ABSTRACT: Ultra-High-Performance Fibre Reinforced Concrete (UHPFRC) has received extensive attention due to its excellent mechanical properties. UHPFRC not only offers a very high compressive strength, but also a unique tensile strain-hardening behaviour. Casting method is critical as it determines the fibre alignment, which in turn affects the performance of UHPFRC. In practice, casting in one single pour requires large equipment, large volume of concrete and it is time consuming and in general difficult to achieve. Also, there is not yet a consensus on the most appropriate bending test configuration to characterise the flexural behaviour of the UHFPRC. This study is focused on the effect of casting and testing methods on the flexural behaviour of UHPFRC specimens. Three different casting strategies and two different flexural test configurations (3PB: three-point bending versus 4PB:
four-point bending) were examined. The flexural strength vs normalized deflection curves, cracking behaviour, and failure modes of beams tested using 3PB and 4PB setups were determined and compared. The results of this study showed that the first-crack and ultimate strengths, and the corresponding deflections are dependent on the casting method and the adverse effect can be mitigated by adopting an appropriate UHPFRC placing technique.
KEY WORDS: UHPFRC; Flexural Strength; Crack pattern; Steel Fibres Orientation.
1 INTRODUCTION
UHPFRC has attracted considerable interest due to its excellent mechanical properties in terms of compressive strength, tensile strength, energy absorption capacity, durability and fatigue resistance, which have been achieved by using a low water-to- binder ratio, very fine mineral admixtures and the addition of micro steel fibres (Yoo and Banthia, 2016). With the addition of fibres, the UHPFRC exhibits unique strain-hardening behaviour that is associated with multiple cracking behaviour (Park et al., 2012). Fibre alignment depends on the flow properties of the composite, such as flow velocity and flow velocity gradient, and these makes the casting method crucial (Wille and Parra-Montesinos, 2012). The flexural performance is considerably affected by the fibre distribution and orientation, which largely depends on the quality and direction of flow during the casting process. To evaluate the flexural behaviour, three-point bending (3PB) and four-point bending (4PB) tests are generally carried out following the methodologies recommended by the material testing standards, i.e. RILEM TC 162-TDF, 2002 and ASTM C 1609/C 1609M- 12, 2012 for 3PB and 4PB tests, respectively.
Both standards require the filling of the mould in one single pour when it comes to highly workable or self-consolidating concrete, which is the case for UHPFRC. However, in practice, casting in one single pour is difficult in some circumstances as it requires large volume of concrete, large equipment and it is also time consuming.
The differences between 3PB and 4PB tests are: a) 3PB requires a notch, cut at the mid-span of the specimen, to force the crack localisation there, b) 4PB results in a constant moment between the loading points with no restraint on the
crack formation and localisation (Wille and Parra-Montesinos, 2012). Therefore, in general, the flexural strength of 4PB tests more accurately represents that of a beam in practice.
For quasi-brittle materials, such as concrete, an unstable progress of the fracture process zone (FPZ) ahead of the crack front dominates the material’s strain softening and strain localization behaviour. The essential characteristics to determine fracture behaviour are FPZ width and the crack opening displacement (COD) in this zone (Dong et al., 2018).
In fact, the fracture energy (Gf) is closely associated to FPZ, and the relationship between the cohesive stress and COD in FPZ is usually used to describe the softening behaviour of concrete and to simulate the crack propagation path.
This study is focused on the effect of casting and testing methods on the flexural behaviour of UHPFRC beams. Three different casting strategies and two different flexural test configurations (3PB vs 4PB) were evaluated.
2 EXPERIMENTAL PROGRAM Materials
The ingredients and mix proportions used in this study to develop UHPFRC are shown in Table 1. Straight high-strength micro-steel fibres with the physical and mechanical properties shown in Table 2, were used. The particle size distribution of silica sand, cement and GGBS (ground granulated blast- furnace slag) from particle size analysis (PSA) are shown in Fig. 1. The rheological properties of the mixture were adjusted carefully to ensure an adequate flowability of the UHPFRC and uniform distribution of the steel fibres during casting.
Effect of casting method and test setup on flexural characterization of UHPFRC
Yuanye He1 2, Esmaeel Esmaeeli3, Marios Soutsos1
1School of Natural and Built Environment, Queen’s University, Belfast, BT7 1NN, Northern Ireland, UK
2 College of Architecture Engineering, Xinjiang University, Urumqi, China
3 Dept of Civil & Environmental Engineering, Brunel University London, London, UK email: [email protected], [email protected], [email protected]
Silica sand, GGBS and cement were first mixed for approximately 2 minutes. Microsilica slurry (MSS) was then gradually added followed with pre-mixed water and superplasticizer (SP). Then, the mixture was thoroughly mixed for 15 to 20 minutes until the concrete developed a workable consistency. Lastly, micro-steel fibres were added to the matrix and mixed until they were homogenously distributed. After mixing, the concrete was taken out of the mixer using a shovel and the moulds were filled to their full capacity. The specimens were covered with plastic sheets and left at room temperature for 24 hours. They were then demoulded and immersed in water curing tank of 90℃ for 24 hours accelerated heat-curing.
Table 1 Mix proportion of UHPFRC (kg/m3) Silica Sand CEM I 52.5N GGBS MSS Water SP Steel fibre
999.4 627.3 399.2 203.7 94.6 32.8 156.8 Table 2 Properties of steel fibres
df (mm) lf (mm) lf /df Density (g/cm3) ft (MPa) Ef (GPa)
0.2 13 65 7.84 2000 200
Notes: df=fiber diameter, lf=fibre length, ft=tensile strength of fibre, and Ef= elastic modulus of fibre.
Figure 1 Particle size distribution
As shown in Fig. 2, three different placing strategies were adopted to prepare the beams, namely (1) firstly pouring at the centre and then both ends, (2) firstly pouring at one end and then at the other, and (3) pouring from both ends simultaneously.
3PB and 4PB tests, according to Table 3, were carried out to evaluate and compare the flexural behaviour of the beams. It should be noted that the specimens were cast with two batches of UHPFRC and in Table 3, the first batch is distinguished from the other one with an asterisk sign. The notch had a constant width of 4 mm for 3PB test specimens.
(a) method 1 (b) method 2 (c) method 3 Figure 2 Casting methods
Compression test
Compression tests were performed in accordance with ASTM C39/39M on three cylinders having 60 mm diameter and 120 mm length. The rough end of specimen was sawed and carefully grinded to flat. A uniaxial load was applied using a universal testing machine with a maximum capacity of
3000kN. Two rigid circular rings were secured to the specimen symmetrically at a distance approximately two-thirds of the cylinder’s height. Four linear voltage differential transducers (LVDTs) were fixed to the specimen (Fig. 3) with a 90 plan configuration to measure the axial deformation of the specimens and further to calculate the strain. Due to the presence of the crack after elastic stage, the clamping screw may rotate which makes it unable to capture the post-cracking behaviour (Hassan, Jones and Mahmud, 2012). A fifth LVDT was placed to measure the movement of the crosshead. The full stress-strain curve was obtained by averaging the strain measured at elastic stage by 4-LVDTs and that of the post- crack was obtained from the crosshead movement. In order to obtain the complete pre- and post-peak stress-strain curves, the load was applied at 0.3 mm/min, similar to that adopted by several authors (Krahl, de Miranda Saleme Gidrão and Carrazedo, 2018).
Table 3 Experimental parameters No. Name Notch depth Test method Casting method
1 SUP1*-1 N/A
(Unnotched) 4PB 1
2 SUP1*-2 3 SUP1*-3 4 SNP1*-1
20 mm 3PB 1
5 SNP1*-2 6 SNP1*-3 7 SNP1-1
20 mm 3PB 1
8 SNP1-2 9 SNP1-3 10 SNP2-1
20 mm 3PB 2
11 SNP2-2 12 SNP2-3 13 SNP3-1
20 mm 3PB 3
14 SNP3-2 15 SNP3-3
Figure 3 Compression test setup Flexural test
All the 3PB and 4PB tests were carried out using a servo- controlled electro-hydraulic machine. The geometry of the flexural specimen and test setups are shown in Fig. 4. The span was 450 mm. The dimensions of the prisms were 500×100×100 mm.
For 3PB, the tests were performed under crack-mouth opening displacement (CMOD) control, at a rate of 0.05 mm/min up to
0.1 1 10 100 1000 10000
0 10 20 30 40 50 60 70 80 90
100 Silica sand Cement GGBS
Pass percentage (%)
Particle size (μm)
a CMOD of 0.1 mm and then continued at a higher rate of 0.2 mm/min until the end of the test, i.e. reach the capacity of LVDT (10mm) or reach 10% of the maximum load (RILEM TC 162-TDF, 2002). Two horizontal LVDTs, i.e. LVDT-H in Fig. 4a, were attached to the specimen’s soffit at the location of the notch and the average of their measurements was used to control the test. Another two vertical LVDTs, i.e. LVDT-V in Fig. 4a, were installed vertically on a yoke secured to the lateral faces of the beam. The vertical LVDTs were used to measure the mid-span deflection of the beam. The yoke shown in Fig. 4 was designed for two purposes: 1) to exclude measurements of deformations other than the absolute mid-span displacement of the beam, such as beam settlement at the supports, and 2) not to obstruct the window of interest (WOI) for digital image correlation (DIC) measurement. Two target marks were bonded on the front yoke as the reference points for DIC analysis. The marks were highlighted by white lines in Fig. 4.
4PB tests were conducted using the same machine and a similar test setup as for 3PB tests. The differences were the loading point and the controlling method. As the specific location of crack cannot be known in advance for specimen under 4PB loading, the 4PB tests were controlled by the deflection measured by the vertical LVDTs (LVDT-V) at the rate of 0.2 mm/min. Two horizontal LVDTs (LVDT-H) were attached to the bottom of the specimen as shown in Fig. 4b. The horizontal LVDTs were staggered to allow for simplified identification of crack localization (Baby et al., 2012). The same yoke and DIC measurements as for 3PB tests were used.
(a) Three-point bending test setup
(b) Four-point bending test setup Figure 4 Bending test setup
3 TESTS RESULTS AND DISCUSSIONS Compression test result
The average compressive stress-strain curve for the UHPFRC is shown in Fig. 5. The maximum compressive strength was 176 MPa. UHPFRC specimens showed linear elastic behaviour up to approximately 70% of their compressive strength followed by strain hardening up to the peak strength. Their post-peak response showed an abrupt drop to approximately 40% of the peak strength and subsequently a gradual load decay in the softening regime. Using linear regression analysis, the Elastic modulus was obtained as 50 GPa.
Figure 5 Compressive stress-strain curve Flexural test result
The flexural behaviour of test specimens was evaluated based on the relationship between the flexural strength and normalized deflection. The normalized deflection was calculated as follows:
𝛿 = 𝛿/𝐿 (1)
where 𝛿 is the deflection; and L is the span length, i.e. 450 mm for both 3PB and 4PB test in this study.
The flexural strength was calculated as follows:
𝜎 =𝑀
𝑆 = 6 × 𝑀
𝑏 × ℎ (2)
where M is the applied moment; S is the section modulus; and b and h are the beam cross-sectional width and height, respectively, excluding the notch depth, if applicable. The flexural strength and the related deflection at first-crack and peak load of all series (15 prisms in total) are shown in Table 4. The mean value, standard deviation (SD), and the coefficient of variation (CoV) are also shown.
The effect of casting method on bending behaviour was evaluated by comparing the 3PB test results of the three series of prisms from the same batch. As shown in Table 4, the casting method 2 showed the highest first-crack strength of 14.9 MPa and the corresponding deflection of 0.19 mm. Casting method 3 not only had the lowest first-crack strength and the corresponding deflection, but also had the largest CoV. The average flexural strength of casting methods 1 and 2 were similar (21.2 MPa and 21.1 MPa respectively) and higher than that of method 3 (16 MPa). The CoV in flexural strength for methods 1, 2 and 3 were 23.5%, 16.9% and 18.4% respectively.
0.0000 0.005 0.010 0.015 0.020 0.025 0.030 20
40 60 80 100 120 140 160 180
Stress (MPa)
Strain linear fitting E=50GPa
The results of each series and their average are shown in Fig. 6.
The average flexural strength-normalized deflection curves of specimens cast using method 1 and method 2 was similar up to their ultimate strength, and superior to that of specimens cast with method 3, what confirms the dependency of flexural behaviour of UHPFRC to the placing strategy adopted.
This difference can potentially be explained by the degree of alignment of fibres bridging the cracks. In the case of casting methods 1 and 2, the self-compacting UHPFRC flows from the centre towards the end of the mould initially or flows from one end towards the other end initially, resulting in a higher tendency of fibres alignment along the specimen’s longitudinal axis in the central part of the prisms as compared to method 3.
In fact, in the case of method 3, at the central region of the mould where the simultaneous flows of UHPFRC converge, the flow disturbance increases the inclination of fibres relative to the longitudinal axis of the specimen.
Since same casting and testing methods were adopted for series SNP1 and SNP1*, their test results are compared first (Fig. 7a).
As can be seen from Table 4, the first-crack strength and the corresponding deflection for SNP1 and SNP1* are similar. The SNP1* showed a slightly higher ultimate strength (+4%) and the corresponding deflection (+13%). The results confirmed the replicability of the experiment.
To evaluate the effect of the test setup, SNP1* and SUP1*
series were tested using notched 3PB and unnotched 4PB test configurations, respectively. The specimens in these series were cast using the first batch and adopting method 1. The results of the unnotched 4PB tests account for material randomness, resulting in critical cracks developing at the weakest sections within the middle third of the span. On the other hand, in the notched 3PB test, the cracking is forced to initiate at the notched section in the middle span, where not only the cross-sectional surface area is reduced but also a high stress concentration at the tip of the notch is expected. As shown in Table 4, the first-crack strength and the corresponding deflection for 3PB are higher than that of 4PB. However, as
shown in Fig. 7b, by comparing the average bending behaviour of series SNP1* and SUP1*, i.e. 3PB_average and 4PB_average, it can been seen there is no significant difference in the flexural behaviour of these two series.
Figure 6 Flexural test results of 3PB tests for different casting method
(a) results of different batches (b) comparison of 4PB and 3PB Figure 7 Flexural test results
Table 4 Flexural strengths and the corresponding deflections
No. Test method
Casting method Name
First-crack strength (MPa) First-crack deflection (mm) Ultimate strength (MPa) Ultimate deflection (mm) Stress
Mean (SD)
[CoV] Deflection
Mean (SD)
[CoV] Stress
Mean (SD)
[CoV] Deflection
Mean (SD) [CoV]
1
4PB 1
SUP1*-1 5.9 8.3
(1.7) [20.7%]
0.04 0.07
(0.02) [34.0%]
18.1 21.9
(3.4) [15.5%]
0.53 0.87
(0.29) [33.0%]
2 SUP1*-2 9.3 0.09 26.3 1.23
3 SUP1*-3 9.7 0.09 21.2 0.84
4
3PB 1
SNP1*-1 16.7 12.7
(2.9) [23.3%]
0.16 0.13
(0.03) [20.0%]
28.1 22.0
(5.1) [23.2%]
0.79 0.76
(0.09) [12.0%]
5 SNP1*-2 9.7 0.15 15.6 0.64
6 SNP1*-3 11.6 0.10 22.4 0.86
7
3PB 1
SNP1-1 9.6 12.0
(1.8) [14.7%]
0.08 0.13
(0.04) [29.4%]
28.2 21.2
(5.0) [23.5%]
0.92 0.67
(0.22) [33.3%]
8 SNP1-2 13.9 0.17 18.4 0.70
9 SNP1-3 12.5 0.16 17.0 0.38
10
3PB 2
SNP2-1 13.8 14.9
(2.8) [19.0%]
0.14 0.19
(0.05) [24.1%]
25.4 21.1
(3.6) [17.0%]
0.72 0.74
(0.06) [8.5%]
11 SNP2-2 12.1 0.19 16.6 0.68
12 SNP2-3 18.8 0.25 21.3 0.83
13
3PB 3
SNP3-1 3.1 7.1
(3.9) [54.7%]
0.02 0.07
(0.05) [75.4%]
12.8 16.0
(2.9) [18.4%]
0.58 0.54
(0.03) [5.3%]
14 SNP3-2 5.8 0.04 15.3 0.51
15 SNP3-3 12.4 0.14 19.9 0.54
Notes: (SD) = standard deviation, [CoV] = coefficient of variation
Failure modes of flexural specimens
Irrespective of casting method, all prisms tested under 3PB showed a similar crack opening and propagation as shown in Fig. 8. It can be seen that the onset of the crack was not exactly from the middle of the notch but from one of its corners. In addition, the crack orientation was influenced by the fibre bridging effects. However, the casting methods used in this study had no clear influence on the failure mode.
As shown in Fig. 9, all prisms tested under 4PB showed multiple micro-cracks with one major localized crack. This is because the fibre starts to take effect after reaching cracking stress. The white dashed lines refer to the position of the supports and loads. It can be seen that there were more cracks within the constant moment area the middle one-third region than 1/3 span region at the ends of prisms. It should be noted that the micro-cracks were not evenly distributed. Instead, several micro-crack clusters could be observed.
Figure 8 Comparison of crack pattern for prisms cast using different methods
Figure 9 Crack patterns of 4PB tests 4 CONCLUSION
This study aimed to experimentally evaluate the effect of casting and testing methods on the flexural behaviour of UHPFRC. According to the bending test results, the placing method of UHPFRC influences the first-crack and ultimate strengths, and the corresponding deflections. Prism specimens
cast by initially pouring UHPFRC at the centre of the mould and then simultaneously or sequentially at its ends, resulted in a comparable behaviour, superior to the flexural performance of the specimens cast simultaneously at both ends.
Independent of the test setup, 3PB or 4PB, all specimens exhibit multiple cracking. The notched specimens had a higher first-crack strength than unnotched ones whilst the flexural strength obtained from both configurations were fairly similar.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support provided by Xinjiang Uygur Autonomous Region Educational Commission and Xinjiang University from China.
REFERENCES
[1] ASTM C 1609/C 1609M-12 (2012) Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete ( Using Beam With Third- Point Loading ). doi: 10.1520/C1609_C1609M-12.
[2] ASTM C39/39M (2014) Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens 1. doi: 10.1520/C0039_C0039M- 17B.
[3] Baby, F. et al. (2012) ‘Proposed flexural test method and associated inverse analysis for UHPFRC’, ACI Materials Journal, 109(5), pp. 545–
555. doi: 10.14359/51684086.
[4] Dong, W. et al. (2018) ‘Investigations on the FPZ evolution of concrete after sustained loading by means of the DIC technique’, Construction and Building Materials. Elsevier Ltd, 188, pp. 49–57. doi:
10.1016/j.conbuildmat.2018.08.077.
[5] Hassan, A. M. T., Jones, S. W. and Mahmud, G. H. (2012) ‘Experimental test methods to determine the uniaxial tensile and compressive behaviour of UHPFRC’, Construction and Building Materials, 37, pp. 874–882. doi:
10.1016/j.conbuildmat.2012.04.030.
[6] Krahl, P. A., de Miranda Saleme Gidrão, G. and Carrazedo, R. (2018)
‘Compressive behavior of UHPFRC under quasi-static and seismic strain rates considering the effect of fiber content’, Construction and Building Materials. Elsevier Ltd, 188, pp. 633–644. doi:
10.1016/j.conbuildmat.2018.08.121.
[7] Park, S. H. et al. (2012) ‘Tensile behavior of ultra high performance hybrid fiber reinforced concrete’, Cement and Concrete Composites, 34(2), pp. 172–184. doi: 10.1016/j.cemconcomp.2011.09.009.
[8] RILEM TC 162-TDF (2002) Bending test.
[9] Wille, K. and Parra-Montesinos, G. J. (2012) ‘Effect of beam size, casting method, and support conditions on flexural behavior of UHPFRC’, ACI Materials Journal, 109(3), pp. 379–388. doi: 10.14359/51683829.
[10] Yoo, D. Y. and Banthia, N. (2016) ‘Mechanical properties of UHPFRC:
A review’, Cement and Concrete Composites. Elsevier Ltd, 73, pp. 267–
280. doi: 10.1016/j.cemconcomp.2016.08.001.
