Journal of Building Engineering 29 (2020) 101167

Available online 7 January 20202352-7102/© 2019 Published by Elsevier Ltd.

Microstructure and structural analysis of polypropylene fibre reinforced reactive powder concrete beams exposed to elevated temperature

Mazin Abdul-Rahman a, Alyaa A. Al-Attar b, Hussein M. Hamada c,*, Bassam Tayeh d

a Tikrit University, Iraq b Northern Technical University, Iraq c University Malaysia Pahang, Malaysia d Islamic University in Gaza, Palestine

A R T I C L E I N F O

Keywords: reactive powder concrete Beam Fire Polypropylene fibres Spalling

A B S T R A C T

Despite the superior properties of reactive powder concrete (RPC), the possibility of spalling under fire condi-tions still exists, which can lead to a significant reduction in the fire endurance of reinforced concrete members. This study investigates the efficiency of using the different percentages of polypropylene fibres (PPFs) for enhancing the fire resistance of reinforced beams made from RPC. Five RPC beams were tested by applying two concentrated loads. One of them (without PPFs) was tested under monotonic load up to failure. The other four beams were subjected to service load, and then controlled fire was applied for 120 min in accordance with the fire temperature vs. time curve prescribed in ASTM E 119. Loading tests were then conducted on the beams that were not completely damaged through fire testing, in order to examine their remaining strength after cooling. Experimental results showed that non-fibrous reinforced RPC beams suffer early spalling during the fire test, thus causing beam failure after 38 min of fire exposure. The addition of PPFs in a low volume percentage (0.25%) decreases spalling and delays beam failure until 115 min, whereas PPFs in high percentages (0.75% and 1.25%) completely prevent the spalling and beam collapse. Moreover, an increase in PPF content reduces the total deflection of a beam and improves the residual strength and ultimate strength of fire-failed beams subsequent to cooling.

1. Introduction

Many types of research have attracted their interest in the study of reactive powder concrete (RPC) due to its high strength and excellent durability [1,2]. Thus, RPC is a promising material in the construction field [3,4]. This material is characterized by a dense and homogeneous microstructure which is attained by coarse aggregate eliminating and optimizing the packing density of the mix using pozzolanic material, cement and very fine sand (0.6 mm) with very low water/cement ratio [5,6]. Numerous studies on the properties of RPC [7–10] have been conducted, and some of these studies have addressed the properties of RPC at high temperatures [11–13]. A previous study has reported that when RPC exposed to high temperatures is susceptible to explosive spalling, the thermal spalling has many effects and the vapour pressure of RPC is one of them [14]. Spalling is mostly caused by the build-up of vapour pressure inside the pores and the thermal stresses produced by

temperature gradients [15,16]. However, many investigations have indicated that pore pressure is an important consideration with regard to the spalling of dense concrete mixes [14,16]. The dense and low permeable microstructures of RPC prevent the migration of interior vapour pressure through RPC matrix at high temperatures, thereby resulting in a rapid rise in vapour pressure; if the vapour pressure caused a stresses greater than the tensile strength of RPC, then spalling will occur [3,17]. Wang et al. [18] concluded that the splitting tensile strength of concrete increases with increasing of volume fraction of polypropylene fibre (PPF) content. The addition of PPFs can effectively reduce the occurrence of spalling [18–20]; however, adding PPF alone can reduce the mechanical properties of RPC at room temperature [20]. PPFs melt at a temperature of approximately 170 �C and create an interconnecting channel of equal space in the RPC with rising temper-ature, thus making possible the movement of high-temperature moisture and releasing the vapour pressure [4]. PPF slightly affects the properties

* Corresponding author. E-mail addresses: mazinburhan@tu.edu.iq (M. Abdul-Rahman), dr.alyaa@ntu.edu.iq (A.A. Al-Attar), enghu76@gmail.com (H.M. Hamada), btayeh@iugaza.edu.ps

(B. Tayeh).

Contents lists available at ScienceDirect

Journal of Building Engineering

journal homepage: http://www.elsevier.com/locate/jobe

https://doi.org/10.1016/j.jobe.2019.101167 Received 30 September 2019; Received in revised form 31 December 2019; Accepted 31 December 2019

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of RPC at standard temperature. These fibres have a relatively low modulus of elasticity and cannot prevent the formation and propagation of cracks in hardened RPC but can effectively inhibit cracks due to shrinkage during the fresh state [21]. For the reinforced concrete members, such as beams, the fire conditions must be taken into consideration when it designed to withstand service loads without collapse due to the reducing in the strength of beam caused by elevated temperature. In addition, these members must resist thermal expansion that causes additional stresses and strains [22]. Spalling is considered the main reason for the failure of the reinforced concrete elements that are subjected to high temperatures. This phenomenon causes layers or pieces to separation from the surface, thereby removing the concrete cover which leads that the reinforcing steel is exposed to rising tem-peratures [23].

In the last years, Hou et al. [24] investigated four hybrid-fibre reinforced RPC beams protected with fire insulation exposed to high temperature. They observed that the failure of reinforced RPC beams was because of the fracture of the longitudinal rebars resulting from direct fire exposure after the collapse of the fire insulation on the RPC beam. Another study by Hou et al. [25] studied the behaviour of RPC beams exposed to high temperatures. They tested the cross-section temperatures at different heights, axial thermal expansion, mid-span deflections, crack patterns, fire resistance, and failure modes of beams. The results show that the addition of 0.2% polypropylene fibres and 2% of steel fibres has an active role to reduce the spalling owing to fire in the reinforced RPC beams. Abid et al. [26] studied the creep behaviour of RPC beams at high temperature. They observed that the short-term creep of RPC improves extremely above 500 �C.

On the basis of the abovementioned factors, a full-scale test of RPC specimens under fire and service loads is essential. The present study aims to investigate the structural advantages of adding PPFs to RPC under fire conditions. Several tests have been carried out to study the influence of PPFs with various volume fractions (0%, 0.25%, 0.75% and 1.25%) on the behaviour of reinforced reactive powder concrete beams subjected to service load and elevated heat exposure, where the heating program was in agreement with ASTM E� 119 standard time- temperature relationship [27]. The sides and bottom faces of loaded beam specimens were exposed to fire inside the furnace for 2 h. Furthermore, the residual strength and the ultimate load capacity of fire-failed reinforced reactive powder concrete beams subsequent to cooling of beams are investigated. This work has been performed to show the resistance of reactive powder concrete against the fire using the Polypropylene Fibre (PPF).

2. Experimental program

2.1. Materials and mix proportions

RPC was manufactured using the following components:(1) ordinary Portland cement (ASTM Type I) and it chemical composition is with the limits the Iraqis specifications No.5/1984; (2) densified micro-silica (MEYCO® MS 610)which conforms to ASTM C1240/03 and contains 91% SiO2; (3) natural siliceous sand with a maximum particle size of 0.6 mm; (4) high-range water-reducing admixture (GLENIUM 51) and (5) monofilament PPF with a melting point of 160 �C. Table 1 presents the properties of the PPF.

In accordance with guidelines from published research [5,28] and several trial mixes, mix proportions were formulated to produce RPC with high strength and favourable workability. Four mixtures with

different amounts of PPF were prepared. The mixtures were non-fibrous mixture (P0) and three mixtures containing PPF with 0.25% (P0.25), 0.75% (P0.75) and 1.25% (P1.25) volume fractions.

2.2. Preparation of samples

The control specimens for each mixture were cast to find the me-chanical properties of RPC at room temperature. The specimens cast were three 70 mm cubes [29], three cylinders with a diameter of150 mm and a length of 300 mm and three prisms measuring 100 � 100 � 500 mm3. In addition, five beams were cast—two for the non-fibrous mixture and one for each of the other mixtures.

The mixing process was performed using a horizontal pan rotating mixer [30]. Firstly, the fine sand was added to the mixer. Secondly, PPFs were gradually added to the rotary mixer to avoid balling of fibres and blended for 5 min. Thirdly, the cement and micro silica were added, and the dry components were blended for 5 min. Fourthly, 50% of water was added and blended for 5min. Finally, the water reducer was dissolved in the remaining water before adding and blending it for another 5 min. The workability was immediately measured after the end of mixing process through the flow table test according to ASTM C1437-01 [31]. The flow results is 110 � 5% for all mixtures. RPC mix is placed and vibrated in the moulds in three layers. The moulds were removed on the next day after casting. The moisture content was measured by weighing the specimens before and after drying to a constant mass at 105 �C until there is no further weight loss. The moulds were removed on the next day after casting. The samples were then cured by immersing them in the water at (1) 75 �C for 3 days and (2) at 25 �C on the 28th day.

2.3. Details of the beams

The tested beams were 2000 mm length, 150 mm width and 200 mm depth and were composed of the same reinforcement and concrete cover. These beams were designed to fail in bending with tensile mode failure. The dimensions and reinforcement details are depicted in Fig. 1. The concrete cover is 30 mm, the distance from the edge of longitudinal rebar to the bottom of beam. Table 2 lists the mix proportions of RPC in detail, all the materials were constant in the concrete mixtures, except the PPF was variable. The longitudinal steel reinforcement in RPC beam has a significant effect to prevent the failure during the fire.

2.4. Mechanical properties tests

Cube compressive strength ðfcuÞ tests were conducted on 28-day-old cube specimens using a hydraulic universal testing machine [32]. Modulus of rupture ðfrÞ and splitting tensile strength ðfctÞ tests were conducted on the prism and cylindrical specimens. For each test, the average of the three specimens’ test results was adopted. The standard test was used according to ASTM C78 [33] to determine the modulus of rupture and tensile strength.

2.5. Fire test

2.5.1. Test setup The beam specimens were tested in the furnace which was designed

to expose the beam specimens (bottom and sides) to standard time-fire temperature curve and subject them to a constant two-point load dur-ing the fire test. This furnace was built using ordinary brick (width and depth ¼ 1250 mm and length ¼ 2000 mm). The walls and floor were covered with refractory bricks and ceramic wool to provide favourable thermal insulation. The furnace top open was covered with two steel plates sheets coated with ceramic wool. The details of the furnace are exhibited in Fig. 2.

Special setup for beam specimen loading was constructed. The sim-ply supported beam (clear span of 1800 mm) was placed above the top open of the furnace horizontally parallel to the furnace axes. The

Table 1 Properties of PPFs.

Fibre length

Fibre diameter

Specific gravity

Tensile strength

Modulus of elasticity

12 mm 175 μm 910 kg/m3 320–400 MPa 3.5–3.9 GPa

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structural loading was performed using a steel I-beam which acts as a lever that transfers the concrete blocks’ weights to the top face of the tested beam at the two points at 600 mm apart from the support (sym-metrical at the midspan section). The deflection was measured at mid-span using a dial gauge with 0.01 mm accuracy. The furnace temperature was raised using a gas burner and measured at three lo-cations near the specimen’s surface using Type K thermocouples and a digital temperature indicator. The temperature inside the beam speci-mens was also measured at specific locations.

2.5.2. Test procedure The fire test was divided into two phases. Firstly, the tested beam was

incrementally loaded by increasing the weight on the beam up to 50%

from the maximum load (which characterize the maximum service load value). At each increment, the midspan deflection was measured. This load level remained constant during the next phase of the fire test, in which both sides and bottom of beams were under a controlled fire scheme that conforms with the ASTM E 119 standard time-temperature relationship plotted in Fig. 3 for 120 min. The temperatures at specific points and the midspan deflection were measured and recorded every 5min. The exposure was maintained until the beam failed or until the test time ended.

3. Results and discussion

3.1. Mechanical properties

The 28-day mechanical properties tests at room temperature for all mixes are summarised in Table 3. The results showed that adding PPFs with different volume fractions slightly decreases the strength of RPC. This phenomenon was due to these fibres have a lesser elasticity modulus than RPC [8,34,35]; therefore, the fibres do not effectively contribute to the beams’ stress resistance [36]. Furthermore, PPF in-creases RPCs’ air content and pore volume [37,38]. The fct and fr of RPC without PPF was less than those of the control sample. Both quantities gradually increased with the PPF contents; this finding is consistent with the results obtained by previous studies [39–41] (see Table 4).

However, the mechanical properties of RPC with PPF still acceptance

Fig. 1. Dimensions and reinforcement of RPC beams.

Table 2 RPC mix proportions (kg/m3).

Mix type

Fine sand

Binder materials Water Water reducer

PPF (%)

Cement Micro silica

P0 1070 963 107 214 32 0.00 P0.25 1070 963 107 214 32 0.25 P0.75 1070 963 107 214 32 0.75 P1.25 1070 963 107 214 32 1.25

Note: PPF (%) is the volume dosage.

Fig. 2. Geometry of the setup Furnace and loading system.

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in terms of using these type of concrete as structural members in the construction buildings [42].

3.2. Microstructure of RPC beams at elevated temperature

The microstructures images of samples taken from RPC beams specimens are analysed after exposing to high temperature using a scanning electron microscope (SEM) technique. As shown in Fig. 4, the microstructure of the RPC sample at high temperature has denser than the control sample. The selected images were 250 � , 500 � , 1000 � , and 2000 � as in Fig. 4. The microstructure of the RPC beam with various fibre contents at high temperature is shown in Fig. 4. The compressive strength test was conducted for all RPC samples with various fibre content at high temperature. The compressive strength decreased slightly due to adding PPF to the cement.

The RPC samples cast with 0.25%, 0.75%, and 1.25% have illus-trated molten channels performed due to melt of polypropylene fibres.

The compressive strength of RPC at 800 C has shown decreased if compared with the control sample. This decrease is due to the high density of the internal structure of RPC as well as the high temperature led to accelerating the reaction of materials to be high-density structure. The test of SEM conducted on the RPC cubes at temperature up to 800 C to test the microstructure. The test shows that there is a significant number of cracks arises because of thermal expansion of the RPC which results in weak bonding between cement and aggregate. Hereafter, the decrease in RPC samples strength is observed at 800 C. The existence of micro-cracks and channel created due to thermal expansion as observed in SEM images of RPC. From Fig. 4, it is noted that RPC with 0.25% PPF content has a melted fibre channels due to exposure to the high tem-perature up to 800 C. While, the RPC with 0.75% and 1.25% PPF has a micro-crack in the surface structure as well as less bond in the interface zone between cement paste and aggregate. Overall, the existence of micro-cracks arises because of increasing vapour pressure. The micro-structure of RPC with 1.25% fibre content has illustrated the good

interface between cement paste and aggregate, however melting of fibre created further of microcracks in channels. Increase the temperature led to weakening continuously, 0.25% fibre content in RPC has illustrated weak porous. The interfacial transition zone (ITZ) between cement paste and aggregate in concrete, it is the zone of the cement paste around the fine and coarse aggregate at 800 C as shown in Fig. 4. Abid et al. [43] tested the RPC cubes exposed to the high temperature by the SEM/EDX and XRD tests. At ambient temperature, the RPC has a very dense microstructure. While appearing the micro-cracks at 300 �C on the RPC surface. Hou et al. [44] reported that in the microstructure test, the melting process of PP fibres in RPC beams result in the safe passage for releasing vapours pressure during high temperature. Therefore, the compressive strength of RPC reduced more than the control sample. The dense microstructure and minimum cracks with melted channels were seen in the mix containing 0.25% PPF in the RPC at 800 C. The micro-structure of RPC with 1.25% fibre content have a compressive strength more than that other PPF content at high temperature. The increasing number and width of cracks are mainly causing to reduce the compressive strength.

3.3. Response of reference beam under ultimate static load

The reference beam (with no fibres) have been tested under for flexural loading until failure to determine its behaviour and strength. Fig. 5 presents the load-deflection of control beam until failure. At initiate of loading, the linear load-deflection relationship is constituting up to the form of the first cracks at the maximum tension zone. Then, more cracks propagation produces additional losses of strength and nonlinear response is noticed until the of reinforcement steel yielding. The next stage starts with the reinforcement steel yielding up to beam failure which occurred by concrete crushing at the compression region, as shown in Fig. 6. Table (4) present the crack load Pcr, yield load Py and ultimate load Pu of the reference beam which tested until failure.

At service loading stage, the beam will be in cracked-elastic state. In the current research, a service load level of 22.14 kN is selected as ser-vice load for the fire test which is approximately 50% of the maximum failure load [42], this service load level is determined using ACI 318–11 [43].

3.4. Fire test results

In order to find the ultimate flexural strength, one of the two non- fibrous reinforced RPC beams was loaded at room temperature until maximum failure load. The crack, yield and ultimate loads were 14.72, 36 and 44.2 KN, respectively.

All the other beam specimens were tested under the same loading condition during the fire test. Fig. 7 presents the load–midspan deflec-tion relationship when two-point increments of the static load was applied to the simply supported beam until it reached 22.14 KN (service load).The first cracking load ðPcrÞ and midspan deflection at service load level ðδsÞ for all beams are summarised in Table 5. The initial linear relationship became non-linear after the cracking in the RPC beam’s tension zone. The addition of PPF slightly affected the load–deflection curve before the first crack but reduced the first crack load value. After the first crack stage (second stage), δs increased by about 41%, 35% and 18% for reinforced RPC beams with PPF ratios of 0.25%, 0.75% and 1.25% respectively. This phenomenon is due to the decreasing in modulus of elasticity and tensile strength caused by presence of PPFs. As reported in literature studies, the tensile strength of reactive powder beams decreases with increasing of temperature [45].

The midspan deflection–fire exposure time curves are plotted in Fig. 8. Moisture content was measured by dehydration mass loss of specimens. The average moisture contents of RPC specimens before fire test were 2.5%–2.7%. All specimens exhibited a sharp increasing in the downward deflection at the beginning of the exposure given the thermal bowing effect caused by the expansion of the lower part of the beam. The

Fig. 3. Standard time-temperature relationship according to ASTM E 119.

Table 3 Mechanical properties at room temperature.

Mix. PPF (%) fcu ðMPaÞ fct ðMPaÞ fr ðMPaÞ

P0 0.00 84 5.9 6.8 P0.25 0.25 81.3 5.1 6 P0.75 0.75 80 5.4 6.14 P1.25 1.25 82.6 5.75 6.45

Table 4 Results of reinforced RPC reference beams tests.

Beam PPF % Pcr (KN) Py (KN) Pu (KN)

B0 0.00 14.72 36 44.1

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fast increase in temperature led to a high thermal gradient in the beams’ cross section, thereby increasing the rate of deflection. The non-fibrous RPC beam (B0) spalled early during the fire test. The sound of spalling and breaking segments was observed after 7 min, in which the cross section of the beam was reduced, and the temperature rise inside the beam increased, thus the reinforcing steel is exposed to elevated

Fig. 4. The SEM specimens were taken from RPC beam.

Fig. 5. Load-deflection relationship of the control reinforced RPC beam.

Fig. 6. Failure mode of control beam subjected to static load.

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temperatures. Spalling phenomena is continued, and the downward deflection increased during fire exposure. The tested RPC beam failed after 38 min. There were some samples of which the top parts spalled into small parts especially with B0.25 due to containing low level of PPT and exposed to the air for cooling. While, the spalling occurred in the bottom zone in the (B0) without FPP was due to exposed the beam to the fire and no PPF melting in the sample.

The addition of PPFs with 0.25% volume fraction reduced the effect of spalling in the RPC. The stiffness of the corresponding reinforced RPC beam (B0.25) gradually declined upon exposure to fire, thus exhibiting less increase in the deflection than the non-fibrous RPC beam. The RPC beam continued to resist loads and rising temperature exposure before failing at the 115 min mark. The addition of PPF with volume fractions of 0.75% and 1.25% prevented spalling during the entire test. Conse-quently, the increase of temperature throughout the beams was mini-mal. The corresponding reinforced RPC beams (B0.75 and B1.25) did not fail during the test. For both beams, the rate of deflection during heating and the total deflection at the end of the test decreased with increasing of fibre volumetric percentage. The temperatures of different points of B1.25 that did not experience spalling are presented in Fig. 9. The appearance of the cooled tested beams is illustrated in Fig. 10.

3.5. Structural strength of the fire-damaged beams

The loading tests were conducted on pre-tested fire-damaged rein-forced beams that did not fail during at the fire test to inspect the re-sidual strength and ultimate load capacity of these specimens after cooling. The test results which demonstrate δS, deflection at ultimate state (δuÞ and the ultimate load ðPuÞ are presented in Table 6. The load–deflection relationships of B0.75 and B1.25 are presented in Fig. 11. The stiffness is lower in the damaged RPC beams than in the undamaged RPC beams, thereby resulting in a higher deflection at ser-vice load level. The decrease in the strength of the fire-damaged beams decreases with the increase in the volume fraction of PPFs. By contrast, the ultimate load capacity increases.

Fibres of 0.75% and 1.25% volume fraction prevented the spalling of the reactive powder concrete beams during the fire test. However, sig-nificant damage was observed given high temperatures. In this study, the bottom and the sides of the beams were exposed to high tempera-tures, whereas the upper face was subjected to free air cooling; thus, severe damage occurred in the tension region, thus reducing the residual tensile stiffness of beam and resulting in a large reducing of bond strength between the rebar and concrete. The residual properties of the rebar slightly changed because the temperature at the bottom rein-forcing bars in B0.75 and B1.25 did not exceed 450 �C. Previous studies have shown that steel bars recover largely of its strength during the cooling after the exposure to this temperature [46,47]. The reduced stiffness of the damaged beams could be attributed to the weak bond between the rebar and RPC.

4. Conclusions and recommendations

The subsequent conclusions are drawn from the current study:

i. Addition of PPF in various ratios (0.25%, 0.75% and 1.25%) slightly reduces the strength of RPC beams at room temperature, while decreased the spalling and delays beam failure until 115 min resulted from the high temperature.

ii. Non-fibrous RPCs have a high risk of explosive spalling under the influence of high temperature. The non-fibrous reinforced RPC beam spalled early during the fire test (38 min of fire exposure).

iii. The non-fibrous reinforced RPC beams (0%) suffer spalling at the early time during the fire test. PPFs with small ratios (0.25%) decrease spalling and thus increase the resistance to the fire for the reactive powder concrete beams and delay the failure of the beam until 115 min. By contrast, PPFs with high volume ratios (0.75% and 1.25%) completely stop spalling and failure of beam. Moreover, the rate of deflection during heating decreases with the fibre ratio increasing.

iv. Fire damage decreases with an increase in the PPF fraction vol-ume. The increase in volume fraction from 0.25% to 0.75% and 1.25% decreases the midspan deflection at service load by 52% and 62.3% and increases the ultimate load by 21.6% and 25.6%, respectively.

Based on the present study, the following recommendations are suggested for future research:

Studying the fire performance of reinforced RPC beam under nega-tive bending moment in order to examine the effect of initial compres-sive stress at the bottom beam face which is exposed to fire on the development of spalling.

Studying the effect of adding polypropylene and steel fibres on fire performance of other RPC structural members such as column, slab and T-beam.

Investigating the effect of adding polypropylene and steel fibres on the thermal conductivity of RPC.

Thermal and structural models developed in the present study could be used to simulate a frame including beams and columns to achieve

Fig. 7. Load–deflection relationship of reinforced RPC beams up to the service load level.

Table 5 Test results of reinforced RPC beams.

Beam PPF % Pcr (KN) δs (mm) δT (mm)

B0 0.00 14.72 2.8 ــــB0.25 0.25 15.7 3.9 ــــB0.75 0.75 18 3.63 16.9 B1.25 1.25 18.9 3.16 14.1

Note: B0 and B0.25 failed during the fire test.

Fig. 8. Deflection–fire exposure time curves of the reinforced RPC beams.

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realistic loading and end conditions or investigate the effect of different fire scenarios (different temperature-time curves) on the performance of RPC structural members.

Interest

None.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jobe.2019.101167.

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