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Experimental investigations on mechanical and radiation shielding properties of hybrid lead–steel fiber reinforced concrete
Nuclear Engineering and Design 239 (2009) 1180–1185
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Experimental investigations on mechanical and radiation shielding properties of hybrid lead–steel fiber reinforced concrete Akanshu Sharma a,∗ , G.R. Reddy a , L. Varshney b , H. Bharathkumar c , K.K. Vaze a , A.K. Ghosh a , H.S. Kushwaha a , T.S. Krishnamoorthy c a b c
Reactor Safety Division, HS&EG, BARC, Mumbai 400085, India Isomed, Radiation Technology Development Section, BARC, Mumbai 400085, India Structural Engineering Research Centre, Chennai 600113, India
a r t i c l e
i n f o
Article history: Received 1 December 2008 Received in revised form 20 February 2009 Accepted 24 February 2009
a b s t r a c t This paper summarizes the results from the investigations carried out on fiber reinforced concrete with steel fibers, lead fibers and a combination of the two (hybrid fibers). The intent of this research was to investigate the effect of the two types of fibers on mechanical and radiation shielding properties of concrete. Compressive strength, split tensile strength and flexural toughness were among the mechanical properties investigated and radiation shielding to gamma rays was investigated by comparing the attenuation provided by different types of concrete against each other and against blank readings without attenuation. The results clearly showed that the hybrid fibers showed a significant enhancement in both mechanical and radiation shielding properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Fiber reinforced concrete (FRC) is a much-investigated material in the recent past. Fibers of various shapes and sizes produced from steel, plastic, glass, and natural materials are used. The glass fibers are mainly used for precast panels, small containers, sewer pipe, thin concrete shell roofs, wall plaster for concrete block, architectural cladding, etc. Carbon fibers are used for single and double curvature membrane structures, boat hulls, scaffold boards. Polypropylene and nylon fibers are used for foundation piles, prestressed piles, facade panels, etc. and the natural fibers for roof tiles, corrugated sheets, pipes, silos and tanks. However, steel fiber is the most commonly used of all the fibers for structural applications which include, seismic-resistant structures, bridge decks, cellular concrete roofing units, pavement overlays, concrete pipe, airport runways, pressure vessels, tunnel linings, ship-hull construction, etc. (Harajli and Rteil, 2004; Soreli et al., 2006; Choi et al., 2007; Climent and Orial, 2008). Steel fiber reinforced concrete (SFRC) is a concrete mix that contains discontinuous, discrete steel fibers dispersed randomly and uniformly distributed. Extensive research and developments have been carried out to study the effects of steel fibers on the mechanical properties of cement mortar and concrete (Bentur and Mindess, 1990; Wafa and
∗ Corresponding author. Tel.: +91 22 25591530. E-mail addresses: [email protected], [email protected] (A. Sharma). 0029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2009.02.017
Ashour, 1992; Kwak et al., 2002; Chen, 2004; Ghosh et al., 2007). Also, several numerical methods have been developed to predict the behavior of reinforced fibrous concrete members (Henager, 1977; Swamy and Al-Taan, 1981; Hassoun et al., 1989; Rossi, 2001; Cucchiara et al., 2004; Shannag et al., 2007). The addition of steelfibers significantly improves many of the engineering properties of mortar and concrete, notably impact strength and toughness, flexural strength, fatigue strength, tensile strength and the ability to resist cracking and spalling. Effect of addition of steel-fibers on compressive strength ranges from negligible to marginal and sometimes up to 25% as reported by Balaguru and Shah (1992). A typical load–deflection behavior of plain and fiber reinforced concrete from Hanna (1977) is shown in Fig. 1 that shows an increase in the abovementioned mechanical properties. The quality and quantity of steel fibers influence the mechanical properties of concrete. Ordinary concrete contains numerous microcracks and the rapid propagation of these microcracks under applied stress is responsible for the low tensile strength of the material. Unlike plain concrete, a fiber reinforced concrete specimen does not break immediately after initiation of the first crack as shown in Fig. 2 (Johnston, 1980). This has the effect of increasing the work of fracture, which is referred to as toughness and is represented by the area under the load–deflection curve. The benefits of using steel fibers become apparent after concrete cracking because the tensile stress is then redistributed to fibers as shown in Fig. 3 (Liu, 2006). Lead is known to be an excellent radiation shielding material and is widely used for the same. In nuclear reactors, heavy weight concrete is popularly used for shielding in areas such as calendria
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2. Research significance Induction of steel fibers in concrete enhances almost all of its mechanical properties. However any effort to investigate the radiation shielding properties of the same is unknown. Lead is a well-known radiation shielding material and an investigation on the radiation shielding properties of concrete with lead fibers is worth an effort. The research conceived with an idea to replace heavy weight concrete in high radioactive areas in the reactor or other nuclear facilities with another concrete based material that will, in addition to good shielding properties, provide enhanced mechanical properties such as toughness, ductility, etc. Such a material can solve the problem of shielding as well as structural safety in the case of events like earthquakes. This research therefore focuses on the following aspects: Fig. 1. Typical load–deflection behavior of plain concrete steel FRC (Hanna, 1977).
vault and other high active areas (Glasstone and Sesonke, 1998; Topcu, 2003; Basyigit, 2006; Pugachev and Popenko, 1999; Gusev, 1956). Also, boron loaded high density concrete is sometimes used as neutron attenuator (Glasstone and Sesonke, 1998). It is well known that concrete is an excellent shielding material that possesses the needed characteristics for both neutron and gamma ray attenuation. But, as seen above, plain concrete is inherently brittle and possess much less toughness as compared to fiber reinforced concrete. This work is an attempt to study the characteristics of concrete with different proportions and mixes of steel fibers, lead fibers and hybrid steel and lead fibers, from mechanical and radiation shielding point of view. The mechanical and radiation shielding properties of different types of concrete were tested and compared.
(i) Mechanical and radiation shielding properties of steel FRC. (ii) Mechanical and radiation shielding properties of lead FRC. (iii) Mechanical and radiation shielding properties of hybrid steel and lead FRC. Any investigation on durability aspects of the same is out of scope of this work. 3. Experimental program 3.1. Concrete mixes The concrete was prepared with Ordinary Portland Cement of 53 grade, river sand available at Chennai having a fineness modulus 2.73 and locally available blue granite crushed stone aggregates of size 20 mm with a dry weight of 1550 kg/m3 and fineness modulus of 7.15. The ratio of cement:sand:coarse aggregate was kept
Fig. 2. Mechanism of increasing flexure toughness of SFRC (Johnston, 1980; Liu, 2006).
Fig. 3. (a) Steel fibers (Dramix 30) and (b) lead fibers.
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Table 1 Series of mixes with percentage of fibers. Series
Steel fibers
Pure lead fibers
Alloy (Pb + 4% Sb) fibers
S1 (control) S2 S3 S4 S5 S6
0% 1.0% 0% 0% 0.5% 0.5%
0% 0% 3% 0% 3% 0%
0% 0% 0% 3% 0% 3%
as 1:2:2.5 with a water/cement ratio of 0.47. To obtain the given degree of workability, chemical admixture, sulphonated naphthalene formaldehyde (viz., CONPLAST SP 430) was used in a quantity of 0.3–1% depending on the type of mix. In general there was a loss in workability when fibers were added in agreement with earlier observations (Endgington et al., 1974). Hence, the dosage of SP 430 was varied for different fiber volume fractions to obtain similar workability. 3.2. Fibers The types of fibers with the specified percentage were used to make different kinds of concrete as given in Table 1. Steel fibers, pure lead fibers and fibers made of an alloy of lead + 4% antimony (Pb + 4% Sb) were used. In general lead FRC mix required higher paste content so as to coat all the fibers effectively leading to workable mix and hence cement of 400 kg/m3 was used in the present study. Trial concrete mixes were carried out with different percentage of lead fibers to arrive at maximum possible lead fibers content. Since the lead fibers are very flexible (bend round the aggregate while mixing) and has higher density, maximum fiber content of 3% could be achieved without balling with super-plasticizer dosage 0.3–1% to obtain workable concrete mixes. Since, a high percentage of lead fibers were added, the percentage of steel fibers was kept low, i.e. at 0.5%. However, higher percentages of fibers (5–20%) can be used in form of slurry infiltrated concrete (Krishnamoorthy et al., 1992; Bharatkumar et al., 2008). Moreover recent investigation on selfcompacting concrete with fiber is reported (Sahmaran et al., 2005; Cauberg, 2005; Barros, 2007) which can be used for shell structures, and structures with highly reinforced section where vibration is difficult. The steel fibers that were used in the present investigation were procured from M/s N.V Bakaert S.A. These fibers were high carbon wire fibers with hooked ends, commercially known as DRAMIX 30 (Fig. 3(a)) with a length of 30 mm and a diameter of 0.45 mm. The ultimate tensile strength of the fibers used was 2000 MPa (from the data supplied by the manufacturers). Lead fibers and alloy fibers were 1 mm in diameter and 50 mm in length (Fig. 3(b)) with a Young’s modulus of 16.6 GPa and a tensile strength of around 880 kPa. Concrete was mixed in a tilting type mixer machine. Lead and steel fibers were added to the concrete as it was getting mixed in the mixer machine. Care was taken to see that concrete was properly placed and compacted using a table vibrator. The sides of the mould were removed 24 h after casting and the test specimens cured in water.
2. Split tensile strength tests using cylinders of 100 mm diameter and 200 mm height. 3. Flexural toughness tests using prisms of 100 mm × 100 mm × 400 mm. The compressive strength and split tensile strength tests were conducted using a 1000 kN Universal Testing Machine (UTM). To determine the flexural strength and toughness, four-point bending test was carried out as per ASTM C1018. A 2500 kN servo-controlled UTM was used for testing the specimens under displacement control at 0.01 mm/min. The displacement was measured using ±5 mm LVDT. The load and displacement values were recorded online. For repeatability, three samples of each series were tested for every property. 3.4. Tests for determination of radiation shielding properties To determine the attenuations of different types of concrete, the tests were carried using cylinders of 100 mm diameter and 200 mm height. The schematic of test setup is shown in Fig. 4. A 15 mm diameter hole was drilled till centre of the concrete cylinder. A cerric-cerrous sulphate dosimeter was lowered through the hole so that it seats in the centre of the specimen. The dosimeter can measure up to 50 kGy using electrochemical cell method. The specimen was subjected to a uniform Cobalt 60 gamma radiation from the sides providing a dose rate of 3 kGy/h (3 kilo Gray/h, 1 kGy = 107 ergs/g of absorbed radiation dose by the material) using Gamma Chamber 5000 at Isomed, BARC, made by Board of Radiation & Isotope Technology (BRIT). Blank dosimeters were exposed in the chamber without attenuation at same position and height of the specimen dosimeter. For repeatability, two samples of each series were tested of each series. 4. Observations, inferences and discussions 4.1. Mechanical properties The mechanical properties of various types of concrete as obtained from the tests are listed in Table 2. It is seen that the density of plain concrete and steel fiber reinforced concrete is practically the same; the density of concrete is increased to the extent of 8–13% with the addition of lead
3.3. Tests for determination of mechanical properties The following tests were conducted on fiber reinforced concrete samples with different types and proportions of lead fibers: 1. Compressive strength tests using cubes of 100 mm × 100 mm × 100 mm.
Fig. 4. Test specimen for studying the radiation shielding properties.
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Table 2 Mechanical properties of different series. Series
Density (kg/m3 )
Compressive strength (MPa)
Split tensile strength (MPa)
S1 S2 S3 S4 S5 S6
2450 2469 2715 2707 2709 2777
40.81 45.56 40.01 41.15 45.33 47.28
2.88 5.10 3.52 2.85 3.97 3.49
fibers and in the case of hybrid lead–steel fiber concrete. This is expected considering the low fraction of steel fibers and relatively high percentage of lead fibers along with the high density of about 11.3 g/cm3 for lead. While comparing the other properties of different series, it is found that the compressive strength and split tensile strength of concrete with pure lead and alloy fibers namely series S3 and S4 are practically similar to that of plain concrete, whereas there is a marginal increase in the flexural strength of S3 and S4 when compared with S1. The compressive strength, split tensile strength and the flexural strength of steel fiber reinforced concrete show a significant increase as compared with plain concrete, in sync with well-established results by previous researchers. However, the most important thing to note from Table 2 is the comparison of hybrid fiber concrete series S5 and S6 with steel FRC series S2. It is very clear that a mixture of lead and alloy fibers with a small quantity of steel fibers lead to mechanical properties very close to that of steel FRC. Fig. 5 shows the load–deflection behavior of series S1 and Fig. 6 shows that for series S2. Three specimens for each series were tested. As can be seen, Figs. 5 and 6 combined look very similar to that shown in Fig. 1. It is clear that the addition of steel fibers increase the flexural strength and toughness both significantly. Fig. 7 shows the load–deflection behavior of series S3 and Fig. 8 shows that for series S4. Again, three specimens for each series were tested. If we compare Figs. 7 and 8 with Fig. 5, we can observe that by adding only lead fibers or only alloy fibers do not increase the flexural strength or toughness to any significant extent. Therefore, only from mechanical properties point of view, it can be inferred
Fig. 5. Load–deflection behavior of S1.
Flexural strength (MPa) Cracking
Peak
3.60 4.77 4.06 4.04 4.55 4.63
3.77 4.84 4.24 4.71 5.27 4.82
Fig. 6. Load–deflection behavior of S2.
that addition of lead fibers or lead alloy fibers do not provide any beneficial effect to the concrete. Fig. 9 shows the load–deflection behavior of series S5 and Fig. 10 shows that for series S6. Again, three specimens for each series were tested. Figs. 9 and 10 are comparable with Fig. 6. We can observe that by adding a combination of lead fibers with steel fibers or lead–antimony alloy fibers with steel fibers do increase the flexural
Fig. 7. Load–deflection behavior of S3.
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Fig. 10. Load–deflection behavior of S6. Fig. 8. Load–deflection behavior of S4.
strength and toughness both in a range quite comparable to that of steel fiber reinforced concrete. Therefore, from mechanical properties point of view, it can be inferred that addition of lead fibers or lead alloy fibers in combination with steel fibers do provide the beneficial effect to the concrete. 4.2. Radiation shielding properties Table 3 summarizes the results of radiation shielding tests carried out on the concrete cylinders. It can be seen that as compared to plain concrete, steel fiber concrete does not provide any additional attenuation to gamma rays, whereas addition of lead fibers and alloy fibers significantly increase the attenuation of the concrete. Also, the concrete with lead or alloy in combination with steel fibers provides significantly higher attenuation to gamma rays as compared to plain concrete or only steel fiber concrete. It can be seen from Table 3 that the average percentage attenuation provided by concrete with lead and steel fibers is up to 36%, which is around 50% more than the attenuation provided by plain or steel fiber reinforced concrete. This proves the efficiency of con-
crete with lead and steel fibers in providing excellent radiation shielding. 5. Conclusions This work aimed at studying a special type of concrete that has both lead and steel fibers from the point of view of both mechanical properties and radiation shielding properties. The following sound conclusions can be drawn from the experimental results of this work: 1. Steel fibers increase all of the mechanical properties of the concrete studied in this work but do not enhance the radiation shielding properties compared to plain concrete. 2. Lead and alloy fibers do not enhance the mechanical properties of the concrete to which they are added to, but very significantly increase the radiation shielding properties as compared to plain concrete. 3. The concrete added with both lead/alloy and steel fibers display a remarkable increase in both the mechanical properties as well as radiation shielding properties. On the one hand, this type of concrete possesses excellent flexural toughness, compressive and tensile strength quite comparable to that of steel fiber reinforced concrete. On the other hand, it also displays significant increase in radiation shielding properties that are up to 50% higher compared to plain concrete. The major outcome of this research from the point of view of application is that hybrid lead–steel fiber reinforced concrete has a high potential to be brought up as an excellent special concrete that can be used for high active areas. This material possess high attenuation combined with enhanced mechanical properties such as toughness, ductility, etc. and can therefore prove to be a better structural concrete type from seismic engineering point of view as well. 6. Scope limitations of this work and recommendations for future work
Fig. 9. Load–deflection behavior of S5.
This work highlighted the possible high usefulness of hybrid lead fiber reinforced concrete in cases where both good mechanical and radiation shielding properties are needed. However, the work is not exhaustive and many more areas need to be explored. A few limitations of this work and recommendations for future work are listed below:
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Table 3 Summary of radiation shielding test results. Series (1)
S1 S2 S3 S4 S5 S6
Sample no. (2)
Average blank dosimeter reading (3)
Attenuated dosimeter reading (4)
Percentage reduction (5) = ((3)–(4)/(3)) × 100
1 2 1 2 1 2 1 2 1 2 1 2
25.3 27.5 25.3 26.4 25.3 27.5 27.5 26.4 25.3 26.4 27.5 26.4
19.18 20.78 19.75 19.41 17.1 17.56 17.45 18.36 16.87 18.01 17.45 16.94
24.18972 24.43636 21.93676 26.47727 32.41107 36.14545 36.54545 30.45455 33.32016 31.7803 36.54545 35.83333
1. This work aimed at only short-term performance of all types of concrete. No durability studies were made under this work. To completely study the usefulness of hybrid lead fiber reinforced concrete, durability studies need to be performed in order to study the long-term behavior of the lead and steel fibers in concrete. Durability studies shall be conducted in order to verify the compatibility of fibers with concrete in long run especially with respect of corrosion and thermal expansion of lead and thermal expansion since the thermal expansion coefficient of lead is almost thrice to that of concrete as against steel whose thermal coefficient of expansion is practically equal to that of concrete. 2. Only one type of composition, with one set of percentage of fibers for all types of concrete was used. Different percentage of fibers and there combinations thereof shall be tested in order to optimize the composition of concrete. It is also suggested by the research that the mix can be designed for a particular application depending on the requirements, e.g. if shielding requirements are higher than the structural requirements, a higher percentage of lead fibers compared to steel fibers may be used and vice versa. 3. Only one size and shape of fibers were used. Different size and shape of fibers may be tested to optimize the design. References Balaguru, N., Shah, S.P., 1992. Fiber Reinforced Cement Composites. McGraw-Hill, New York, pp. 179–214. Barros, J., 2007. Lightweight panels of steel fiber-reinforced self-compacting concrete. Journal of Materials in Civil Engineering 19 (April (4)), 295– 304. Basyigit, C., 2006. The physical and mechanical properties of heavyweight concretes used in radiation shielding. Journal of Applied Sciences 6 (4), 762–766. Bentur, A., Mindess, S., 1990. Fiber Reinforced Cementitious Composites. Elsevier, UK. Bharatkumar, B.H., Udhayakumar, V., Balasubramanian, K., Krishnamoorthy, T.S., Lakshmanan, N., Akanshu Sharma, Reddy, G.R., 2008. Investigations on the lead–steel hybrid fibre reinforced concrete. In: Seventh International RILEM Symposium on Fibre Reinforced Concrete: Design and Applications, BEFIB 2008, Chennai, September, pp. 1089–1097. Cauberg, N., 2005. Fiber reinforced self-compacting concrete. In: SCC 2005, 1st International Symposium on Design, Performance and Use of Self-Consolidating Concrete, RILEM Publications, pp. 481–490. Chen, S., 2004. Strength of steel fiber reinforced concrete ground slabs. In: Proceedings of the Institute of Civil Engineers, Structures and Buildings (157), SB2, pp. 157–163. Choi, K., Taha, M.M.R., Park, H., Maji, A.K., 2007. Punching shear strength of interior concrete slab-column connection reinforced with steel fibres. Cement & Concrete Composites 29, 409–420.
Average percentage reduction (6) 24.31 24.21 34.28 33.50 32.55 36.19
Climent, M., Orial, A., 2008. Experimental and analytical study of SFRC segmental tunnel lining in service conditions. In: Seventh International RILEM Symposium on Fibre Reinforced Concrete: Design and Applications, BEFIB 2008, Chennai, September, pp. 1055–1064. Cucchiara, C., Mendola, L.L., Papia, M., 2004. Effectiveness of stirrups and steel fibres as shear reinforcement. Cement & Concrete Composites 26, 777–786. Endgington, J., Hannant, D.J., Williams, R.I.T., 1974. Steel fiber reinforced concrete. Current Paper CP 69/74 Building Research Establishment Garston Watford. Ghosh, S., Bhattacharjya, S., Chakraborty, S., 2007. Compressive behaviour of shortfibre-reinforced concrete. Magazine of Concrete Research 59 (October (8)), 567–574. Glasstone, S., Sesonke, A., 1998. Nuclear Reactor Engineering—Reactor Design Basics, vol. 1., 4th edition CBS Publishers, New Delhi, pp. 360–361. Gusev, N.G., 1956. Reference Book on Radioactive Radiations and Protections. Medgiz, Moscow. Hanna, A.N., 1977. Steel Fiber Reinforced Concrete Properties and Resurfacing Applications. Portland Cement Association, Skokie, IL, Report RD049.01P. Harajli, H.M., Rteil, A.A., 2004. Effect of confinement using Fibre—Reinforced Polymer or Fiber Reinforced Concrete on Seismic performance of gravity load designed columns. ACI Structural Journal, vol. 101, January–February, pp. 47–56. Hassoun, M.N., Behdad, H., Sawalma, A., 1989. Ultimate strength of reinforced lightweight fibrous concrete beams. In: International Conference on Recent Developments in Fiber Reinforced Cements and Concretes, Cardiff, pp. 467–478. Henager, C.H., 1977. Ultimate strength of reinforced steel fibrous concrete beams. In: Proceedings of the Conference on Fiber Reinforced Materials: Design and Engineering Applications, ICE (London), pp. 151–160. Johnston, C.D., 1980. Progress in concrete technology. In: Malhotra, V.M. (Ed.), CANMET, Ottawa, pp. 452–504. Krishnamoorthy, T.S., Balasubramanian, K., Parameswaran, V.S., 1992. Slurry Infiltrated Fibrous Concrete (SIFCON), Civil Engineering and Construction Review, July, pp. 31–35. Kwak, Y.K., Eberhard, M.O., Kim, W.S., Kim, J., 2002. Shear strength of steel fiber reinforced concrete beams without stirrups. ACI Structural Journal 99 (July–August (4)). Liu, C., 2006. Seismic Behavior of Beam-Column Joint Subassemblies Reinforced with Steel Fibers. PhD Thesis. Department of Civil Engineering, University of Canterbury Christchurch, New Zealand. Pugachev, G.D., Popenko, V.A., 1999. Radiation protection of the electron linear accelerator in production of 99Mo, Problems of Atomic Science and Technology. Nuclear Physics Research 4 (35), 104. Rossi, P., 2001. Ultra-high-performance fiber-reinforced concretes. Concrete International 33 (12), 46–52. Sahmaran, M., Yurtseven, A., Yaman, I.O., 2005. Workability of hybrid fiber reinforced self-compacting concrete. Building and Environment 40, 1672–1677. Shannag, M.J., Farsakh, G.A., Dyya, N.A., 2007. Modeling the cyclic response of fiber reinforced concrete joints. Engineering Structures 29 (11), 2960–2967. Soreli, L.G., Meda, A., Plizzari, G.A., 2006. Steel fiber concrete slabs in grade: a structural matter. ACI Structural Journal 103 (4), 551–558. Swamy, R.N., Al-Taan, S.A., 1981. Deformation and ultimate strength in flexure of reinforced concrete beams made with steel fiber concrete. ACI Journal Proc 78, 395–405. Topcu, I.B., 2003. Properties of heavyweight concrete produced with barite. Cement and Concrete Research 33, 815–822. Wafa, F.F., Ashour, S.A., 1992. Mechanical properties of high strength fibre reinforced concrete. ACI Materials Journal 89 (5), 445–455.
Contents lists available at ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Experimental investigations on mechanical and radiation shielding properties of hybrid lead–steel fiber reinforced concrete Akanshu Sharma a,∗ , G.R. Reddy a , L. Varshney b , H. Bharathkumar c , K.K. Vaze a , A.K. Ghosh a , H.S. Kushwaha a , T.S. Krishnamoorthy c a b c
Reactor Safety Division, HS&EG, BARC, Mumbai 400085, India Isomed, Radiation Technology Development Section, BARC, Mumbai 400085, India Structural Engineering Research Centre, Chennai 600113, India
a r t i c l e
i n f o
Article history: Received 1 December 2008 Received in revised form 20 February 2009 Accepted 24 February 2009
a b s t r a c t This paper summarizes the results from the investigations carried out on fiber reinforced concrete with steel fibers, lead fibers and a combination of the two (hybrid fibers). The intent of this research was to investigate the effect of the two types of fibers on mechanical and radiation shielding properties of concrete. Compressive strength, split tensile strength and flexural toughness were among the mechanical properties investigated and radiation shielding to gamma rays was investigated by comparing the attenuation provided by different types of concrete against each other and against blank readings without attenuation. The results clearly showed that the hybrid fibers showed a significant enhancement in both mechanical and radiation shielding properties. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Fiber reinforced concrete (FRC) is a much-investigated material in the recent past. Fibers of various shapes and sizes produced from steel, plastic, glass, and natural materials are used. The glass fibers are mainly used for precast panels, small containers, sewer pipe, thin concrete shell roofs, wall plaster for concrete block, architectural cladding, etc. Carbon fibers are used for single and double curvature membrane structures, boat hulls, scaffold boards. Polypropylene and nylon fibers are used for foundation piles, prestressed piles, facade panels, etc. and the natural fibers for roof tiles, corrugated sheets, pipes, silos and tanks. However, steel fiber is the most commonly used of all the fibers for structural applications which include, seismic-resistant structures, bridge decks, cellular concrete roofing units, pavement overlays, concrete pipe, airport runways, pressure vessels, tunnel linings, ship-hull construction, etc. (Harajli and Rteil, 2004; Soreli et al., 2006; Choi et al., 2007; Climent and Orial, 2008). Steel fiber reinforced concrete (SFRC) is a concrete mix that contains discontinuous, discrete steel fibers dispersed randomly and uniformly distributed. Extensive research and developments have been carried out to study the effects of steel fibers on the mechanical properties of cement mortar and concrete (Bentur and Mindess, 1990; Wafa and
∗ Corresponding author. Tel.: +91 22 25591530. E-mail addresses: [email protected], [email protected] (A. Sharma). 0029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2009.02.017
Ashour, 1992; Kwak et al., 2002; Chen, 2004; Ghosh et al., 2007). Also, several numerical methods have been developed to predict the behavior of reinforced fibrous concrete members (Henager, 1977; Swamy and Al-Taan, 1981; Hassoun et al., 1989; Rossi, 2001; Cucchiara et al., 2004; Shannag et al., 2007). The addition of steelfibers significantly improves many of the engineering properties of mortar and concrete, notably impact strength and toughness, flexural strength, fatigue strength, tensile strength and the ability to resist cracking and spalling. Effect of addition of steel-fibers on compressive strength ranges from negligible to marginal and sometimes up to 25% as reported by Balaguru and Shah (1992). A typical load–deflection behavior of plain and fiber reinforced concrete from Hanna (1977) is shown in Fig. 1 that shows an increase in the abovementioned mechanical properties. The quality and quantity of steel fibers influence the mechanical properties of concrete. Ordinary concrete contains numerous microcracks and the rapid propagation of these microcracks under applied stress is responsible for the low tensile strength of the material. Unlike plain concrete, a fiber reinforced concrete specimen does not break immediately after initiation of the first crack as shown in Fig. 2 (Johnston, 1980). This has the effect of increasing the work of fracture, which is referred to as toughness and is represented by the area under the load–deflection curve. The benefits of using steel fibers become apparent after concrete cracking because the tensile stress is then redistributed to fibers as shown in Fig. 3 (Liu, 2006). Lead is known to be an excellent radiation shielding material and is widely used for the same. In nuclear reactors, heavy weight concrete is popularly used for shielding in areas such as calendria
A. Sharma et al. / Nuclear Engineering and Design 239 (2009) 1180–1185
1181
2. Research significance Induction of steel fibers in concrete enhances almost all of its mechanical properties. However any effort to investigate the radiation shielding properties of the same is unknown. Lead is a well-known radiation shielding material and an investigation on the radiation shielding properties of concrete with lead fibers is worth an effort. The research conceived with an idea to replace heavy weight concrete in high radioactive areas in the reactor or other nuclear facilities with another concrete based material that will, in addition to good shielding properties, provide enhanced mechanical properties such as toughness, ductility, etc. Such a material can solve the problem of shielding as well as structural safety in the case of events like earthquakes. This research therefore focuses on the following aspects: Fig. 1. Typical load–deflection behavior of plain concrete steel FRC (Hanna, 1977).
vault and other high active areas (Glasstone and Sesonke, 1998; Topcu, 2003; Basyigit, 2006; Pugachev and Popenko, 1999; Gusev, 1956). Also, boron loaded high density concrete is sometimes used as neutron attenuator (Glasstone and Sesonke, 1998). It is well known that concrete is an excellent shielding material that possesses the needed characteristics for both neutron and gamma ray attenuation. But, as seen above, plain concrete is inherently brittle and possess much less toughness as compared to fiber reinforced concrete. This work is an attempt to study the characteristics of concrete with different proportions and mixes of steel fibers, lead fibers and hybrid steel and lead fibers, from mechanical and radiation shielding point of view. The mechanical and radiation shielding properties of different types of concrete were tested and compared.
(i) Mechanical and radiation shielding properties of steel FRC. (ii) Mechanical and radiation shielding properties of lead FRC. (iii) Mechanical and radiation shielding properties of hybrid steel and lead FRC. Any investigation on durability aspects of the same is out of scope of this work. 3. Experimental program 3.1. Concrete mixes The concrete was prepared with Ordinary Portland Cement of 53 grade, river sand available at Chennai having a fineness modulus 2.73 and locally available blue granite crushed stone aggregates of size 20 mm with a dry weight of 1550 kg/m3 and fineness modulus of 7.15. The ratio of cement:sand:coarse aggregate was kept
Fig. 2. Mechanism of increasing flexure toughness of SFRC (Johnston, 1980; Liu, 2006).
Fig. 3. (a) Steel fibers (Dramix 30) and (b) lead fibers.
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Table 1 Series of mixes with percentage of fibers. Series
Steel fibers
Pure lead fibers
Alloy (Pb + 4% Sb) fibers
S1 (control) S2 S3 S4 S5 S6
0% 1.0% 0% 0% 0.5% 0.5%
0% 0% 3% 0% 3% 0%
0% 0% 0% 3% 0% 3%
as 1:2:2.5 with a water/cement ratio of 0.47. To obtain the given degree of workability, chemical admixture, sulphonated naphthalene formaldehyde (viz., CONPLAST SP 430) was used in a quantity of 0.3–1% depending on the type of mix. In general there was a loss in workability when fibers were added in agreement with earlier observations (Endgington et al., 1974). Hence, the dosage of SP 430 was varied for different fiber volume fractions to obtain similar workability. 3.2. Fibers The types of fibers with the specified percentage were used to make different kinds of concrete as given in Table 1. Steel fibers, pure lead fibers and fibers made of an alloy of lead + 4% antimony (Pb + 4% Sb) were used. In general lead FRC mix required higher paste content so as to coat all the fibers effectively leading to workable mix and hence cement of 400 kg/m3 was used in the present study. Trial concrete mixes were carried out with different percentage of lead fibers to arrive at maximum possible lead fibers content. Since the lead fibers are very flexible (bend round the aggregate while mixing) and has higher density, maximum fiber content of 3% could be achieved without balling with super-plasticizer dosage 0.3–1% to obtain workable concrete mixes. Since, a high percentage of lead fibers were added, the percentage of steel fibers was kept low, i.e. at 0.5%. However, higher percentages of fibers (5–20%) can be used in form of slurry infiltrated concrete (Krishnamoorthy et al., 1992; Bharatkumar et al., 2008). Moreover recent investigation on selfcompacting concrete with fiber is reported (Sahmaran et al., 2005; Cauberg, 2005; Barros, 2007) which can be used for shell structures, and structures with highly reinforced section where vibration is difficult. The steel fibers that were used in the present investigation were procured from M/s N.V Bakaert S.A. These fibers were high carbon wire fibers with hooked ends, commercially known as DRAMIX 30 (Fig. 3(a)) with a length of 30 mm and a diameter of 0.45 mm. The ultimate tensile strength of the fibers used was 2000 MPa (from the data supplied by the manufacturers). Lead fibers and alloy fibers were 1 mm in diameter and 50 mm in length (Fig. 3(b)) with a Young’s modulus of 16.6 GPa and a tensile strength of around 880 kPa. Concrete was mixed in a tilting type mixer machine. Lead and steel fibers were added to the concrete as it was getting mixed in the mixer machine. Care was taken to see that concrete was properly placed and compacted using a table vibrator. The sides of the mould were removed 24 h after casting and the test specimens cured in water.
2. Split tensile strength tests using cylinders of 100 mm diameter and 200 mm height. 3. Flexural toughness tests using prisms of 100 mm × 100 mm × 400 mm. The compressive strength and split tensile strength tests were conducted using a 1000 kN Universal Testing Machine (UTM). To determine the flexural strength and toughness, four-point bending test was carried out as per ASTM C1018. A 2500 kN servo-controlled UTM was used for testing the specimens under displacement control at 0.01 mm/min. The displacement was measured using ±5 mm LVDT. The load and displacement values were recorded online. For repeatability, three samples of each series were tested for every property. 3.4. Tests for determination of radiation shielding properties To determine the attenuations of different types of concrete, the tests were carried using cylinders of 100 mm diameter and 200 mm height. The schematic of test setup is shown in Fig. 4. A 15 mm diameter hole was drilled till centre of the concrete cylinder. A cerric-cerrous sulphate dosimeter was lowered through the hole so that it seats in the centre of the specimen. The dosimeter can measure up to 50 kGy using electrochemical cell method. The specimen was subjected to a uniform Cobalt 60 gamma radiation from the sides providing a dose rate of 3 kGy/h (3 kilo Gray/h, 1 kGy = 107 ergs/g of absorbed radiation dose by the material) using Gamma Chamber 5000 at Isomed, BARC, made by Board of Radiation & Isotope Technology (BRIT). Blank dosimeters were exposed in the chamber without attenuation at same position and height of the specimen dosimeter. For repeatability, two samples of each series were tested of each series. 4. Observations, inferences and discussions 4.1. Mechanical properties The mechanical properties of various types of concrete as obtained from the tests are listed in Table 2. It is seen that the density of plain concrete and steel fiber reinforced concrete is practically the same; the density of concrete is increased to the extent of 8–13% with the addition of lead
3.3. Tests for determination of mechanical properties The following tests were conducted on fiber reinforced concrete samples with different types and proportions of lead fibers: 1. Compressive strength tests using cubes of 100 mm × 100 mm × 100 mm.
Fig. 4. Test specimen for studying the radiation shielding properties.
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Table 2 Mechanical properties of different series. Series
Density (kg/m3 )
Compressive strength (MPa)
Split tensile strength (MPa)
S1 S2 S3 S4 S5 S6
2450 2469 2715 2707 2709 2777
40.81 45.56 40.01 41.15 45.33 47.28
2.88 5.10 3.52 2.85 3.97 3.49
fibers and in the case of hybrid lead–steel fiber concrete. This is expected considering the low fraction of steel fibers and relatively high percentage of lead fibers along with the high density of about 11.3 g/cm3 for lead. While comparing the other properties of different series, it is found that the compressive strength and split tensile strength of concrete with pure lead and alloy fibers namely series S3 and S4 are practically similar to that of plain concrete, whereas there is a marginal increase in the flexural strength of S3 and S4 when compared with S1. The compressive strength, split tensile strength and the flexural strength of steel fiber reinforced concrete show a significant increase as compared with plain concrete, in sync with well-established results by previous researchers. However, the most important thing to note from Table 2 is the comparison of hybrid fiber concrete series S5 and S6 with steel FRC series S2. It is very clear that a mixture of lead and alloy fibers with a small quantity of steel fibers lead to mechanical properties very close to that of steel FRC. Fig. 5 shows the load–deflection behavior of series S1 and Fig. 6 shows that for series S2. Three specimens for each series were tested. As can be seen, Figs. 5 and 6 combined look very similar to that shown in Fig. 1. It is clear that the addition of steel fibers increase the flexural strength and toughness both significantly. Fig. 7 shows the load–deflection behavior of series S3 and Fig. 8 shows that for series S4. Again, three specimens for each series were tested. If we compare Figs. 7 and 8 with Fig. 5, we can observe that by adding only lead fibers or only alloy fibers do not increase the flexural strength or toughness to any significant extent. Therefore, only from mechanical properties point of view, it can be inferred
Fig. 5. Load–deflection behavior of S1.
Flexural strength (MPa) Cracking
Peak
3.60 4.77 4.06 4.04 4.55 4.63
3.77 4.84 4.24 4.71 5.27 4.82
Fig. 6. Load–deflection behavior of S2.
that addition of lead fibers or lead alloy fibers do not provide any beneficial effect to the concrete. Fig. 9 shows the load–deflection behavior of series S5 and Fig. 10 shows that for series S6. Again, three specimens for each series were tested. Figs. 9 and 10 are comparable with Fig. 6. We can observe that by adding a combination of lead fibers with steel fibers or lead–antimony alloy fibers with steel fibers do increase the flexural
Fig. 7. Load–deflection behavior of S3.
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Fig. 10. Load–deflection behavior of S6. Fig. 8. Load–deflection behavior of S4.
strength and toughness both in a range quite comparable to that of steel fiber reinforced concrete. Therefore, from mechanical properties point of view, it can be inferred that addition of lead fibers or lead alloy fibers in combination with steel fibers do provide the beneficial effect to the concrete. 4.2. Radiation shielding properties Table 3 summarizes the results of radiation shielding tests carried out on the concrete cylinders. It can be seen that as compared to plain concrete, steel fiber concrete does not provide any additional attenuation to gamma rays, whereas addition of lead fibers and alloy fibers significantly increase the attenuation of the concrete. Also, the concrete with lead or alloy in combination with steel fibers provides significantly higher attenuation to gamma rays as compared to plain concrete or only steel fiber concrete. It can be seen from Table 3 that the average percentage attenuation provided by concrete with lead and steel fibers is up to 36%, which is around 50% more than the attenuation provided by plain or steel fiber reinforced concrete. This proves the efficiency of con-
crete with lead and steel fibers in providing excellent radiation shielding. 5. Conclusions This work aimed at studying a special type of concrete that has both lead and steel fibers from the point of view of both mechanical properties and radiation shielding properties. The following sound conclusions can be drawn from the experimental results of this work: 1. Steel fibers increase all of the mechanical properties of the concrete studied in this work but do not enhance the radiation shielding properties compared to plain concrete. 2. Lead and alloy fibers do not enhance the mechanical properties of the concrete to which they are added to, but very significantly increase the radiation shielding properties as compared to plain concrete. 3. The concrete added with both lead/alloy and steel fibers display a remarkable increase in both the mechanical properties as well as radiation shielding properties. On the one hand, this type of concrete possesses excellent flexural toughness, compressive and tensile strength quite comparable to that of steel fiber reinforced concrete. On the other hand, it also displays significant increase in radiation shielding properties that are up to 50% higher compared to plain concrete. The major outcome of this research from the point of view of application is that hybrid lead–steel fiber reinforced concrete has a high potential to be brought up as an excellent special concrete that can be used for high active areas. This material possess high attenuation combined with enhanced mechanical properties such as toughness, ductility, etc. and can therefore prove to be a better structural concrete type from seismic engineering point of view as well. 6. Scope limitations of this work and recommendations for future work
Fig. 9. Load–deflection behavior of S5.
This work highlighted the possible high usefulness of hybrid lead fiber reinforced concrete in cases where both good mechanical and radiation shielding properties are needed. However, the work is not exhaustive and many more areas need to be explored. A few limitations of this work and recommendations for future work are listed below:
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Table 3 Summary of radiation shielding test results. Series (1)
S1 S2 S3 S4 S5 S6
Sample no. (2)
Average blank dosimeter reading (3)
Attenuated dosimeter reading (4)
Percentage reduction (5) = ((3)–(4)/(3)) × 100
1 2 1 2 1 2 1 2 1 2 1 2
25.3 27.5 25.3 26.4 25.3 27.5 27.5 26.4 25.3 26.4 27.5 26.4
19.18 20.78 19.75 19.41 17.1 17.56 17.45 18.36 16.87 18.01 17.45 16.94
24.18972 24.43636 21.93676 26.47727 32.41107 36.14545 36.54545 30.45455 33.32016 31.7803 36.54545 35.83333
1. This work aimed at only short-term performance of all types of concrete. No durability studies were made under this work. To completely study the usefulness of hybrid lead fiber reinforced concrete, durability studies need to be performed in order to study the long-term behavior of the lead and steel fibers in concrete. Durability studies shall be conducted in order to verify the compatibility of fibers with concrete in long run especially with respect of corrosion and thermal expansion of lead and thermal expansion since the thermal expansion coefficient of lead is almost thrice to that of concrete as against steel whose thermal coefficient of expansion is practically equal to that of concrete. 2. Only one type of composition, with one set of percentage of fibers for all types of concrete was used. Different percentage of fibers and there combinations thereof shall be tested in order to optimize the composition of concrete. It is also suggested by the research that the mix can be designed for a particular application depending on the requirements, e.g. if shielding requirements are higher than the structural requirements, a higher percentage of lead fibers compared to steel fibers may be used and vice versa. 3. Only one size and shape of fibers were used. Different size and shape of fibers may be tested to optimize the design. References Balaguru, N., Shah, S.P., 1992. Fiber Reinforced Cement Composites. McGraw-Hill, New York, pp. 179–214. Barros, J., 2007. Lightweight panels of steel fiber-reinforced self-compacting concrete. Journal of Materials in Civil Engineering 19 (April (4)), 295– 304. Basyigit, C., 2006. The physical and mechanical properties of heavyweight concretes used in radiation shielding. Journal of Applied Sciences 6 (4), 762–766. Bentur, A., Mindess, S., 1990. Fiber Reinforced Cementitious Composites. Elsevier, UK. Bharatkumar, B.H., Udhayakumar, V., Balasubramanian, K., Krishnamoorthy, T.S., Lakshmanan, N., Akanshu Sharma, Reddy, G.R., 2008. Investigations on the lead–steel hybrid fibre reinforced concrete. In: Seventh International RILEM Symposium on Fibre Reinforced Concrete: Design and Applications, BEFIB 2008, Chennai, September, pp. 1089–1097. Cauberg, N., 2005. Fiber reinforced self-compacting concrete. In: SCC 2005, 1st International Symposium on Design, Performance and Use of Self-Consolidating Concrete, RILEM Publications, pp. 481–490. Chen, S., 2004. Strength of steel fiber reinforced concrete ground slabs. In: Proceedings of the Institute of Civil Engineers, Structures and Buildings (157), SB2, pp. 157–163. Choi, K., Taha, M.M.R., Park, H., Maji, A.K., 2007. Punching shear strength of interior concrete slab-column connection reinforced with steel fibres. Cement & Concrete Composites 29, 409–420.
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