Use of large volumes of Construction and Demolition waste based on the concept of reduce, reuse and recycle, encourages sustainable construction practices; for instance, the feasibility of using C&D waste for rigid pavement construction, reduces the overburden to a great extent.
Bharathi Ganesh, Prathima G, Professor, Nitte Meenakshi Institute of Technology, Bangalore
Sharada Bai H, Retd. Professor, UVCE, Bangalore, Nagesh Puttaswamy, Srinivasa G, Ultratech Cements Ltd. Bangalore
With the rapid change in global urbanization and industrialization, there is an unpredicted growth in the population and intense development in developing countries like India. The main two concerns are the requirement of large quantity of construction materials (both natural and artificial) and major environmental issue of managing wastes generated from new constructions and demolition of structures.
After China, India is the second largest populated nation with over 1.29 billion population contributing 17.6% of the total population worldwide. In contrast, India is sharing 5% of the world’s total area book-keeping for 3,185,263-sq.km of which 68% lives in the countryside and 32% in built-up zones (cities, towns, etc). Handling the requirement of the huge quantity of construction materials and managing / disposing off the waste generated from construction is the key for successful urban planning.
C & D Waste Generation & Composition
Infrastructure services include management of water supply, sanitation, solid waste, land and urban environment etc. Studies show that about 25% of Municipal Solid Waste is contributed by construction and demolition. India produces about 165-170 MT of construction debris annually. By the year 2047, municipal solid waste generation in India is expected to reach 300 MT and the land requirement for disposal of this waste would be 169.6km2 as against the 20.2km2 in 1997 for management of 635 48MT.
One of the best possible solutions is the extraction of Aggregates called Recycled Coarse Aggregates [RCA] and Recycled Fine Aggregates [RFA], in general Recycled Aggregates [RA] from C&D waste. Composition of C&D waste (Fig.1) and various contributors of C&D waste from construction industry (Fig.2).
Processed C&D waste is obtained from various methods adopted to segregate waste: clearing the mortar on the surface of recycled aggregates and such processes make aggregates to behave in line with that of natural aggregates.
Sources of C&D Waste
C&D waste materials include the waste generated during new constructions, renovations, or demolition of RCC structures, roads, bridges etc. Segregation of RA [RCA and RFA] from other components of C&D waste like wood, asphalt shingles, metal, cardboard, bricks, plastics, soil, etc, is a major task which consumes time and finances.
The use of C&D waste in building construction is yet to gain popularity in India. Creating awareness on use of RA in concrete constructions is one of the environment friendly solutions for sustainable construction, and is the need of the hour.
The notification of Central Public Work Department (CPWD) in 2016 has made it mandatory for the CPWD and National Building Construction Company (NBCC) to recommend utilization of Recycled Concrete Aggregates (RCA) generated within a radius of 100km of construction in Lean Concrete, Plain Cement Concrete (PCC) and Reinforced Cement Concrete [RCC]. A clause recommends partial replacement of natural coarse aggregates with Recycled Concrete Aggregates; 25% for plain Cement Concrete, 20% only up to M25 grade RCC and 100% in Lean Concrete in less than M15 grade.
Various initiatives by the Government through recommendations for reducing solid waste include the notification of the Ministry of Environment and Forests dated March 29th 2016, and the advisory of CPWD on usage of recycled products:
- All Delhi Govt. agencies tenders shall mandate the use of 2% C&D waste recycled products in building works and 10% in road works.
- Urban Local Bodies shall mandate builders during plan approval to use 5% C&D waste recycled products in non-structural applications.
C & D Waste Processing & Utilization
One of the potential use of processed aggregates (processed with various techniques and applications) is in concrete pavement construction. In order to make the construction activity more sustainable, it is important to use recycled aggregates in various construction materials in concretes and also in concrete products such as precast concrete blocks, interlocking pavement blocks, precast compound wall panels, panels for canal lining to name a few. Aggregates form about 60 to 75% of the total volume of concrete. If the availability of large volume of good quality processed C&D waste and huge requirement of aggregates for concrete and rigid pavement applications are matched, both the problems will be addressed simultaneously.
Surface Treatment Methods for Aggregate Beneficiation
One of the major factors that lowers the quality of recycled aggregates is the mortar that adheres to the surfaces of aggregates. It may be removed using mechanical, thermal or chemical treatment processes of beneficiation or a combination of these, depending upon the requirements and property of the aggregates. Each method has its own merits and demerits. The latest method is the microwave assisted beneficiation technique developed at the National University of Singapore.
Thermal Beneficiation: Conventional Heating
The particles of aggregates are heated to a temperature of about 5000c for nearly two hours to generate thermal stress, which results in thermal expansion, resulting in fracture of adhered mortar.
This method works upon generating thermal stresses through thermal expansion to fracture and remove away the adhered mortar. In this method, recycled concrete aggregates particles are heated at about 500°C for about two hours. Moreover, when concrete is heated at temperatures higher than 300°C, mortar is made brittle due to dehydration; lowering its resistance against the thermal stresses developed. The efficiency of this method could be increased by pre-saturating the mortar before heating, because it can lead to pore pressure development which may result in the more efficient removal of mortar.
Immersing the heated recycled concrete aggregates in cold water immediately after heating could lead to development of increased differential thermal stresses, which could further add to mortar removal.
This technique uses mechanical forces to grind and remove away the adhered mortar. Japanese researchers have proposed two techniques for the mechanical beneficiation:
- Eccentric-shaft rotor method of grinding: In this, there are two concentrically placed cylinders and crushed concrete lumps are passed downward between the outer and inner cylinder that rotates at high speed to separate the adhered mortar from the aggregate particle.
- Mechanical grinding: In this method, the mortar is removed by the abrasive action of iron balls in a rotating drum partitioned into a number of small compartments.
This method is an application of a combination of mechanical stresses generated through rubbing and thermal stresses generated through conventional heating adopted successfully to remove mortar adhered to aggregates. In 1999, Shima et al. proposed a thermal-mechanical treatment technique known as “heating and rubbing” in which, the concrete debris is heated at 300°C in a vertical furnace to make the cement paste brittle due to dehydration. Further to remove the mortar, the heated concrete debris is fed into the rubbing equipment. Here, the heated concrete waste is rubbed against steel balls and the dislodged mortar portion is discharged through a screening system. Inventors of this method claimed that it can increase the quality of recycled concrete aggregates to comply with the JCI (Japan Concrete Institute) standards for high quality, recycled concrete aggregates.
Beneficiation through soaking in Acid solution
More recently, Tam et al. proposed a new method to remove the mortar by pre-soaking recycled concrete aggregates in 0.1 molar acidic solutions for 24 hours. They considered three different acidic solutions (HCL, H2SO4, H3PO4) in this study and reported that the water absorption of recycled concrete aggregates after treatment reduced, showing improvements in the range of 7.27% to 12.17%. However, it was found that the treated aggregates had higher content of chlorides and sulphates. This increase in chloride and sulphate content may pose serious durability issues.
Abbas et al. proposed to use combined chemical degradation through exposure of recycled concrete aggregates to sodium sulphate solution and mechanical stresses created through subjecting recycled concrete aggregates to freeze-and-thaw action to separate mortar from recycled concrete aggregates. However, the main objective of their study was focused on quantifying the amount of mortar present for use in recycled concrete aggregates classification. The technique is considered not suitable for full scale recycled concrete aggregates production.
This novel microwave-assisted beneficiation technique developed at the National University of Singapore, takes advantage of the differences in electromagnetic properties and water absorption of natural aggregates and mortar to heat them at considerably different heating rates. The differential heating of natural aggregates and mortar may lead to development of high differential thermal stresses within the mortar, especially at its interface.
While microwave heating heats up only the recycled concrete aggregates volumetrically, it is significantly more energy efficient compared to conventional heating, which heats the entire heating chamber together with its contents. The efficiency of microwave heating in the current state of art systems reported may be as high as 90%. This requires significantly shorter duration, lower energy consumption, in addition to shorter heating duration and the lower temperatures (200°C). Due to the result reached compared to conventional heating methods, the quality of the original coarse aggregates, e.g. granite is more likely to remain unaffected after processing.
Properties of C&D Waste/RCA
The basic properties of RCA as an ingredient of concrete in comparison with that of natural aggregates, the main variation is attributed to a few common features of RCA, namely residual adhered mortar on RCA, minute fracture with each aggregate, stresses due to loading of the structure etc. The properties are listed as follows:
Density (Loose Bulk Density or Rodded Bulk Density): The density of RCA is found to be lesser than that of virgin aggregates in most of the cases because of the presence of mortar of lesser density. The variation in density is dependent on the specific aggregate source and is lower than that of virgin aggregates approximately by 7 to 17%.
Specific Gravity: The specific gravity gives valuable information on the quality and properties of aggregate and it is seen that the higher values of specific gravity indicates that the aggregate is harder and stronger. Various studies show that the specific gravity values of recycled aggregates are in line with the virgin aggregates.
Porosity and Water Absorption: Increased porosity and water absorption of RCA in comparison with that of natural aggregates is mainly due to the presence of mortar around the aggregate particles. Depending upon the source of C&D waste, the variation in water absorption was found to be 3.5-9.2% against 1.5 to 2.5% for natural aggregates.
Crushing value and Abrasion value: The general trend of crushing and the Los Angeles abrasion test values of recycled concrete aggregates are higher than that of natural aggregates. The variation recorded in crushing tests resulting in values of 23.1% to 42.7% for RCA from different sources against 11-19.4% for natural aggregates.
The behaviour of recycled concrete aggregates in crushing and abrasion tests shows the weakness may be due to the presence of unseen cracks (due to loading) within the aggregates.
Aggregate Impact Value: As discussed by various researchers, the aggregate impact value of virgin coarse aggregate was18.5%, whereas for the recycled coarse aggregate the same value was 17.04%, showing the impact value of recycled concrete aggregates to be similar to that of virgin aggregates.
It is advocated to enhance the utilisation/application of RCA in various constructions so as to reduce the burden of solid waste disposal. In this regard, the processed C & D wastes are encouraged to be used in concrete as a sustainable construction practice.
The use of C&D waste as sustainable materials in appropriate and adequate quantities in possible products has a socio-economic impact by reducing waste and pollution, which is the responsibility of the construction industry.
Works carried out in the recent past
The applications of RCA in rigid pavements started around 1980’s with both satisfactory and unsatisfactory results. Since the material properties of RCA is source-based, the mix design gets affected and hence the performance of concrete in terms of its fresh, hardened (strength and durability) properties were also varying in nature. The main cause of concern is strength reduction at higher level replacement and also the expected durability performance is not met with.
Hence, a study for standardisation of material properties for each major city and the need for establishing pavement structural behavioural.
Aggregates in concrete pavement
The most effective properties of aggregate that can be used to reduce the volume change in pavement and, thus, improve the concrete properties are type, strength, grain shape, size distribution, the maximum grain size, elasticity module, coefficient of expansion, fine material ratio and alkali-aggregate reaction.
The influence of aggregate properties on the characteristics and performance of the concrete pavements mixes under a prevailing traffic loading and environmental conditions are explained briefly. The durability and loading ability of pavement depends on suitable aggregate selection, where the aggregate properties affected strength, density, permeability, and shrinkage of rigid pavement. Therefore, proper selection of the aggregate properties affects the characteristics and performance of the concrete pavement.
Pavement Performance Parameters
Jointed plain concrete pavements (JPCP), jointed reinforced concrete pavements (JRCP), and continuously reinforced concrete pavements (CRCP) are the three common types of Portland cement concrete PCC pavements. Pavement performance is generally expressed in terms of the extent of distresses that adversely affect the functional efficiency of the pavement.
Table 2 lists the various distresses for each pavement type, as recognized by the Long-Term Pavement Performance distress identification manual. The properties of materials (aggregates) and even resulting distresses have a critical effect on pavement performance. The distresses such as alkali- aggregate reaction, blow-ups, D-cracking, longitudinal cracking, roughness, spalling, surface friction, and transverse cracking occur in all pavement type, whereas corner breaks and transverse joint faulting occur only in concrete pavements joints; and punch-outs occurs only in CRCP.
Table 1 provides a ready reckoner for identifying aggregates for pavements. The classification is to help improve the selection of coarse aggregate based on a few significant properties which affect performance. Properties related to rigid pavement performance are summarized in a flow chart (Fig 2). Aggregate properties that have a significant impact on each concrete pavement distress are summarized in Table 3.
Types of Distresses in Concrete Pavements
Strength and durability parameters of concrete pavements
The following properties are to be assessed while using new materials such as C&D waste as Aggregates in pavement concrete.
- Porosity, absorption, and permeability
- Properties of microfines
- Shape, angularity, and texture
- Thermal expansion
- Abrasion resistance (hardness)
- Elastic modulus
- Chemical Constituents
- Durability Properties
- Alkali-aggregate reaction (AAR)
Durability of concrete pavement is the ability to resist weathering influences or to remain fully functional over an extended period under prevailing exposure conditions. The durability of concrete plays an important role in the service life of rigid pavement. Few important exposure conditions and deterioration mechanisms in concrete pavement are:
- Freezing and thawing resistance
- Effect of Climatic Changes
- Chloride Ion Diffusion and Abrasion Resistance
- Alkali-Aggregate Reaction Process
- When water freezes, it expands about 9%. As the water in moist concrete freezes, it produces pressure in the pores of the concrete. If the pressure developed exceeds the tensile strength of the concrete, the pore will stretch and rupture.
- Chemicals used in pavements include sodium chloride, calcium chloride, magnesium chloride, and potassium chloride. These chemicals reduce the freezing point of the rain as it falls on pavements.
- D-Cracking of concrete pavements is caused by the freeze-thaw deterioration of the aggregate. D-cracks are closely spaced crack formations parallel to transverse and longitudinal joints that later multiply outward from the joints toward the center of the pavement panel. This problem can be reduced by the installation of effective drainage systems, and by selecting aggregates that function.
- Literature studies on properties of recycled concrete aggregates have mixed results with several studies discussing increase in properties of concrete using recycled aggregates upon incorporating some inert material such as fly ash or silica fumes etc.
- The mixes prepared using recycled concrete aggregates require more water than concrete prepared using primary aggregates to maintain the same slump without the use of admixtures. This affects the quality and strength of the concrete, resulting in lower concrete strength.
- The mortar adhered to the surface of recycled aggregates increases the porosity of recycled aggregates, thus proper method should be adopted for its removal.
- Recycled concrete aggregate reduces the need for primary aggregates, thereby reducing the environmental impact of the aggregate quarrying process.
- The transportation requirements for the project are significantly reduced by removing the need for both the new material production needs and the waste disposal.
- Recycled aggregates are much more cost effective as compared to normal aggregates, also if recycled near the project site, the transportation and hauling costs could also be reduced effectively.
- In addition to the resource management aspect, recycled concrete aggregates absorb a large amount of carbon dioxide from the environment.
The study is conducted on a reassessment of the rigid pavement performance parameters and aggregate properties that affect these parameters. The most important concrete pavement performance parameters that are affected by aggregate selection include:
- Blowups, D-cracking, longitudinal cracking, roughness, spelling, surface friction, and transverse cracking of all pavement types.
- Corner breaks and faulting of JCP.
- Punch outs of CRCP.
- The properties of RCA vary from source of generation to source. Recycled concrete aggregate may be of a better quality than some virgin aggregates where no distresses, including spelling, cracks and punch outs, have been observed in CRCP using 100% recycled aggregates. In addition, the transverse crack distributions are similar to those in concrete with natural aggregate.
- The aggregate properties that mostly affect the performance parameters of concrete pavements include physical properties (e. g. absorption, gradation, properties of microfines, shape, angularity, texture and thermal expansion); mechanical properties (such as abrasion resistance, elastic modulus, polish resistance, and strength); chemical properties (for example, mineralogy); and durability properties (including alkali-aggregate reactivity and freezing and thawing resistance).
- Aggregates type has a remarkable effect especially on the compressive strength and modulus of elasticity of rigid pavement. Because aggregate gradation has a little effect on the mechanical and environmental properties, larger sized coarse aggregates can be used without clearly negative effects and, therefore, pavement producers can get more flexibility in creating cost-effective mixes. Gapgraded aggregates generally show higher drying shrinkage, creep, and lower strength than mixtures with dense-graded aggregates. Furthermore, the inclusion of microfines to the mixture improved most of the durability properties such as shrinkage cracks, abrasion and permeability resistance.
- Traffic-loading factors such as volume of traffic or truck weight and axle configuration have a significant impact on the performance of concrete pavements. Higher-quality aggregates are specified for pavements subjected to heavy traffic loadings. The load transfer across cracks depends on crack width, aggregate type, aggregate top size and aggregate gradation. Thus, the aggregate should be wisely selected to improve the fracture energy or the cracking resistance of pavement which increases in relation to harder aggregates, especially trap rock. A dense against gap gradation had only a minor effect on the pavement fracture energy. The larger aggregate size increased the fracture energy at both early and older pavement ages.
- The durability of rigid pavement is found to be associated with aggregates soundness, which means the resistance to weathering, including freezing and thawing, wetting and drying, and heating and cooling. On the other hand, higher coefficient of thermal expansion of aggregate increases the probability of occurrence of thermal stresses, drying shrinkage cracking and decrease both strengths and durability of rigid pavement.
After studying different technologies and research on utilisation of C&D waste, it is concluded that C&D waste reuse and recycling has a great business potential. Its success depends on the incentives by the government and formation of standards, strict compliance of regulations and better technology for recycling of the waste.
- It has generally been seen that the use of C&D waste is restricted for use in green buildings only which require certification.
- In developing countries, lack of regulations lead to illegal dumping, causing environmental degradation.
- It is seen that there is very high diversification in properties of C&D waste available across the location. Hence, the problem in classification and standardisation.
- Lot of mixed C&D waste has large amount of clay based material in it which makes is unsuitable for recycling as cost of processing shoots up. This type of C&D waste goes to landfills only.
- The problem of C&D waste utilisation can be managed easily provided all sections of society come forward to take a pledge for protection of the environment.
The authors acknowledge various references and sources for compiling the information in this article. Please click www.nbmcw.com to view the unedited article along with the references.