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X-ray computed tomography harnessed to determine 3D spacing of steel fibres in self compacting concrete (SCC) slabs
Construction and Building Materials 74 (2015) 102–108
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
X-ray computed tomography harnessed to determine 3D spacing of steel fibres in self compacting concrete (SCC) slabs Tomasz Ponikiewski a, Jacek Katzer b,⇑, Monika Bugdol c, Marcin Rudzki c a
Silesian University of Technology, Faculty of Civil Engineering, Gliwice, Poland Koszalin University of Technology, Faculty of Civil Engineering Environmental and Geodetic Sciences, Koszalin, Poland c Silesian University of Technology, Faculty of Biomedical Engineering, Gliwice, Poland b
h i g h l i g h t s Engineered steel fibre are important type of concrete reinforcement. SCC and FRC are combined into one type of cement composite. We examine fibre spacing in FRC–SCC slabs. X-ray computed tomography is harnessed to conduct the tests. Distribution and spacing analysis of fibre has been performed.
a r t i c l e
i n f o
Article history: Received 16 June 2014 Received in revised form 26 September 2014 Accepted 14 October 2014
Keywords: Concrete Porosity Fresh mix Steel fibres Self-compacting concrete SCC SFRC X-ray computed tomography Image processing
a b s t r a c t The conducted research programme was focused on fibre spacing in SFR-SCC slabs. SCC slabs with two steel fibres types and different casting points were examined. A distribution and spacing analysis of fibres has been performed. Their number and volume were evaluated along the slab main axis. The distribution of fibres in slabs cast using different techniques have been compared. The amount of fibres in the upper and lower slab halves has been analysed. The uniformity of spacing of fibres was assessed. The angles between the fibres and the beam main axis were examined. Graphical visualization using 4D spherical histograms for quick assessment of fibres orientation is also presented. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Steel fibre mixed into the concrete can provide an alternative to the provision of conventional re-bars or welded fabric in many applications of concrete. The concept has been in existence since 1874 when the first patent was applied for in California by Berard [23]. During 1970s the commercial use of steel fibre reinforced concrete (SFRC) began to gather momentum, especially in Western Europe, USA and Japan. Current increasing widespread of SFRC is observed due to its hassle free casting while preserving mechanical properties similar to traditionally reinforced concrete using bars
⇑ Corresponding author. http://dx.doi.org/10.1016/j.conbuildmat.2014.10.024 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
and stirrups. There are many areas of civil engineering where SFRC is the main construction material. It has been widely adopted for industrial floors (both ground-supported and pile supported), external pavements, sprayed concrete and precast elements [12,23]. The newest trend in development of technology of SFRC is creation of a self-compacting SFRC. The first self-compacting concrete (SCC) also known as self-consolidating concrete or selflevelling concrete was pioneered in late 1970s and early 1980s in Germany, Italy and Japan [24]. This flowing concrete is able to completely fill the formwork (even in the presence of dense reinforcement) whilst maintaining homogeneity and consolidate under its own weight without the need of any additional compaction [10,20,22,35]. Combining both non-conventional concretes (SFRC and SCC) into a new type of fibre cement composite
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(SFR-SCC) would create a material opening wide range of future applications, especially in case of structures vulnerable to blast, harmonic and fatigue loading [4,5,16,32,33]. The new composite potentially gives previously unknown flexibility in designing and creating concrete structures. It could also be viable in purely economic terms due to eliminating both most laborious processes: placing bar-stirrup reinforcement and consolidating fresh mix. To fully utilize the composite its technology and mechanical properties have to be thoroughly tested and described. The properties of fresh mix and hardened SFR-SCC strongly depend on fibre spacing within cast element [7,13,14,27]. The dependence of casting methods and mix composition on fibre spacing and mechanical properties of SFR-SCC is a subject of multiple on-going research programmes [3,15]. The core idea behind the research programme was to assess fibre spacing of hardened SFR-SCC. To achieve this goal slabs were cast using only one type of SFR-SCC. The only variable was the location of the concrete casting points (CCP). The aim of the research programme was to denominate the correlation between the location of CCP and fibre spacing in hardened SFR-SCC element. The authors decided to harness X-ray computed tomography (CT) as an efficient and flexible tool in the non-destructive high-resolution characterization of the microstructural configuration of materials [19,21,36]. CT technology widely used in medicine allows to assess the air voids spacing and porosity characteristics of the concrete specimens.
2. Used materials, mix design and specimens The test were conducted on the SFR-SCC modified by one type of steel fibres. Crimped steel fibres characterized by length of 50 mm were chosen as the most suitable for planned research programme. The choice of fibres was based on previous experience with steel fibre reinforced concrete and SCC. The commercial availability and commonness of civil engineering applications of specific fibre types were also studied and taken into consideration [18]. The geometry and other parameters of used steel fibres are presented in Table 1. The composition of the mix characterized by W/C ratio equal to 0.41, was fixed for all castings (see Table 2). CEM I 42.5R was utilized as a binder and its content was equal to 490 kg/m3. Such cement content was successfully harnessed by multiple researchers [3,7,25]. The cement was characterized by initial and final setting time of 170 min and 250 min respectively. Fineness of the cement was equal to 3400 cm2/g. There were used two admixtures: superplasticizer and stabilizer. The superplasticizer based on polycarboxylate ether (concentration 20%) and characterized by density equal to 1.07 g/cm3 was applied into the mix in the amount of 3.5% (of the mass of cement) [9]. The stabilizer in a form of a synthetic co-polymer and characterized by density equal to 1.01 g/cm3 was added in the amount of 0.4% (of the mass of cement). Utilized aggregate was in a form of natural sand (median diameter dm = 0.435 mm [17]) and natural subrounded gravel.
The authors applied mix proportioning system described by Okamura and Ozawa [26] which assumes general supply from ready-mixed concrete plants. The coarse and fine aggregate contents are fixed so that self-compatibility is achieved easily by adjusting only the water-powder ratio and superplasticizer dosage [25]. The details of mixing procedure is presented in Fig. 1. The specimens were in a form of slabs (150 mm 1210 mm 1240 mm). The fresh mix was poured to the forms from two different concrete casting points. Slab C1 was prepared while pouring concrete on the edge of a mould. Slab C2 was prepared while pouring concrete in a centre of a mould. Both slabs were reinforced by addition of 1% of fibres (80 kg/m3). The location of both CCP and mould/specimen sizes are presented in Fig. 2. After hardening, slabs were cut into 16 beams with dimensions 150 mm 150 mm 600 mm each which were non-destructively tested using X-ray computed tomography.
3. Research programme The research programme consisted of three main stages. Stage one covered properties of fresh mix such as density tested according to PN-EN 12350-6: 2011 [29] and consistency assessed with the help of the slump-flow test according to RILEM TC 145-WSM [1]. During the slump flow test two parameters were measured: the diameter (SFD in mm) of the concrete pat after removal of the slump cone and the time (T500 in seconds) in which the flowing mix formed a 500 mm concrete pat. Stage two of the research programme covered non-destructive tests harnessing X-ray computed tomography conducted on beams. The applied computed tomography scanner was equipped with 64 rows of detectors, and the thickness of a series of reconstructed native CT scan was 0.625 mm. The penetration factor was an X-ray beam. The examined surface of each layer of concrete was 150 mm 150 mm. For each beam the result consisted of a native series written in DICOM (Digital Imaging and Communications in Medicine) format with at least 950 images, and reconstructed series with at least 1500 images taking into account the interval in the range 50 80% of the thickness of the native layer. Parameters of acquisition were not less than: 140 kV lamp voltage and 400 mA s current strength. Acquired CT volumetric images were processed by in-house built software using C++ libraries for
Fig. 1. Mixing procedure. (S – sand, FA – fine aggregate, C – cement, W – water, F – fibre, SP – superplasticizer, ST – stabilizer).
Table 1 Characteristics of steel fibres. Shape
Material
Tensile strength (N/mm2)
Number of fibres per kg
Low carbon steel wire
800 ± 15%
1128
Fibre type
Length (mm)
Width (mm)
Cross section
F 50
50 ± 10%
2.30/2.95
Segment of a circle
Cement CEM I 42.5R (kg/m3)
Sand (0–2 mm) (kg/m3)
Fine aggregate (2–8 mm) (kg/m3)
Water (kg/m3)
Steel fibres (% by volume) (kg/m3)
Superplasticizer (kg/m3)
Stabilizer (kg/m3)
W/C
C 490
S 807.3
FA 807.3
W 201
F 80 (1.02)
SP 17.2
ST 1.96
– 0.41
Table 2 Composition of SFR-SCC mix.
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Slab C1 – I stage
Slab C1 – II stage
Slab C1 – III stage
Slab C2 – I stage
Slab C2 – II stage
C2 – III stage
Fig. 2. The orientation of angles and axis and methodology of casting SFR-SCC slabs (1240 mm 1200 mm 150 mm).
medical image processing ‘‘The Insight Toolkit’’ [11]. Obtained images were subjected to complex image processing involving: determination of VOI (‘‘volume of interest’’ conducted according to the procedure described in [11]), background removal, segmentation, labelling, determination of geometric features (coordinates of object centre, object volume, object diameters, object direction etc.). Image processing steps are presented using exemplary beam in cross-sections after VOI selection and after fibres segmentation in Fig. 3. For each segmented steel fibre the coordinates of its centre, its orientation and the distance to the infusion point have been calculated. Data retrieved during the image analysis consisted of: label defining each fibre, coordinates of the fibre centre in 3D,
Image aer
eigenvector of the fibres’ rotation matrix, corresponding to the smallest eigenvalue and volume of the fibre. The orientation of angles and axis is presented in Fig. 2. Using these data spherical histograms were generated following procedure used by Rudzki et al. [31]. Harnessed transformation of orientation vector to spherical coordinates is briefly presented in Fig. 4. Stage three of the research programme covered tests of strength properties such as basic compressive tests and three-point bending tests. The tests were following the general guidelines of European code (EN 14651-2005). The load – CMOD (crack mouth opening displacement) relation was recorded and used for calculation of fR1, fR2, fR3, fR4, fLOP.
Image aer fibres segmentaon
VOI selecon Fig. 3. Cross sections of an exemplary specimen.
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Fig. 4. Transformation of orientation vector to spherical coordinates.
Table 3 Properties of fresh mix. Slab
C1 C2
Slump flow test T500 (s)
SFD (mm)
2.5 2.5
760 765
Density (kg/m3)
Air content (%)
2383 2389
2.5 2.4
4. Test results Properties of fresh mix are presented in Table 3. The achieved results were compared with EFNARC [6] requirements for SCC. According to EFNARC a diameter value of at least 650 mm is required to call a mix a SCC. The guidelines also suggest that there is no generally accepted advice on what are reasonable tolerances about a specified value, though the value of 650 mm ± 50 mm, as used in the flow table test, might be appropriate. On the other hand multiple researchers agree with the required rheological parameters summarized in Table 4. All cast compositions gave acceptable flow diameters and flow times to classify them as SCC. Fibres distribution along the X, Y, Z axis, computed during the second stage of the research programme are presented in Fig. 5. In case of X axis (length of the slabs) fibres in the lower half of both slabs are distributed rather uniformly. In slab C1 there is clearly visible descent of the number of fibres (NF) after passing 1000 mm. The same phenomenon is mirrored in the upper part of the slab with a visible descent starting after passing 750 mm. In lower part of slab C2 the number of fibres is on average the same all along the X axis (NF = 1150 ± 150). In the upper part of the slab C2 one can see a clear pick of NF near point of 550 mm descending in both directions. In case of both slabs, there is a visible wall effect for X from 0 mm to 30 mm. Slab C1 is also characterized by the strong wall effect at the other side of the formwork (NF is smaller than 1000 for 1110 mm 6 X 6 1200 mm). The differences in value of NF is getting smaller and smaller along the X axis. In case of Y axis (depth of the cast slabs) the distribution of the NF is very similar. The smallest amount of fibre is present at the top of the slabs (starting from 0 mm). The NF is getting larger and larger along the increasing depth to achieve its maximum value of
over 3500 close to the bottom of the slab. NF gets larger than 1000 at the depth of 30 mm and 40 mm for slab C1 and slab C2 respectively. The last 30 mm of the depth of both slabs are characterized by a very steep descend in the NF. This wall effect at the bottom of the formwork is characterized by very similar thickness and value of NF, comparing to the ‘‘horizontal’’ one along X axis. Axis Z describes width of the slabs. As it is in case of axis X, there is clearly visible difference in the NF between the lower and upper part of both slabs. One can also distinguish the location of CCPs by significantly lower NF in these places (by 60% in both slabs). In Fig. 6, there are presented fibre distributions along the angle H and U. Both angles are presented in Fig. 2. The angle U describes the fibre orientation with respect to the horizontal plane. Slab C1 is characterized by uniform fibre orientation (NF is larger than 1500 for almost all values of U) in case of its lower part. This tendency is mirrored in the upper part of the slab where NF is larger than 2200. For both parts of slab C1, the NF is significantly smaller for U > 80°. It means that fibres tend to be oriented along the direction of flowing mix (X-axis). There is only a small amount of fibres which are oriented along Z-axis. General characteristics of fibre orientation in slab C2, with respect to the horizontal plane is similar to slab C1. The only relevant difference is associated with the peak in NF for U from 6° to 23°. It means that fibre are more oriented along the direction of flowing mix (X-axis) than in case of slab C1. Analysing charts for angle H one can see a tendency of fibres to be oriented horizontally. NF reaches its maximum for H values not larger than 20° from horizontal plane. The amount of fibre oriented horizontally is 10–12 times larger than the amount of fibre oriented vertically H = 90°. The described phenomenon is noticeable in both slabs. On the basis of the achieved load – CMOD diagram (according to EN 14651), four different values of the residual strengths (fR1, fR2, fR3, fR4) were calculated for both composites in question. Strength properties of hardened composite are presented in Table 5.
5. Discussion The residual strengths corresponding to different values of the CMOD are difficult to incorporate to fibre reinforced concrete design procedures. Therefore the residual strengths fR1 and fR3 which are significant for service and ultimate conditions are commonly assumed to characterize the global residual strength. This strength can be harnessed for serviceability limit states (SLS) analysis and ultimate limit states (USL) analysis. In ‘‘fib Bulletin 55, Model Code 2010’’ it was proposed that material behaviour at ULS will be related to the behaviour at SLS employing the fR3/fR1 ratio. Basically, in order to classify the post-cracking strength of SFR-SCC a linear elastic behaviour can be assumed by considering the characteristic residual strength significant for service (fR1) and ultimate (fR3) conditions. According to this procedure SFR-SCC post-cracking residual strength is described by two parameters:
Table 4 Required rheological parameters of SCC mix according to [34]. Slump flow – diameter of a concrete pat 550–650 mm
660–750 mm
760–850 mm
Over 850 mm
Elements with no or nearly no reinforcement, high vertical elements (posts, walls) cast from top, small horizontal elements (plates). Hindered finishing surface of the elements
Forming horizontal and vertical elements with regular reinforcement and arbitrary measurements
Forming horizontal elements with dense reinforcement, of complex shapes, when the mix is pumped. Not recommended for high vertical elements (high pressure on formworks). Aggregation should not be bigger than 16 mm
As in the 760–850 spread, in special cases. Aggregation should not be bigger than 12 mm. High risk of segregation of the mix
Flow time T500 – up to 500 mm radius (plastic viscosity) Up to 2 s
Over 2 s
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CCP
Slab C2
Slab C1
Slab C1
CCP
Slab C2
CCP
CCP
Slab C1
CCP
Slab C2
CCP
Fig. 5. Fibres distribution along the X, Y, Z axes in tested slabs.
namely fR1 (representing the strength interval) and a letter a, b, c or d (representing the ratio fR3/fR1). This classification properly represents the most common cases of SFR-SCC softening and hardening. Traditional reinforcement substitution is enabled if relationships 1 and 2 are fulfilled. Full classification of tested SFR-SCC according to ‘‘fib Bulletin 55, Model Code 2010’’ is summarized in Table 6.
f R1 =f LOP > 0:4
ð1Þ
f R3 =f R1 > 0:5
ð2Þ
Achieved relations of fibre distribution along the X, Y, Z axes in tested slabs are very helpful in analysing uniformity of cast elements. The most interesting relations are for Y axis (depth of a slab) and Z axis (width of a slab). The phenomenon of steel fibre ‘‘drowning’’ in SCC mix is clearly visible while analysing fibre distribution along Y axis. In case of slab C1 the number of fibre at the depth of 20 mm is around 500. While going deeper and deeper into the slab the number of fibre is getting larger and larger. The peak of fibre number is at the depth from 120 mm to 140 mm. At this depths the number of fibre is over 3500. In case of slab C2 this relation
is very similar, reaching similar fibre numbers at similar depths. This phenomenon was described in literature for ordinary SFRC [12,23]. The differences in fibre volumes in the upper and in the lower part of specimens, were much smaller for SFRC comparing to tested SFR-SCC. Very flowable character of fresh SFR-SCC mix favours the tendency of steel fibre to settle down in the cast element. In this way the produced concrete elements are characterized by different volume of reinforcement in the top and bottom part of the any given cross-section. So far traditional SFRC were recognized as being reinforced almost uniformly throughout their volume. A large number of SFRC applications were based on this simple assumption. In case of tested composites the bottom and top side of cast slabs are characterized by very different mechanical properties thus traditional ‘‘SFRC approach’’ to cast elements cannot be implemented. Fibre distribution along the Z axes in tested slabs shows the influence of CCP on number of fibres. The place where the mix was poured is characterized by a significantly smaller number of fibre than other parts of the slabs. Concerns should be risen about homogeneity of the SFR-SCC based on this phenomenon. The
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Slab C1
Slab C2
Slab C2
Slab C1
Fig. 6. Fibres orientation distribution of the angle H and U in tested slabs.
Table 5 Properties of hardened composites. Slab
fcm,28d (MPa)
NF (per slab)
fLOP
fR1
fR2 (MPa)
fR3
fR4
C1 C2
80.0 77.3
65,336 66,602
2.63 2.74
6.18 5.21
4.58 4.85
10.76 10.06
2.11 2.79
Table 6 SFRSCC classification in compliance to ‘‘fib Model Code 2010’’. Slab
fR1/fLOP
fR3/fR1
Class
Reinforcement substitution
C1 C2
2.35 1.91
1.74 1.93
6e 5e
Enabled Enabled
number of fibre in CCP is two times smaller comparing to the rest of the slab. In hardened element the CCP would be characterized by significantly lower mechanical properties than the rest of the slab. The authors are convinced that the future research programmes should cover the processes of concrete casting and placement of reinforcement. Saving large amounts of energy and time, during these technological steps would result in making the whole erecting process much greener, especially while harnessing recycled fibre [8] and waste aggregate [30] for its production. Compacting fresh concrete mix through vibrating and laborious placement of re-bars and stirrups significantly increase the amount of energy needed for erecting a concrete structure. In the authors’ opinion the solution for both problems is SFR-SCC, which addresses multiple technological [2] and environmental issues [28]. 6. Conclusions The following conclusions can be drawn, based on the experimental data obtained in this-investigation. – The location of CCP influences fibre spacing in cast SFR-SCC slabs.
– There are significant differences in fibre volume between the lower and the upper part of SFR-SCC slabs. – The number of fibres reaches its maximum in the range of 25 to 45 mm from the bottom of the mould. – The number of fibre at the depth of 25 mm from slab surface is almost four times smaller than the number of fibre at 25 mm from the bottom of the mould. – The wall effect was observed along all four walls and at the bottom of a formwork. – The magnitude of the wall effect is similar for both slabs. – The wall effect influences the fibre number in layer of 30 mm. – In both slabs fibres tend to be oriented horizontally and along direction of flowing mix. – Tendency of fibre to be oriented horizontally is almost the same in both slabs. – Fibres in slab C2 are characterized by stronger tendency to be oriented along direction of flowing mix then fibres in slab C1. – At the CCPs the number of fibre is significantly lower comparing to the rest of the volume of SFR-SCC slabs. – The test should be conducted using different types of fibre and geometric shapes of cast elements.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
X-ray computed tomography harnessed to determine 3D spacing of steel fibres in self compacting concrete (SCC) slabs Tomasz Ponikiewski a, Jacek Katzer b,⇑, Monika Bugdol c, Marcin Rudzki c a
Silesian University of Technology, Faculty of Civil Engineering, Gliwice, Poland Koszalin University of Technology, Faculty of Civil Engineering Environmental and Geodetic Sciences, Koszalin, Poland c Silesian University of Technology, Faculty of Biomedical Engineering, Gliwice, Poland b
h i g h l i g h t s Engineered steel fibre are important type of concrete reinforcement. SCC and FRC are combined into one type of cement composite. We examine fibre spacing in FRC–SCC slabs. X-ray computed tomography is harnessed to conduct the tests. Distribution and spacing analysis of fibre has been performed.
a r t i c l e
i n f o
Article history: Received 16 June 2014 Received in revised form 26 September 2014 Accepted 14 October 2014
Keywords: Concrete Porosity Fresh mix Steel fibres Self-compacting concrete SCC SFRC X-ray computed tomography Image processing
a b s t r a c t The conducted research programme was focused on fibre spacing in SFR-SCC slabs. SCC slabs with two steel fibres types and different casting points were examined. A distribution and spacing analysis of fibres has been performed. Their number and volume were evaluated along the slab main axis. The distribution of fibres in slabs cast using different techniques have been compared. The amount of fibres in the upper and lower slab halves has been analysed. The uniformity of spacing of fibres was assessed. The angles between the fibres and the beam main axis were examined. Graphical visualization using 4D spherical histograms for quick assessment of fibres orientation is also presented. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Steel fibre mixed into the concrete can provide an alternative to the provision of conventional re-bars or welded fabric in many applications of concrete. The concept has been in existence since 1874 when the first patent was applied for in California by Berard [23]. During 1970s the commercial use of steel fibre reinforced concrete (SFRC) began to gather momentum, especially in Western Europe, USA and Japan. Current increasing widespread of SFRC is observed due to its hassle free casting while preserving mechanical properties similar to traditionally reinforced concrete using bars
⇑ Corresponding author. http://dx.doi.org/10.1016/j.conbuildmat.2014.10.024 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.
and stirrups. There are many areas of civil engineering where SFRC is the main construction material. It has been widely adopted for industrial floors (both ground-supported and pile supported), external pavements, sprayed concrete and precast elements [12,23]. The newest trend in development of technology of SFRC is creation of a self-compacting SFRC. The first self-compacting concrete (SCC) also known as self-consolidating concrete or selflevelling concrete was pioneered in late 1970s and early 1980s in Germany, Italy and Japan [24]. This flowing concrete is able to completely fill the formwork (even in the presence of dense reinforcement) whilst maintaining homogeneity and consolidate under its own weight without the need of any additional compaction [10,20,22,35]. Combining both non-conventional concretes (SFRC and SCC) into a new type of fibre cement composite
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(SFR-SCC) would create a material opening wide range of future applications, especially in case of structures vulnerable to blast, harmonic and fatigue loading [4,5,16,32,33]. The new composite potentially gives previously unknown flexibility in designing and creating concrete structures. It could also be viable in purely economic terms due to eliminating both most laborious processes: placing bar-stirrup reinforcement and consolidating fresh mix. To fully utilize the composite its technology and mechanical properties have to be thoroughly tested and described. The properties of fresh mix and hardened SFR-SCC strongly depend on fibre spacing within cast element [7,13,14,27]. The dependence of casting methods and mix composition on fibre spacing and mechanical properties of SFR-SCC is a subject of multiple on-going research programmes [3,15]. The core idea behind the research programme was to assess fibre spacing of hardened SFR-SCC. To achieve this goal slabs were cast using only one type of SFR-SCC. The only variable was the location of the concrete casting points (CCP). The aim of the research programme was to denominate the correlation between the location of CCP and fibre spacing in hardened SFR-SCC element. The authors decided to harness X-ray computed tomography (CT) as an efficient and flexible tool in the non-destructive high-resolution characterization of the microstructural configuration of materials [19,21,36]. CT technology widely used in medicine allows to assess the air voids spacing and porosity characteristics of the concrete specimens.
2. Used materials, mix design and specimens The test were conducted on the SFR-SCC modified by one type of steel fibres. Crimped steel fibres characterized by length of 50 mm were chosen as the most suitable for planned research programme. The choice of fibres was based on previous experience with steel fibre reinforced concrete and SCC. The commercial availability and commonness of civil engineering applications of specific fibre types were also studied and taken into consideration [18]. The geometry and other parameters of used steel fibres are presented in Table 1. The composition of the mix characterized by W/C ratio equal to 0.41, was fixed for all castings (see Table 2). CEM I 42.5R was utilized as a binder and its content was equal to 490 kg/m3. Such cement content was successfully harnessed by multiple researchers [3,7,25]. The cement was characterized by initial and final setting time of 170 min and 250 min respectively. Fineness of the cement was equal to 3400 cm2/g. There were used two admixtures: superplasticizer and stabilizer. The superplasticizer based on polycarboxylate ether (concentration 20%) and characterized by density equal to 1.07 g/cm3 was applied into the mix in the amount of 3.5% (of the mass of cement) [9]. The stabilizer in a form of a synthetic co-polymer and characterized by density equal to 1.01 g/cm3 was added in the amount of 0.4% (of the mass of cement). Utilized aggregate was in a form of natural sand (median diameter dm = 0.435 mm [17]) and natural subrounded gravel.
The authors applied mix proportioning system described by Okamura and Ozawa [26] which assumes general supply from ready-mixed concrete plants. The coarse and fine aggregate contents are fixed so that self-compatibility is achieved easily by adjusting only the water-powder ratio and superplasticizer dosage [25]. The details of mixing procedure is presented in Fig. 1. The specimens were in a form of slabs (150 mm 1210 mm 1240 mm). The fresh mix was poured to the forms from two different concrete casting points. Slab C1 was prepared while pouring concrete on the edge of a mould. Slab C2 was prepared while pouring concrete in a centre of a mould. Both slabs were reinforced by addition of 1% of fibres (80 kg/m3). The location of both CCP and mould/specimen sizes are presented in Fig. 2. After hardening, slabs were cut into 16 beams with dimensions 150 mm 150 mm 600 mm each which were non-destructively tested using X-ray computed tomography.
3. Research programme The research programme consisted of three main stages. Stage one covered properties of fresh mix such as density tested according to PN-EN 12350-6: 2011 [29] and consistency assessed with the help of the slump-flow test according to RILEM TC 145-WSM [1]. During the slump flow test two parameters were measured: the diameter (SFD in mm) of the concrete pat after removal of the slump cone and the time (T500 in seconds) in which the flowing mix formed a 500 mm concrete pat. Stage two of the research programme covered non-destructive tests harnessing X-ray computed tomography conducted on beams. The applied computed tomography scanner was equipped with 64 rows of detectors, and the thickness of a series of reconstructed native CT scan was 0.625 mm. The penetration factor was an X-ray beam. The examined surface of each layer of concrete was 150 mm 150 mm. For each beam the result consisted of a native series written in DICOM (Digital Imaging and Communications in Medicine) format with at least 950 images, and reconstructed series with at least 1500 images taking into account the interval in the range 50 80% of the thickness of the native layer. Parameters of acquisition were not less than: 140 kV lamp voltage and 400 mA s current strength. Acquired CT volumetric images were processed by in-house built software using C++ libraries for
Fig. 1. Mixing procedure. (S – sand, FA – fine aggregate, C – cement, W – water, F – fibre, SP – superplasticizer, ST – stabilizer).
Table 1 Characteristics of steel fibres. Shape
Material
Tensile strength (N/mm2)
Number of fibres per kg
Low carbon steel wire
800 ± 15%
1128
Fibre type
Length (mm)
Width (mm)
Cross section
F 50
50 ± 10%
2.30/2.95
Segment of a circle
Cement CEM I 42.5R (kg/m3)
Sand (0–2 mm) (kg/m3)
Fine aggregate (2–8 mm) (kg/m3)
Water (kg/m3)
Steel fibres (% by volume) (kg/m3)
Superplasticizer (kg/m3)
Stabilizer (kg/m3)
W/C
C 490
S 807.3
FA 807.3
W 201
F 80 (1.02)
SP 17.2
ST 1.96
– 0.41
Table 2 Composition of SFR-SCC mix.
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Slab C1 – I stage
Slab C1 – II stage
Slab C1 – III stage
Slab C2 – I stage
Slab C2 – II stage
C2 – III stage
Fig. 2. The orientation of angles and axis and methodology of casting SFR-SCC slabs (1240 mm 1200 mm 150 mm).
medical image processing ‘‘The Insight Toolkit’’ [11]. Obtained images were subjected to complex image processing involving: determination of VOI (‘‘volume of interest’’ conducted according to the procedure described in [11]), background removal, segmentation, labelling, determination of geometric features (coordinates of object centre, object volume, object diameters, object direction etc.). Image processing steps are presented using exemplary beam in cross-sections after VOI selection and after fibres segmentation in Fig. 3. For each segmented steel fibre the coordinates of its centre, its orientation and the distance to the infusion point have been calculated. Data retrieved during the image analysis consisted of: label defining each fibre, coordinates of the fibre centre in 3D,
Image aer
eigenvector of the fibres’ rotation matrix, corresponding to the smallest eigenvalue and volume of the fibre. The orientation of angles and axis is presented in Fig. 2. Using these data spherical histograms were generated following procedure used by Rudzki et al. [31]. Harnessed transformation of orientation vector to spherical coordinates is briefly presented in Fig. 4. Stage three of the research programme covered tests of strength properties such as basic compressive tests and three-point bending tests. The tests were following the general guidelines of European code (EN 14651-2005). The load – CMOD (crack mouth opening displacement) relation was recorded and used for calculation of fR1, fR2, fR3, fR4, fLOP.
Image aer fibres segmentaon
VOI selecon Fig. 3. Cross sections of an exemplary specimen.
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Fig. 4. Transformation of orientation vector to spherical coordinates.
Table 3 Properties of fresh mix. Slab
C1 C2
Slump flow test T500 (s)
SFD (mm)
2.5 2.5
760 765
Density (kg/m3)
Air content (%)
2383 2389
2.5 2.4
4. Test results Properties of fresh mix are presented in Table 3. The achieved results were compared with EFNARC [6] requirements for SCC. According to EFNARC a diameter value of at least 650 mm is required to call a mix a SCC. The guidelines also suggest that there is no generally accepted advice on what are reasonable tolerances about a specified value, though the value of 650 mm ± 50 mm, as used in the flow table test, might be appropriate. On the other hand multiple researchers agree with the required rheological parameters summarized in Table 4. All cast compositions gave acceptable flow diameters and flow times to classify them as SCC. Fibres distribution along the X, Y, Z axis, computed during the second stage of the research programme are presented in Fig. 5. In case of X axis (length of the slabs) fibres in the lower half of both slabs are distributed rather uniformly. In slab C1 there is clearly visible descent of the number of fibres (NF) after passing 1000 mm. The same phenomenon is mirrored in the upper part of the slab with a visible descent starting after passing 750 mm. In lower part of slab C2 the number of fibres is on average the same all along the X axis (NF = 1150 ± 150). In the upper part of the slab C2 one can see a clear pick of NF near point of 550 mm descending in both directions. In case of both slabs, there is a visible wall effect for X from 0 mm to 30 mm. Slab C1 is also characterized by the strong wall effect at the other side of the formwork (NF is smaller than 1000 for 1110 mm 6 X 6 1200 mm). The differences in value of NF is getting smaller and smaller along the X axis. In case of Y axis (depth of the cast slabs) the distribution of the NF is very similar. The smallest amount of fibre is present at the top of the slabs (starting from 0 mm). The NF is getting larger and larger along the increasing depth to achieve its maximum value of
over 3500 close to the bottom of the slab. NF gets larger than 1000 at the depth of 30 mm and 40 mm for slab C1 and slab C2 respectively. The last 30 mm of the depth of both slabs are characterized by a very steep descend in the NF. This wall effect at the bottom of the formwork is characterized by very similar thickness and value of NF, comparing to the ‘‘horizontal’’ one along X axis. Axis Z describes width of the slabs. As it is in case of axis X, there is clearly visible difference in the NF between the lower and upper part of both slabs. One can also distinguish the location of CCPs by significantly lower NF in these places (by 60% in both slabs). In Fig. 6, there are presented fibre distributions along the angle H and U. Both angles are presented in Fig. 2. The angle U describes the fibre orientation with respect to the horizontal plane. Slab C1 is characterized by uniform fibre orientation (NF is larger than 1500 for almost all values of U) in case of its lower part. This tendency is mirrored in the upper part of the slab where NF is larger than 2200. For both parts of slab C1, the NF is significantly smaller for U > 80°. It means that fibres tend to be oriented along the direction of flowing mix (X-axis). There is only a small amount of fibres which are oriented along Z-axis. General characteristics of fibre orientation in slab C2, with respect to the horizontal plane is similar to slab C1. The only relevant difference is associated with the peak in NF for U from 6° to 23°. It means that fibre are more oriented along the direction of flowing mix (X-axis) than in case of slab C1. Analysing charts for angle H one can see a tendency of fibres to be oriented horizontally. NF reaches its maximum for H values not larger than 20° from horizontal plane. The amount of fibre oriented horizontally is 10–12 times larger than the amount of fibre oriented vertically H = 90°. The described phenomenon is noticeable in both slabs. On the basis of the achieved load – CMOD diagram (according to EN 14651), four different values of the residual strengths (fR1, fR2, fR3, fR4) were calculated for both composites in question. Strength properties of hardened composite are presented in Table 5.
5. Discussion The residual strengths corresponding to different values of the CMOD are difficult to incorporate to fibre reinforced concrete design procedures. Therefore the residual strengths fR1 and fR3 which are significant for service and ultimate conditions are commonly assumed to characterize the global residual strength. This strength can be harnessed for serviceability limit states (SLS) analysis and ultimate limit states (USL) analysis. In ‘‘fib Bulletin 55, Model Code 2010’’ it was proposed that material behaviour at ULS will be related to the behaviour at SLS employing the fR3/fR1 ratio. Basically, in order to classify the post-cracking strength of SFR-SCC a linear elastic behaviour can be assumed by considering the characteristic residual strength significant for service (fR1) and ultimate (fR3) conditions. According to this procedure SFR-SCC post-cracking residual strength is described by two parameters:
Table 4 Required rheological parameters of SCC mix according to [34]. Slump flow – diameter of a concrete pat 550–650 mm
660–750 mm
760–850 mm
Over 850 mm
Elements with no or nearly no reinforcement, high vertical elements (posts, walls) cast from top, small horizontal elements (plates). Hindered finishing surface of the elements
Forming horizontal and vertical elements with regular reinforcement and arbitrary measurements
Forming horizontal elements with dense reinforcement, of complex shapes, when the mix is pumped. Not recommended for high vertical elements (high pressure on formworks). Aggregation should not be bigger than 16 mm
As in the 760–850 spread, in special cases. Aggregation should not be bigger than 12 mm. High risk of segregation of the mix
Flow time T500 – up to 500 mm radius (plastic viscosity) Up to 2 s
Over 2 s
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CCP
Slab C2
Slab C1
Slab C1
CCP
Slab C2
CCP
CCP
Slab C1
CCP
Slab C2
CCP
Fig. 5. Fibres distribution along the X, Y, Z axes in tested slabs.
namely fR1 (representing the strength interval) and a letter a, b, c or d (representing the ratio fR3/fR1). This classification properly represents the most common cases of SFR-SCC softening and hardening. Traditional reinforcement substitution is enabled if relationships 1 and 2 are fulfilled. Full classification of tested SFR-SCC according to ‘‘fib Bulletin 55, Model Code 2010’’ is summarized in Table 6.
f R1 =f LOP > 0:4
ð1Þ
f R3 =f R1 > 0:5
ð2Þ
Achieved relations of fibre distribution along the X, Y, Z axes in tested slabs are very helpful in analysing uniformity of cast elements. The most interesting relations are for Y axis (depth of a slab) and Z axis (width of a slab). The phenomenon of steel fibre ‘‘drowning’’ in SCC mix is clearly visible while analysing fibre distribution along Y axis. In case of slab C1 the number of fibre at the depth of 20 mm is around 500. While going deeper and deeper into the slab the number of fibre is getting larger and larger. The peak of fibre number is at the depth from 120 mm to 140 mm. At this depths the number of fibre is over 3500. In case of slab C2 this relation
is very similar, reaching similar fibre numbers at similar depths. This phenomenon was described in literature for ordinary SFRC [12,23]. The differences in fibre volumes in the upper and in the lower part of specimens, were much smaller for SFRC comparing to tested SFR-SCC. Very flowable character of fresh SFR-SCC mix favours the tendency of steel fibre to settle down in the cast element. In this way the produced concrete elements are characterized by different volume of reinforcement in the top and bottom part of the any given cross-section. So far traditional SFRC were recognized as being reinforced almost uniformly throughout their volume. A large number of SFRC applications were based on this simple assumption. In case of tested composites the bottom and top side of cast slabs are characterized by very different mechanical properties thus traditional ‘‘SFRC approach’’ to cast elements cannot be implemented. Fibre distribution along the Z axes in tested slabs shows the influence of CCP on number of fibres. The place where the mix was poured is characterized by a significantly smaller number of fibre than other parts of the slabs. Concerns should be risen about homogeneity of the SFR-SCC based on this phenomenon. The
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Slab C1
Slab C2
Slab C2
Slab C1
Fig. 6. Fibres orientation distribution of the angle H and U in tested slabs.
Table 5 Properties of hardened composites. Slab
fcm,28d (MPa)
NF (per slab)
fLOP
fR1
fR2 (MPa)
fR3
fR4
C1 C2
80.0 77.3
65,336 66,602
2.63 2.74
6.18 5.21
4.58 4.85
10.76 10.06
2.11 2.79
Table 6 SFRSCC classification in compliance to ‘‘fib Model Code 2010’’. Slab
fR1/fLOP
fR3/fR1
Class
Reinforcement substitution
C1 C2
2.35 1.91
1.74 1.93
6e 5e
Enabled Enabled
number of fibre in CCP is two times smaller comparing to the rest of the slab. In hardened element the CCP would be characterized by significantly lower mechanical properties than the rest of the slab. The authors are convinced that the future research programmes should cover the processes of concrete casting and placement of reinforcement. Saving large amounts of energy and time, during these technological steps would result in making the whole erecting process much greener, especially while harnessing recycled fibre [8] and waste aggregate [30] for its production. Compacting fresh concrete mix through vibrating and laborious placement of re-bars and stirrups significantly increase the amount of energy needed for erecting a concrete structure. In the authors’ opinion the solution for both problems is SFR-SCC, which addresses multiple technological [2] and environmental issues [28]. 6. Conclusions The following conclusions can be drawn, based on the experimental data obtained in this-investigation. – The location of CCP influences fibre spacing in cast SFR-SCC slabs.
– There are significant differences in fibre volume between the lower and the upper part of SFR-SCC slabs. – The number of fibres reaches its maximum in the range of 25 to 45 mm from the bottom of the mould. – The number of fibre at the depth of 25 mm from slab surface is almost four times smaller than the number of fibre at 25 mm from the bottom of the mould. – The wall effect was observed along all four walls and at the bottom of a formwork. – The magnitude of the wall effect is similar for both slabs. – The wall effect influences the fibre number in layer of 30 mm. – In both slabs fibres tend to be oriented horizontally and along direction of flowing mix. – Tendency of fibre to be oriented horizontally is almost the same in both slabs. – Fibres in slab C2 are characterized by stronger tendency to be oriented along direction of flowing mix then fibres in slab C1. – At the CCPs the number of fibre is significantly lower comparing to the rest of the volume of SFR-SCC slabs. – The test should be conducted using different types of fibre and geometric shapes of cast elements.
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