A TRIAL OF REDUCING AUTOGENOUS SHRINKAGE BY RECYCLED AGGREGATE

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1 A TRIAL OF REDUCING AUTOGENOUS SHRINKAGE BY RECYCLED AGGREGATE Ippei Maruyama and Ryoichi Sato Dept of Social and Environmental Engineering, Hiroshima University, Hiroshima, Japan Abract A possibility of reducing autogenous shrinkage by recycled aggregate which is much more absorbable than normal aggregate is inveigated. Autogenous shrinkage as well as self-induced ress of recycled concrete with water to cement ratio of.25 is compared with those of companion normal concrete experimentally. The experiment shows that recycled aggregate is effective in reducing the autogenous shrinkage as well as its induced ress, while shrinkage rain of recycled concrete after exposed to the air is more significant than that of normal concrete. 1. Introduction and objective High-rength concrete (HSC) has been widely udied during the la decade, and increasingly applied in order to enhance durability performance and ructural performance of concrete ructures. However, such concrete with low water to cement ratio(w/c) has already been known to shrink significantly at early ages, which is caused by self-desiccation and may result in cracking [1-4]. In order to avoid this type of cracking, several methods to reduce the autogenous shrinkage have been udied. From the chemical point of view, expansive additive [5,6], shrinkage-reducing agent [7], Portland cement containing higher C 2 S content [8] and the combination of these materials [9] are reported as effective admixtures for reducing autogenous shrinkage. Additionally, from the physical aspect, internal water curing of concrete, which enable to counteract self-desiccation by partly replacing normal weight aggregate with pre-saturated lightweight aggregate [1,11] or adding super-adsorbent polymer particles as concrete admixture [12], have been reported. On the other hand, concrete wae is rongly required to be utilized as an aggregate for ructural concrete to reduce environmental load. A trial experiment is reported in this chapter on how recycled aggregate is effective in reducing autogenous shrinkage in low W/C concrete, which was performed paying attention to high absorption of recycled aggregate like lightweight aggregate. 2. Experiment 2.1 Materials Ordinary Portland cement is used for all the concrete in this udy. Aggregates used for original concrete (OC) to produce recycled aggregates and virgin concrete (VC), which is to be compared with recycled aggregate concrete (RAC), are the same. The fine aggregate is river sand and coarse aggregate is crushed hard sandone. Two kinds of

2 recycled coarse aggregate CR45 and CR6 and those of recycled fine aggregate FR45 and FR6 are produced from two kinds of original concrete of W/C=.45 and W/C=.63, respectively. Recycled coarse aggregate includes coarse aggregate made of mortar only. Compressive rength of these original concrete cured in water of 2 o C are 5 N/mm 2 in case of W/C=.45 and 3 N/mm 2 in case of W/C=.63 at the age of 28 days. In Table 1, details of aggregates are shown. This table also gives the ratios of mortar content to total coarse aggregate in recycled coarse aggregate (CR) and cement pae content to total fine aggregate in recycled fine aggregate (FR) at ages at crushing. The ratios of mortar and cement pae contents were made higher up to 3%-5%, in attempt to recycle concrete wae as effectively as possible. These values are obtained by a te method for insoluble residue in recycled aggregate using hydrochloric acid. Table 2 shows mix proportions of high rength concrete with virgin aggregate (HVC) and high rength concrete with recycled aggregate (HCFRC). Their water to cement ratios is.25. HCFRC denote concrete made of both recycled coarse aggregate and recycled fine aggregate in the proportion that CR45:CR6 is 2:1 for coarse aggregate and that FR45:FR6 is 2:1 for fine aggregate. Table 1 Properties of aggregate. Kinds of aggregate Age at crushing (days) Density ρ sat Density ρ dry Absorption (%) Finess modulus Mortar/Pae content in mass (%) Virgin fine aggregate Virgin coarse aggregate Recycled coarse aggregate-45 (CR45) Recycled coarse aggregate-6 (CR6) Recycled fine aggregate-45 (FR45) Recycled fine aggregate-6 (FR6) Table 1 Mix proportion of concrete. W/C s/a Unit content (kg/m 3 ) SP* Pigment W C S G (g/m 3 ) (g/m 3 ) HVC HCFRC *:Superplaicizer

3 2.2 Specimens and experiments Sealed specimens of φ 1 x 2 mm for compressive rength and Young s modulus are prepared and teed at the age of 3, 7, 14, 28 days. Splitting tensile rength tes are carried out with sealed specimens of φ 2 x 15 mm at the age of 28 days. These specimens are cured in room temperature. In order to inveigate the effects of recycled coarse and fine aggregates on shrinkage rain, 3 specimens for each concrete are prepared with the cross section of 15 mm x 2 mm and length of 5 mm, which are sealed until 28 days and then exposed to drying. Embedded rain gauges whose size is φ 2 14 mm and reference length is 1 mm with low elaic modulus of 39 N/mm 2 are used for measuring. Two different RC beams, namely series B and S, are prepared for each HVC and HCFRC in order to measure the shrinkage induced ress. Curing condition of these beams are the same as that of shrinkage specimens. Aluminum adhesive tape is used for sealed condition. The details of the each specimen are shown in Figure 1. The B series beam specimens have the reinforcement ratio of 1.6 % and the S series have the value of 2.39 %. Figure 1 Details of reinforce concrete beams. (unit:mm) 3. Experimental results 3.1 Compressive rength, Young s modulus and tensile splitting rength Developments of compressive rength and Young s modulus of HVC and HCFRC are shown in Figure 2 and Figure 3, respectively. In these figures, compressive rength and Young s modulus of HVC and HCFRC cured in a condition of saturated and 2 o C. Until the age of 7 days, HVC and HCFRC under sealed condition show almo the same development of compressive rength, though HCFRC shows the slightly smaller increase at later ages. The compressive rength of HCFRC is 8 % smaller than that of HVC in sealed and room temperature condition, while it is 23 % smaller in saturated and 2 o C condition [13,14]. Young s modulus of HCFRC is also smaller than those of HVC in sealed and room temperature condition as well as saturated and 2 o C condition. 15 % difference in sealed

4 Compressive rength (N/mm 2 ) HVC sealed, room temperature 2 HCFRC sealed, room temperature HVC saturated, 2 o C HCFRC saturated, 2 o C Age (days) Figure 2 Developments of compressive rength of HVC and HCFRC. Young's modulus (kn/mm 2 ) HVC sealed, room temperature HCFRC sealed, room temperature HVC saturated, 2 o C HCFRC saturated, 2 o C Age (days) Figure 3 Developments of Young s modulus of HVC and HCFRC. Splitting tensile rength (N/mm 2 ) Saturated Sealed -5 Deformation ( x1-6 ) VC (.25) CFRC (.25) HVC HCFRC sealed drying Age (t+1 days) Figure 4 Comparison of tensile splitting te of HVC and HCFRC in saturated and sealed conditions Figure 5 Deformation of HVC and HCFRC. Sealing is removed after 28 days. and room temperature condition and 17 % difference in saturated and 2 o C condition are observed. Tensile splitting rength of HVC and HCFRC are compared in Figure 4. In this figure, 36% difference in tensile splitting rength is marked in the condition of saturated and 2 o C condition, while 3% difference is marked in the condition of sealed and room temperature condition [13,14].

5 Shrinkage-induced ress (N/mm 2 ) Shrinkage-induced ress (N/mm 2 ) 3 HVC-B HCFRC-B 3 HVC-S HCFRC-S Sealed Age (t+1 days) drying sealed Age (t+1 days) 28 drying Figure 6 Shrinkage induced ress of HVC-B and HCFRC-B (Reinforcement ratio: 1.6 %) Figure 7 Shrinkage induced ress of HVC-S and HCFRC-S (Reinforcement ratio: 2.39 %) 3.2 Deformation Figure 5 shows the time-dependent shrinkage rain of HCFRC before and after drying compared with that of HVC. Autogenous shrinkage rain develoed more rapidly and largely in HCFRC than that in HVC within the age of 1 day. This rather rapid development is due to higher water supply, which promoted the hydration. Autogenous shrinkage of HCFRC is about 4 % smaller than that of HVC at 28 days. This compensation of autogenous shrinkage can be attribute to the higher absorption of recycled aggregate. Absorbed water may be supplied to the cement pae matrix. This empirical fact indicates that recycled aggregate with the higher rate of absorption has eminent properties to control autogenous shrinkage at early ages. On the contrary, under drying environment after removing the seal, the whole shrinkage rain of HCFRC developed rapidly and exceeds HVC s rain at 12 days after drying, and the former reaches 12x1-6 at 11 days after drying which is 2% larger than that of the latter. This could be resulted from the increase of porosity and the smaller Young s modulus of HCFRC. 3.3 Shrinkage induced ress Figures 6 and 7 illurate shrinkage induced ress of concrete on the bottom of RC beams, which is calculated by using nominal cross sectional area and Young s modulus of 2 kn/mm 2 for reinforcing bar. Self-induced ress in concrete at the extreme bottom fiber due to reraint of reinforcing bar is determined by considering equilibrium requirement between concrete and reinforcing bars as well as the assumption of linear rain diribution:

6 ( )( ) σ c = P A c 1 Ac d Cg h Cg / I + c P = AEε where P : force in the tensile reinforcing bars, A c : Cross sectional area of concrete in section of beam, A : Cross sectional area of tensile reinforcing bars, E :Young s modulus of tensile reinforcing bars, ε : rain in tensile reinforcing bars, σ c : ress of concrete at extreme bottom fiber. Nominal cross sectional area and Young s modulus of 2 kn/mm 2 are used for calculation of ress in concrete. Self-induced ress in HCFRC-B and HCFRC-S is larger than in HVC-S and HVC-B in response to the rapid shrinkage development before 1 day. However, the later ress developed slowly. At 28 days, the ress in HCFRC is smaller than that in HVC in both B and S series. The difference is about.5 N/mm 2 in B series, while about 1 N/mm 2 difference is obtained in S series. After drying arted, shrinkage ress in HCFRC was produced with approximately the same rate as that in HVC and the former resulted in 7% of the latter at 3 days after drying in both series. The reason for this could be explained by small Young s modulus and large creep of recycled concrete, compared with those of virgin concrete. This confirms that recycled aggregate would make drying shrinkage increase and rerained ress decrease to the contrary. (1) 4. Summary and conclusions The possibility of reducing autogenous shrinkage by the absorbed recycled aggregate, which has much pore than normal aggregate, is examined. The following conclusions were drawn within the limit of the present trials. 1. HCFRC shrunk 8 % larger at the age of.8 day and 6 % smaller at the age of 28 days than HVC in sealed condition. This empirical fact indicates the effectiveness of recycled aggregate on controlling autogenous shrinkage at early ages. 2. Under dry environment after removing the seal, the total shrinkage rain of HCFRC reached almo the same shrinkage rain as HVC after 12 days drying period. And HCFRC reaches 12x1-6 at 11 days after drying which is 2% larger than that of HVC. 3. Shrinkage-induced ress in HCFRC by reinforcing eel bars with the ratio of 2.39 % on the bottom of RC beam was 1.1 N/mm2 and that in HVC was 2. N/mm 2 at the age of 28 days, while.8 and 1.2 N/mm2 are marked with HCFRC and HVC at the ratio of 1.6, respectively. Even after drying arted, though the rate of shrinkage in HCFRC is twice of that in HVC, the rate of ress development is

7 approximately the same and HCFRC shows smaller shrinkage-induced ress. This could be explained by the smaller Young s modulus and the larger creep rain of recycled concrete. This confirms that recycled aggregate would make drying shrinkage increase and rerained ress decrease to the contrary. 4. The optimum quantity of absorbed recycled aggregate is the problem for future udy. This problem should be solved from several points of view, i.e., durability properties and mechanical properties. References [1] Paillere, A.M., Buil, M. and Serrano, J.J., Effect of fiber addition on the autogenous shrinkage of silicafume concrete. ACI Materials Journal, 86(2), pp (1989). [2] Tazawa, E. and Miyazawa, S., Autogenous shrinkage of cement pae with condensed silica fume. Fourth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Ianbul, Turkey, SUPPLEMENTARY PAPERS, pp (1992). [3] Schrage, I., Mangold, M. and Sticha, J., An approach to high-performance concrete in Germany. Fourth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Ianbul, Turkey, SUPPLEMENTARY PAPERS, pp (1992). [4] Tazawa, E., Matsuoka, S., Miyazawa, S. and Okamoto, S., Effect of autogenous shrinkage on self ress in hardening concrete. International RILEM Symposium on Thermal Cracking in Concrete at Early Ages, pp (1994). [5] Tazawa, E. and Miyazawa,S., Autogenous shrinkage caused by self desiccation in cementitious material. 9th International Congress on the Chemiry of Cement, Vol.4, New Delhi, India, pp (1992b). [6] Hori, I., Morioka, M., Sakai, E. and Daimon, M., Influence of Expansive Additives on Autogenous Shrinkage. International Workshop on Autogenous Shrinkage of Concrete, JCI, Edited by Tazawa, E., Hiroshima, Japan, E & FN SPON, pp (1998). [7] Weiss, W.J., Borichevsky, B.B. and Shah, S.P., The Influence of a Shrinkage Reducing Admixture on Early-Age Shrinkage Behavior of High Performance Concrete. 5th International Symposium on Utilization of High Strength/High Performance Concrete, Vol.2, Sandefjord, Norway, pp (1999). [8] Tazawa,E. and Miyazawa,S., Influence of Cement and Admixture on Autogenous Shrinkage of Cement Pae. Cement and Concrete Research, 25 (2), pp (1995). [9] Sato, R., Tanaka, S., Hayakawa, T., and Tanimura, M., Experimental Studies on Reduction of Autogenous Shrinkage and Its Induced Stress in High-rength Concrete, Proceedings of the Second International Research Seminar in Lund, pp (1999). [1] Philleo, R., Concrete Science and Reality. In: J.P. Skalny and S. Mindess Editors, Materials Science of Concrete II, American Ceramic Society, Weerville, OH, USA, pp. 1 8 (1991).

8 [11] Weber, S., and Reinhardt, H.W., A Blend of Aggregates to Support Curing of Concrete, Proceedings of International Symposium on Structural Lightweight Concrete, Edited by I. Holand, T.A. Hammer and F. Fluge, Sandefjord, Norway, pp (1996). [12] Jensen, O.M. and Hansen, P.F., Water-Entrained Cement-Based Materials: I. Principle and Theoretical Background, Cement and Concrete Research, 31, pp (21). [13] Sato, R., Kawai, K., and Baba, Y., Mechanical performance of reinforced recycled concrete beams, Proceedings of International Workshop on Recycled Concrete, JSPS76 Committee on Conruction Materials, Tokyo, Japan, pp (2). [14] Sato, R., Kawai, K., and Baba, Y., Mechanical properties of reinforced concrete members made of recycled aggregate, Cement Science and Concrete Technology, No. 54, pp (2).