Investigation of the Properties of Concrete Modified with Various Fibres

Transcription

1 Investigation of the Properties of Concrete Modified with Various Fibres Ronald Nsubuga Mukalazi 1 Chun Qing Li 2 ABSTRACT The use of concrete modified with fibres requires understanding of its mechanical properties and behaviour, in particular in long terms. Research on the degradation of the mechanical properties under long-term aggressive environment is limited. An experimental program is carried out to study key factors, such as dosage, fibre type, and water to cement ratio that affect the mechanical properties of concrete modified with fibres. Compressive strength, flexural strength, tensile strength and flexural toughness are determined for concrete modified with fibres. Tests, including direct uniaxial tension, three point bending, toughness and compressive strength, are carried out to produce results on important properties of concrete modified by steel and synthetic fibres. Test results presented in the paper are from short term tests which are part of a comprehensive long-term experimental program on steel fibre reinforced concrete exposed to marine environment. Empirical relationships between strengths and test variables are identified from short-term tests. Based on the test results models are developed for mechanical properties of concrete modified with fibres, which are expressed as a function of basic design variables. The developed models are verified with existing data. Modification and potential application of the developed models to prediction of deterioration of fibre concrete strength in long-term marine environment exposure is explored. KEYWORDS Fibres, Properties, Durability, Long-term, Empirical. 1 School of Engineering, University of Greenwich, Kent, UNITED KINGDOM. r.n.mukalazi@gre.ac.uk 2 School of Engineering, University of Greenwich, Kent, UNITED KINGDOM. c.q.li@gre.ac.up.uk

2 Ronald Nsubuga Mukalazi and Chun Qing Li 1 INTRODUCTION The use of steel fibres for the reinforcement of concrete in civil engineering is becoming commonplace. Macro or structural steel fibres are of the order of diameter > 0.5 mm and length of mm [BS EN [2006]]. At typical bulk fibre dosages, steel fibres added to concrete improve post-cracking tensile and flexural strengths, toughness and reduce cracking sensitivity [Romualdi and Mandel [1964]; Shah and Rangan [1971]; Concrete Society [2007a]]. To date various investigations on basic contributing factors on the effect of fibre reinforced concrete (FRC) strength have been performed in different engineering applications. Kwak et al. [2002] have studied the effect of various steel fibre dosages and compressive strengths on shear strength of steel fibre reinforced concrete (SFRC) and reported a change from shear to flexural failure mode with increase in dosage. Krustulovic-Opara and Naaman [2000] have studied and reported the use of fibres in cementitious composites to self-develop prestress stresses. Plizzari et al. [2000] have found that steel and carbon FRC under cyclic tensile loads are more effective in improving fatigue life in high strength concrete (HSC) than in normal strength concrete (NSC). Banthia and Soleimani [2005] have studied and found synergy between fibres in some FRC hybrid composites in enhancement of their flexural toughness. Synthetic FRC has been studied under simulated elevated temperature settings to establish the durability and benefit of their use e.g. Sahmaran et al. [2010], Li et al. [2004]. Rodezno and Kaloush [2010] found that FRC round panel tests best captured the effect (toughness) of the addition of fibres to concrete; a value-added benefit was reported for polypropylene fibres. Bernard [2010] studied and concluded that post-cracked creep of SFRC was small in comparison with macro-synthetic FRC. Various advantages of using synthetic fibres in reducing shrinkage cracking have been investigated e.g. Najm and Balaguru [2002], Naaman et al. [2005], Voigt et al. [2004]. However, behaviour of steel fibre reinforced concrete (SFRC) in the long term is not altogether clear or certain. For SFRC exposed to marine environment, various attempts have been made to study its long term behaviour affected by aggressive environment. Kosa and Naaman [1990] reported a decrease in tensile strength and toughness after 6 months of accelerated testing under corrosive environment. Lambrechts et al. [2003] performed long-term corrosive tests on cubes, performed the wedge-splitting test, and reported no loss in strength after 18 months exposure. Zhang et al. [2001] investigated the performance of SFRC panels after 10 years of exposure in Arctic marine environment and established the chloride ion-penetration in SFRC; but strengths of the SFRC structures were not investigated. From the review of research literature, it becomes clear that a systematic comprehensive test program under controlled environment is necessary to investigate important mechanical strengths of SFRC material and structures exposed to marine environment. This paper aims to study and assess the effects of important design variables on mechanical strengths of fibre reinforced concrete (FRC), both steel and synthetic fibres, in particular the tensile strength and toughness. As part of a large ongoing long-term test program, this paper reports the cylinder compression, flexural strength and toughness for FRC under short-term conditions. Results for 28-day strengths are analysed and presented and will be compared with results from long-term marine environment exposure tests on SFRC. Based on the test results models are developed for mechanical strength of concrete modified with these fibres, which are expressed as a function of basic design variables. 2 EXPERIMENTAL PROGRAM Details of the standard test specimens are given in Table 1. The respective number of specimens for each test under normal and aggressive environment conditions and using water cement ratios (w/c) = 0.47, 0.5 and 0.6 is shown. The weights of the constituent materials per m 3 (Table 2) were calculated according to the methods described by Teychennè et al. [1997]. Crimped steel fibres with a round cross-sectional area and length 60 mm were used. 2 XII DBMC, Porto, PORTUGAL, 2011

3 Fibre Reinforced Concrete Table 1. Specimens used in the test. Strength property Specimen Dimensions Number of specimens per mix for each test (mm) Normal exposure Marine environment Cylinder compression Cylinder 150 φ Flexural strength Beams Flexural Toughness Round Panel 800 φ Direct uni-axial tension Notched Cylinder 150 φ Table 2. Mixture proportions of steel fibre reinforced concrete in kg per m 3. (add other proportions you used) Water (kg) Cement (kg) Aggregate (kg) Fibre Dosage (kg) Steel fibre dosages 35, 40 and 60 kg/m 3 were used. Macro/ mono-filamented polypropylene synthetic (HPP) fibres were added to the concrete at dosages of 2, 5, 6, 10 kg/m 3. Details of the fibres used are summarised in table 3. Dosages used are normal values recommended in the range of kg/m 3 for steel fibres [Concrete Society [2007a]] and a range of 2-12 kg/m 3 for macro-synthetic fibres [Concrete Society [2007b]] for current bulk field applications to enhance post-cracking strength properties. Table 3. Typical properties of fibres. Fibre Type Steel URW 1050 Poly-propylene Synthetic HPP45 Fibre Length Lf mm Fibre Diameter φf mm Specific gravity Lf/φf Aspect Ratio Reinforcing Index RI = Vf*Lf/φf Elastic Modulus GPa Tensile Strength GPa Ultimate Elongation % The cylinders were tested for compression. Beams were tested for flexure according to BS EN [2005] (Fig. 1) and round panels (rdp s) were tested for flexural toughness according to ASTM C 1550 [2005] (Fig.2). The notched cylinders were tested for maximum direct uniaxial tension at failure (without post-peak strain softening) according to RILEM TC 162-TDF [2003] (Fig.3). Ordinary sharp sand, 10 mm and 20 mm crushed aggregate were used. Ordinary Portland fly-ash cement was used. The experiment rational is to establish the effect of different test variables in the short-term in normal strength concrete modified with various fibres. Strengths of identical samples modified with steel fibres only and exposed to marine environment will be determined. Due to time required for long-term exposure, this paper reports initial assessment and findings for short term testing. Key variables in the test program are concrete strength, fibre type, fibre dosage, and time of exposure for long term testing. Three average characteristic concrete strengths with w/c of 0.47, 0.5, and 0.6 were designed and tested. Two commercially available fibre types were used i.e. steel (URW1060) and synthetic (Enduro HPP-45) fibres. Figure 1. Figure 2. Figure 3. Figures 1, 2, 3 Typical test set up for flexural toughness (Fig.1, Fig.2) and direct tension (Fig.3). XII DBMC, Porto, PORTUGAL,

4 Ronald Nsubuga Mukalazi and Chun Qing Li 3 RESULTS AND ANALYSIS 3.1 Flexural Strength (f fl ) and Toughness (f flt ) Figure 4 and Figure 5 show plots of load verses crack mouth-opening displacement (CMOD) for fibre reinforced concrete for w/c=0.5 and 0.6. Maximum load for concrete reinforced with various fibres at various dosages is approximately the same. Plain concrete results in Fig. 4 appear to be higher than those for the same concrete mix modified with fibres. General trend of the plots of load verses displacement are similar. Area under load verses deflection curves (to 3 mm) is representative of flexural toughness of specimens tested. Figure 4. Figure 5. Figures 4 and 5 Load verses crack mouth-opening displacement for FRC beams (CMOD). At w/c=0.5, flexural toughness (f flt ) increased by % for an increase in steel fibre volume fraction (V f ) from 0.5 to A 202 % increase in f flt for w/c=0.5 was observed for use of steel fibres at V f =0.5 in comparison with macro-synthetic polypropylene fibres (PPF) at V f =0.55. At w/c=0.6, f flt increased by % for an increase in steel fibres from V f = 0.45 to A % increase in f flt for w/c=0.6 was observed for use of steel fibres at V f =0.45 in comparison with PPF at V f =0.55. Flexural strength (f fl ) at w/c=0.5 increased by % for an increase in steel fibre volume fraction V f = 0.5 to A % increase in f flt for w/c=0.5 was observed for use of steel fibres at V f =0.5 in comparison with PPF at V f At w/c 0.6, f flt increased by 7.16 % for an increase in steel from V f =0.45 to An 8.06 % increase in f flt for w/c=0.6 was observed for use of steel fibres at V f =0.45 in comparison with PPF at V f =0.55. An increase of % in f fl was observed for use of HPP fibres at V f =0.55 for concrete mixes of w/c=0.5 and w/c=0.6. A corresponding enhancement of 5.33 % in the flexural toughness was observed. An increase of % in f fl was observed between mix 7 with steel fibre V f = 0.77 (w/c = 0.5) and mix 10 (w/c=0.6, V f = 0.77). A corresponding enhancement of % in the flexural toughness was observed. 3.2 Flexural Toughness (Tj) Figures 6, 7 and 8 depict plots for load verses displacement (to 40 mm) for concrete reinforced with steel and PPF at various dosages. At a w/c =0.47 (Fig. 6) the use of a lower steel fibre V f =0.5 in comparison with synthetic fibre V f =1.1 increased the toughness marginally by 3.98 %. For w/c=0.47, increase in V f for synthetic fibre from 0.22 to synthetic-fibre V f =0.66 lead to % increase in flexural toughness. However, for w/c=0.47 increasing V f of synthetic fibres from 0.22 to syntheticfibre V f =0.66 only lead to % increase in flexural toughness. 312 % difference in flexural toughness was realised between synthetic fibres V f =1.1 and XII DBMC, Porto, PORTUGAL, 2011

5 Fibre Reinforced Concrete At w/c =0.5 (Fig. 7) % difference in flexural toughness between steel fibres at V f =0.77 and synthetic-fibre V f =0.55 was observed % increase in flexural toughness was noted for an increase in steel fibre V f from 0.5 to 0.77 for w/c=0.5. Similarly, at w/c= 0.6 (Fig. 8) % difference in flexural toughness between steel fibres at V f =0.77 and synthetic-fibre V f =0.55 was observed % increase in flexural toughness was noted for an increase in steel fibre volume fraction from V f =0.45 to 0.77 for w/c=0.6. As a general observation, flexural toughness in concrete mix with w/c=0.5 was % greater for steel fibre V f =0.77 than steel fibre V f =0.77 in concrete mix with w/c=0.6. Figure 6. Figure 7. Figure 8. Figures 6, 7 and 8 Flexural toughness in fibre reinforced concrete (FRC) round panels. 3.3 Tensile Strength From the test data in this study (Table 4), plain concrete at w/c=0.5 appears to exhibit a superior ultimate tensile stress value in direct uniaxial tension in comparison with steel fibre reinforced concrete (SFRC). Difference in direct ultimate tensile stress (f t ) between plain concrete, w/c=0.5 and SFRC, w/c=0.5 with V f =0.77 was substantial at 22.7 %. A similar loss in tensile strength of % was observed between plain concrete w/c=0.5 and polypropylene fibre reinforced concrete (PPFRC), w/c=0.5 and V f =0.55. However, a marginal 6.6 % increase in direct tensile strength was observed between steel fibre V f =0.5 and steel V f =0.77. For the mix w/c=0.5, SFRC mix (V f =0.5) exhibited the lowest ultimate tensile strength value. Table 4. Average values of ultimate tensile cylinder strength. Mix Fibre Dosage (kg/m3) Average Ultimate Tensile Stress (MPa) SFRC SFRC PPFRC Plain MODEL DEVELOPMENT AND DISCUSSIONS Cylinder compressive strength (f cy ) for polypropylene fibre reinforced concrete (PPFRC) and steel fibre reinforced concrete (SFRC) increased marginally. f cy in PPFRC and SFRC increased with V f and reduced with increase in w/c. Multiple regression of the data for f cy, w/c and V f in this study gives equation 1 for SFRC and equation 2 for PPFRC. f cy = w/c + 5.0V f (1) f cy = w/c V f (2) BS EN [2005] computes flexural strength from equation 3 below which shows the dependence of flexural strength on maximum load sustained in the beam in flexure, XII DBMC, Porto, PORTUGAL,

6 Ronald Nsubuga Mukalazi and Chun Qing Li f fl = FL/bh 2 (3) where f fl is flexural strength, L is length of beam, b is width of beam, and h is beam depth. Maximum load the beam can carry is marginally increased by the modification of concrete with fibres. Dosages of fibres are not directly represented in equation 3 and therefore the advantage of modifying concrete with various fibres is not evident. Plain concrete in Fig. 4 appears to sustain a larger load in comparison to fibre reinforced concrete of identical mix design. JSCE-SF4 [1984] test was used to determine the flexural strength and toughness of plain concrete. Specimens are not notched therefore a more representative maximum load in flexure and flexural strength are realised However, the load is not related to the crack width. A crack width corresponding to a mid-span deflection of 3 mm will vary considerably depending on the position of the crack. Load deflection curves are therefore not completely defined by the toughness [Concrete Society [2007a]]. Concrete is notch sensitive and BS EN [2005] stipulates that beam specimens are notched at mid-span in testing for flexural strength and toughness of fibre reinforced concrete. While this ensures that fibre reinforced concrete beams crack at mid-span, it reduces the maximum load F and consequently reduces the flexural strength, equation 3. This test is recommended for testing fibre reinforced concrete. Attempts to test plain concrete for comparison with FRC specimens was impractical due to the instant failure of notched plain samples. Figures 9 and 10 show the relationship of flexural strength (f fl ) with w/c and V f determined using SFRC beams. f fl decreases with increase in w/c. f fl increases significantly with V f. Equation 4 for the multiple regression of data from this study relates f fl to w/c and V f for SFRC as, f fl = w/c V f (4) Figure 9. Figure 10. Figure 11. Figure 12. Figures 9 and 10. Relationship of flexural strength (SFRC beams) with w/c and V f. Figures 11 and 12. Relationship of flexural toughness (SFRC beams) with w/c and flexural strength with V f. Flexural toughness of FRC using beam specimens is represnted by the area under the load deflection curves to a mid-span deflection of 3 mm. Enhancement of concrete toughness by addition of fibres increases with increase in fibre dosage. Toughness in steel fibres at lower dosages of 35 kg/m 3 is more significant in comparison to toughness in synthetic fibres at mid-range dosages of 5 kg/m 3 at the same w/c. Figures 11 and 12 show the relationship of flexural toughness (f flt ) determined using SFRC beams. f flt decreases with increase in w/c. f flt increases significantly with increase in V f. Equation 5 for the multiple regression of data from this study relates f flt to w/c and V f for SFRC as, f flt = w/c V f (5) Flexural toughness (Tj) as determined by FRC round panels indicate an increase in Tj with increase in V f (Figs. 13 and 15) and decrease in Tj with increase in w/c (Figs. 14 and 16). Steel fibres tend to exhibit higher fexural toughness values in comparison with PPFRC. However from Fig. 3 above, at a central displacement between mm, FRC at higher PPF dosages of 10 kg/m 3 exhibit superior flexural toughness qualities to those of SFRC at a dosage of 60 kg/m 3. This can be attribted to the high ultimate % elongation of PPF in comparison to steel fibres (Table 3). Equation 6 for the multiple regression of data from this study relates Tj to w/c and V f for SFRC and equation 7 relates Tj to w/c andv f for PPFRC as, 6 XII DBMC, Porto, PORTUGAL, 2011

7 Fibre Reinforced Concrete (6) (7) Figure 13. Figure 14. Figure 15. Figure 16. Figures 13, 14, 15 and 16 Relationship of flexural toughness for FRC round panels with w/c and flexural toughness with V f. Tensile strength values are unclear. Bentur and Mindess [2007] reported situations in which the addition of fibres to concrete has lead to no enhancement in direct tensile strength of FRC. The importance of post cracking tensile strength as toughness in strain softening or strain hardening as area under the load deflection plot in accounting for the advantage of fibre addition is evident. Ideally tensile toughness plots should be assessed. 5 CONCLUSION It has been found that addition of steel fibres at dosages 35 kg/m 3 to concrete marginally increases the cylinder compressive strength of normal strength concrete at water cement ratios Loss in compressive strength of synthetic reinforced concrete compared to plain and steel fibre reinforced concrete is possible. Fibres at normal dosages do not significantly enhance the maximum load or flexural strength. However, at these dosages flexural toughness is considerably enhanced to values of the order of 200%. Flexural strength and toughness of plain concrete at normal strengths is low. A general trend is established showing an increase in area under the plots (toughness) with an increase in fibre dosage. Use of steel fibres instead of macro-synthetic polypropylene fibres enhances toughness more at most dosages. However, higher dosages of macro-synthetic polypropylene fibres (10 kg/m 3 ) significantly enhance flexural toughness in comparison with steel fibre reinforced concrete at a lower dosage (35-40 kg/m 3 ). Strength of FRC reduces with increase in w/c and strength increases with increase in dosage or fibre volume fraction (V f ). Relationships between strengths of FRC and key variables for short-term tests in this study are established. REFERENCES American Society for Testing and Materials, ASTM C 'Standard Test Method for Flexural Toughness of Fibre Reinforced Concrete (Using Centally Loaded Round Panel))', West Conshohocken, Pennsylvania, USA: ASTM. Banthia, N., & Soleimani, S. M 'Flexural response of hybrid fibre reinforced cementitious composites', ACI, Materials Journal, 102 [6], Bentur, A., & Mindess, S. 2007, Fibre Reinforced Cementitious Composites, Second Edition. London: Taylor and Francis. Bernard, E. S. 2010,' Influence of fibre type on creep deformation of cracked fibre-reinforced shortcrete panels', ACI, Materials Journal, 107[5], XII DBMC, Porto, PORTUGAL,

8 Ronald Nsubuga Mukalazi and Chun Qing Li BRITISH STANDARD INSTITUTION, BS EN , 'Test methods for metallic fibered concrete- Measuring the flexural strength (limit of propotionality(lop), residual)', London: BSI. Concrete Society. 2007a, 'Guidance for the Design of Steel-Fibre-Reinforced Concrete, Technical Report No. 63', Camberley, UK: The Concrete Society. Concrete Society. 2007b, 'Guidance on the use of macro-synthetic-fibre-reinforced concrete, Technical Report No. 65', Camberley, UK: The Concrete Society. JSCE-SF4 1984, 'Methods of Tests for Flexural Strength and Flexural Toughness of Steel Fibre Reinforced Concrete. Concrete Library of Japanese Society of Civil Engineers, Japanese Society of Civil Engineers (JSCE), 3, Tokyo, Japan, pp Kosa, K., & Namaan, A. E. 1990, 'Corrosion of Steel Fibre Reinforced Concrete, ACI Materials Journal, 87[1], Kwak, Y. K., Eberhard, M. O., Kim, W. S., & Kim, J. 2002, 'Shear strength of steel fibre reinforced concrete beams without stirrups, ACI, Structural Journal, 99[4], Lambrechts, A., Nemegeer, J., Vanbrabant, J., & Stang, H. 2003, 'Durability of Steel Fibre Reinforced Concrete', ACI Special Publication, 212 [42], Naaman, A. E., Wongtanakitcharoen, T., & Hauser, G. 2005, 'Influence of different fibres on plastic shrinkage cracking of concrete', ACI, Materials Journal, 102[1], Plizzari, G. A., Cangiano, S., & Cere, N. 2000, 'Post-peak behaviour of fibre reinforced concrete under cyclic tensile loads', ACI, Materials Journal, 97[2], RILEM TECHNICAL COMMITTEES 2003, 'RILEM TC 162-TDF: Test and Design method for Steel Fibre Reinforced Concrete. Round-robin analysis of the RILEM TC 162-TDF uni-axial tensile test: Part 1', Materials and Structures, 36, Rodezno, M. C., & Kaloush, K. E. 2010, 'Effect of different dosages of polypropylene fibres in thin whitetopping concrete pavements', ACI, Material Journal, 107[1], Romualdi, J. P., & Mandel, J. A. 1964, 'Tensile Strength of Concrete Affected by Uniformly Distributed and Closely Spaced Short Lengths of Wire Reinforcement', ACI Journal Proceedings, 61[6], Sahmaran, M., Lachemi, M., & Li, V. C. 2010, 'Assessing mechanical properties and microstruture of fire-damaged engineered cementitious composites', ACI, Materials Journal, 107 [3], Shah, S. P., & Rangan, V. 1971, 'Fibre Reinforced Concrete Properties', ACI, Materials Journal, 68[2], Teychennè, D. C., Franklin, R. E., & Erntroy, H. C. 1997, 'Design of normal concrete mixes', BRE Report 331. Watford, UK: Building Research Establishment Ltd. Zhang, M. H., Bilodeau, A., & Malhotra, V. M. 2001, 'Performance of Concrete after Ten Years of Exposure in the Arctic Marine Environment', ACI Special Publication, 200[39], XII DBMC, Porto, PORTUGAL, 2011