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Procedía Engineering
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Procedía Engineering 65 (2013) 290 - 295
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Concrete and Concrete Structures 2013 Conference
Properties of Steel Fibre Reinforced Self-compacting Concrete for Optimal Rheological and Mechanical Properties in Precast Beams
In the paper the basic influence trends of different composition and properties of steel fibres on fresh mixture and mechanical properties (compressive and flexural strength researching in different length of precast beams) of Self-Compacting Concrete (SCC). Discussion about the results covers mechanism of fibres influence on mechanical effectiveness of moulding precast beams. Presented relations can be used for selecting of optimal admixtures in presence of steel fibres, as well as for control of properties of fresh SCC containing fibres. The research has shown the negative influence of fibres added to concrete mix on its rheological properties and workability. Optimal addition of superplasticizers (SP) improves its properties, and becomes positive as an additive to concrete mixes. The current state of knowledge is not sufficient to effectively control the mixtures with fibres according to rheological and mechanical properties. It compels to recognize the real nature of workability and to determine the impact of SP and fibres on phenomena taking place in fresh and hardened self-compacting concrete. The goal of this work was to determine the distribution and orientation of steel fibres in self-compacting concrete, cast of small beams by filling the mould with mixture in one edge, and to investigate their mechanical properties.
©2013TheAuthors. Published by Elsevier Ltd.
Selectionandpeer-review underresponsibilityofUniversityofZilina,FacultyofCivilEngineering,DepartmentofStructures andBridges
Keywords: self-compacting concrete; steel fibres; rheology; precast beams; flexural strength; X-ray investigation; Computed tomography;
1. Introduction
Little known research area is the influence of formation methods of self-compacting mixtures with the addition of steel fibres (SFRSCC) on the behavior of assumed technological and mechanical properties of concrete in the design process. There is no information on the actual dispersion and orientation of fibres in the mixture in the course of technological processes. The effect of steel fibres on the self-compactibility of concrete has been investigated
* Tel.: +48-32-237-22-19; fax: +48-32-237-27-37. E-mail address: Tomasz.Ponikiewski@polsl.pl
Tomasz Ponikiewskia*9 Jacek Golaszewskr
'Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland
Abstract
1877-7058 © 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review under responsibility of University of Zilina, Faculty of Civil Engineering, Department of Structures and Bridges doi: 10. 1016/j .proeng .2013.09.045
already [1-3J, including the examination of its mechanical properties at the same time [4-6]. However, studies on development of this technology, including the deployment of fibre in the mixture, are far less advanced. This is due to the complexity of such advanced research methods and limited methods for verifying such concrete structures. The addition of dispersed reinforcement enhances to varied degree various concrete properties, at the same time it creates difficulties in preparing a composition, which satisfies the conditions of a self-compacting, homogeneous matrix in the whole volume, and also significantly improves the physical-mechanical properties [7-8]. The most important issue is to determine the fibre distribution dispersed in various formed structural elements depending on the method of their self-compacting mixture's forming, taking into account the rheological properties of the mixture, as well as the volume ratio and geometrical parameters of steel fibres. The main objective of this study was to determine distribution and orientation of steel fibres in tested SFRSCC for a chosen structural model and its effect on mechanical properties. The aim of presented research was to determine the distribution of steel fibres in SFRSCC in specific case of the corner beam concreting of the structural element and the effect of the fibres on concrete's mechanical properties. Beam structural elements are specific but commonly performed concrete structures, hence the authors' proposal to analyze SFRSCC properties in suggested elements in terms of selected technological factors.
2. Research methodology
As a structural model, concrete beams with dimensions 1200*150x150 mm and 1800*150x150 mm were chosen, where concrete mixture was introduced to the form in one of the edge points (Fig. 1). After hardening, elements were cut into two or three samples with dimensions 600*150*150 mm.
Fig. 1. Methodology of forming concrete in case of concrete beam 1800x150x150 mm
Obtained beams were analyzed by computed tomography, described earlier in the work [9] and measured their strength to tensile tests in bending, according to RILEM recommendations [10]. The composition of tested self-compacting mixtures is presented in Table 1. Two types of steel fibres were considered on three levels of volume ratio. The properties of used steel fibres are presented in Table 2. The self-compactibility of the mixture met the criterion throughout the interval of varying fibre content. The flow diameter of the reference mixture was SF=770 mm. The addition of SW35 and SW50 fibres did not effect significantly the value of SF, due to good plastic viscosity of the reference sample SCC mixture. The beams were deformed after 24 hours, and then cut into two or three parts of 600 mm in length. Indications I, II, III were assigned, depending on the increasing distance from the position of mixture's introduction. The influence of steel fibres type, fibres content and kind of forming beam on the flexural strength of SFRSCC are presented in Table 3.
Table 1. Composition of SFRSCC mix
CementCEM 142,5R mS„and , (0-2 mm) Aggregate (2-8 mm) Steel fibres Superplasticizer Stabilizer W/C Slump-flow
kg/m3 kg/m3 490 804 kg/m3 804 kg/m3 40-80-120 kg/m3 12.25 kg/m3 1.96 0.41 SF3
Table 2. The type of used steel fibres*
Description Length (mm) Width (mm) The cross-section Material Tensile strength (N/mm2)
SW 35/1.0 35±10% 2.30-2.95 Part of the circle Low carbon steel 880±15%
SW 50/1.0 50±10% 2.30-2.95 Part of the circle Low carbon steel 880±15%
* Remark: fibres with straight-shaped bars
Table 3. The influence of steel fibres type, fibres content and kind of forming beam on the flexural strength of SFRSCC ffl [MPa]
Type of fibres SW50 SW50 SW35
Fibres content (kg/m3) 80 120 40 80 120 40 80 120
I specimen Beam I II specimen 9.17 7.95 9.10 8.59 4.96 4.63 6.56 6.35 5.97 5.52 3.54 4.55 7.12 3.81 3.75 6.11
I specimen Beam II II specimen 8.41 10.80 8.64 11.97 4.62 4.15 7.92 8.85 7.52 9.98 3.73 4.52 6.99 3.37 4.65 6.82
I specimen Beam III II specimen 8.80 12.19 8.26 13.52 _ _ _ _ _
The scope of the study included:
• Marking the consistency class after 10 minutes of the mixing process finish,
• forming (Figurel),
• distribution of fibres with computed tomography (14 days after forming),
• marking tensile strength in bending fn (28 days after forming).
The computed tomography scanner used for testing was described in earlier authors' work [9]. 3. The results and discussion
The results of tests for beams formed in various distances from the point of formation (Fig. 1). Obtained results allowed to test the effect of the distance from the point of formation of SFRSCC on the distribution of fibres in matrix and mechanical parameters of beams. Beams of 600x150x150 mm cut from 1200x150x150 mm beams, were marked in such way that positioned closer to the point of formation is labeled as beam I, and the beam formed respectively further as beam II. Beams of 600x150x150 mm cut from 1800x150x150 mm beams, were labeled respectively as beam I, beam II and beam III, the last one was positioned the furthest from the place of mixture's introduction. In research, the 2D and 3D images of real distribution of steel fibres in tested self-compacting concrete beams were obtained due to the use of computed tomography method. Fig. 2 shows a selection of 2D and 3D images of steel fibres in tested SCC with the addition of variable content of SW50 fibres. It has been shown, that the increase in bending strength occurs with increasing content of SW35 fibres in SCC, what could have been expected. There were no significant differences in the bending force as a function of beam deflection noticed between beams I and II for SCC with steel fibres SW35. The SFRSCC mixtures and concrete with SW35 fibres are characterized by similar fibre orientation, their distribution and mechanical properties, regardless of the distance from the point of
forming the SFRSCC. There is also similar, uneven distribution of fibres in the cross-sections of all beams. The amount of SW35 fibres increases towards the bottom section of beams. The orientation of beams parallel to the direction of flow of the mixture at formation of SFRSCC is observed. Additionally, the increase in amount of the air macro-pores with the increase of fibres content in the mixture is observed. On the basis of conducted observation, it could be concluded that the hypothesis of decreasing by fibres from deaeration of SFRSCC mixture is possible, especially in places with greater density of fibres in concrete matrix. In case of beams formed from the element of 1200x150x150 mm, the effect of SW50 fibres content on tensile strength in bending (Fig. 2.) is analogous to the first series of samples with SW35 fibres. Also in these beams, there is the increase of bending force with the increase of SW50 fibres content. Additionally, there is an increase in bending force in case of beams II in relation to beams I with the increase in volume content of SW50 fibres. When the content of SW50 fibres is 120 kg/m3, the increase in bending force in beam II, located at the further distance from the place of mixture's introduction to the form, is greater by 80% than in case of beam I. In case of the SW50 fibres content in SCC of 40 kg/m3, there was no difference between the bending forces of beam I and beam II; whereas, in case of SW50 fibres content of 80 kg/m3, there was the increase in bending force in beam II by 35% in relation to beam I. The second series of measurements confirmed these trends. Figure 2 presents the flexural strength fa - yield value g curves for SFRSCC with all detected steel fibres according Viskomat XL measuring as well. In general, increasing yield value g causes linear increase of flexural strength /a of SFRSCC with high coefficient of determination.
Fig. 2. The effect of steel fibres SW50 on tensile strength in bending of beams I and II in the first (up) and second (down) measurement; X-ray 2D and 3D images of steel fibres in tested SCC; flexural strength /fl - yield value g curves for SW35 and SW50 steel fibres in SFRSCC
For the formed elements, cut from beams of 1800x150x150 mm, the effect of SW50 fibres content on tensile strength in bending of tested SFRSCC, is illustrated on Fig. 3. (up - formation and the first measurement; down -formation and the second measurement). It has also been shown the increase in bending force with the increase of SW50 fibres content in SFRSCC.
Fig. 3. The effect of steel fibres SW50 on tensile strength in bending of beams I, II and III, the first beams (up) and second beams (down).
Additionally, there was the increase in bending force for beams II and I with the increase of SW50 fibres content in tested SFRSCC. For SW50 fibres content of 120 kg/m3 in SFRSCC, there was the increase of bending force in beam III (furthest one) by 65% in relation to bending force in beam I (closest one). For SW50 fibres content of 80 kg/m3 in SFRSCC, there was no difference between bending forces in beams I, II and III. The repeated tests on beams' formation and measurements confirmed these trends. Presented tests results show significant differences in strength parameters of SFRSCC beams in relation to the point of their formation. The increase in distance of SFRSCC mixture's formation significantly improves their strength parameters. This is definitely related to the directional orientation of the fibres and their perpendicular position to the applied bending force and their parallel anchoring in bending element. Fibres as dispersed reinforcement but at the same time as directional reinforcement significantly improve concrete's mechanical parameters with their addition. Longer SW50 fibres "work" much better as directional dispersed reinforcement as a function of distance from the point of SFRSCC formation. In paper, there are no results on measurements of SFRSCC comprehensive strength due to small effect of steel fibres on this parameter, what has been confirmed in Glinicki's [11] and Brandt's [12] studies. On the basis of previous studies [13-15], the authors showed small changes in comprehensive strength with the increase of volume ratio of steel fibres in SCC.
Also, the parallel positioning of fibres was confirmed, in accordance with the direction of movement of the mixture in the form, especially in case of SW50 fibres. Only few fibres were perpendicular to this direction.
The radial arrangement of fibres in the vicinity of the corners of beams' cross-section is associated with the slower movement of the mixture close to the form's walls. This is due to higher frictional resistance occurring in those areas.
4. Summary and conclusions
The study confirms technological problems connected with uneven distribution of steel fibres in SCC matrix. At the same time, fibres are generally parallel placed to the direction of concrete mixture's flow in the form. This phenomenon intensifies in elements with longer fibers (SW50) and with the increase of their volume ratio in the mixture. However, such orientation of fibres caused the improvement of strength in bending of beams with the addition of longer fibres (SW50). The increase in flexural strength was even up to 80% in beam II in case of SW50 fibres content of 120 kg/m3, in relation to beam I, cut form the element of 1200x150x150 mm.
Similar, but somewhat smaller increase in resistance were found in beams III in relation to beams I cut from elements of 1800x150x150 mm. Obtained results may form a basis for the development of methods of designing self-compacting reinforced fibres, including the form of structural elements. The core of the problem is to determine changes in distribution of dispersed reinforcement in various structures depending on used technology of mixture's forming and its rheological properties, and also the volume and geometrical forms of steel fibres. Obtained results may also be used for modeling mechanical properties of selected structural elements.
Acknowledgements
The authors would like to thank the National Science Centre for funding this research under grant N N506 236039: Effect of forming methods SFRSCC (Steel Fibre Reinforced Self-Compacting Concrete) on the distribution and orientation of the reinforcement distributed in selected structural elements.
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