Energy Absorbing Concrete for Impact Loading R. Sri Ravindrarajah * and M. C. Lyte ** *Senior Lecturer, **Former Student, Centre for Built Infrastructure Research University of Technology, Sydney, P O Box 123, Broadway, NSW 2007, Australia R.Ravindra@uts.edu.au Abstract Two structural grades, compressive strength of 30 and 45MPa, normal weight concrete and lightweight polystyrene concrete were tested for their impact response under 75kg weight test through 462mm drop. The impact response was monitored through load-time plot and the results were used to characterise the concrete for its impact response. Peak load and contact time are considered as important measurable quantities in identifying the energy absorbing capacity of concrete. The results showed that polystyrene aggregate concrete outperformed the normal weight concrete in its impact resistance due to inherent energy absorbing quality of embedded expanded polystyrene beads. Under the tested impact loading condition, polystyrene concrete having the compressive strength of 30MPa showed 28% increase in the contact time and 18% reduction in the peak load compared to the similar grade normal weight concrete. 1. INTRODUCTION Impact properties of concrete are of interest where there is the possibility of impact loading in the foreseeable service life of a concrete structure. Wide ranges of situations exist where structures may be subject to some type of impact loading. These include many different forms of missile impact, gas explosions, construction accidents, vehicle impacts and pile driving. Impact from an object colliding with a concrete structure can be divided into hard and soft impact depending upon the relative characteristics of the projectile (or striking object) and the target (or structure). Different forms of
damage can occur during hard impact of concrete structure. The contact zone is subject to intense dynamic stresses producing crushing, shear failure and tensile fracturing. These may result in spalling and crater formation, penetration, back-face scabbing, perforation or shear failure and flexural failure of the target (Hughes (1984) and Brown and Perry (1989)) Green (1964) studied the number of blows of a ballistic pendulum which plain concrete cube specimens could withstand before reaching a no-rebound condition. This condition was chosen to indicate a definite state of damage and reported a large variability in data, much greater than usually occurred in static tests. He concluded that in standard compression tests, there is some relief of highly stressed weak zones due to creep. On the other hand under dynamic impact testing, no redistribution of stress is possible due to sudden short duration of load and inherent weaknesses, therefore, have a greater influence on impact results. The presence of various types of microcracks and air voids in concrete system and pores in the hardened cement paste component in concrete are weak zones which could initiate the growth of cracks, lowering impact strength. Generally, impact strength of concrete increases with the compressive strength and concrete with higher compressive strength resulted in lower energy absorbed per blow before cracking (Green (1964)). An ideal energy absorbing material should have a low crushing strength and capacity for large deformation. During hard impact kinetic energy is absorbed by crushing the impacted material. Bischoff et. al. (1989) experimented with the impact properties of concrete incorporating expanded polystyrene beads and reported that this concrete did exhibit similar properties to an ideal energy absorbing material. Concrete containing varying amounts of polystyrene beads were tested using a drop weight device to impact small concrete slabs of varying thicknesses. This investigation showed that the concrete containing highest percentage of expanded polystyrene beads significantly prolonged the impact period and reduced contact force. Compressive strength of concrete tested ranged from 4 to 16 MPa. The static tests showed that once peak load had reached, large
deformation followed while the load remained constant. Once concrete become compacted under the static load, the load increased until failure, similar to strain hardening effect. Polystyrene aggregate concrete did not fail by cracking which occurs in standard normal weight concrete but rather localized crushing under the head of impact tup. Hoff (1970) reported similar findings for concretes with expanded polystyrene beads. Tests were conducted using explosive to test the energy absorbing characteristic of polystyrene aggregate concrete of low compressive strengths. The results showed that this type of concrete effectively dissipated shock and absorb kinetic energy through deformation and concrete failed through localized crushing with little or no cracking. Sabaa and Sri Ravindrarajah (2000) reported impact resistance of polystyrene concrete, having the densities ranging from 1600 to 2100 kg/m 3, with and without polypropylene fibres. They concluded that the impact resistance of concrete is improved by the incorporation of expanded polystyrene aggregate. The energy absorption capacity of concrete is increased by increased level of polystyrene aggregate content and the amount of energy required to cause damage and energy dissipation increased with an increased polystyrene aggregate content. The addition of polypropylene fibres of 0.9% by weight of cement increased the impact strength of polystyrene aggregate concrete to produce first crack by 13 to 40% and to cause ultimate failure by 36% to 119%. The main purpose of this paper is to report the results of an investigation into the impact properties of two strength grades of polystyrene aggregate concrete compared to normal weight concrete. The polystyrene concrete was produced by incorporating varying amounts of expanded polystyrene beads in normal weight concrete mix.
EXPERIMENTAL DETAILS Materials and mix compositions General purpose cement, Type GP, complying with AS 3972 and low calcium fly ash were used as binder materials. Single-sized crushed basalt aggregate (20 mm and 10 mm) was used, in equal weight proportion, as coarse aggregate. River sand was used as fine aggregate. Commercially available chemically coated expanded polystyrene graded beads, having a mean diameter of 3 mm, were used. Naphthalene formaldehyde based superplasticiser was used to produce workable concrete mixes. The percentage passing the standard 4.75 and 2.36 mm sieves were 100%, and 10%, respectively, for the expanded polystyrene beads used in some mixes. Three normal weight Grades 32, 40 and 50 concrete mixes were used as reference concretes (Mixes 1, 2 and 3) and the details of the mix composition was supplied by a local ready-mixed concrete producer. Mix 4 is a normal weight concrete and Mixes 5 and 6 were polystyrene aggregate concrete containing 30% and 60% of the fine aggregate volume replaced with expanded polystyrene, respectively. Table 1 shows the compositions of the concrete mixtures studied in this investigation. Mixing of concrete and casting and curing test specimens The concrete mixtures were produced in a pan-type of mixer. All the concrete ingredients were batched by weight. Following mixing sequence was adopted to achieve proper mixing of the ingredients. Fine aggregate and coarse aggregate were added to the mixing pan with one litre of mixing water and mixed for one minute and subsequently let stand for 5 minutes. Then, cement and fly ash were added and mixed for two minutes. Remaining of water and superplasticiser were added gradually and subsequently mixed for further 1 minute. The wet mix was allowed to stand for 10 minutes and then mixed again for 1 minute. For Mixes 5 and 6, the mixing sequence was the same except the polystyrene beads were added together with water and superplasticiser. For each concrete, the following standard test specimens were cast in steel moulds: 100 mm diameter by 200 mm high cylinders (9 Nos.) and 150 mm diameter by 300 mm high cylinders (2 Nos.). The specimens were demoulded after 24 hours of casting and stored in water at 20 o C until testing. Bearing capacity of hardened concrete was determined on 150mm diameter by 100mm high concrete specimens, obtained through cutting of 150 mm diameter by 300 mm cylinder into three pats using an electric diamond
tipped concrete saw. The cylinders were cut two days before testing and the cut specimens were returned to water curing. These specimens were taken out from the curing tank after a day and allowed to air dry and capped with dental plaster prior to testing under bearing load at the age of 28 days. Testing of fresh and hardened concrete Immediately after mixing, fresh concrete samples were taken and tested for slump and unit weight in accordance with the procedures given in AS1012. At the ages of 3, 7 and 28 days, the standard cylinders were tested in uniaxial compression. For each testing age, three identical 100 mm diameter by 200mm high cylinders were tested. At 28 days, a total of nine 150 mm diameter by 100 mm high cylindrical specimens, obtained from three identical cylinders, was used for impact testing. The impact test was conducted using a drop weight rig. A 75kg drop weight called a tub, was dropped freely and impact on the bearing plate (75mm diameter by 230mm thick high carbon steel) over a concrete cylindrical specimen placed on the loading plate. Tup was made from hardened steel with a flat milled striking face. The height of the drop was maintained at 462mm giving the impact velocity of 3.01m/s. The impact load was recorded using twin light gates that read an encoded strip. All data was recorded using a 14 bit analog-to-digital analysing recorder at 0.02µs intervals for the duration of 360µs. The signal response was amplified by a DC amplifier board before being converted and stored in the transient recorder. RESULTS AND DISCUSSION Fresh Concrete Properties Table 2 shows the wet density and compressive strength at 1, 7 and 28 days for all six concrete mixtures. The wet density of Mixes 5 and 6 with expanded polystyrene beads is lower than with that for the normal weight concrete mixes without the beads. The density decreased with the increase in the polystyrene beads content. When 30% and 60% of the fine aggregate volume was replaced with the polystyrene beads, the density was reduced from 2395 to 2130 and 1900 kg/m 3, respectively. This corresponds to the density reductions of 11.1% and 21.0%, respectively. Mixes 1, 2 and 3 had collapsed slump indicating their very high workability. Mixes 4, 5 and 6, having water to cement ratio of 0.27, showed very low workability, even though an increased amount of superplasticiser was used as indicated in Table 1.
Compressive Strength of Normal Weight and Lightweight Concretes Table 2 summarises the mean cylinder strength at the ages of 1, 7 and 28 days for all six concrete mixes. The strength of concrete mixes relative to their corresponding 28-day strength is also included Table 2. As expected the increased water cement ratio from 0.27 to 0.41 showed the reduction in 28-day cylinder strength from 69.2 MPa to 32.8 MPa. Fig. 1 shows the density of concrete has significant influence on the compressive strength of concrete at 1, 7 and 28 days. Since the potential strength of polystyrene concrete is limited by its density rather than age, the difference between the lightweight and normal weight had increased with the increase in the age of concrete as seen from Fig. 1. The 1-day strength of polystyrene aggregate concrete is over 40% of its 28-day strength compared to 31% for the highstrength concrete at the corresponding age. The compressive strength after one day for the polystyrene concrete with the density of 1900 kg/m 3 is 11.8 MPa compared to 5 MPa for the normal weight concrete with the water to cementitious materials ratio of 0.49. Impact Response of Normal Weight and Lightweight Concretes Figures 2 and 3 show the typical load-time plots for normal weight concrete (Mix 2) and lightweight polystyrene concrete (Mix 6). Table 3 summarises the peak load and contact time under drop hammer tests for normal weight concretes (Mixes 1 and 2) and lightweight polystyrene aggregate concretes (Mixes 5 and 6). The statistical analysis of the multiple test results indicated that the standard error of the mean values for peak load and contact time was below 10%. Mixes 1 and 6 had comparable compressive strength of about 30MPa and Mixes 2 and 5 had the strength of about 45MPa. The mean contact time for Mix 1 is 1.10µs compared to 1.41µs for Mix 6 under the same impact loading. Therefore, for the same grade concrete, polystyrene concrete showed an increase of 28% in contact time. Mixes 2 and 5 having higher grade concrete showed similar trend. The mean contact time increased from 0.99µs to 1.15µs, an increase of 16% of contact time for polystyrene aggregate concrete. The results also indicate that decrease in the density of polystyrene concrete caused the improved contact time. The reduction in the density of polystyrene concrete from 2130 kg/m 3 to 1900 kg/m 3, increased the mean contact time from 1.15µs to 1.41µs (an increase of 23%). The results given in Table 3 shows that the mean peak load for low strength grade polystyrene concrete (Mix 6) was 386 kn compared to 472 kn for a
similar strength grade normal weight concrete (Mix 1). Therefore, under the same impact loading a structural grade (about 30MPa) polystyrene concrete outperformed the normal weight concrete of similar strength as evident from 18% reduction in peak load together with 28% increase in contact time. For the 45 MPa concrete, once again polystyrene concrete showed peak load reduction from 522 kn to 463 kn (11% reduction) together with 23% increase in the contact time. The results obtained in this study are consistent with those reported by Bischoff et. al. (1989) who concluded that increasing polystyrene content improves impact performance through prolonged impact duration thus reducing the peak impact force. Under impact loading the visual damage varied according to the compressive strength and polystyrene aggregate content. For Mix 1 specimens cracks appeared around the edge of the bearing plate. Usually 3 to 5 cracks evenly distributed and radiating outwards from the edges of the bearing plate appeared. The indentation caused by impact is insignificant to measure. The high grade normal weight concrete (Mix 2) did not show any visible signs of cracking or damage however, a slight indentation under the bearing plate was noted. Concrete test specimens with Mix 5 displayed no visible damage and a slight indentation under the bearing plate was noted. For the low grade polystyrene concrete (Mix 6) specimens cracks were evident similar to those noted with Mix 1. A slight indentation was noted under the bearing plate. CONCLUSION The impact response of structural concrete having 30MPa and 45MPa normal weight and lightweight (containing expanded polystyrene beads) were studied under drop weight test. The results clearly showed the polystyrene concrete out performed the normal weight concrete under impact due to energy absorbing capacity of the expanded polystyrene beads. The impact response of the polystyrene concrete was increased with the increase in the polystyrene content. Under the tested impact loading condition, polystyrene concrete having the compressive strength of 30MPa showed 28% increase in the contact time and 18% reduction in the peak load compared to the similar grade normal weight concrete.
REFERENCES Bischoff, P. H., Yamura, K., and Perry, S. H. (1989), Polystyrene aggregate concrete subjected to hard impact, Proc. of the Institution of Civil Engineers, Part 2, Vol. 89, London, pp. 225-239. Brown, I. C., and Perry, S. H. (1989), Development of a new assessment method, Part B, Assessment of impact damage caused by dropped objects on offshore structures, HMSO, London, pp. 79-151. Green, H. (1964), Impact strength of concrete, Proc. of the Institution of Civil Engineers, Vol. 28, London, pp. 383-396. Hoff, G. C. (1970), New applications for low density concrete, Lightweight Concrete, ACI Special Publication SP-29, USA, pp. 181-220. Hughes, G. (1984), Hard missile impact on reinforced concrete, Nuclear Engineering and Design, Vol. 66, pp. 22-36. Sabaa, B. A., and Sri Ravindrarajah, R. (2000), Impact resistance of polystyrene aggregate concrete with and without polypropylene fibres, Proc. of the Second International Symposium on Structural Lightweight Aggregate Concrete, Norway, 2000, pp. 719-728. Table 1: Mix composition in kg/m 3 of the Standard and Polystyrene Aggregate Concretes Materials Mix 1 Mix 2 Mix 3 Mix 4 Mix 5 Mix 6 Cement (C) 260 350 430 450 450 450 Fly Ash (F) 100 100 110 102 102 102 20mm Basalt 700 700 700 - - - 10m Basalt 280 280 280 943 943 943 River sand 820 760 670 771 540 309 Free water (W) 175 180 185 140 140 140 Superplasticiser (l) 3.6 4.5 5.4 11.0 11.0 11.0 W/(C + F) 0.49 0.41 0.35 0.27 0.27 0.27
Mix Table 2: Density and Compressive Strength (MPa) of Hardened Concrete W/C Ratio Wet density (kg/m 3 ) 1-day (Rel. to 28d strength) 7-day (Rel. to 28d strength) 28-day 1 0.49 2325 5.0 (15%) 19.3 (59%) 32.8 2 0.41 2360 11.0 (26%) 28.9 (64%) 45.0 3 0.35 2365 14.8 (28%) 38.7 (73%) 53.2 4 0.27 2395 21.1 (31%) 50.9 (74%) 69.2 5 0.27 2130 18.2 (42%) 38.5 (90%) 43.0 6 0.27 1900 11.8 (41%) 21.8 (75%) 29.0 Table 3: Peak load and contact time under impact loading Mix Density 28d strength No. of Mean Peak Mean contact (kg/m 3 ) (MPa) tests Load (kn) time (µs) 1 2325 32.8 8 471 + 17 1.10 + 0.06 6 1900 29.0 5 386 + 18 1.37 + 0.09 2 2360 45.0 6 522 + 18 0.99 + 0.03 5 2130 43.0 5 463 + 31 1.15 + 0.16 Cylinder strength (MPa) 80 28-day 60 40 7-day 20 1-day 0 1800 1900 2000 2100 2200 2300 2400 2500 Density (kg / cu. m.) Fig 1 Cylinder strength of standard and polystyrene concretes
Load (kn) 600 500 400 300 200 100 0 38.0 38.5 39.0 39.5 40.0 Time (microseconds) Fig 2 Impact response of polystyrene aggregate concrete (1900kg/m 3 ) Load (kn) 600 500 400 300 200 100 0 38.0 38.5 39.0 39.5 40.0 Time (microseconds) Fig 3 Impact response of normal weight concrete (2360kg/m 3 )