DURABILITY OF CONCRETE MADE FROM RECYCLED AGGREGATES

Transcription

1 DURABILITY OF CONCRETE MADE FROM RECYCLED AGGREGATES Jiri Brozovsky (1), Jiri Zach (1) and Jiri Brozovsky, Jr. (2) (1) Brno University of Technology, Faculty of Civil Engineering, Technology Institute of Building Materials and Elements, Brno, Czech Republic (2) VSB-Technical University of Ostrava, Faculty of Civil Engineering, Department of Building Mechanics, Ostrava, Czech Republic Abstract This study describes findings of monitoring concerning the sulphate/acid resistance of concrete made from various volumes of fine/coarse recycled concrete aggregates. Corroding medium used: Na 2 SO 4 concentrated solution to 10,000 mg/l, or diluted HCl with ph 3±0.2 in some cases. Test samples have been exposed to this medium for a period of 18 months. Natural aggregates have been replaced with recycled concrete ones in various proportions; monitoring findings show that there are concretes resistant to sulphate/acid aggressive environment as follows: (i) reference concrete (etalon); (ii) concrete made from fine (100 % natural) aggregate and coarse aggregate (50 % natural, 50 % recycled aggregate); (iii) concrete made from fine (100 % natural) aggregate and coarse aggregate (100 % recycled aggregate); (iv) concrete made from fine (50 % natural, 50 % recycled aggregate) aggregate and coarse aggregate (100 % natural). Parameters of other concretes under monitoring hit their maximum levels after 6 to 12 months of exposure being followed by a subsequent decrease; that is why they are not resistant to said aggressive environment. 1. INTRODUCTION In the nineties, the Czech Republic recorded considerable expansion of concrete technology making use of demolished concrete/reinforced-concrete constructions, using this material as aggregates for concrete mixing. Checking of their utilization aimed in particular to testing of physical-mechanical characteristics under various proportions of these aggregates in concrete along with monitoring of its durability, as well as low temperature resistance and stability to an attack of deicing chemicals [8-10, 13,14]. However, this building practice using recycled aggregate does not protect these constructions from various aggressive media their attack has been examined abroad, for example [11,12] nevertheless, the Czech Republic has neglected these questions. Some of these corroding media may be sulphate or liquid acid aggressive medium. That is why it is necessary to know in addition to basic physical-mechanical

2 characteristics of these concretes also their resistance to aggressive media. The durability of concrete made from recycled aggregates has been tested, taking into account most frequent concrete classes corresponding to C16/20 C25/30 concrete in accordance with EN [1]. The recorded findings within the scope of this examination [15] are described in this paper. 2. BASIC CHARACTERISTICS OF INPUT BASE MATERIAL AND CONCRETES Concretes have been made of material as follows: Aggregates: natural gravel aggregate (high level of feldspar from 30 to 50 %) 0/4 mm; natural crushed aggregate 4/8 mm (granodiorite), and recycled aggregate 0 to 4 and 4 to 8 mm (lab-made from concrete C20/25 to C25/30 respectively) Cement: CEM II/B-S 32,5 R Mix water: potable water from water-supply network (conformable to EN 1008 [2]) Admixture: ADDIMENT BV3 plasticizer Concretes include a constant volume of cement, mixing water and plasticizer. Only one variable was aggregate type natural and recycled fine/coarse aggregate ones. Natural gravel aggregates have been replaced by recycled aggregate one in the volume as follows: BS1-0 % BS2-50 % of 4-8 mm BS3-100 % of 4-8 mm BS4-50 % of 0-4 mm BS5-50 % of 0-4 mm and 50% of 4-8 mm BS6-50 % of 0-4 mm and 100% of 4-8 mm BS7-100 % of 0-4 mm and 100% of 4-8 mm See Table 1 for basic parameters of concrete exposed to aggressive media: Table 1: Basic parameters of concrete assigned to monitoring of resistance to aggressive media Parameter under monitoring BS1 BS2 BS3 BS4 BS5 BS6 BS7 compression strength - 7 days [MPa] tensile bending strength -7 days [MPa] mass density 7 days [kg/m 3 ] 2,335 2,258 2,144 2,331 2,168 2,103 2,050 compression strength -28 days [MPa] tensile bending strength -28 days [MPa] mass density - 28 days [kg/m 3 ] 2,331 2,229 2,179 2,268 2,111 2,143 2,133

3 3. PROCEDURE OF EXPERIMENTAL WORKS Concrete resistance to aggressive media has been tested on specimens mm, each exposed to an acid aggressive medium, i.e. sodium sulphate solution to 10,000 mg/l, or diluted HCl with ph 3±0.2 in some cases. After manufacturing, the test samples have been stored in a humid environment for 24 hours and after demolding stored in a water bath at 19 to 21 C. After 28 days of concrete hardening, the test samples have been exposed to an aggressive medium for a period of 18 months. Parameters under monitoring are conformable to ČSN [7]. The following parameters have been monitored on test samples: Appearance (visually - [7]) Mass density (EN [6]) Ultrasonic pulse velocity (EN [3]) Dynamic modulus of elasticity as measured through the ultrasonic pulse method (ČSN [4]) Compression strength (EN [5]) Tensile bending strength (EN [5]) 4. RESULTS OF CONCRETE RESISTANCE TESTING See Figs. 1 to 4 for the acid aggressive environment testing results concerning selected parameters of concrete in which a portion of natural aggregates has been replaced with recycled aggregate ones within an exposure time period from zero to 18 months Tensile bending strength [MPa] Exposure time period [months] BS1 BS2 BS3 BS4 BS5 BS6 BS7 Fig. 1: Tensile bending strength curves depending upon the time period exposure to acid aggressive medium

4 Ebu [MPa] BS1 BS2 BS3 BS4 BS5 BS6 BS Exposure time period [months] Fig. 2: Compression strength curves depending upon the time period exposure to acid aggressive medium 70 Compression strength [MPa] Exposure time period [months] BS1 BS2 BS3 BS4 BS5 BS6 BS7 Fig. 3: Dynamic modulus of elasticity curves depending upon the time period exposure to acid aggressive medium

5 12 Tensile bending strength [MPa] Exposure to sulfate environment [months] BS1 BS2 BS3 BS4 BS5 BS6 BS7 Fig. 4: Tensile bending strength curves depending upon the time period exposure to sulphate environment Compression strength [MPa] Exposure to sulfate environment [months] BS1 BS2 BS3 BS4 BS5 BS6 BS7 Fig. 5: Compression strength curves depending upon the time period exposure to sulphate environment

6 Ebu [MPa] BS1 BS2 BS3 BS4 BS5 BS6 BS Exposure to sulfate environment [months] Fig. 6: Dynamic modulus of elasticity curves depending upon the time period exposure to sulphate environment 5. CONCLUSION To state whether concrete is resistant to an aggressive medium (liquid sulphate and acids in this case) under ČSN [7] with its 12-months evaluation criterion, it is necessary to establish that no one monitored parameter has declined within the time period under monitoring. Monitoring in line with ČSN [7] findings show that there are concretes (with various volumes of fine/coarse aggregate replacement by recycled aggregate ones) resistant to acid aggressive environment as follows: Reference concrete (only natural aggregate),

7 Concrete made from fine (100 % natural) aggregate and coarse aggregate (50 % natural, 50 % recycled aggregate), and resistant to sulphate aggressive environment as follows: Concrete made from fine (100 % natural) aggregate and coarse aggregate (100 % recycled aggregate), Concrete made from fine (50 % natural, 50 % recycled aggregate) aggregate and coarse aggregate (100 % natural). Parameters of other concretes under monitoring hit their maximum levels after 6 to 12 months of exposure being followed by subsequent decrease; that is why they are not resistant to said aggressive environment. Test results prove that concrete made from fine aggregate are not resistant to a sulphate/acid aggressive media. The reason is that fine aggregate namely its finest fraction contains a considerable portion of hydrated cement. This hydrated cement shows a various rate of corrosion; surface films contain CaCO 3 due to carbonation. Coarse recycled aggregate allows production of concrete resistant to the aforementioned sulphate aggressive medium subject to using of acceptable fine aggregate (preferably a natural one). Its use is limited by the size of maximum aggregate particle. Particles over 32 mm can hold hydrated cement to a great extent; this hydrated cement impacts to the durability of concrete in an utmost manner. As to concrete under acid aggressive medium, use of coarse recycled aggregate is inadvisable. The above facts imply a constatation that recycled aggregates are of limited utilization for concrete of constructions in an aggressive media. However, practically it is necessary to keep in view that the crushed source concrete is of a different origin (variety of buildings); that it has been exposed to different environments and media, and that it consists of different material of various composition and structure. These factors can significantly impact the resistance of concrete made from recycled aggregate; that is why when designing constructions exposed to aggressive media it is necessary to proceed ponderously to a great extent. ACKNOWLEDGEMENTS The work was supported by the MSM plan: Progressive Building Materials with Utilization of Secondary Raw Materials and their Impact on Structures Durability and the GAČR 103/04/0169 project. REFERENCES [1]. EN Concrete Part 1 : Specification, Performance and Conformity [2]. EN 1008 Mixing Water for Concrete. Specification for Sampling, Testing and Assessing the Suitability of Water, Including Water Recovered from Processes in the Concrete Industry, as Mixing Water for Concrete [3]. EN Testing concrete Part 4: Determination of Ultrasonic Pulse Velocity [4]. CSN Method of Ultrasonic Pulse Testing of Concrete [5]. EN Methods of Testing Cement - Part 1: Determination of Strength

8 [6]. EN Testing Hardened Concrete Part 7: Density of Hardened Concrete [7]. CSN Concrete Constructions. Tests of Corrosion Resistance of Concrete. General Requirements [8]. Ahmad S.H., Fisher D. and Sackett K., Mechanical Properties of Concretes with North Carolina Recycled Aggregate, Integrated Design & Environmental Issues in Concrete Technology, 1996, pp [9]. How J.C., Yen T and Kuan H.C., Use of Building Rubbles as Recycled Aggregates, Cement and Concrete Research 33, 2003, pp [10]. Morel A., Gallias J.L., Bauchard M., Mana F. and Rousseau E., Practical Guidelines for the Use of Recycled Aggregate in Concrete in France and Spain, Proc. of 3rd Int. RILEM Symposium, 1994, pp [11]. Mulheron M. and O'Mahony M., The Durability of Recycled Aggregates and Recycled Aggregate Concrete, Proc. of 2nd Int. RILEM Symposium, 1988, pp [12]. Nishibayash S. and Yamura K., Mechanical Properties and Durability of Concrete from Recycled Coarse Aggregate Prepared by Crushing Concrete, Proc. of 2nd Int. RILEM Symposium, 1988, pp [13]. Pytlik, P., Beckerova, L: Durability of Recycled Aggregate Concrete The 1 st Conference on Concrete and Reinforced Concrete, Moscow, Russia, September 2001, p , ISBN [14]. Beckerova, L., Pytlík.P.: Evaluation of Recycled Aggregate Class C30/37, 2003 Conference: on Ways and Perspective of Building Waste Recycling for Resources of Full-featured Raw Materials, Brno , 2003, p , ISBN (in Czech) [15]. Drochytka R.: et al.: Progressive Building Materials with Utilization of Secondary Raw Materials and their Impact on Structures Durability. Brno University of Technology, Final Report of VVZ CEZ MSM: Project, Brno Brozovsky J.: Subtask 3 Concrete Durability. (in Czech).