Strengthening of Recycled Coarse Aggregate using Different Surface Treatment Techniques

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Introduction
In recent years, it has been recognized that increasing energy consumption from all sources could contribute to the global climate change both directly and indirectly. Not only the global climate change but also another very serious problem faced by the modern society is the depletion of non-renewable resources due to their extensive use. It is generally agreed that current and most often used construction materials as well as building methods are unsustainable, having large environmental impacts such as CO 2 emissions, energy consumption, pollution, dusts, depletion of natural resources, generating considerable amount of solid waste [1].
Among all existing construction materials, concrete is the most used material because of many technical and economic factors. Apart from cement, aggregate accounts for 65-80% of the total volume of concrete and could significantly affect the environmental load of concrete and its level of sustainability. Aggregates for concrete are whether collected as gravel or produced as a result of crushing rocks. In both cases, the excessive use of natural aggregate to respond to the high demand of construction leads to depletion of non-renewable resources. Introducing secondary aggregate from different sources such as aggregates collected from demolished buildings and subjected to crushing and screening and then used in new concrete production has many environmental advantages [1].
It takes many years for a waste management system to develop into a sustainable, reliable, skilful and marketable industry, encouraging the reuse and recycling of components and materials. It is necessary that all parties involved (i.e. clients, contractors, planners and manufactures) play their role in achieving a more sustainable approach. This can be done by extending the life cycle of materials, components and resources. The use of recycled materials in high rather than low-grade applications must also be a priority in the near future. In order to be successful in this approach, the correct choice of materials, recycling procedures and manufacturing processes is fundamental [2].
Over the last decade the concept and development model of Circular Economy has been gaining a growing attention. It aims to provide an alternative to the traditional and dominant model featured at consuming resources and then disposing it. Circular Economy emerges through three main actions, namely reduction, reuse, and recycle. Waste management, as a recovery of resources and environmental impact prevention, has become an important subsector of Circular Economy. Around 30% to 40% of the urban solid waste come from construction and demolition (C&D) activities. The overwhelming amount of C&D wastes generated in the forms of concrete, bricks, and tiles are causing pressures on the limited urban landfill space. On the other hand, limited natural resources, such as virgin aggregates, call for the utilization of recycled alternatives to meet the construction industry needs [3].
Many studies have been conducted on recycled aggregate. The results of those works reveal that the quality of recycled aggregate is usually lower than that of natural aggregate due to adhering mortar particles, surface cracks, and higher water absorption and porosity of recycled coarse aggregate (RCA). This will cause negative effects on the mechanical properties, workability, air content and durability of fresh and hardened concrete. To overcome this issue, several studies were conducted on improving the performance of RCA through enhancement treatment methods. These approaches and techniques can be fundamentally classified into two categories. The first category mainly includes removing the adhered mortar on the surface, whereas the second category is extensively focused on modifying and improving the quality of adhered mortar. The adhered mortar can be removed by using different treatments that include ball milling, heating and then rubbing and ultrasonic cleaning method. The modification of the quality of adhered mortar includes surface coating with different materials such as water glass, pozzolanic materials such as fly-ash and silica fume, polyvinyl alcohol emulsion and bio-deposition [4].
The present study aims to evaluate the performance of concrete made with treated recycled coarse aggregate (RCA). In this study, RCA were obtained from crushed concrete structures with an unknown strength and were subjected to treatment. The specific objectives of the study are (1) To determine the physical and mechanical properties of recycled aggregates before and after the surface treatment. (2) To find the optimum percentage of treated recycled concrete aggregates by casting and testing cube specimens. This scheme provides a more industriallyreliable approach that promotes the use of RCA in the construction industry.

Treatment techniques
In order to strengthen the weak adhered mortar and improve the RCA surface, four different types of treatment methods were adopted.

Pre-soaking in Pozzolan Slurry
For the first treatment method using pozzolan slurries ,two different slurries i.e. Fly ash and Cement slurry (FA&C) and Alccofine and Cement slurry (AF&C) were used. The two strengthening materials of each slurry were equally used with 40% of the total weight of RCA. In making slurries, the two different types of strengthening slurries were prepared by blending the strengthening materials with water (twice the weight of RCA taken) for 2 minutes. Then, the RCA was added into each slurry and soaked for a soaking time of 4 hrs. After that, the RCA was removed from the slurry bath and dried at the room temperature for 3 days. Finally, the hardened RCA was sieved again to remove the excessive materials before conducting the different laboratory tests. This mechanism involves cement reaction together with the pozzolanic reaction forming additional (C-S-H) gel [6], as shown in Eqs. (1) and (2) below Cement reaction:

Acid Treatment
For the treatment of aggregates with acid, RCA samples were soaked in 0.1 molar HCl solution for 24 hours at room temperature. Low concentration of acidic solution was chosen to provide a suitable acidic environment for RCA without an influence on its quality. After 1 day presoaking, the aggregates were washed in normal water to wash off the adhered mortar and then the treated RCA was dried. Treatment could remove adhered mortar which helps to improve the weak ITZ between the RCA and adhered mortar. The mechanism of this treatment is that the hydration products of cement are dissolved in the acid solution. Acid treatment requires safety precautions while handling the acids and during final washing by fresh water after treatment.

Pre-soaking in Polyvinyl Alcohol (PVA)
PVA pellets were added into boiled water to prepare the polymer solutions, such that the concentration of PVA solution was 10%. After the PVA pellets were dissolved, the polymer solution was allowed to cool down to room temperature. The aggregates were soaked in the polymer solution for 24 hrs. Then aggregates were removed from solution and air dried. Polymer emulsions have adhesive properties and can solidify in a short period of time. The polymer molecules can fill the pores of the adhered mortar and seal the surfaces of the RCA.

Optimized Triple Mixing Method
The pretreating process would lead to a higher cost, thus limiting its application in the construction industry [5]. So a special mixing method called optimized triple mixing method (OTM) as shown in Fig.1   'Wa' equals 60-80 percent of the product that the weight of RA multiplied by its water absorption ratio. 'Wb' equals residual water the SCM weight percent in the total cementitious material. 'Wc' equals mixing water-Wb-Wa. 40% of cement was replaced by flyash and GGBS in the ratio 7:3.

Experimental investigation
A series of laboratory tests were conducted to investigate the macro properties of RCA before and after using the above mentioned surface treatment methods. In order to assess the effect of the treatment on the physical and mechanical properties of RCA, different tests were conducted according to the following standards: Later, cube specimens were cast using treated recycled aggregates in order to find the compressive strength. Apart from using treated recycled aggregates, optimized triple mixing was done using untreated recycled aggregate, which would promote densification of the ITZ in recycled aggregate concrete (RAC). From the results obtained, best treatment method was identified and its optimum replacement percentage in concrete mix was found.

Water Absorption
The water absorption of RCA increases as the amount of attached mortar increases. The test is done as per (IS 2386-1963) and result is tabulated in Table 1. The results show that aggregate treated with polyvinyl alcohol has shown better results in this regard.

Aggregate Impact Value
The impact resistance test for coarse aggregate represents the toughness of aggregate and its ability to resist fracture. The experimental results of the impact resistance test are presented in Table 3, where the better aggregate shows the lower impact value. The PVA treated recycled aggregate has obtained a lower impact value compared to untreated RA.

Aggregate Crushing Value
The aggregate crushing value gives a relative measure of the resistance of an aggregate to crushing under a gradually applied compressive load. Low aggregate crushing value indicates better aggregates. The experimental results of the aggregate crushing value test are presented in Table 4. The PVA treated recycled aggregate has obtained a lower crushing value compared to untreated RA.

Specific Gravity
Specific gravity of an aggregate is considered to be a measure of strength or quality of the material. Aggregates having low specific gravity are generally weaker than those with higher specific gravity values. The results are shown in Table 5.

Void Ratio
Void ratio is defined as the ratio of void volume to volume of solids. The test results are shown in Table 6.

Bulk Density
Bulk density is the weight of material in a given volume. The bulk density of an aggregate is affected by several factors including the amount of moisture present and the amount of effort introduced in filling the measures. The test results are shown in Table 7.

Compressive Strength
Compressive strength was taken as the main parameter in deciding the best treatment method. The results obtained are shown in Table 8. A comparison of the results is shown in Fig. 2. The result shows that the concrete prepared using recycled aggregate treated with fly ash and cement slurry has higher compressive strength.
The strengthening technique by using pozzolanic materials works to fill the pores and voids in the weak adhered mortar. The pozzolanic materials react with the hydrous compounds (CH) in the adhered mortar that leads to the formation of new hydrated compounds such as C-S-H formation by the pozzolanic reaction. Also comparing to other methods of treatment, treatment with fly ash and cement slurry is both economical and effective.  In general, polymer treatment can improve the quality of RCA and reduce water absorption, impact value etc. However, they cannot enhance the compressive strength of the concrete. The reason for reduction of compressive strength may be attributed to the process by which the positive polymer groups permeate into the cement paste and make it hydrophobic, which hinders the hydration of the unhydrated cement in the paste. At the same time, the formation of water-repellent film weakens the bonding strength between the aggregate and cement matrix [6].

Optimum percentage replacement of RCA treated with fly ash and cement slurry
Optimum percentage replacement of treated recycled aggregate was found out. Cubes were cast with 0%, 20%, 40%, 60% and 100% replacement of natural coarse aggregate with recycled aggregate treated using fly ash and cement slurry. The results obtained are shown in Table 9. A comparison of the results is shown in Fig. 3.

Conclusions
Treated recycled aggregate can perform better than untreated recycled aggregate and contribute to significant improvement in compressive strength of concrete. Among the treatment methods adopted in the present study, treatment of recycled aggregate with fly ash and cement slurry soaking is found to be an effective technique, with advantages in terms of both low cost and the enhancement effects on recycled aggregate. The optimum percentage replacement of treated recycled aggregate was found out to be 60% based on compressive strength. Thus, a sustainable and environment friendly concrete with adequate structural capacity can be proposed which can also address the issues of disposal of construction and demolition waste.

Disclosures
Free Access to this article is sponsored by SARL ALPHA CRISTO INDUSTRIAL.