Experimental parametric study on the effectiveness of polypropylene fibres at mitigating heat-induced concrete spalling

Cristian Maluk1, Luke Bisby2, and Giovanni Terrasi3

1 Postdoctoral Research Associate, School of Engineering, University of Edinburgh, UK2 Arup Professor of Fire and Structures, School of Engineering, University of Edinburgh, UK

3 Head, Mechanical Systems Engineering, EMPA Dübendorf, Zurich, Switzerland

Abstract: Many modern concrete structures incorporating high-strength concrete may be susceptible to heat-induced explosive concrete spalling during fire. This presents a major challenge for contemporary concrete designers who often wish to use high-strength concrete in various structural engineering applications. The concrete industry is only just beginning to grapple with the implications of increased spalling of modern concrete mixes. There is widespread disagreement on the relative importance of the physical mechanisms which may trigger or exacerbate heat-induced concrete spalling, and while it has been shown that introducing polypropylene (PP) fibres into the fresh concrete reduces the likelihood of spalling during furnace tests, the reasons for PP fibre effectiveness remain a matter of debate within the research community. This article presents a comprehensive experimental study on heat-induced concrete spalling, with an emphasis on assessing the effectiveness of various types and doses of PP fibres. High-strength, self-consolidating concrete mixes in which PP fibre type, cross section, length, supplier, and dose were varied, were tested under simulated standard furnace exposures. It is shown that increased PP fibre dose, which is currently the sole parameter prescribed by available design guidelines, mitigates spalling. However PP fibre cross-section and length may also be important for PP fibre effectiveness in mitigating heat-induced concrete spalling during fire.

1 HEAT-INDUCED CONCRETE SPALLINGThe building design industry has traditionally, and to large extent justifiably, relied on the perceived ‘inherent’ fire safety features of concrete (e.g. non-combustible, non-flammable, high thermal inertia) to assure the structural fire safety (fire resistance) of concrete structures (Bilow and Kamara 2008). Fire resistance design of concrete structural elements currently relies almost universally on prescribing minimum member dimensions and minimum concrete cover to the steel (or other) reinforcement and presumes satisfactory response during fire on this basis (typically without explicit consideration of full structural response in fire by structural designers).

Advances in concrete technology, driven mainly by factors other than fire safety (i.e. architectural design, economics, ease and speed of construction, sustainability, etc.) have promoted the use of new structural systems and construction techniques, many of which use modern, high-strength concrete mixes. Such mixes tend to suffer from an increased propensity for heat-induced concrete spalling (Bentz 2000) as compared with more traditional, normal-strength concrete mixes. Failure to account for this increased propensity for spalling in modern building designs could potentially lead to unexpected structural failures during fires, if not properly considered during building design, construction, and operation.

Heat-induced concrete spalling is a phenomenon wherein heated concrete separates from the heat-exposed surface in a more or less violent manner (see Figure 1). This results in a reduction of the concrete cover to the internal reinforcement and a reduction in the effective size of the overall cross section and adversely affects its fire resistance. More than a century of research studies into the occurrence of heat-induced concrete spalling have led to the conclusion that spalling is a sudden and stochastic phenomenon characterized by its dependency upon multiple influencing parameters (Maluk 2014). For instance, spalling is known to be influenced by (at

least): concrete strength, moisture condition, age, aggregate type and grading, certain admixtures, mechanical loads, mechanical restraint, heated area, element thickness, severity of thermal exposure, and the inclusion of steel and/or polypropylene (PP) fibres. Spalling of modern mixes presents a serious challenge in the context of the historically deterministic approach to structural fire safety wherein standard fire resistance tests (furnace tests) on isolated structural elements, along with historical evidence of reasonable performance in fires, is relied on (Maluk 2014).

The number of factors known to influence spalling make its prediction difficult, if not impossible, in practice. Thus, the purpose of the study presented herein was to develop a test method that could be used to perform comparative studies of various candidate high-strength, self-consolidating concrete mixes, in an attempt to determine the optimal PP fibre type and dose that could ‘guarantee’ no spalling under standard fire conditions.

Figure 1. Photograph showing heat-induced concrete spalling on the underside of a reinforced concrete slab after a standard furnace test (photo courtesy Ieuan Rickard).

1.1 Mechanisms of SpallingTwo main mechanisms are widely considered to contribute to the occurrence of heat-induced concrete spalling. First is a thermo-hydraulic mechanism associated with the transport and/or evaporation of free water (or capillary water) within the concrete microstructure; this leads to generation of steam pressures and a ‘moisture clog’, and eventually to spalling. It is almost universally agreed that higher moisture content results in increased heat-induced spalling, all other factors being equal (e.g. Bailey and Khoury 2011). Second is a thermo-mechanical mechanism defined at both micro- and macro-scale. At the micro-scale, this mechanism is associated with internal mechanical stresses resulting from incompatibilities in the thermal and thermo-mechanical behaviour of the various components within the concrete matrix (e.g. coarse and fine aggregates, cement paste, chemically bound water, etc). At the macro-scale, this mechanism is linked to internal mechanical stresses resulting from external loading, restraining forces, and/or differential thermal stresses arising due to uneven heating, thermal gradients, and/or the presence of cold areas. The relative importance of the above mechanisms for a particular concrete mix under a particular thermal exposure in a given application are not well known, although it is now widely agreed that both mechanisms must be considered in any attempt to understand or predict the occurrence of heat-induced concrete spalling.

1.2 Polypropylene FibresMore than three decades of experimental studies have convincingly shown that polypropylene (PP) fibre inclusion in fresh concrete reduces the propensity for heat-induced concrete spalling (e.g. Chandra et al. 1980, Hertz and Sørensen 2005, Jansson 2013). PP fibres are theorised to ‘alter’ the transient moisture migration and/or evaporation processes within heated concrete, thus reducing the risk of heat-induced spalling (particularly when the thermo-hydraulic mechanism is dominant). While the mechanisms behind PP fibres’ effectiveness remain poorly understood,

three potential mechanisms are widely proposed that involve the PP fibres generating: (1) discontinuous reservoirs, (2) continuous channels, and/or (3) vacated channels (Khoury 2008).

During heating, rapid volumetric changes of the PP fibres cause micro-cracks within the concrete matrix surrounding the fibres, thus creating discrete reservoirs that enhance moisture migration within concrete. PP fibre inclusion also promotes the formation of discrete reservoirs by increasing air entrainment. During heating, continuous channels are formed at the interfaces between the PP fibres and the concrete matrix due to a poor interfacial adhesion and/or a relatively more porous region at the interface. This phenomenon, coined Pressure-Induced Tangential Space (PITS) by Khoury (2008), may also enhance moisture migration within concrete. Finally, enhanced moisture transport may also be driven by the formation of vacated channels left behind by melted/pyrolized PP fibres during heating. This is the most widely quoted mechanism used to describe the effect of PP fibres in on heated concrete Khoury (2008), however there is little direct evidence for it. Despite decades of research, the relative importance of the above mechanisms remains a matter of debate. Physical mechanisms aside, it is reasonable to assume that an optimum (or most ‘effective’) PP fibre and dose should exist to mitigate spalling under a given set of conditions.

PP fibres are commercially available in a range of types and sizes. The most common are monofilament, multifilament, and fibrillated (Figure 2). Monofilament and multifilament fibres are both manufactured through an extrusion process, with nominal diameters in the range of 10-40 microns. Monofilament fibres are manufactured from a single strand of fibre, while multifilament fibres are made from multiple, combined strands. While the diameter of fibrillated fibres is in the range of monofilament and multifilament fibres, these are manufactured in the form of films that are slit in such a way that they can be expanded into an open network (Kumar et al. 2013). Fibres of all types can be cut to the desired length, commonly in the range of three to 20 mm. More recently, the use of fibres made out of alternative materials (e.g. polyvinyl alcohol, cellulose, nylon, jute) has been considered, although their effectiveness has yet to be convincingly demonstrated (e.g. Heo et al. 2011).

Figure 2. Photographs of (a) 6 mm monofilament (32 μm diameter), (b) 12 mm multifilament (32 μm diameter), and (c) 20 mm fibrillated (37×200 μm2) PP fibres.

1.3 Designing Against SpallingWhilst the precise mechanisms by which PP fibres mitigate heat-induced concrete spalling remain unknown (Maluk 2014), current design and construction guidance for spalling prevention

(e.g. CEN 2004a, CCAA 2013) is based on a one size fits all understanding where a dose of PP fibres (i.e. mass of PP fibres per volume of concrete) is prescribed for concrete mixes or end use conditions for which spalling is likely to occur (e.g. high-strength concrete, high in-service moisture content, etc). For example, CEN (2004a) recommends including more than 2 kg/m3 of monofilament PP fibres for certain types of concrete – high-strength (>55 MPa cube compressive strength), high moisture content (>3% by mass) and/or with high inclusion of silica fume (>6% by mass of cement) – whereas CCAA (2013) states that addition of 1.2 kg/m3 of 6 mm long monofilament PP fibres has a “dramatic effect in reducing the level of spalling”. The above guidance is based on available experimental research on heat-induced concrete spalling, and can only be viewed as a means of reducing, rather than eliminating, the risk of spalling.

2 AIMS OF THE CURRENT STUDYDue to the complexity and uncertainty of the mechanisms contributing to spalling, the inability to credibly computationally model spalling, and the considerable expense of performing large-scale furnace tests on concrete elements to demonstrate adequate non-spalling behaviour in practice, the goal of the current project was to develop a test method that could be used to interrogate the behaviour of candidate concrete mixes using medium-scale specimens, at low economic and temporal cost and with good repeatability. It was also desired to experimentally study the relative performance of various PP fibre types and doses in preventing spalling.

The study included 11 candidate high-strength, self-consolidating concrete mixes for which PP fibre type, cross section, length, supplier, and dose were systematically varied. Compressive strength and the workability of the fresh concrete (slump flow) were maintained as constant as possible for the various mixes due to the requirements of an industrial partner (a precast concrete manufacturer). The influence of externally imposed pre-compressive stress was also studied since the candidate mixes were intended for end use applications involving pretensioned elements.

Rather than seeking to unravel and understand the precise mechanisms contributing to spalling, the current study was focused on validating the capability of a repeatable test to demonstrate the ability of a candidate concrete mix to ‘guarantee’ non-spalling properties under credible worst-case heating and mechanical loading conditions during a standard furnace test.

3 TEST METHODOLOGYA novel fire testing methodology was developed with the goal of simulating the thermal exposure to which similar concrete test specimens were exposed during a standard fire resistance test. The method is named the Heat-Transfer Rate Inducing System (H-TRIS), and has been presented elsewhere (Maluk 2014). H-TRIS uses a mobile array of propane-fired radiant panels with a mechanical linear motion system (Figure 3) to impose a prescribed time-history of incident heat-flux (i.e. thermal exposure) on a target exposed surface (test specimen). Essentially, by moving the radiant panel array toward or away from the specimen, using a pre-calibrated distance versus incident heat flux relationship, the specimen is subjected to through thickness thermal gradients that are identical to those experienced by a similar specimen when tested in a fire testing furnace; repeatedly, quickly, and at a fraction of the cost of furnace testing. During the current study the thermal exposure was based on that experienced by concrete test elements during large-scale fire resistance tests by the authors (Terrasi et al. 2012). A loading and restraining frame was also used to induce constant compressive mechanical load during testing.

Mobile radiant panel arrays have previously been used for thermal testing of construction materials (e.g. Mowrer 1998, Skji et al. 2008). However, the current work is the first time that such an approach has been taken to simulate the time-history of heat flux in a standard fire resistance test based on an inverse model of through thickness temperature measurements obtained during furnace tests of effectively identical specimens (Maluk 2014).

Figure 3. Heat-Transfer Rate Inducing System (H-TRIS).

Figure 4 shows a comparison of through-thickness temperature measurements recorded for concrete specimens during a standard large-scale furnace test at the Swiss Federal Laboratories for Materials Science and Technology (Terrasi et al. 2012) against those measured at the same locations in tests using H-TRIS (programmed to simulate the furnace exposure). The shaded areas represent the scatter of through-thickness temperature measurements in five effectively identical large-scale specimens tested simultaneously during a single furnace test, whereas the black lines represent the equivalent measurements during a series of individual tests on effectively identical specimens in H-TRIS (three individual thermocouple readings are shown for each specimen at 10, 20 and 45 mm from the exposed surface). The figure shows that the H-TRIS testing methodology is able to accurately replicate the thermal gradients experienced by concrete specimens during a standard furnace test. The added advantages of lower economic and temporal cost, along with superior repeatability as compared with furnace testing, are evident.

Figure 4. Comparison of variability in selected through-thickness temperature measurements recorded at various distances from the heated surface in concrete specimens during furnace tests

(shaded areas) and using H-TRIS (solid lines).

4 SPALLING TEST PROGRAMMEConstrained by requirements on minimum strength and self-compaction of the candidate concrete mixes (imposed by the industrial partner) the concrete strength (C90 according to CEN 2004b) and workability (slump flow of 750 mm according to CEN 2010) were maintained as constant as possible for all candidate mixes (Maluk 2014). Mix labels shown in Table 1 have no inherent meaning but were defined by the industrial partner. Parameters assessed amongst the mixes were: PP fibre cross section (18 or 32 μm diameter circular cross sections and 37 × 200 μm2

rectangular cross section); PP fibre length (3, 6, 12, or 20 mm); PP fibre supplier (Bekaert, Propex, or Vulkan)*; PP fibre type (monofilament, multifilament, or fibrillated); and dose (between 0.68 and 2.34 kg of PP fibres per m3 of concrete).

Table 1. Testing matrix (all specimens tested in triplicate).

Mix Label

PP Fibre Parameters Like-for-like Comparison

Supplier(type)

Cross-section Length

Dose[kg/m3]

Total fibresurface area

[m2]

Total fibrelength[km]

Total number of individual fibres[mill. of fibres]

042 None -132 Bekaerta

(monofilament)18 μm

6 mm0.68 165 2915 486

142 32 μm 1.20 165 1640 273341

Propexb

(multifilament) 32 μm

3 mm1.20 165 1640 547

342 2.00 275 2733 911345

6 mm1.20 165 1640 273

343 1.40 192 1913 319344 12 mm 1.20 165 1640 137241

Vulkanc

(fibrillated)37 × 200

μm2 20 mm1.20 84 178 15

242 2.00 141 297 25243 2.34 165 348 29

a www.bosfa.com/products/duo-mix-fire.aspxb www.fibermesh.com/product/microsynthetic.htmlc http://www.en.krampeharex.com/pdf/Kunststofffaser_PF.pdf

The fibre doses given in Table 1 were decided to provide like-for-like comparisons assessing: (1) the total fibre mass, (2) the total fibre surface area, (3) the total fibre length, (4) the total number of individual fibres per unit volume of concrete; all while maintaining consistent compressive strength and self-consolidating properties (Maluk 2014). For each of the 11 mixes unreinforced medium-scale specimens (45 × 200 × 500 mm3) were tested using H-TRIS in a vertical orientation with heating from one side only (Figure 3).

Recognising that scaling of test specimens in structural fire resistance testing is debated on various grounds (Maluk 2014), dimensions in the direction of the principal heat flow were the same as in the prior furnace testing (Terrasi et al 2012); specimens were cast with cross-sections 45 × 200 mm2. The length of test specimens in the current study was 500 mm, as compared with 3300 mm in the prior furnace tests, with cold overhangs of 50 mm; the thermally exposed surface was thus 400 × 200 mm2.The moisture content of the concrete at the time of testing was between 4.0 and 5.0% (by mass) at an the age of between 13 and 16 months. Cube compressive strengths at 28 days were 94 to 104 MPa. Specimens were tested under sustained compressive load or

* Specific PP fibre suppliers are named purely for the purposes of factual accuracy.

under a free-to-expand (unrestrained) condition. Specimens tested under load were subjected to a concentric vertical stress of 12.3 MPa. All tests were performed in triplicate.

5 TEST RESULTS AND DISCUSSIONMore than 60 individual spalling tests were performed during a period of only 30 days; clearly demonstrating the low temporal cost of testing in H-TRIS as compared with conventional fire testing furnaces. This would likely have taken a year or more in a standard fire testing furnace.

Spalling occurred for four of the 11 candidate concrete mixes: 042, 341, 241, and 242 in Table 1. None of the other mixes experienced any spalling. Convincingly, when spalling occurred for a given mix it occurred for all three identical repeat tests. Likewise, if no spalling was observed for a particular mix then this result held for all three repeat tests. Figure 5 shows typical post-test photographs of three specimens demonstrating various increasing amounts of spalling (i.e., surface, mild, and destructive). A summary of relevant test data for mixes that experienced spalling is given in Table 2 (non-spalling mixes are not included).

Figure 5. Typical specimens after tests using H-TRIS, demonstrating(a) surface, (b) mild, and (c) destructive spalling.

The data given in Table 2 demonstrate the following behaviours in correspondence with the various concrete mix parameters: PP fibre cross section – Inclusion of PP fibres with smaller cross sections appears to have a

positive influence in mitigating spalling for both loaded and free-to-expand specimens. For instance, neither mixes 132 (18 µm diameter PP at a dose of 0.68 kg per m3 of concrete) and 142 (32 µm diameter at a dose of 1.20 kg per m3 of concrete) experienced spalling, although mix 132 had a much lower dose. The cross section and dose for these mixes were decided to provide a comparison of the total PP fibre surface area, with all other parameters (e.g. PP fibre length, supplier, type, etc.) essentially unchanged (refer to Table 1).

Table 2. Selected results for specimens that experienced spalling.

Fibre length – Concrete mixes cast with relatively short (3 mm long) monofilament PP fibres exhibited a higher risk of spalling than practically identical mixes (equivalent PP fibre doses) with longer fibres (6 or 12 mm long). Thus, longer PP fibres (presumably up to some optimal length) appear to be more effective at mitigating heat-induced spalling. For instance, a comparison of test results from mixes 341, 345, and 344, which included fibres 3, 6 and 12 mm long, respectively (all at a dose of 1.20 kg of PP fibres per m3 of concrete), suggests a negative influence of using very short (3 mm) PP fibres since Mix 341 is the only one which experienced spalling.

Mix label

PP fibre parameters

Sustained compressive

stressTime-to-spall

Absorbed heat density2

[kJ/cm2]

Mass spalled3

[%]

042

0 MPa

10.9 2.28 2.0 %

24.7 5.99 6.3 %

13.9 3.01 10.7 %

12.3 MPa

12.5 2.66 12.1 %

11.0 2.31 6.3 %

13.1 2.81 11.2 %

341

0 MPa

17.1 3.85 8.4 %

14.6 3.19 2.4 %

16.5 3.67 4.0 %

12.3 MPa

16.3 3.64 28.5 %

13.0 2.80 18.7 %

14.4 3.16 24.4 %

241

0 MPa

12.4 2.66 3.8 %

7.9 1.61 0.9 %

11.7 2.49 2.2 %

12.3 MPa

7.3 1.48 2.3 %

14.3 3.11 1.9 %

12.5 2.66 7.2 %

242

0 MPa

- - -

- - -

- - -

12.3 MPa

9.9 1 2.07 4.1 %

9.4 1 1.93 4.0 %

10.6 1 2.22 14.3 %1 For the specimens tested during the set of furnace tests (Terrasi et al. 2012), spalling first occurred between 9.2 and 10.3 minutes from the start of the test.2 Absorbed heat density (or total thermal energy absorbed per unit volume of concrete in the specimen) was calculated as the area under the absorbed heat flux versus time curve experienced by each of the specimens tested using H-TRIS, up to the moment of spalling. This ratio potentially allows for rational comparison between tests under varied thermal exposures; i.e. tested under different time-histories of incident heat flux (Maluk 2014).3 Mass spalled was calculated by subtracting the mass of the tested specimen (after cooling) from the initial mass of the specimen. It is noteworthy that no distinction was made between mass lost due to the spalled concrete and the mass lost due to dehydration of the specimen during heating.

Fibre supplier – PP fibre supplier has no obvious influence on test results, all other factors being equal.

Fibre type – Because the dimensions (cross section and length) of monofilament or multifilament, and fibrillated PP fibres assessed in this study diverged significantly, it is not possible to independently assess the influence of PP fibre type on the occurrence of spalling. Nonetheless, no obvious influence was observed for mixes cast with monofilament or multifilament PP fibres, all other factors being equal. It can be concluded that fibrillated PP fibres are less effective than monofilament PP fibres.

Fibre Dose – As expected, high doses of PP fibres had a positive influence in mitigating the occurrence of spalling; however some very low doses (e.g. Mix 132) were also effective at mitigating spalling. The reasons for this are not yet known and additional research is underway.

Sustained Pre-Compressive Stress – Tested specimens for which spalling occurred under sustained compressive load also suffered from spalling when tested under a free-to-expand condition (with exception of Mix 242 which spalled only under compressive load). Thus, only a very mild influence of pre-compressive stress was observed in the current study. It is noteworthy that while the sustained compressive load was maintained constant, the thin test specimens (45 mm thick) bowed due to differential thermal expansion during heating.

It was also observed that for the concrete mixes assessed in this study, the inclusion of PP fibres showed a negative effect on slump flow (i.e. unfavourable effects on self-compacting and workability). PP fibres with reduced cross section and/or large individual lengths showed a more significant negative influence on slump flow, whereas inclusion of PP fibres showed no obvious influence on moisture content or compressive strength measurements.

Furthermore, during tests using H-TRIS, it was possible to accurately quantify the time-to-spall, the mass spalled, and the absorbed heat density of the tested specimens (refer to Table 2). Spalling consistently occurred between 7 and 25 minutes from the start of heating. Finally, the occurrence of heat-induced concrete spalling for specimens tested with H-TRIS was in reasonable agreement – in terms of time-to-spalling (i.e. ±2 minutes) – with effectively identical concrete specimens tested during furnace tests (Terrasi et al 2012).

6 CONCLUSIONSThe inclusion of PP fibres of whatever type appears to have a positive influence on mitigating heat-induced concrete spalling. PP fibre dose is currently the sole parameter prescribed by design guidance to counter spalling; however, the current study has revealed that fibre cross section and individual fibre length may also have important influences on PP fibre effectiveness in mitigating the occurrence of heat-induced concrete spalling. Additional research is needed to better understand the necessary mix design parameters to reliably prevent heat-induced concrete spalling under a range of service conditions and credible design fire exposures.

The current study represents the first attempt to use the H-TRIS testing methodology to rationally replicate the thermal exposures (i.e. through thickness thermal gradients) experienced by concrete specimens in a standard furnace test through repeatable medium-scale testing with mobile radiant panels. The capability of H-TRIS to inexpensively, accurately and repeatably quantify the thermal (and to some extent mechanical) conditions during a fire resistance test has been demonstrated.

Future studies will use H-TRIS to replicate a broad range of relevant thermal and mechanical exposures, from those encountered in a standard fire resistance test to those encountered in any ‘real’ design fire, including hydrocarbon and tunnel fire exposures relevant to precast concrete tunnel lining segments. It is hoped that forthcoming research projects and developments using the

H-TRIS methodology will promote an industry-wide move away from pass/fail large-scale furnace testing and towards rational assessment of materials and structural elements under tightly controlled and repeatable conditions.

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