Functional microfibre reinforced ultra-high performance concrete (FMF-UHPC)

Cement and Concrete Research 130 (2020) 105993

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Functional microfibre reinforced ultra-high performance concrete (FMFUHPC)

T

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Maximilian Schleitinga, , Alexander Wetzela, Philipp Krooßb, Jenny Thiemickec, Thomas Niendorfb, Bernhard Middendorfa, Ekkehard Fehlingc a

University of Kassel, Department of Building Materials and Construction Chemistry, Kassel, Germany University of Kassel, Institute of Materials Engineering, Kassel, Germany c University of Kassel, Department of Structural Concrete, Kassel, Germany b

A R T I C LE I N FO

A B S T R A C T

Keywords: E. High-performance concrete E. Fibre reinforcement A. Rheology A. Thermal treatment Shape memory alloys

The present paper proposes a novel class of fibre reinforced concrete, realized by using shape memory alloy (SMA) fibres as reinforcement in ultra-high performance concrete (UHPC). SMAs can change their geometry imposed by a solid to solid phase transformation triggered by thermal activation or stress. This effect gives the possibility to use one geometry of fibres giving a minimum negative impact on the rheological properties and workability of concrete and a second geometry of the fibres for maximum positive impact on final mechanical properties. The present work highlights the influence of fibres with various shapes on the workability and the mechanical properties. The impact of SMA fibres in fresh and hardened UHPC compared to UHPC prepared with standard steel fibres is discussed. In light of the advances related to the unique properties of the SMA fibres, this novel concrete is referred to as “Functional Microfibre reinforced Ultra-High Performance Concrete” (FMFUHPC).

1. Introduction Ultra-high performance concrete (UHPC) is defined by its high durability and high strengths, in particular compressive strength. These characteristics of UHPC are mainly based on a low water/binder-ratio of about 0.20 to 0.25 and a high packing density, which is gained by calculating the amount of fines like cement, silica fume and quartz powder. Due to the increased specific surface of the fines and the low water content, the use of high amounts of superplasticizers is needed [1,2]. Micro-fibres, mainly steel fibres, are used to improve the mechanical characteristics under tensile forces [1–3]. Without fibres, the UHPC shows a brittle cracking behaviour after reaching its maximum compressive strength. This effect of spontaneous and brittle failure is more pronounced with increasing compressive strength [4]. Usually a fibre content between 1 and 2 vol%, sometimes up to 4 vol%, of straight steel fibres with a length of 6–50 mm and a diameter of 0.15–0.5 mm, is used in UHPC [2]. Due to an increase in bonded length, fibres with a high aspect-ratio (length/diameter) show a higher impact on the tensile strength. Steel fibres are characterised by a high tensile strength between 1000 and 1400 N/mm2 and a Young's modulus of about 200 kN/ mm2 [5]. It is shown that the tensile strength of UHPC can be increased

from 5 to about 9 N/mm2, while the flexural strength can be increased up to 22 N/mm2 upon adding 2.5 vol% fibres with a fibre aspect-ratio of 60–90 [2]. In contrast to the enhanced mechanical properties of the concrete, however, the workability of the fresh concrete deteriorates with increasing content of fibres (Fig. 1). Especially fibres with a high aspect-ratio lead to decreasing flowability and workability of the concrete and to agglomerations of fibres [6,7]. Concomitantly, this leads to an inhomogeneous distribution of fibres within the concrete and an enhanced amount of air-voids are entrained during mixing process [8]. Thus, the compressive strength might be decreased. In the late 1990s and early 2000s shape memory alloy (SMA) fibres came into the focus of civil engineering and the building industry [9,10]. SMA fibres can be used as reinforcement in concrete like any other fibre material. Additionally, new abilities like prestressing without anchorage, self-healing and a higher seismic resistance can be added to the composites due to the unique properties of SMAs which are also known as “smart materials” [9,11–14]. Their smart behaviour is based on the shape memory effect, which has been revealed for various alloys. Many of these alloys are based on copper, iron or nickel‑titanium [15]. However, even in these well-known alloys the shape memory effect can occur differently in term of type of

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Corresponding author. E-mail addresses: [email protected] (M. Schleiting), [email protected] (A. Wetzel), [email protected] (P. Krooß), [email protected] (J. Thiemicke), [email protected] (T. Niendorf), [email protected] (B. Middendorf), [email protected] (E. Fehling). https://doi.org/10.1016/j.cemconres.2020.105993 Received 2 May 2019; Received in revised form 17 January 2020; Accepted 18 January 2020 0008-8846/ © 2020 Elsevier Ltd. All rights reserved.

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transforms to austenite again and the fibre returns to its imprinted geometry at the Af-temperature (Marker 4 in Fig. 2) [9,16]. The second shape memory effect is the so called “two-way shape memory effect”. This effect highlights the ability of the SMA to remember two different geometries which can be linked to two different temperature levels. To exploit this effect, a special thermomechanical training of the material is needed [9,16]. The third shape memory effect is the “superelasticity”, also called “pseudoelasticity”. In this case, the transformation from austenite to martensite occurs just by applying stress at a temperature above the Aftemperature. This transformation concomitantly leads to deformation of the material. By releasing the stress, the martensite retransforms to austenite and the material returns into its original shape [9,16]. The functional micro-fibre ultra-high performance concrete (FMFUHPC) is proposed to combine the superior mechanical properties of the hardened concrete gained by a high content of fibres and a good workability similar to a fibre-free/low fibre content concrete. To achieve this aim, functional micro-fibres (FMF) made of SMAs instead of standard steel fibres can be used. Generally, these SMA-fibres can be made of nickel and titanium (NiTi) or any other SMA, e.g. an iron-based alloy in regards to economic factors. The general idea makes use of the ability of SMA fibres to “remember” an imprinted geometry induced by thermal activation (one-way shape memory effect) [9,16,17]. Consequently, two different geometrical shapes of the same fibre can be exploited at different stages of production. The tailored shape of the FMF in the hardened state of the concrete depends on the type of concrete and the application field of the final structural element. Anyhow, this geometry is set by coining the fibres at a certain temperature depending on SMA-type used. In the present work, the NiTifibres used were coined for 15 min at 350 °C (Fig. 3). Subsequently, the geometry favourable for workability of the fresh concrete is set by mechanical deformation. This geometry should have a low length/ diameter-ratio as this ratio significantly affects the rheology of the fresh concrete [18,19]. During mixing, the FMFs being compact in shape only show minor interaction with each other. Therefore, fibre aggregates are avoided. Only after mixing and pouring into the mould, the shape memory effect is triggered by thermal activation while the concrete still remains flowable for a given period of time. The concrete is heated at 80 °C for 1 h resulting in an internal concrete temperature of about 50 °C. Due to the increase of temperature to values above the transformation temperature of the SMA, the fibres transform into their imprinted geometry, finally enhancing the mechanical properties of the hardened concrete (Fig. 4). As a first step, workability investigations have been conducted in

Fig. 1. Influence of the fibre content on the viscosity of UHPC. Symbols are measured values; grey fields indicate approximate flexural strengths for different fibre contents.

transformation and/or transformation temperature. Most importantly, effects are strongly influenced by numerous alloy characteristics, e.g. chemical composition. Three different types of the shape memory effects are known. The first one is the “one-way shape memory effect”, also referred to as “pseudoplastic effect” (Fig. 2). Alloys tailored to show this effect have the ability to transform into an imprinted geometry imposed by thermal treatment, i.e. the so-called shape setting. The related characteristic behaviour is based on the temperature dependent phase transformation of austenite to martensite and vice versa. Above a specific temperature (Af = Austenite finish) only austenite is stable. In this state the fibre can be formed and the material will “remember” this imprinted geometry (later on triggered by heating). However, before other steps have to be accomplished. By cooling the SMA, austenite begins to transform into a self-accommodated martensite at a specific temperature (Ms = Martensite start). At a (slightly) lower temperature (Mf = Martensite finish) only twinned martensite prevails (Markers 1 and 5 in Fig. 2). Imposed by external stress, the martensitic fibre changes its geometry as the twinned martensitic structure changes to a detwinned structure (Marker 2 in Fig. 2). By releasing the external stress (Marker 3 in Fig. 2) and heating the SMA fibre above the Astemperature (As = Austenite start) afterwards, the martensite

Fig. 2. Phase transformation and corresponding stress-strain-temperature path related to the “one-way shape memory effect” (based on Kaack [14]). Mf = Martensite finish; Ms = Martensite start; As = Austenite start; Af = Austenite finish. 2

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Fig. 3. Coining device for imprinting the endhook geometry of the fibres.

the present work using steel fibres of various shapes to reveal the impact of a given shape of a fibre. Here, circular shaped steel fibres are compared to straight ones. Furthermore, the mechanical properties of the NiTi-fibres are investigated and compared to conventional steel fibres. The bond strength between the fibre and cementitious matrix as well as the efficiency of the shape memory effect in a high viscosity material are investigated as both aspects are essential for the final mechanical properties of the hardened concrete [13,20,21]. So far, those aspects are only comprehensively discussed for “conventional” concrete and high performance concrete (HPC) [20,22,23]. Data of Yoo et al. [22] indicate that the bond strength between fibre and concrete increases with increasing compression strength of the concrete. Therefore, UHPC, being characterised by a superior compression strength of > 150 N/mm2, seems to be most eligible for the use of fibre reinforcement. In summary, the present work introduces and details important aspects to be considered for realization of a functional microfibre reinforced UHPC (FMF-UHPC). Results obtained and discussed so far clearly reveal the feasibility of the approach and further introduce remaining challenges towards final products.

Table 1 Compounds of the M3Q mixture and their amounts in kg/m3 and wt%.

CEM I 52 R HS/NA Silica fume Quartz sand Quartz powder Superplasticizer Water w/b-ratioa a

kg/m3

wt%

797 169 966 199 27 187 0.21

34.0 7.2 41.2 8.5 1.1 8.0

w/b-ratio: water/binder ratio.

Table 2 Characteristics of the used fibres. Abbreviation

Fibre material

Length [mm]

Diameter [mm]

Aspect-ratio

Geometry

S10_0.4 S10_0.4_c S13_0.2_w

Steel Steel Steel (brass Steel (brass Steel (brass Steel (brass Steel (brass NiTi NiTi NiTi

10 10 13

0.4 0.4 0.2

25 (25) 65

Straight Circular Waved

13

0.2

65

End hooks

13

0.25

52

Straight

17

0.2

85

Waved

17

0.15

113

Waved

13 13 13

0.2 0.2 0.5

65 65 26

Straight End hooks End hooks

S13_0.2_e

2. Material and methods

S13_0.25

2.1. Material

S17_0.2_w S17_0.15_w

A standard mixture based on the formulation M3Q known quite well from former investigations (SPP 1182; [24]) is used for all measurements. The compounds and their amounts are shown in Table 1. The general mix design is similar for all mixtures, however, fibre addition is adjusted for the individual investigations in regards to fibre type (steel and SMA) and shape (straight, waved and circular). The length, diameter, shape and aspect-ratio of the used fibres are given in Table 2, images of some of the used fibres are shown in Fig. 5. The FMF fibres used were cut from SMA wire obtained from the company “Memry”. The SMA considered here (NiTi) is also known as Nitinol characterised by an Af-temperature of 46 °C. The concrete is produced using an intensive mixer R05T (by

SMA13_0.2 SMA13_0.2_e SMA13_0.5_e

coated) coated) coated) coated) coated)

[Note] Abbreviations are given by the following scheme: “Fibre material (S = Steel, SMA = Shape Memory Alloy/FMF)“_”Fibre length in mm“_”Fibre diameter in mm“_”Fibre shape (c = circular, w = waved, e = endhooked, without any letter = straight)”.

Fig. 4. High-Volume fibre-reinforced UHPC with low viscosity using functional micro fibres: Shape Memory Alloy fibres with an imprinted geometry (fibres with endhooks) are formed into a geometry favourable for the rheology of the UHPC. After mixing, the concrete is thermally treated to activate the shape memory effect and to transform the fibres into a geometry which influences the mechanic properties of the UHPC positively. 3

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Fig. 5. Images of the used fibres with different shapes. a) S13_0.25; b) S13_0.2_w; c) S13_0.2_e; d) SMA13_0.2_e; e) S10_0.4_c. Fibres with abbreviations that are not shown, have one of the here shown shape but with another length and/or diameter of the fibre.

“Eirich”), which has a maximum mixing volume of 40 l.In case of all mixtures, the dry compounds were homogenised for 1 min before adding water and superplasticizer. While adding the fluid compounds, the mixing speed was increased to accelerate the turning over of the concrete. After 3 min, mixing was paused to unstick residual dry compounds from the mixing container and tool. In case of the workability investigations and tensile strength measurements, fibres were added in the mixing break via an inclined vibrating chute to ensure an optimum separation of the fibres. For the fibre pullout tests, the fibres were placed directly in the cube formworks. Details of the procedure are given in Section 2.2.2. After the break all components were mixed again for 5 min with a mixing speed being similar to that at the beginning of the mixing process. 2.2. Methods 2.2.1. Workability investigations of fresh UHPC regarding fibre geometry To estimate general rheological properties and, therefore, the workability of the fresh concrete three different methods were used: 1. Measurement of the slump flow based on DIN EN 1015-3:2006 [25] 2. Coarse-grain rotation-rheometer (20 l, “eBT2”) 3. Fine-grain rotation-rheometer (350 ml, “Viskomat NT”). The measurement of the slump flow of fresh concrete is a practical examination procedure based on DIN EN 1015-3:2006 [25]. This method is used to get information about the flowability, workability and ductility of fresh concrete. The slump flow of UHPC mixtures without fibres and with 1 vol% and 2.5 vol% of fibres with different shapes (waved and endhooked) was measured. The viscosity measurements of the fresh concrete in regards to the fibre content (0 vol%, 1 vol% and 2.5 vol%) were also conducted with waved and endhooked steel fibres. Additionally, to investigate the influence of circular shaped fibres, rheological tests of those were compared to straight steel fibres. A rotational fine-grain rheometer “Viskomat NT” and a coarse-grain concrete rheometer “eBT2” (both by “Schleibinger Geräte”; Fig. 6) were used. The coarse-grain concrete rheometer applied usually is used to measure the rheological properties of the fresh concrete containing fibres. Due to the influence of fibres on the rheology of the concrete, a fine-grain rheometer should generally not be used for those tests with fibre-concrete. However, due to the limited number of fibres available and, thus the limited amount of material, the tests with circular shaped fibres had to be done with the fine-grain rheometer. In case of the present study, this procedure is justified, as the characteristic length of the circular fibres is the ring diameter, which is small as compared to the ball probe [26]. In case of the coarse-grain concrete rheometer, the measurement method is based on a ball measurement system operating based on the Searle principle [27], i.e. the measurement balls rotate in a fixed container. In contrast, the fine-grain rheometer operates based on the Couette principle [27], i.e. the container, including the material, rotates around the fixed measurement balls. The rheometers deliver information about the rotation speed [m/s] and torque [Nm]. For the practical

Fig. 6. Sketches of the used rheometers. a) Sketch of the measurement unit of the rotational rheometer “Viskomat NT”. b) Sketch of the measurement unit of the coarse-grain concrete rheometer “eBT2”. Size indication in mm.

determination of the flow curve, a ramp profile with increasing and decreasing speed is used. Based on the downward curve of the speed profile, the physical plastic viscosity and yield stress are obtained by a simulation-based method proposed by Gerland et al. [26,28], using the following equation in absolute manner:

τy l ⎤a2 ⎞ M = l2μΩD ⎜⎛a0 + a1 ⎡ ⎟, ⎢ ⎣ μΩD ⎥ ⎦ ⎠ ⎝

(1)

with the torque M, the ball's circular path l, the plastic viscosity μ, the rotational velocity Ω, the yield stress τy as well as the geometry-specific coefficients a0, a1, a2. From the results of the slump test, the absolute yield stress based on Roussel's and Coussot's relation was calculated [29]:

τy =

225ρgV 2 128π 2R5

(2)

with the fresh concrete's density ρ, the gravity g, the slump's volume V, and the slump flow's spreading distance R. As all methods applied deliver objective Bingham properties, a direct quantitative comparison of 4

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Fig. 7. Setup for the fibre pullout tests based on a universal testing machine (150kN Zwick/Roell). a) Front-view of the setup. b) Side-view of the setup. Size indication in mm.

distributed inconstantly over the embedded fibre length. This behaviour is, however, not determinable using the pullout testing setup. Most importantly, the presently considered kind of interpretation, i.e. the force/slip relationship, is widely accepted and published [20,22,30], while there are hardly any experimental or modelling studies implying inconstant shear stress [30,31]. As the embedded length of the fibre decreases due to fibre pullout, the actual pullout stress evolution of the fibre according to the slip is calculated by the following term:

the rheological properties of all mixes was possible. All tests were performed about 10 min after the end of mixing. 2.2.2. Fibre pullout tests – bond strength The fibre pullout tests were carried out using a custom-built setup based on a universal testing machine (150 kN Zwick/Roell; Fig. 7). Straight, brass coated steel fibres (13 mm length, 0.25 mm diameter) were compared to straight FMF made of NiTi (Table 2). These were cut manually from a wire to a length of 13 mm. The diameter of the FMF was 0.2 mm. For the single fibre pullout tests, 10 UHPC slabs for FMF and 9 UHPC slabs for steel fibres were fabricated to analyse the bonding behaviour between fibre and matrix. The slabs had a size of 10 × 10 × 3 cm3. The embedded length of the fibres was 5 ± 0.5 mm for all tests resulting in a free fibre length of about 8 mm. The free fibre length was clamped as closely as possible to the concrete surface (< 0.5 mm) to minimise elastic strain. In contrast to the common posttreatment after pouring into the mould, no vibration in regards to deaeration was applied. The FMF were stuck into hard foam slabs, which filled out 10 × 10 × 10 cm3 cube formworks up to a height of 7 cm, with a defined depth to guarantee for an adequate bond length of the fibre. After demoulding, the samples were stored under standard conditions (20 °C, 65% rel. hum.) until testing after 7 days of curing. No thermal curing was applied considering the thermal sensitivity of the FMF. Fibre pullout tests were done at room temperature to guarantee that the FMF were fully martensitic during pullout (see Section 3.1). To determine the bond strength between fibre and the cementitious matrix, the fibre has to be pulled out without ripping. The maximum pullout stress and, thus, the bond strength can be calculated based on the maximum pullout load and the area of the fibre that is embedded in the matrix using the following equation:

τ bmax =

Fmax Lb × π × df

τb (s ) =

F (s ) (Lb − s ) × π × df

(4)

where τb(s) is the bond stress at slip s, and F(s) is the pullout force at slip s. As the pullout of the fibre is intended, the fibre tensile stress induced by fibre pullout has to be lower than the fibre tensile strength. The fibre tensile stress can be calculated by the following equation:

σ fmax =

Fmax π × df2/4

(5)

The relation of the maximum fibre tensile stress and the strength of the fibre is a critical factor, as an appropriate fibre type in regards to the strength of the concrete has to be used to prevent plastic deformation or failure of the fibre. 2.2.3. Flexural strength of fibre reinforced UHPC The fibre reinforcement has no significant impact on the compressive strength of concrete; so reference tests in this regard were not conducted. To investigate the influence of different fibre content and geometry on the flexural strength of the concrete, UHPC-prisms with a variation of fibre reinforcement (in terms of geometry waved and endhooked as well as straight steel fibres) were fabricated. The flexural strength of the material was tested in a universal testing machine (150 kN Zwick/Roell) by a four-point bending test according to DIN EN 12390-5 [32]. The samples were stored under standard conditions (20 °C, 65% rel. hum.) until they were tested 7 days after casting, with a testing speed of 0.01 mm/s. Specimen had a size of 160 × 40 × 40 mm3 (norm size).

(3)

where Fmax is the maximum pullout load, Lb is the embedded fibre length and df is the fibre diameter. This approach implies a constant shear stress over the whole embedded length of the fibre until Fmax is reached, i.e. no (local) deboning should have occurred. However, it is likely that in fact the shear stress is 5

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2.2.4. Microstructural investigations high-resolution computed tomography (μ-CT) and scanning electron microscopy (SEM) In order to evaluate and verify the retransformation of the FMF to their imprinted geometry without destroying the samples, μ-CT investigations were conducted. This method allows imaging the very fine and complex structures of fibre reinforced concrete [33]. For this study a Xradia 520 Versa high-resolution computed tomography from Carl Zeiss Microscopy GmbH was used. In case of FMF-UHPC components, exact knowledge about the location and shape of the transformed fibres is required. It is essential that the fibres transform back to their imprinted shape as their positive influence on the mechanical properties is only effective in this form. To investigate these aspects and to visualise a more complex fibre geometry, fibres with endhooks (SMA13_0.2_e) were used. The endhookgeometry was imprinted at 350 °C for 15 min. After cooling to a temperature below Mf-temperature, the fibres were formed manually into a circular shape before adding to the concrete. After thermal treatment (following the procedure detailed above, Fig. 4) and hardening of the concrete, small specimens were cut out of UHPC prisms/cubes to be able to analyse the whole volume by μ-CT. Based on the experimental approach considered in present work, it was possible to obtain quantitative information about the absolute number of fully retransformed fibres. For evaluation, the absolute distance from one end to the other end was analysed for each individual fibre. The fully retransformed fibres were assumed to be those being characterised by an absolute distance being equal to the initial length of the fibre upon shape setting. If a fibre is not fully retransformed, absolute distance from end to end has to be smaller. In addition to the μ-CT measurements, fibres were washed out of the fresh concrete directly after initial heating, providing for the opportunity to directly compare the activated fibres before and after hardening of the concrete. This was done with fibres with a diameter of 0.2 mm and, additionally, with fibres with a diameter of 0.5 mm. To correlate the results of the fibre pullout tests with the different fibre materials, their surface textures and the ongrown CSH phases were investigated using an environmental scanning electron microscope (ESEM) after fibre pullout (Quanta FEG 250 from the company FEI). Images were taken in secondary electron (SE) mode.

Fig. 8. a) Stress-strain response of NiTi wires at 23 °C and b) at 70 °C.

deformation. The plateaus indicate the stress induced phase transformation in case of the sample tested at 70 °C (well above Af) on the one hand as well as a martensite variant reorientation in case of the sample tested at 23 °C (well below Ms) on the other hand. In the latter case, the reorientation starts at a stress level of around 640 N/mm2. at 70 °C the critical stress for stress-induced phase transformation is at around 800 N/mm2. In both cases an elastic deformation of the detwinned martensite occurs afterwards until final failure is seen.

3. Results

3.2. Workability investigations of fresh UHPC regarding fibre geometry

3.1. FMF properties

A decrease of the flow spread of the fresh concrete with increasing fibre content (1 vol% and 2.5 vol%) is clearly visible (Fig. 9). A higher aspect ratio of the fibres intensifies this effect. The effect is most significant for mixtures with a higher (2.5 vol%) fibre content. The fibres

The transformation temperature As (30 °C) of the NiTi wire with a diameter of 0.2 mm used in the present study was provided by the company Memry. Additional DSC analysis of the NiTi wire revealed Mfand Af-temperatures of 22 °C and 46 °C, respectively. Thus, at room temperature the wire in its initial stage was primarily martensitic. Above 46 °C (without superimposed stresses) the wire fully transforms into austenite. However, it has to be mentioned that these temperatures are only valid for the NiTi-alloy used in the present work. With regard to future applications the temperature windows can be adjusted by changes in the alloy composition, by microstructure manipulation induced by thermomechanical treatment during processing as well as by the absolute value of superimposed mechanical load according to the Clausius-Clapeyron relationship [9]. In consequence, for the intended future application of the proposed concept, the transformation temperatures could be tailored in order to avoid unwanted phase changes. In this regard, the prevailing phases affect various mechanical properties of the SMA simultaneously, e.g. stiffness, ultimate strength and elongation to fracture. Fig. 8a and b show two different stress-strain curves of the NiTi wires. The elongation to failure is found to be at around 30% at a stress of 1800 N/mm2 at 23 °C (Fig. 8a). For the sample that was tested at 70 °C the elongation to failure is at around 25% at a stress level of around 1700 N/mm2 (Fig. 8b). At both temperatures samples show a stress plateau upon initial elastic

Fig. 9. Slump flow of fresh fibre reinforced concrete with different amounts and geometries of fibres. Descriptions of the abbreviations are given in Table 2. 6

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Fig. 10. Results of the workability investigations. a) Yield stress of fresh UHPC and b) plastic viscosity of fresh UHPC in regards to measurement system, fibre content and fibre shape.

Generally, the yield stress that is obtained by the slump flow in accordance with the analytical solution of Roussel and Coussot [29] fits the results gained by the two rheometers. At least, a good correlation is seen for the fibre free reference and the mixtures containing fibres with a length of 13 mm. The yield stress determined by the rheometers for mixtures containing fibres with a length of 17 mm are somewhat higher than the yield stress obtained by Roussel's and Coussot's method. This could be due to the fact that this analytical solution was proposed for highly flowable concrete being characterised by large spread. In consequence, as the fibre-induced yield stress increases and the slump flow reduces, the validity of Eq. (2) is limited.

with the highest aspect ratio (S17_0.15_c; aspect ratio: 113) show a more significant decrease of the flow spread for mixtures containing 2.5 vol% fibres as compared to mixtures containing 1 vol% fibres. Adding fibres with endhooks leads to an even slightly lower flow spread of the concrete as compared to mixtures with fibres of the same length and diameter without endhooks. This finding holds true for both fibre contents, i.e. 2.5 vol% and 1 vol%, being in focus of the present work. Fig. 10 shows the results of the rheometer measurements as a function of different fibre contents and shapes. The results obtained by both measurement systems are similar and fit in line with those of the flow spread tests. Clearly, the plastic viscosity and the yield stress increase with increasing fibre content and increasing aspect ratio of the fibres. The mixtures containing fibres with a length of 17 mm could not be tested with the “Viskomat NT” as the homogeneity of the sample was disturbed by heavy fibre agglomerations and wall slippage was clearly dominating the flow. For the same reasons, the mixture containing fibres with the highest aspect ratio (S17_0.15_w) could also not be tested with the “eBT2”. To clearly reveal the positive effect of circular fibres on the plastic viscosity, measurements using UHPC containing 2 vol% circular steel fibres were done and results compared to data from UHPC containing 2 vol% straight steel fibres. The results clearly show that a circular fibre geometry is favourable as this geometry results in a lower plastic viscosity as well as a lower yield stress. Decrease of plastic viscosity of about 33% and yield stress of about 36% was achieved when introducing the same fibre content but a circular geometry (compared to straight fibres). Therefore, the workability of the fresh concrete is clearly improved by the circular shape of the fibres. All results highlighting the properties of the fresh concrete are summarised in Table 3.

3.3. Flexural strength of prisms with different fibre contents and fibre geometry The flexural strength of the fibre reinforced UHPC prisms increases with increasing length and aspect-ratio of the fibres (Fig. 11). This can be seen for both, 1 vol% and 2.5 vol% fibre content, however, the increase in flexural strength induced by the fibre aspect-ratio is more significant for mixtures with higher fibre contents. Furthermore, in mixtures with low fibre contents, fibres with endhooks lead to similar results as regular, straight fibres. In contrast, mixtures containing 2.5 vol% fibres are significantly influenced by the endhook-geometry of the fibres. In summary, the influence of fibre aspect-ratio and geometry is more significant with increasing fibre content. 3.4. One-way shape memory effect in UHPC and μ-CT measurements To evaluate the shape memory effect of the FMF in fresh concrete, 7

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Table 3 Summary of the results highlighting the properties of the fresh concrete, i.e. the flow spread and workability investigations. Fibre content [vol%]

1

Fibre type

S13_0.2_w S13_0.2_e S17_0.2_w S17_0.15_w S13_0.2_w S13_0.2_e S17_0.2_w S17_0.15_w S10_0.4 S10_0.4_c

2.5

2

Slump flow [cm]

27.9 28.1 27.0 26.6 25.7 23.3 26.8 24.6 23.6 13.7 – –

eBT2

Viskomat NT

Roussel [29] (calculated)

μ [Pa*s]

τy [Pa]

μ [Pa*s]

τy [Pa]

τy [Pa]

100 92 100 143 170 245 171 246 347 – – –

4 4 4 7 13 19 5 10 15 – – –

130 93 125 148 – – 155 270 – – 375 253

5 3 4 5 – – 5 15 – – 11 7

4.5

two different methods were used. On the one hand, the activated fibres were washed out of the fresh concrete after heating (Fig. 12a and b). Obviously, NiTi-fibres with a diameter of 0.2 mm and 0.5 mm are able to return into their imprinted geometry upon applying temperatures of 50 °C and higher. However, some fibres showed irreversible kinks in the middle section of the straight part and some endhooks did not form out perfectly (Fig. 12b). The μ-CT measurements clearly shows transformed SMA fibres in the hardened concrete as depicted in Fig. 12c. It is revealed by μ-CT analysis that several fibres (0.5 mm diameter) almost completely transformed into their imprinted geometry upon heating the fresh concrete. However, again some fibres show irreversible kinks as well as not perfectly formed endhooks.

5.5 6.0 7.1 11.6 5.9 9.1 11.2 170.1 – –

in average). Both fibre types show an increasing pullout stress with increasing normalised slip of the fibre. This is a result of the fibre geometry, often being characterised by “flattened” ends due to the cutting process during the fibre production. This shape leads to an anchorage effect imposed by the fibre end. The average slip at maximum pullout load differs for the fibre types tested. In case of the FMF, the average fibre slip at maximum pullout load was about 0.5 mm while in case of steel fibres the average fibre slip at maximum pullout load was about 1.9 mm. Furthermore, in case of the steel fibres the fibre slip at maximum pullout load varied much more as compared to FMF. The bond stress of the FMF decreased constantly after reaching maximum bond strength. In contrast, the bond strength of the steel fibres remained on an almost constant level until a slip of 3 cm and only decreased abruptly afterwards. The highest reached tensile stress value reached by the fibres was 383 N/mm2 and 468 N/mm2 for steel fibres and FMF, respectively. These values were much lower than the fibre tensile strengths, hindering rupture of both fibre types (at given fibre lengths). Secondary electron (SE) images detailing all features being characteristic for the surfaces of the embedded parts of the pulled out fibres

3.5. Fibre pullout tests The results of the pullout tests conducted for straight steel fibres as well as the straight FMF (Fig. 13, Table 4) highlight that the average maximum pullout stress τbmax of FMF (1.9 N/mm2 in average) is lower than the average maximum pullout stress of the steel fibres (2.9 N/mm2

Fig. 11. Flexural strength of UHPC prisms with different fibre contents and fibre shapes. Descriptions of the abbreviations are given in Table 2. 8

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Fig. 12. a) Washed out NiTi-fibres with a diameter of 0.2 mm after heating to 50 °C of the fresh concrete. b) Washed out NiTi-fibres with a diameter of 0.5 mm after heating to 50 °C of the fresh concrete. c) μCT-Image of transformed SMA-fibres in hardened concrete. The imprinted geometry of the fibres is nearly completely regained, however, kinks in the centre of the fibres are visible.

fibres leads to a decrease of plastic viscosity and yield stress by about one third. In consequence, the workability was improved significantly, indicating that higher fibre contents and/or higher aspect ratios of fibres could be achievable by the use of circular fibres in UHPC. However, the circular fibres are not adequate to significantly improve final mechanical performance. As shown in the present work, a key to a significant performance increase is the exploitation of the shape memory effect (Fig. 15). The fibres have to be able to change their shape inside the concrete matrix eventually enhancing the flexural strength while preserving the workability of the material. Even a general proof of concept is established in the present study, numerous factors influence workability in parallel and, thus, further research work has to be conducted [28,34]. The circular shaped FMF are able to transform back into their imprinted geometry triggered by heating of the still flowable concrete. The applied temperature of 50–60 °C is quite common for precast concrete element production [35]. However, it still has to be proven that this temperature level is also reached in the core of a bigger concrete element within a given period of time, in which the concrete is still workable and the shape memory effect can lead to a shape change

are shown in Fig. 14. In case of both fibre types, cementitious matrix material (CSH phases) on the surface of the embedded part of the fibre (near to the boundary between the embedded part and the blank fibre) are visible. These CSH phases are located only within the first mm of the embedded part of the fibre (Fig. 14a) in case of the FMF. In all areas below this region only single spots of cementitious material are found (Fig. 14b). In case of the steel fibres, matrix material is located all over the embedded part of the fibre (Fig. 14c and d). Furthermore, failure cones made of matrix material are visible on the steel fibres. 4. Discussion and conclusion The results obtained for the fresh concrete revealed that an increase of the fibre content leads to a higher plastic viscosity and lower slump flow and, thus, to a deteriorated workability of the UHPC. Fibres with endhooks and a higher aspect-ratio further promote detrimental effects, especially in case of fibre contents > 1 vol%. Results are in line with data published in literature [6,7]. In direct comparison to the straight fibres, the plastic viscosity of fresh concrete containing circular fibres was significantly enhanced. It could be revealed that the use of circular

Fig. 13. Results of the fibre pullout tests. a) Average fibre pullout force and slip curves for steel fibres and functional microfibres. b) Average shear stress and normalised slip for steel fibres and functional microfibres. 9

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Table 4 Results of the single fibre pullout tests for straight steel- and SMA fibres. Fmax [N]

Smax [mm]

τbmax [MPa]

σfmax [MPa]

S13_0.25_1 S13_0.25_2 S13_0.25_3 S13_0.25_4 S13_0.25_5 S13_0.25_6 S13_0.25_7 S13_0.25_8 S13_0.25_9

16.1 12.6 15.0 2.5 18.8 7.8 8.0 14.9 7.8

1.1 3.0 1.2 2.6 3.4 1.1 1.1 3.2 2.5

4.1 3.2 3.8 0.6 4.8 2.0 2.0 3.8 2.0

328 256 305 51 383 159 163 304 159

Average

11.5

1.9

2.9

234

SMA13_0.2_1 SMA13_0.2_2 SMA13_0.2_3 SMA13_0.2_4 SMA13_0.2_5 SMA13_0.2_6 SMA13_0.2_7 SMA13_0.2_8 SMA13_0.2_9 SMA13_0.2_10 Average

Fmax [N]

Smax [mm]

τbmax [MPa]

σfmax [MPa]

14.7 11.9 7.1 6.8 4.3 2.7 4.6 7.9 2.8 5.1 5.9

0.05 0.2 0.1 0.2 1.3 0.4 0.6 1.1 0.3 0.4 0.5

4.7 3.8 2.3 2.2 1.4 0.9 1.5 2.5 0.9 1.6 1.9

468 378 226 218 136 86 147 251 91 163 216

[Note] Fmax: Maximum pullout load, Smax: Fibre pullout at maximum pullout load, τbmax: maximum pullout stress, σfmax: maximum fibre tensile stress.

strength, revealed that an increase of the fibre content enhances the flexural strength of the concrete significantly, as it was expected according to literature [1–3]. This clearly indicates the advantage of using FMF in order to increase the fibre content and, eventually, improve mechanical properties. However, due to the lack of a sufficient number of FMF, it was not possible to produce FMF reinforced prisms in order to evaluate the flexural strength of FMF-UHPC, yet. The fibre pullout tests indicate that the bond strength between FMF (NiTi) and the cementitious matrix is about 34% lower than the bonding strength between steel fibre and the cementitious matrix. This result is further supported by the fact that less cementitious matrix material is visible on the FMF surface after the fibre pullout as compared to steel fibres (Fig. 14). Therefore, at the current stage of research, the flexural strength of FMF reinforced UHPC is supposed to be lower compared to steel fibre reinforced UHPC, at least considering straight FMF made from NiTi. It can be expected, that the imprinted geometry of the FMF will have a significant impact not only on the workability of the fresh concrete but also on the mechanical properties. Enabled by the shape memory effect, novel and innovative fibre shapes may become

of the fibre. Furthermore, the activation temperature in fresh concrete may differ from the results obtained by DSC measurements as even the fresh concrete has to be seen as an obstacle for the phase transformation and shape change, respectively. Thus, transformation temperatures may increase as a function of the viscosity of the concrete as well as of fibrefibre interaction according to the Clausius-Clayperon relationship [15]. As temperature gradients may affect the functional properties of the SMA, further studies are needed focusing on the robustness of the approach introduced. Furthermore, it has to be mentioned that kinks seen in some fibres and missing endhooks after transformation of the FMF, as highlighted e.g. by μ-CT measurements, already point at issues to be addressed in future work. Irreversible deformation could be induced by local plasticity imposed by manufacturing of the fibres. The fibres were formed manually into a circular geometry in present work. It is assumed that individual fibres were bended too much at this specific point and, therefore, this part of the fibre experienced plastic deformation. Another crucial aspect to be investigated in the future and not addressed in present work is fibre-fibre interaction. In order to account for this aspect, the number of FMF in the concrete has to be drastically increased. The characterisation of the mechanical properties, i.e. the flexural

Fig. 14. Secondary electron images of the embedded parts of the pulled out fibres. a) Boundary between the embedded and blank part of a SMA fibre. b) Lower embedded part of a SMA fibre. c) Boundary between the embedded and blank part of a steel fibre. d) Lower embedded part of a steel fibre. Images in low vacuum mode, voltage is 15 kV, working distance 10 mm.

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Fig. 15. Aimed enhancement of the flexural strength due to the use of functional micro fibres (FMF made of NiTi) based on first approaches in concrete-viscosity optimisation by circular fibres for M3Q UHPC-Composition. The dashed, black line indicates plastic viscosity trend for different straight-fibre contents, the dashed, grey line indicates decrease in plastic viscosity of about 1/3 for the use of circular fibres.

Alexander Wetzel: Project administration, Writing - review & editing. Philipp Krooß: Writing - original draft, Writing - review & editing. Jenny Thiemicke: Writing - review & editing. Thomas Niendor: Writing - review & editing. Bernhard Middendorf: Supervision. Ekkehard Fehling: Writing - review & editing.

realisable, e.g. complex shaped endhooks possibly improving post cracking behaviour. However, the results of the current study only confirm that an endhook geometry significantly improves the flexural strength at higher fibre contents as was already indicated in [36]. In future studies, a correlation between the fibre shape and the post cracking behaviour, in which the bonding between fibre and matrix is an essential factor [3,21], will be established. As can be deduced from the results of the fibre pullout tests, the bond strength between the FMF made of NiTi and UHPC is less strong as compared to the bond strength between regular steel fibres and UHPC. Thus, the mixture (fibre and/or cementitious matrix) has to be further developed in future works to achieve the level of bond strength of a steel fibre reinforced UHPC in case of FMF-UHPC. Furthermore, additional surface treatment of the FMF may have a positive impact on the mechanical properties by affecting the bonding between the fibres and the matrix [36,37]. Fibre pullout tests also have shown that the fibre shear stress due to fibre pullout is lower than the critical stress for detwinning (about 640 N/mm2, Fig. 6b) and much lower than critical stress related to failure of the fibre (about 1800 N/mm2, Fig. 6b). The latter relation is very important as rupture of the fibre would lead to an abrupt failure of the concrete, which is not favourable. In any case, when the NiTi fibres were heated to return to their imprinted shape in the fresh concrete, some still retained high levels of plastic deformation, potentially improving post cracking behaviour. In view of the results of the current study, the application of functional microfibres in combination with UHPC elements paves a potential pathway to significantly increased contents of fibre reinforcement, potentially leading not only to a significant enhancement of the static mechanical material properties but also to an improvement of the post cracking behaviour as already known for steel fibre reinforcement [1,2]. Even if a general proof of concept is provided in present work, component test with FMF reinforcement have to be conducted in future research. Moreover, future work will focus on the enhancement of the bond strength of the FMF and the concrete matrix as well as on the development of novel beneficial endhook geometries.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank Florian Gerland, Niels Wiemer, Leon Franken and Johanna Frenck for their support in the investigation and sample measuring. Financial support by University of Kassel within the SmartCon framework is gratefully acknowledged. References [1] T. Leutbecher, E. Fehling, Tensile behavior of ultra-high-performance concrete reinforced with reinforcing bars and fibers: minimizing fiber content, ACI Struct. J. 109 (2012) 253–263. [2] R. Bornemann, M. Schmidt, E. Fehling, B. Middendorf, Ultra-Hochleistungsbeton UHPC - Herstellung, Eigenschaften und Anwendungsmöglichkeiten, Beton- und Stahlbetonbau 96 (7) (2001) 459–467. [3] A. Naaman, Engineered steel fibers with optimal properties for reinforced cement composites, J. Adv. Concr. Technol. 1 (3) (2003) 241–252. [4] Y. Kusumawardaningsih, E. Fehling, M. Ismail, A. Aboubakr, Tensile strength behavior of UHPC and UHPFRC, Procedia Engineering 125 (2015) 1081–1086. [5] M. Schulz, Einfluss auf den Beton - Stahlfasern: Eigenschaften und Wirkungsweisen, beton, (2000), pp. 382–387. [6] R. Weber, Guter Beton: Ratschläge für die richtige Betonherstellung, Bau + Technik, 2010. [7] S. Illguth, D. Lowke, C. Gehlen, Rheology of fibre reinforced fine-grained high performance concrete for thin-walled elements - effect of type and content of steel fibres, Rheology and Processing of Construction Materials – 7th RILEM International Conference on Self-Compacting Concrete and 1st RILEM International Conference on Rheology and Processing of Construction Materials, 2013. [8] R. Wang, X. Gao, Relationship between flowability, entrapped air content and strength of UHPC mixtures containing different dosage of steel fiber, Appl. Sci. 6 (2016) 216–229. [9] L. Janke, C. Czaderski, M. Metavalli, J. Ruth, Application of shape memory alloys in

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