FLEXURAL PROPERTIES OF REINFORCED RECYCLED CONCRETE BEAMS

Transcription

1 FLEXURAL PROPERTIES OF REINFORCED RECYCLED CONCRETE BEAMS Ippei Maruyama (1), Masaru Sogo (1), Takahisa Sogabe (1), Ryoichi Sato (1), and Kenji Kawai (1) (1) Hiroshima University, Japan Abstract ID Number: 315 Author contacts Authors Fax Postal address Ippei Maruyama Masaru Sogo Takahisa Sogabe Ryoichi Sato Kenji Kawai Contact person for the paper: Ippei Maruyama Presenter of the paper during the Conference: Ippei Maruyama 11 Total number of pages of the paper (this one excluded): 1 zip: Hiroshima University Eng. A2-52 Higashi-hiroshima, Hiroshima, Japan zip: Hiroshima University Eng. A2-525 Higashi-hiroshima, Hiroshima, Japan zip: Hiroshima University Eng. A2-52 Higashi-hiroshima, Hiroshima, Japan Page

2 FLEXURAL PROPERTIES OF REINFORCED RECYCLED CONCRETE BEAMS Ippei Maruyama (1), Masaru Sogo (1), Takahisa Sogabe (1), Ryoichi Sato (1), and Kenji Kawai(1) (1) Hiroshima University, Japan Abstract Flexural properties of reinforced recycled concrete (RRC) beams are investigated experimentally. Their parameters are water-cement ratio, type, combination of aggregates and usage of expansive additive. Flexural properties of RRC beams, i.e. cracking moment, maximum crack spacing, maximum crack width, deflection under serviceable load and plastic deflection are discussed. Experimental results indicate the mechanics-based possibility of utilizing recycled concrete for reinforced concrete structures under the proper design as well as within the proper limit of application. Key words: Reinforced recycled concrete beam, Expansive additive, Flexural properties 1. INTRODUCTION According to the white report of 2 by ministry of the environment [1], concrete waste amounted to 35Mt and 1.3Mt out of them are disposed. This indicates that the 9% of concrete waste is recycled in Japan. But almost all the recycled use of concrete waste is for pavement base or back filling for retaining wall, which does not necessarily require high performance compared with structural concrete. The limited usage for concrete waste is attributable to not only unclear quality of the original concrete but also low and scattering quality due to high porosity and impurity. But reuse of recycled aggregate to pavement base will decrease in near future. Consequently utilization to structural concrete should be indispensable in order to enhance the rate of reuse as well as to compensate for lack of natural aggregate. In utilizing recycled aggregate as a structural aggregate, the inferiority in concrete quality is a major concern. The main reason is that recycled aggregate contains significant amount of porosity in clinging mortar or paste. Some studies concerning concrete and reinforced concrete (RC) made of recycled aggregate have been performed [2-]. However, those are not sufficient for making clear behavior of structural members leading to establishing design method. The aim of this contribution is to investigate the applicability of recycled aggregate containing high amount of mortar or cement paste as a structural aggregate by comparing the Page 1

3 flexural properties of RC members produced from recycled aggregate concrete with those of RC members with virgin concrete. 2. MATERIALS AND SPECIMENS Ordinary Portland cement is used for all the specimens. Virgin coarse aggregate is crushed hard sandstone from Yamaguchi whose absorption is.1% and density is 2.8 g/cm 3, and virgin fine aggregate is land sand from Ogasa whose adsorption is 1.% and density is 2.57 g/cm 3. Recycled coarse aggregate is producud in jaw-crusher and impact-crusher from building concrete. This has absorption of.18% and density of 2. g/cm 3 with 3.% mortar in weight. Recycled fine aggregate which is produced by the same way as that of recycled coarse aggregate has adsorption of 12.5% and density of 2.23 g/cm 3 with 19.% cement in weight. The original concrete structure from which recycled coarse and fine aggregates are -years old building and the members of the structure are located under the ground. The mixture proportions are listed in Table 1. Parameters for concrete are mainly watercement ratio (W/C) and usage of recycled aggregate, i.e. virgin coarse and fine aggregate (), recycled coarse aggregate and virgin fine aggregate (), and recycled coarse and fine aggregate (). As to specimens with water-cement ratio of.5, specimens to which lime type expansive additive (EX) is added are also prepared aiming for enhancement of RC performance. The compressive strength as well as Young s modulus of cylinder concrete with the size of φ1 mm 2 mm was measured according to JIS A 118. Spilitting tensile strength test of specimens with the size of was φ15 mm 2 mm was conducted at the ages of 28 days and age at loading. Averaged values among three specimens were evaluated at each age. After Table 1: Mixture proportions of concrete Concrete W/C /a Unit Weight kg/m 3 fc (W/B) (%) W C Ex *1 S G pigment antiformer SP *2 AE *3 AEWR * *5 Age * (N/mm 2 ) (days) EX5 (.5) EX5 (.5) EX5 (.5) *1 Lime type expansive additive, *2 Super plastisizer, *3 Air-entraining agent, * Air-entraining and water reducer agent *5 Compressive strength at loading, * Concrete age at loading Page 2

4 reinforcement ratio : 1. % Youngs moudlus of 187 kn/mm 2 Stress at yield point 331 N/mm 2 Figure 1: Details of reinforced concrete beams(unit:mm) demolding at the age of one day, the specimens were cured under 2 o C and saturated condition as well as room temperatute and wet condition. Details of RC beam are shown in Fig. 1. One specimen was prepared for each mixture proportion. The beams are 15 mm wide, 2 mm high and 28 mm long. Reinforcing bars (RBs) with measured Young s modulus of 187kN/mm 2 are 13 mm in nominal diameter and were arranged at the depth of 1 mm from the extreme compressive fiber. Wire strain gauges were attached on the reinforcing bars as well as on the extreme compressive fiber of concrete at mid-span. The specimens were cured under room temperatur and wet condition. 3. EXPERIMENTAL RESULTS 3.1 Properties of concrete Compressive strength, Young s modulus and splitting tensile stregth of concrete at 28 days, which were cured under 2 o C and saturated condition, are shown in Figs. 2, 3 and, respectively. Ratios of compressive strength of and to that of are not affected by water-cement ratio and they are 85% and 5% respectively. Young s modulus and splitting tensile strength of and also have the same tendency. Young s modulus of and indicate the ratio of 8% and % respcetively while % and 5 % are marked in the case of splitting tensile strength. In all cases physical properties of of W /C=.3 which is normal to high leads rather large ratios. In Fig. 5, data of compressive strength are plotted as a function of cement-water ratio, where cement includes expansive additive. There is linear relationship between compressive strength and cement-water ratio, but the tangent values are different according to the usage of aggregate. Although shows the largest tangent and the shows the smallest tangent, compressive strength of concrete with recycled aggregate is controllable as the same as W/C=.3 W/C=.5 W/C=.5 W/C=. 5 W/C=.3 W/C=.5 W/C=.5 W/C=. 8 W/C=.3 W/C=.5 W/C=.5 W/C= Figure 2: Compressive strength Figure 3: Young s modulus Figure : Splitting tensile strength Page 3

5 Compressive strength (N/mm 2 ) days Age at loading -EX -EX -EX Cement-water ratio (C/W) Splitting tensile strength (N/mm 2 ) t=.23*f'c 2/3 (JSCE eq.) 28 days Age at loading -EX -EX -EX Compressive strength (N/mm 2 ) Proposed Values by JSCE 3 28 days Age at loading 2 1 -EX -EX -EX 5 1 Compressive strength (N/mm 2 ) Young's modulus (kn/mm 2 )5 Figure 5: Compressive strength as a function of C/W Figure : Splitting tensile strength as a function of compressive strength Figure 7: Young s modulus as a function of compressive strength ordinary concrete by cement-water ratio. In Figs and 7, splitting tensile strength and Young s modulus are plotted as a function of compressive strength. Effect of usage of recycled aggregate on the relationship between comressive strength and splittinge tensile strength is rather small while, and tend to become low in turn. On the other hand, Young s modulus is affected by the aggregate type. while the increasing rate of Young s modulus with increase of comressive strength is the same. Compressive strength of and are 15 % and 3% smaller than that of respectively. 3.2 Cracking behaviors Fig. 8 shows cracking patterns of all the beams. This figure indicates that there is no noticeable difference in cracking patterns among reinforced recycled concrete (RRC) beams and virgin concrete beams, even the crack spacings are rather small and larger number of cracks can be find in shear span in RRC beams. Table 2 summarizes flexural properties when reinforcing bar (RB) stress reaches 2 N/mm 2 including cracking moment (M cr ), averaged and maximum crack spacing (l av, l max ), and crack width (w av, w max ). Cracking moment is calculated using experimental results of splitting tensile strength and Young s modulus of concrete at loading test. The results of crack spacing and width evaluated by the following JSCE s method [5]. The stress in RB is assumed to be 2 N/mm 2 in cracked section. l = 1.1k k k c+.7 c φ (1) max 1 2 3{ ( s )} ( σ / ε ) w = l E + max max se s csd where, l max is the crack spacing, k 1 is a coefficient for an effect of surface shape of RB and 1.3 is for deformed bar, k 2 is a coefficient for an effect of concrete properties, which calculated by: k2 = 15/ ( f c + 2) +.7 where f c is compressive strength in positive value, k 3 is a coefficient for the number of layers of tension RB ( n ) which is calculated by k3 = 5( n+ 2 )/( 7n+ 8), c is cover (mm), c s is distance between centroids of RBs, φ is the nominal diameter of RB, wmax is the maximum crack width, σ se is the increased stress in RB to compensate the stress in concrete at the same depth of RB, E s is Young s modulus of RB, ε is a value for the effect of shrinkage and creep in concrete. csd Page

6 Figure 8: Cracking pattern Table 2: Flexural properties under serviceability condition when stress in RB reaches 2 N/mm 2 M cr l (mm) w (mm) Concrete W/C (knm) l av l max w av w max (mm) Exp. Cal. Exp. Exp. Cal. Exp. Exp. Cal. Exp. Cal EX EX EX Page 5

7 The deflection of beam is calculated by the Branson s equation []: For the concrete with expansive additive, the deflection must be smaller than that of ordinary concrete beam because the difference between tensile stress in RB before loading and the stress in RB determined by the equilibrium requirement at loading is smaller than that of ordinary concrete. Hence, the effective flexural stiffness will be given as [7]: σ s / Es σ c / E σ c s, ex / Es σ c, ex / Ec (2) EI c cr= M/ ϕ, ϕ = ϕex +, ϕex = d d where ϕ ex is the incremental deflection by concrete expansion, σ s, ex and σ cex, are the stress in RB and the extreme compressive fiber at the state that concrete stress at the same depth as tension RB is zero respectively, d is the distance from the extreme compressive fiber to the RB, σ s and σ c are the stress produced by applied bending moment M in RB and the extreme compressive fiber respectively. The cracking moment of RRC beam is smaller than that of conventional RC beam. These tendency can be explained by smaller splitting tensile strength, while the ratio of experimental results to calculation results for all beams are scattered between.8 and 1. except for 3 (Fig. 9). Fig. 1 shows maximum crack spacings for all types of beams are shown compared with calculated results by JSCE methods. Maximum crack spacings of RRC beams are smaller l max by exp. (mm) M cr by exp. (knm) 8-2% 2 -EX -EX -EX 2 8 M cr by calc. (knm) Figure 9: Comparison of experimetal results and calculation of M cr EX -EX -EX mm mm l max by calc. (mm) Figure 11: Comparison of experimental results and calculation for l max Maximum crack spacing (mm) W/C=.3 W/C=.5 W/C=.5 W/C=. Figure 1: Maximum crack spacings of beams. Maximum crack width (mm) W/C=.3 W/C=.5 W/C=.5 W/C=. Figure 12: Maximum crack width of beams Page

8 than those of conventional RC beams. Regarding this aspect, Fig. 11 shows the comparison of experimental data of l max with calculation values. All the experimental results of RRC beams are below calculation except for 3 (see also Table 2). Maximum crack width of beams at 2 N/mm 2 in RB stress is shown with calculated results by JSCE method in Fig. 12. RC beams with recycled aggregate shows wider cracking width than conventional RC beams. And from Figs w max by calc. (mm) and 12, RC beams with recycled aggregate marked wider crack width though the crack spacing is larger. This tendency indicates that tensile strength is predominating factor for crack spacings rather than the effect of decrease in bond strength. But from the practical point of view, it is not really a problem because the crack width of RRC beams is smaller than the empirical crack width which is calculated by JSCE methods by about.1 mm (see Fig. 13). 3.3 Tension stiffening In Fig. 1 calculated stain of RB neglecting concrete stress in tension is plotted as a function of averaged strain in pure M zone with 7 mm length. This averaged strain is measured by PI gauges attached both sides of beam. The tension stiffening of RRC is slightly smaller than that of conventional RC of W/C=.3 and.5, while significant difference is observed in the case of W/C=.. This tendency implies that the bond of RRC is inferior to that of conventional RC and this causes the wider crack width of RRC with the smaller cracking spacing. In the case of concrete with expansive additive, the averaged strain is determined as increased strain before to after loading, and this gives the high tension stiffening. w max by exp. (mm) EX -EX -EX.1 mm Figure 13: Comparison of experimetal results with calculation of w max -EX -EX -EX at cracked section Stress in RB (N/mm 2 ) 2 1 W/C=.5 W/C=.3 W/C=.5 With Ex W /C= Averaged strain at the same depth of RB( 1 - ) Figure 1: Tension stiffening: stain of RB versus averaged strain of concrete at the same depth of RB. Page 7

9 1 8 2 W/C=.3 W/C=.5 W/C=.5 W/C=. Deflection (mm) 1 8 -EX 2 -EX -EX Young's modulus (kn/mm 2 ) Deflection by exp. (mm) 2 -EX -EX -EX +2% -2% 2 Deflection by calc. (mm) Figure 15: Deflection Figure 1: Deflection as a function of Young s modulus Figure 17: Comparison of experiment with calc. result in Deflection Table 3: Plastic deflection and corresponding moment of flexural beams My δy Mu u Concrete W/C (knm) (mm) (knm) (mm) u/δy Exp. Exp. Exp. Exp. Exp EX EX EX Deflection under serviceability load Fig. 15 shows the deflections of beams when the stress in RB is 3 N/mm 2 with calculated deflection by Branson s equation. The deflections of RRC is larger than that of conventional RC. Adding expansive additive decreases the deflection but its effect is rather small. Decrease of Young s modulus of concrete is dominant to the deflection than the effect of expansion of concrete and this can be seen in Fig. 1 as strong correlation between Young s modulus and deflection. And the deflections of RRC are predictable comparably to that of conventional RC which is shown in Fig. 17. Page 8

10 3. Plastic behaviors Yielding moment (My), deflection at yielding (δy ), ultimate moment (Mu), deflection just before sudden drop of moment (δu) and ductility factor which is the ratio of deflection just before sudden drop of load to that at yielding of steel bars obtained from all the beams are tabulated in Table 3. Ultimate moment was calculated assuming cracked section using measured yielding stress of steel bars. According to the Table, experimentally obtained ultimate moments are greater than calculated results by 1-2%. Ductility factors, which is defined as the ratio of the ultimate deflection to the deflection at yielding of RB, are affected by using recycled aggregate. Using recycled aggregate decrease ductility factor. But all the experimental values of ductility factor are beyond 5.. Hence, it could be concluded that flexural capacity of RRC will not become an issuer for practical application of determinate structure under the condition that anchorage performance is sufficient and the steel bars yield before failure of concrete in compression. 11. CONCLUSIONS Recycled concrete with saturated and 2 o C curing decreases in compressive strength, Young s modulus, and splitting tensile strength compared with normal concrete of the same water-cement ratio. The decrease in ratio is dependent on the usage of recycled aggregate. But compressive strength of recycled concrete has the liner relationship with cement-water ratio like normal concrete, and Young s modulus and splitting tensile strength are controllable by compressive strength. Crack widths of reinforced recycled concrete (RRC) beams with wet curing are wider than those of conventional reinforced concrete (RC) when the stress in RB is 2 N/mm 2, while crack spacings of RRC beams are smaller than those of RC beams. However, the difference of crack widths between both is small from the engineering point of view and, furthermore, they are still overestimated by the design equation of JSCE by.1mm. Expansive additive contributed to decreasing the crack widths, the magnitude of the effectiveness in reduction is 2-3% in this present study. The deflections of RRC beams, when the stress in RB is 3 N/mm 2, are larger than those of conventional RC beams. This tendency can be predicted by considering the difference of Young s modulus, even in the case of adding the expansive additive. Measured ultimate moments are larger than the calculated results using measured yield stress of RB by 1-2%. From the experimental results, it could be concluded that flexural capacity does not decrease by using recycled aggregate under the condition that anchorage performance is sufficient and the steel bars yield before failure of concrete in compression. Ultimate deflections of RRC are smaller than those of RC beams. But the minimum ductility factors that are measured in the case of with W/C of. are greater than 5., which should not be the problem for determinate structures. REFERENCES [1] White reports of the environment, Ministry of the environment, 2 [2] Mukai, T., Kikuchi, M., Studies on utilization of recycled concrete for structural members (Part1, Part2), Summaries of technical papers of annual meeting, Architectural Institute of Japan, (1978) 85-8, Page 9

11 [3] Nanba, A., Abe, M., Tanano, H., and Maeda, H., An experiment for improvement of qualities of recycled aggregate concrete, Proc. Japan Concrete Institute, 17(2) (1995) [] Sato, R., Kawai, K., and Baba, Y.,. Mechanical properties of reinforced concrete members made of recycled aggregate, Cement Science and Concrete Technology, (2) [5] JSCE, STANDARD SPECIFICATIONS FOR CONCRETE STRUCTURES-22, Structural Performance Verification, (22) 1 [] Branson, D.E., Instantaneous and Time-Dependent Deflection of Simple and Continuous RC Beams, Alabama Highway Research Report, 7 (Bureau of Public Roads, 193). sited in Ibid. [5] 19 [7] Tanimura, T., Sato, R., Hiramatsu, Y., and Hyodo, H., A GENERAL METHOD FOR EVALUATING THE EFFECT OF LENGTH CHANGE AT EARLY AGES ON CRACK AND DEFORMATION OF RC FLEXURAL MEMBERS, Journal of materials, concrete structures and pavement, JSCE, No.7/V-3, (2) Page 1