Post-heating strength of fiber-reinforced concretes

Fire Safety Journal 49 (2012) 100–106

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Post-heating strength of fiber-reinforced concretes ¨ Gyorgy L. Bala´zs, E´va Lublo´y n Budapest University of Technology and Economics, Department of Construction Materials and Engineering Geology, Pf. 1521, Budapest, Hungary

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 July 2010 Received in revised form 1 January 2012 Accepted 5 January 2012 Available online 5 February 2012 Keywords: High temperature Hot strength Residual strength Polypropylene fibers Steel fibers

The strength reduction of high-strength concrete can be different from that of normal-strength concrete. The investigation was directed toward the study of the residual compressive strength and surface cracking of fiber-reinforced concretes subjected to high temperatures. Six different concrete mixes were tested over a compressive strength range from 60 to 74.7 N/mm2. The test variables were concrete composition, type of fiber reinforcement (polypropylene fibers ؼ 0.032 mm, ‘ ¼ 18 mm; or ؼ 1.1 mm, ‘ ¼ 40 mm; steel fibers: ؼ 1.1 mm, ‘ ¼ 18 mm; ؼ 0.9 mm, ‘ ¼35 mm; ؼ 0.3 mm, ‘ ¼12.5 mm) and maximum temperature (20 1C, 50 1C, 150 1C, 200 1C, 300 1C, 400 1C, 500 1C, 600 1C, 800 1C). The test results indicate that the advantageous effects of polypropylene and steel fibers in concrete subjected to high temperatures are mainly observed for thin fibers and not for thick fibers. Strength reduction and surface cracking are detailed for the various tested fiber-reinforced concretes. & 2012 Elsevier Ltd. All rights reserved.

1. Introduction

a form to the b form at a temperature of 573 1C [11]. This

The properties of concrete may be considerably influenced by high temperatures [1,2]. The extent to which these properties are modified depends on the maximum temperature and the composition of the concrete: water-cement ratio (w/c), type of cement, type of aggregate and porosity [3–8]. The deterioration of concrete at high temperatures is manifested in two ways: (1) the deterioration of the material itself and (2) the deterioration of the structural performance. 1.1. Deterioration of the material Chemical changes can be studied by thermogravimetrical analysis (TG/DTG/DTA). The following chemical transformations can be observed with increasing temperature. At approximately 100 1C, weight loss indicates water evaporation from micropores. The dehydration of ettringite (3CaOAl2O3  3CaSO4  32H2O) occurs between 50 1C and 110 1C. At 200 1C, there is further dehydration, which causes slight weight loss. The weight lost at various moisture contents has been observed to be different until the local pore water and the chemically bound water are gone. Further weight loss is not perceptible at approximately 250–300 1C [9,10]. During heating, the endothermic dehydration of Ca(OH)2 occurs between 450 and 550 1C (Ca(OH)2-CaOþ H2Om[9]). In concretes with quartz–gravel aggregates, another influencing factor is the crystalline conversion of quartz from the n

Corresponding author. Tel.: þ0036 463226; fax: þ0036 4633450. E-mail addresses: [email protected] (G.L. Bala´zs), [email protected] (E´. Lublo´y).

0379-7112/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2012.01.002

transformation is followed by a 5.7% volumetric increase. The dehydration of calcium-silicate-hydrates has been observed to occur at 700 1C [7]. 1.2. Deterioration of the structural performance: In the case of tunnel fires, in addition to reductions in loadbearing capacity, the spalling of concrete cover produces further difficulties [12]. The probability of concrete-cover spalling increases with the increasing strength of concrete [13]. Special care is required to avoid the spalling of concrete cover. A series of experiments on normal-strength concrete suggest that the application of polymeric fibers considerably reduces the probability of concrete-cover spalling [13–16]. Experiments with tunnel segments (length 11 m, height 2 m) carried out by M¨orth, Haberland, Horvath and Mayer [17] indicate that the cover of polypropylene-fiberreinforced concrete did not spall. In Austria, a group of researchers [18] reported the same finding. They tested reinforced concrete slabs that were loaded along their planes. The spalling of concrete cover was observed for conventional reinforced concrete slabs without polymeric fibers. However, no spalling was experienced in slabs prepared with 1 to 3 polypropylene fibers per volume. Silfwerbrand [19] suggests that the application of polypropylene fibers is also suitable for high-strength concrete. The utilization of polypropylene fibers not only reduces the probability of concrete-cover spalling but may also reduce the residual compressive strength of concrete [20]. Using concrete cylinders (|¼100 mm, ‘ ¼200 mm), Horiguchi [21,22] experimentally proved that the addition of polymeric or steel

100

900

90

800

80

700

70

600

Tempereture(°C)

Residual compressive srength (MPa)

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60 50 40

A 0 V%

30

B 0.5 V% PP

20

C 0.5 V% S

10

D 0.25 V% PP+ 0.25 V% S

101

2

1

3

500 400

test

300 200 100 0

0 0

100

200 300 Temperature (°C)

400

0

500

Fig. 1. Residual compressive strength of high strength concrete with or without fibers.

fibers alters the residual compressive strength of concrete. Specimens were heated at a rate of 10 1C/min up to 200 1C or 400 1C and held for 1 h at high temperature; they were then tested at room temperature (Fig. 1). The water-to-cement ratio was 0.3 (with 583 kg/m3 cement and with 175 ‘ water). Mix A: prepared without fibers, Mix B: with 0.5 V% polypropylene fibers, Mix C: with 0.5 V% steel fibers, Mix D: with 0.25 V% polypropylene and 0.25 V% steel fibers. Horiguchi [22] concluded that hybrid fibers (polypropylene and steel fibers) improve the residual compressive strength of highstrength concrete subjected to a temperature of 200 1C or 400 1C. The residual strength of polypropylene-fiber-reinforced concreted showed the greatest losses after heating to 200 1C and 400 1C. The purpose of the present study was to monitor the influence of various polypropylene fibers on the residual compressive strength and surface cracking of relatively high-strength concretes subjected to high temperatures. A group of experiments were also carried out on steel-fiber-reinforced concrete subjected to high temperatures.

60

120

180

240

300

t (min) Fig. 2. Schematic representation of experimentally applied temperature histories.

Table 1 Concrete compositions.

Cement, kg/m3 CEM I 42.5 N Water, kg/m3 Aggregate 0–4 mm, kg/m3 Aggregate 4–8 mm, kg/m3 Aggregate 8–16 mm, kg/m3 Superplasticizer, kg/m3 Fibers, kg/m3

Mix 1

Mix 2

Mix 3

Mix 4

Mix 5

Mix 6

350 151 912 485 544 1.4 –

350 151 912 485 544 1.4 1

350 151 912 485 544 1.4 1

350 151 912 485 544 1.4 35

445 144 818 363 636 8.9 –

445 144 818 363 636 8.9 –

Remark: aggregates were quartz gravel and sand.

Table 2 Characteristics of the applied fiber types. Characteristic

Fiber 1

Fiber 2

Fiber 3

Fiber 4 Fiber 5

Material Length (mm) Diameter (mm) Fiber volume (mm3) Density (kg/m3) Melting point (1C) Decomposition temperature

pp 40 1.1 37.99 910 171 360

pp 18 0.032 0.014 910 160 365

steel 35 0.9 22.76 7850 – –

steel 35 0.75 15.50 7850 – –

2. Test parameters 2.1. Temperature history In experiments performed at high temperatures, the temperature history and the testing time are very important. The first part of the heating curves was observed to be similar to that of the standard fire curve for buildings up to 800 1C [23]. After heating to specified temperatures (see

in Fig. 2), speci-

mens were held at these maximum temperatures for 2 h(

) (20 1C,

50 1C, 150 1C, 200 1C, 300 1C, 400 1C, 500 1C, 600 1C, 800 1C). Specimens were then cooled down (

) under laboratory conditions.

Specimens were then tested at room temperature.

steel 12.5 0.3 0.88 7850 – –

Mix 3: same as Mix 1 with 1 kg/m3 added polypropylene fibers type 2 (see Table 2) – Mix 4: same as Mix 1 with 80 kg/m3 added steel fibers type 3 (see Table 2) – Mix 5: same as Mix 1 with 80 kg/m3 added steel fibers type 4 (see Table 2) – Mix 6: same as Mix 1 with 80 kg/m3 added steel fibers type 5 (see Table 2). –

2.2. Concrete composition Compressive strength tests were carried out on cubes with 150-mm sides. The experimental concrete compressive strength range was 60–90 N/mm2. The concrete compositions are shown in Table 1, where – Mix 1: the reference concrete without fibers – Mix 2: same as Mix 1 with 1 kg/m3 added polypropylene fibers type 1 (see Table 2)

The same type of Portland cement (CEM I 42.5 N) as well as the same type of aggregate (quartz sand and gravel, ragg ¼2.64 kg/m3)

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102

were used in all of the specimens. Altogether, there were 162 specimens, and four measurements were performed for every combination of parameters. The characteristics of the applied fiber types are presented in Table 2. After removing the specimens from the formwork, they were stored in water for 7 days and then kept under laboratory conditions until testing. The specimens were 28 days old by the testing time. The moisture contents of the different mixes were as follows: Mix 1: 3.80%; Mix 2: 3.75%; Mix 3: 3.85%; Mix 4: 8%; Mix 5: 3.76%, Mix 6: 3.81%; these contents were measured as the differences between the original weights and weights after drying at 60 1C and are expressed as percentage values. 2.3. Test method After leaving the specimens to cool, the specimens first investigated for the development of surface cracking were investigated; compressive strength tests were then carried out. In addition, the structures of the heat-treated concrete samples were studied by optical microscopy and electron microscopy.

3. Results for concretes without fibers 3.1. Observations with microscopy

by the relatively high pore water content of the tested concretes at an age of 28 days. The valley ends at a relative compressive strength close to 1.00. 2. This valley is followed by a decrease in compressive strength at higher maximum temperatures. This can be explained by the physical and chemical changes of the hardened cement paste and those of the aggregates (see Chapter 1). 3. The residual compressive strength at 800 1C was 0.23 for Mix 1.

3.3. Visual observations In the case of Mix 1, a considerable number of surface cracks could be observed on the specimens after loading to a maximum temperature of 800 1C (Fig. 5b). In Mix 1 (fcm ¼64 N/mm2, no fibers), chemical and physical changes resulted in surface cracks that were distributed rather evenly.

4. Results for concretes with fibers The probability of concrete-cover spalling can be reduced by the application of polymeric fibers. Fiber geometry can affect both surface cracking and strength reduction. Concrete mixes with two types of polypropylene fibers were tested (see Table 2):

The structure of the cement stone on the surfaces of the aggregate particles was analyzed by optical microscopy and electron microscopy. In Fig. 3a, a layer of crystals on the surface of the quartz gravel is shown (more detailed analysis of the contact zone was carried out with scanning electron microscopy). This contact layer consists of portlandit (Fig. 3). Investigations show that the portlandit decomposes between 400 1C and 500 1C, causing a significant strength reduction in the concrete discussed in the following chapters.

fiber 1 with ؼ0.032 mm and fiber 2 with ؼ1.1 mm.

3.2. Compressive strength The average compressive strength of concrete from Mix 1 was 64 N/mm2. Fig. 4 shows the measured residual compressive strength values of concrete as a function of maximum temperature up to 800 1C. The following conclusions can be drawn from Fig. 4: 1. A strength valley seems to appear (also observed by other researchers, such as Schneider and Lebeda [24]) at relatively low maximum temperatures, i.e., a small strength decrease is followed by a small increase between 20 1C to 200 1C for Mix 1 (average strength 64 N/mm2). This valley might be explained

Fig. 4. Measured compressive strengths for concrete mixes without fibers as a function of maximal temperature (every point is average of 4 measurements).

A A

P

P

Fig. 3. Bond layer of aggregate (quartz gravel) to cement stone at temperature of 20 1C. Notation: A: aggregate, P: portlandit. (a) Optical microscopic observation. (b) Electron microscopic observation.

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melted fibers on the surface exposed to 200°C

Fig. 5. Concrete surfaces after heating at 800 1C for 2 h and cooling down. (a) With polypropylene fibers (Mix 2). (b) Without fibers (Mix 1).

103

signs of burning of thicker fibers exposed to 400°C

Fig. 6. Surface of Mix 3 specimens with thicker polypropylene fibers after temperature loading.

4.1. Concrete with polypropylene fibers The average compressive strength of Mix 2 was 63 N/mm2 and that of Mix 3 was 70 N/mm2 at 201C without temperature loading at 28 days.

4.1.1. Visual observations In the concrete with thinner fibers (Mix 2, Fig. 5a), practically no surface cracks were observed up to 8001C. In the reference concrete without fibers (Mix 1), we observed surface cracks in specimens heated up to 800 1C (Fig. 5b). A comparison between Fig. 5a and b indicates the beneficial influence of thinner polypropylene fibers at high temperatures. Mix 3, with thicker fibers, showed considerably different behavior. Thicker fibers close to the surface flowed onto the surface (at 2001C) and were then burnt off (Fig. 6), producing a characteristic color on the surface (at 4001C). In some places, small holes also appeared. These fibers were rather parallel to the concrete surface and were burnt off in this position. This could be avoided using thinner fibers (such fiber 1) instead of thicker fibers (like fiber 2) [25,26]. The difference can be explained by the different volumes of fibers incorporated into the concrete formulations.

4.1.2. Compressive strength Compressive strengths measured as a function of maximum temperature are presented in Figs. 11 and 12 for Mix 1, Mix 2, Mix 3 and Mix 5. The following conclusions can be drawn from these figures: 1. The overall tendency of strength reduction with increasing maximum temperature was similar for the concrete without fibers (Mix 1) and the concrete with polypropylene fibers (Mix 2, fiber 1; and Mix 3, fiber 2). 2. The residual compressive strengths at the maximum temperature of 8001C were 20%–30% of the strength observed at 20 1C for Mix 2 and Mix 3 with fibers as well as Mix 1 and Mix 5 without fibers. 3. The most considerable reduction in compressive strength occurred between 4001C and 800 1C in all cases. 4. Between 201C to 4001C, the compressive strengths were different for Mix 1, Mix 2 and Mix 3. a. For the reference concrete (Mix 1) with no fibers (Fig. 4) as well as in the concrete with thinner polypropylene fibers (Mix 2) (Fig. 7), the reductions in strength observed between 501C and 1701C were followed by increases in strength up to 2001C and then gradual reductions up to

Fig. 7. Measured compressive strengths of concrete with fiber 1 (see Table 2) as a function of temperature (every point is the average of 4 measurements).

4001C (see discussion on strength valley in Section 3.2 Chapter). b. For concrete with thicker polypropylene fibers (Mix 3) (Fig. 8), the early reduction in compressive strength was followed by a plateau from 4001C to 5001C. Our test results indicate that the advantageous effect of polymeric fibers on concrete subjected to high temperatures is observed mainly for thin fibers and not for thick fibers. 4.1.3. Observations with microscopy Scanning electron microscopy images of concrete mixes featuring polymeric fibers were acquired to observe the modification of the fibers due to temperature loading. These pictures show that: – the polypropylene fibers are embedded into the cement stone at 201C (Fig. 9a), – the polypropylene fibers are still visible at 501C (Fig. 9b), – the polypropylene fibers are melted at 1501C (Fig. 9c). 4.2. Results for concrete with steel fibers We also tested fiber-reinforced concrete specimens featuring 80 kg/m2 steel fibers (Ø ¼1.1 mm, Mix 4; Ø 0.9 mm, ‘ ¼35 mm,

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Mix 5; Ø 0.3 mm, ‘ ¼12.5 mm, Mix 6) subjected to high temperatures.

spalling of cover were influenced by the geometry and volume of the fibers.

4.2.1. Visual observations In steel-fiber-reinforced concrete with fiber type 3 (Mix 4), a special type of failure occurred during the heating process over the temperature range between 600 and 8001C: some corners of the cube specimens were cracked off. In steel-fiber-reinforced concrete with fiber types 4 and 5 (Mixes 5, 6), practically no surface cracks were observed up to 800 1C. According to our observations, surface cracking and the

4.2.2. Compressive strength The average compressive strengths of concrete Mixes 4, 5 and 6 were 74.7 N/mm2, 67.2 N/mm2 and 70.5 N/mm2, respectively. Fig. 10 shows the measured relative residual compressive strengths of steel-fiber-reinforced concrete as a function of maximum temperature up to 8001C. The following conclusions can be drawn from Fig. 10:

5

700°C decomposition and new formation of CSH

4

573°C crystalline conversion of quartz

3

450°C decomposition of portlandit Ca(OH)2

2

365°C fiber decomposition

1

171°C fiber melting point 1

2

3

4

1. According to our observations, the residual compressive strength was influenced by the geometry and volume of the fibers. 2. The most favorable residual compressive strength was obtained with fiber type 5 (Mix 6). 3. The least favorable residual compressive strength was obtained with fiber type 3 (Mix 4). 4. The residual compressive strengths at the maximum temperature of 800 1C were between 45% for Mix 6 (fiber 5) and 17% for Mix 4 (fiber 3). If steel fibers are applied, our test results suggest the use of thin and short fibers to impart residual compressive strength on concrete after processing at high temperatures.

5

80

Compressive strength (N/mm2)

70 60 50 40 30 20 10 with fiber 2 0 0

200

400 600 Maximal temperature (°C)

800

1000

Fig. 8. Measured compressive strengths of concrete with fiber 2 (see Table 2) as a function of temperature (every point is the average of 4 measurements).

fiber

Fig. 10. Measured compressive strengths for concrete mixes with steel fibers as a function of maximal temperature (every point is average of 3 measurements), relative strength values (related to those at 20 1C).

fiber

fiber 50°

20°

150°C

Fig. 9. Polymeric fibers (ؼ0.032 mm fiber type 1, see Table 2) after temperature loading.

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5.1. Mixes without fibers The compressive strength of Mix 1 was 64 N/mm2 at room temperature. A strength valley seems to appear at relatively low maximum temperatures, i.e., small decrease followed by a small increase between 20 1C and 200 1C for Mix 1. This valley might be explained by the pore water content of the tested concretes at an age of 28 days. The valley ends with a relative compressive strength close to 1.0. This valley is followed by a decrease in the compressive strength at higher maximum temperatures. The residual compressive strength at 800 1C was 23% for Mix 1. 5.2. Mixes with polypropylene fibers

Fig. 11. Measured compressive strengths for concrete mixes without fibres and with polypropylene fibres as a function of maximal temperature (every point is average of 3 measurements), relative strength values (related to those at 20 1C).

The probability of concrete-cover spalling can be reduced by applying polymeric fibers. We tested two types of polypropylene fibers (fiber 1 with Ø ¼0.032 mm, ‘ ¼18 mm; fiber 2 with ؼ1.1 mm, ‘ ¼40 mm). Our observations indicate that the advantageous effect of polymeric fibers on concrete subjected to high temperatures is mainly observed for thin fibers and not for thick fibers. The overall tendency of strength reduction with increasing temperatures was similar with and without the use of plastic fibers (fibers 1 and 2). Between 20 1C and 400 1C, the compressive strengths of the 4 mixes were different. Scanning electron microscopy images were acquired for concrete samples with polymeric fibers to observe the modified surfaces of fibers after temperature loading. It was observed that the fibers are melted above 150 1C. 5.3. Mixes with steel fibers

Fig. 12. Measured compressive strengths for concrete mixes with steel fibres as a function of maximal temperature (every point is average of 3 measurements), relative strength values (related to those at 20 1C).

For steel-fiber-reinforced concrete with fiber type 3 (Mix 4), a special type of failure occurred during the heating process over the temperature range between 600 and 800 1C; some corners of the cube specimens were cracked off. For steel-fiber-reinforced concrete with fiber types 4 and 5 (Mixes 5 and 6), practically no surface cracks were observed up to 800 1C. The most considerable reduction in compressive strength took place between 400 1C and 800 1C in all cases. Our test results suggest that thin and short steel fibers contribute to the residual compressive strength after high-temperature loading. For steel-fiber-reinforced concrete featuring fiber types 4 and 5 (Mixes 5 and 6), practically no surface cracks were observed up to 800 1C. References

5. Conclusions The purpose of our study was to monitor the effects of polypropylene fibers on relatively high-strength concrete grades subjected to high temperatures with considerably different fiber geometries. The studied parameters included surface cracking and residual compressive strength. A group of experiments was also carried out on steel-fiber-reinforced concrete subjected to high temperatures. Five different fiber-reinforced concrete mixes were tested over a compressive strength range between 60 and 74.7 N/mm2. The test variables included concrete composition, type of fiber reinforcement (polypropylene fibers Ø 0.032 mm, ‘ ¼18 mm; or Ø 1.1 mm, ‘ ¼ 40 mm; steel fiber: Ø 1.1 mm, ‘ ¼18 mm) and maximum temperature (20 1C, 50 1C, 150 1C, 200 1C, 300 1C, 400 1C, 500 1C, 600 1C, 800 1C). A total of 180 specimens were tested from 5 different concrete mixes. During our tests, the specimens were heated to the given maximum temperature and then held at this temperature for two hours; they were then cooled under laboratory conditions. Visual observations and compressive tests were carried out at room temperature. The following conclusions can be drawn.

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