Behaviour of Steel Fibre Reinforced Concrete Pavements on A Single Fibre Level

. Concrete is a popular construction material used all around the globe. It is strong in compression and weak in tension. To mitigate this tensile weakness, various reinforcement methods have been used over the years, of these, fibre reinforced concrete (FRC) has gained popularity. Fibres not only improve the tensile strength of the concrete matrix, but also control crack propagation through crack bridging action. The goal of this study is to investigate the long-term behaviour of FRC pavements through conduction of static and fatigue meso-scale level tests using different steel fibres. To achieve this goal, the influence of different fibre geometries and embedment angles on the pull-out behaviour of fibres in FRC was investigated first. This was carried out by examining 60mm Dramix 3D steel and 5D steel fibres at single fibre level. The embedment depth was kept constant at half the fibre length, and the fibre embedment angles were tested at 0 ° , 15 ° and 30 ° . The average maximum pull-out load, A max , was determined first through static tests then fatigue tests were carried out using 85% of the A max value on pre-damaged samples. These tests indicated that an increase in fibre embedment angle and number of hooks leads to additional anchorage of the fibre. Fibre pull-out was found to be the dominant fibre failure mechanism for the static single fibre pull-out tests (SFPTs).


Introduction
Popular for its versatility and durability as a construction material, concrete is well known for its high compressive strength and weakness in tensile loading.To combat this weakness, various techniques in Reinforced Concrete (RC) have been employed globally.Of these techniques, the use of steel rods has been the most implemented.However, these steel rods present some disadvantages as reinforcement such as its high susceptibility to corrosion and its labour-intensive nature during construction that make the solution rather expensive [1].To mitigate some of the issues presented by steel rods, Fibre Reinforced Concrete (FRC) has gained popularity. FRC refers to concrete containing randomly dispersed fibres [2].Varying concrete binder formulation as well as the fibre material type, fibre geometry, fibre distribution, fibre angle and fibre concentration affect the characteristics and performance of FRC [2].It is therefore important to understand the effects of these factors on FRC performance.
FRC has a broad array of uses in the construction field which include pavements, tunnel linings, bridge decks and wind energy towers.These different uses of FRC may be subject to static and fatigue loading.Within the structures, the dead load of the elements and materials act as static loads that the structure has to resist [3].In FRC, this static load is therefore resisted by the fibres when in tension.This consequently leads to a need to understand the behaviour of FRC under static loading.The applications of FRC may also be exposed to fatigue loading, which can occur in different forms such as environmental fatigue, corrosion fatigue and thermal fatigue [4].According to Fataar [5], these forms of fatigue loading may occur independently or in conjunction with applied fatigue loading, in the form of cyclic, variable uniaxial or multiaxial loading.Sustained exposure to static and fatigue loads on a structure therefore affects the serviceability of a structure as the stiffness of the component materials is compromised.
In its utilisation, FRC presents a number of benefits to the performance of concrete structures.Fibres in FRC work by not only carrying the tensile load but by controlling the propagation of cracks formed during loading.Contrary to their steel rod counterparts, the fibres in FRC inhibit fast progression of cracks because they are activated at the initiation of the crack [6].This is due to their random dispersed nature in FRC elements as some of the fibres can be found close to the surface unlike in traditional RC where a cover distance to the steel rods has to be maintained.The random dispersion of fibres is also favourable because the location of cracks may vary under different types of loading therefore there is better control of cracking in FRC.The residual tensile strength of the concrete is therefore increased during the cracked phase of the FRC [6].
This study aims to investigate the static and fatigue behaviour of fibre reinforced concrete pavements.Fibre reinforced concrete pavements are one application of FRC that may be exposed to static and fatigue loading in the form of self-weight and heavy traffic loading respectively.The investigation is carried out at a single fibre level by performing single fibre pull-out tests to assess the bond strength in the fibre-matrix interface.Two Dramix fibres, the 3D steel fibres and 5D steel fibres are tested statically and under cyclic loading at different fibre embedment angles.The two fibres tested differ in terms of their tensile strength and hooked end deformation.The Dramix 5D steel has a higher tensile strength and is double hook ended while the 3D steel has a lower strength and is single hook ended.These differences affect the degrees of resistance and anchorage of the fibres in FRC [7].
In the tests carried out, the effect of fibre embedment angle at 0°, 15 °and 30° is assessed for both steel fibres extensively in static loading and an indication of the behaviour under fatigue loading is also provided.Since the fibres in FRC are only active after crack initiation, for the fatigue tests, the single fibres were pre-slipped under load control at 85% of the peak static pull-out loads of the specimens.The fatigue tests were maintained at the 85% load level and concluded at 2 million cycles or at failure.Computed Tomography (CT) micro x-ray scans of the specimens were taken to provide an insight to the internal damage after testing.

Experimental Program
To investigate the static and cyclic behaviour of FRC pavements, quantitative and qualitative data collection was carried out through different experiments and data analysis.The experimental study investigated the influence of fibre geometry and fibre embedment angle on the static and fatigue behaviour of FRC at single fibre level (meso-scale).

Mix design
For both the static and fatigue SFPTs carried out, the concrete mix design detailed in Table 1  The study investigated the behaviour of two steel fibres, namely, the Dramix 3D-65/60-BG steel fibres and the Dramix 5D-65/60-BG steel fibres.These fibres were selected due to their material strength and distinct end hook deformations that not only enhance FRC performance but are also significantly easier to cast in concrete.The material properties of these fibres are detailed in Table 2.

Sample Preparation
In this study, one specimen size was used for both the static and cyclic tests with the fibre geometry and embedment angle as the two variables of interest.The fibre embedment length (30 mm) remained constant throughout the tests.Fibre preparation was carried out before concrete mixing commenced.First, the fibres were selected, cleaned using industrial tissue paper, and the glued steel fibres separated into single fibres.Next, the fibres were marked at half their length and the one end of the fibre to be left exposed beyond the top surface of the specimen was cut off to prepare for fibre gripping during testing.The steel fibres were either kept straight or bent to 15° and 30° and inserted up to the marked point into Styrofoam pieces that would hold the fibres in place during casting.The bent fibres before placement into concrete are illustrated in Figure 1.Wooden moulds were customised to allow eight 40 × 100 × 100 mm specimens to be casted per mould as shown in Figure 2.These moulds were coated with waterproof acrylic paint to avoid sample water loss.The concrete was sieved using a 4.75 mm sieve on a shaker table to filter out the aggregates from the mix to obtain fresh mortar for casting.The mix was sieved to obtain mortar from a single mix design to be used in in further macro-scale investigations thus allowing comparisons and modelling for both meso-scale and macro-scale.The MATEC Web of Conferences 364, 05018 (2022) https://doi.org/10.1051/matecconf/202236405018ICCRRR 2022 mortar was then cast into oiled wooden moulds with no fibres present in the mix at this juncture.The mortar was used to allow easier fibre insertion and prevented fibre damage.The moulds were vibrated for 180 seconds to get rid of entrapped air.For each sample, a single fibre was then embedded at half its length into at the centre of each sample with the Styrofoam holding it in place.The moulds were vibrated for another 10 seconds.

Figure 2: Single fibre concrete samples
The concrete samples were then placed in a climatecontrolled room for the first 24 hours then demoulded.The Styrofoam pieces were removed, and the steel fibres were coated with grease to protect against corrosion during the curing process.The samples were placed in curing water tanks maintained at 24 ± 1°C.After 26 additional days, the samples were removed from the curing tanks and allowed to air dry for 24 hours prior to testing on the 28 th day.

Control Tests
In this investigation, multiple concrete specimens were cast and to maintain the required quality, control tests were carried out.The slump test was used to test the fresh state concrete mix and the cube compression tests were carried out for the hardened concrete.
The single point slump tests were carried out according to BS EN12350-2:2009 [11].The fresh concrete was required to have a true slump of 140 to 160 mm to ensure sufficient workability and consistency.The compression concrete cube tests were carried out on 100 x 100 x 100 mm cubes using the King Test 2 MN Contest machine.The compressive strength of cubes casted for every concrete batch was recorded to be assessed for validity.

Static Tests
Prior to the fatigue testing of the concrete specimens, it was imperative to carry out static single fibre pull-out tests (SFPT).The static tests were done to obtain the peak load that each of the fibres could sustain at the three different embedment angles.
The static tests were carried out using the MTS Criterion Model 43 uniaxial testing machine that has a 5kN capacity internal loadcell.The test setup with the specimen is indicated in Figure 3.The specimen was placed at the centre of the bottom MTS fixture and clamped tight on the 100×100 mm sides.The upper fixture of the MTS was carefully lowered with the clamps open, then the fibre was clamped tight when at 3 ± 1mm from the top surface of the specimen.Cautiously, to avoid sample damage, the clamps were carefully tightened to avoid any slippage or fibre rotation once testing commenced.

Figure 3: MTS static test setup
The MTS was used in conjunction with the TWE Elite software.The test loading regime was defined in this software.For the static tests, the clamped fibres were pulled out at a rate of 2.5 mm/min until complete fibre pull-out, or fibre rupture.The load-pull-out was recorded as the fibre pulled out and the peak load was recorded as Amax to be used in the fatigue tests.

Fatigue Tests
For the fatigue tests, a 50kN servo-controlled hydraulic actuator was used.The actuator was secured to a stiff steel frame that was fixed to the laboratory concrete floor.The setup also consisted of a bottom fixture with clamps that was bolted to the frames, a top fibre clamp, and an external linear variable differential transducer (LVDT).The specimen to be tested was placed at the centre of the bottom fixture and clamped tight on the 100 x 100 mm sides.The LVDT was attached to the test set up to keep track of the actuator displacement.Figure 4 indicates the fatigue test setup.

Figure 4: Instron SFPT fatigue test setup
Following the static tests carried out, the average peak loads (Amax) were calculated for each specimen type.These values were used to determine the maximum load to be applied on each specimen during cyclic loading.The maximum load for the cyclic loading was taken as 85% of the average static test peak load.This load was used in conjunction with a minimum load (Amin) of 20 N to maintain tensile conditions on the fibre.The cyclic tests were conducted at a testing frequency of 6 Hz.The cyclic test consisted of three stages which were pre-defined in the Instron Wave-Matrix software.These included the fibre pre-slip, loading/unloading and the cyclic loading.The pre-slip step was load controlled and was carried out to activate the fibres.To achieve a sinusoidal loading and unloading curve in the cyclic stage of the tests, the second step involved increasing or decreasing the load applied on the specimen to reach a mean value that lies at the midline between two amplitudes, Amax, and Amin.Beginning at the mean value, the cyclic loading was applied between Amax, and Amin, in the third step, then the number of cycles to failure recorded.The test concluded at two million cycles, or fibre failure in pull-out or rupture.

Results and discussion
In this section, an indication of the static SFPTs is provided for the samples tested at the three embedment angles.The cyclic performances of the fibres are also discussed later in the section.

Control Tests
The slump tests carried out for the concrete mix design gave an average slump of 150mm.This slump value was sufficient as it was within range.The hardened concrete compression tests carried out at 28 days gave an average compressive strength of 58 MPa.

Static SFPT
Static single fibre pull-out tests were carried out for concrete specimens with 3D steel and 5D steel fibres embedded at the centre.These were embedded at half their length and oriented at 0°, 15° and 30°.For each fibre type and angle, at least three static SFPTs were conducted.

Effect of fibre embedment angle on pull-out behaviour
The load-pull-out graphs obtained for the static 3D steel and 5D steel fibres' SFPTs at 0°, 15° and 30° are as indicated in Figure 5 and Figure 6 respectively.It is evident from the graphs that an increase in the fibre embedment angle of the 3D steel leads to an increase in the peak pull-out load.For the 0° specimens, the greatest peak load obtained was 502 N, while it was 551 N and 667 N for the 15° and 30° specimens respectively.This increase in peak load with fibre embedment angle can be attributed to the increased anchorage of the fibre when pulling out.The fibre angle creates an additional hook the pull-out force needs to overcome.A similar trend was obtained for the 5D steel fibres as illustrated in Figure 6 as the average peak loads are 812.8N, 934.7 N and 1096.4N for the 0°, 15° and 30° respectively.
The fibre angle also affects the failure mechanisms of the fibres during pull-out.The load-pull-out graphs show that the dominating failure mechanism is fibre-pullout.However, with an increase in fibre angle, the chances of fibre rupture are increased.The 30° samples had more fibre ruptures than the other two tested angles.During the static loading, the 3D steel 30° specimens were more susceptible to fibre rupture than the 5D steel 30° specimens.This is due to the 5D steel fibres having greater tensile strength than the 3D steel fibres.

Effect of fibre type on pull-out behaviour
Another variable considered was the fibre type and its effects on the pull-out behaviour.Considering the 0° fibre angle, the 3D steel had a lower average peak pull-out load compared to the 5D steel fibre as tabulated in Table 3.A similar trend is observed for the fibres oriented at 15° and 30°.This difference is due to the varying material properties of the fibres.The 5D steel fibres have generally have superior properties compared to the 3D steel fibres.This is due to its greater tensile strength and the double hook deformation which enhances the fibre anchorage.

Evaluation of failure modes
The failure modes obtained for the three types of fibre pull-out tests at the three different fibre angles are provided in Table 3. Considering all the specimens tested, 71% of the tested specimens had fibre pull-out and was therefore the dominating failure mode.Figure 7 illustrates the failure modes found for the SFPTs.Figure 8 compares the typical pull-out behaviour of the 3D steel and the 5D steel at 30°.From Figure 8, it is evident that both fibre pull-outs follow the fibre pull-out behaviour model proposed by Fataar [8].The fibre pullout can be split into five phases, namely, fibre-matrix debonding initiation (Phase1), complete debonding (Phase 2), fibre hook deformation (Phase 3), fibre straightening (Phase 4) and fibre pull-out (Phase 5).
From Figure 8 it can be seen that the two fibres exhibit all five pull-out phases at different loads and pullout.Considering Points A on both Figure 8 (a) and Figure (b), Phase 1, the initial debonding peak, of the 3D and 5D occur at 213 N and 225 N respectively both at approximately 0.5 mm pull-out.This shows that the debonding strength is similar for both the 3D and 5D steel.Point A occurs at around 220 N for both fibres.And this correlates to Phase 1 of the Fataar [8] model.It takes approximately double that load to reach the completion of the fibre debonding at Point B (Phase 2) for the 3D steel since the peak load is almost 400 N.For the 5D steel, the peak load is at about 1200N which is six times the load at the debonding initiation at Point A.
Another noticeable factor on the pull-out behaviour of the fibres is the linear increase in the fibre pull-out load that occurs after the first debonding peak.This linear increase has different typical gradients as shown in Figure 8.The linear increase is steeper for the 3D steel fibre than it is for the 5D steel.This implies that the fibre type and its material properties affect the rate to which the fibre pull-out load is reached.Since the 5D steel fibre has greater tensile strength than the 3D, this difference in behaviour is expected.The increase fibre hooks in the 5D also plays a role because of the increased anchorage.
Considering Phase 3 of the Fataar [8] model, which is the peak pull-out load of both the 3D and 5D steel at 30°, it is evident that the pull-out load of the 5D steel is almost three times higher than that of the 3D steel.Both fibres exhibited a fibre straightening peak, Phase 4, and a final pull-out load (Phase 5).However, it is evident that the loads remain higher for the 5D steel than the 3D steel because the 5D steel has higher resistance at the straightening out phase.Fibre rupture failure mode Figure 5 and Figure 6 show that 29% of the tested samples failed in fibre rupture.Of all the ruptured fibres, the cutoff point of the rupture was either at the surface of the specimen or at a few millimetres that had pulled out of the specimen.This indicates that the fibre clamping mechanism did not contribute to the fibre rapturing.This failure type occurred mostly with an increase in the fibre angle.This is mostly evident in the 3D 30° samples in Figure 5(c).In all its occurrences, this failure mechanism was exhibited after the peak load had been reached.This indicates that following the peak load, the fibre has been weakened significantly by elongation and necking therefore the chances of its rupture before hook resistance is activated are increased.The maximum fibre tensile stress, σf, of the two steel fibres is greatest at the larger angles as given in Table 3.This explains the increased chances of fibre fracture at these angles.At these higher angles, the maximum bond strength, τmax, is also higher, and this shows that with greater angle the greater the adhesion to the concrete matrix.

SFPT X-ray CT Scans imagery
To illustrate the fibre behaviour that cannot be discerned from the exterior of the sample, X-ray Computed Tomography (CT) scans of the specimens were taken.Figure 9 (a) and (b) illustrate the CT scans of 3D fibres specimens at 15° angles before and after the static SFPT respectively.
The undeformed specimen in Figure 9 (a) shows the fibre embedded to half its length into the mortar.The hook in this case is still intact and no disturbance of the specimen has taken place.Figure 9 (b) represents the deformed state and has notable differences.The first being the translation of the steel fibre.It is evident in Figure 9 (b) that the pulling out of the fibre causes the fibre to straighten out completely as it is then pulled out along the fibre path as shown.This movement has left a black trace in the mortar that represents the void that has been formed.This void has a shape that corresponds to an undisturbed 3D steel fibre.This indicates that during the static pull-out of the fibre, there was no spalling of the concrete internally.However, the spalling is observed at the top surface of the specimen.This indicates that the angle of the fibre increases the anchorage by creating an additional "hook".This additional hook must be overcome during pull-out and in the process and as a result spalling occurs at the surface.Figure 11(a) and (b) illustrate the load-pull-out responses of the 3D and 5D steel fibres respectively.The cyclic loading was carried out in three phases that are illustrated in the graphs in Figure 11.These were Phase F1, fibre preslip, Phase F2, loading/unloading and Phase F3, the cyclic loading.The pre-slip step was carried out at 85% of the peak pull-out load obtained for each fibre in the static tests.Since Figure 11 is based on the results of the static tests, it shows that the loads resisted by the 5D steel fibre is still significantly higher than that of the 3D steel.
Phase F1 was carried out to activate the fibre anchorage by de-bonding the fibre from the matrix prior to the fatigue loading.In Figure 11 (a), Phase F1 for the 3D fibre occurs at a lower rate than it does for the 5D steel in Figure 11 (b).This is seen in the slope of Phase F1 for the 5D steel fibre is steeper than that of the 3D steel fibre.This suggests that when 85% of the peak static load is applied, the 3D steel pulls out more (at 1 mm) while the 5D steel pulls out less (0.5 mm) at 85% of its peak static value.This can be attributed to the presence of two hooks in the 5D fibre so there is more anchorage of the fibre during the debonding stage, phase F1.The 3D steel exhibited a graph similar to past literature such as [9].The 5D steel response however exhibited an intermediate loading and unloading during Phase F2 before the programmed cyclic commences in Phase F3.This kink may be due to the additional resistance brought by the extra hook at the fibre end.
Both fatigue tests in Figure 11 ended due to fibre failure; complete pull-out for the 3D steel and rupture for the 5D steel.The 3D steel fibres sustained more load cycles (24,063 cycles) in phase F3 than the 5D steel fibres (6,992 cycles).Due to the high anchorage of the 5D steel at the hook ends, the fibre ruptured at the intermediate length of the fibre.Figure 10 (b) shows this as the exposed length post rupture was only 5 mm.The 3D fibre however straightened and ruptured during the cyclic loading, as seen in Figure 10 (a) because the measured length was 16mm after fibre pull-out.A smaller fraction of the length was left embedded in the specimen for the 3D (14 mm) than in the 5D specimen (25 mm).

Conclusions
Static and fatigue single fibre pull-out tests (SFPTs) were conducted on 3D and 5D steel single fibre specimens with fibres embedded at 0°, 15° and 30°.All the fibres were embedded at half their total length.The static SFPTs were ended when the fibres had pulled out at half the embedment depth while the fatigue tests were terminated at 2 million cycles or at fibre failure in fibre pull-out or rupture.The following conclusions can be drawn from the results obtained: • Fibre hook ends increase the peak pull-out loads that can be resisted by fibres during SFPTs as is evident from greater loads resisted by the double hook ended 5D steel than the single hook 3D steel.• The significantly higher peak pull-out loads of the 30° specimens compared to the 0° and 15° specimens suggest that the greater the fibre embedment angle, the greater the fibre anchorage and resistance to pullout.• For static SFPT, fibre pull-out is the dominating failure mode in these tests since 71% of the specimens failed as such.• In the static SFPTs, the specimens that failed by fibre rupture, experienced this failure mechanism post peak pull-out load due to the weakening and straightening of fibre hooks.• 5D steel fibres can resist higher fatigue loading than the 3D steel fibres since they are based on the peak static loads.• The presence of additional hooks in 5D steel fibres leads to more anchorage during the cyclic loading.This additional anchorage by the hooks creates more internal anchorage leaving the rest of the fibre length susceptible to rupture.

Figure 7 :
Figure 7: SFPT failure modesFibre pull-out failure Of the tested 3D steel samples at 0° angle, only one out of the five tested specimens had failed in rupture whereas the other four specimens completely pulled out.Fibre pullout was the dominating mechanism exhibited by most of the tested specimens.The pull-out behaviour of the 3D steel and the 5D steel fibres follow a similar curve with a few differences.

Figure 9 :
Figure 9: SFPT Specimens for 15° 3D fibres (a) undeformed (b) deformed 3.3 Cyclic Test results and analysis Fatigue single fibre pull-out tests were carried out for the 3D and 5D steel fibres.The steel fibres exhibited different failure modes as well as load-pull-out responses.Figure 10 illustrates the two possible failure modes, fibre pullout in Figure 10 (a) and fibre rupture in Figure 10 (b).