gauges is very important since higher pore pressures were measured for specimens with gauges filled with silicon oil compared to specimens with empty gauges for all different types of pressure gauges used.

Pore pressure measurements for gauges without silicon oil resulted in unstable pore pressure results. REFERENCES

1. Kodur V.K.R., “spalling in High strength Concrete Exposed to Fire – Concerns, causes, Critical Parameters and cures”, ASCE, 2000.

2. Suhaendi S.L. and Horiguchi T., “Explosive spalling mitigation mechanism of fiber reinforced high strength concrete under high temperature condition”, Proceedings of the International fib Workshop on Fire Design of concrete structure (2008), p. 189 – 197.