Recycled Concrete Aggregates: Its Classification, Properties & Uses

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How concrete manufactures ?

Concrete is produced by combining cement, gravel, sand and water.

NA =  Natural aggregates,

RCA = Recycled Concrete Aggregate.

What is Recycled Concrete Aggregates (RCA) ?

Recycled concrete aggregates are manufactured from waste concrete or demolished concrete of various constructional works. Simply we can say that, it is the byproduct of the concrete building demolition or concrete demolition. First the demolished concrete is cleaned from dirt and other loose particles. After that, it is broken into smaller pieces to produce aggregates which is known as recycled concrete aggregates. It is also known as crushed concrete aggregates. These are available in size of 20mm to 50mm. RCA concrete type had compressive strength up to 22% greater than the natural aggregate concrete.

Recycled Concrete Aggregates

Recycled Concrete Aggregates

The quality of recycled concrete mainly depends on two things:-

1. The characteristic of the original aggregate

2. The condition of the demolished concrete

Classification of Recycled Concrete Aggregates (RCA) :-

According to the ‘BRE Digest 433’ (1998) (Building Research Establishment-UK), recycled concrete aggregates are classified into three types :-

A). Class RCA-1 :-

The origin of RCA-1 is brickwork. It contains about 100% of brick content or block masonry by its weight. It is the lowest quality material which contains high amount of impurities and has low strength. Its source was derived from crushing concrete from decommissioned municipal sidewalk, curb and gutter structures. It have most roughened surface texture. It has less amount of adhered mortar (about 46% by weight).

B). Class RCA-2 :-

The origin of RCA-2 is crushed concrete. It contains about 90% of concrete and significant amount natural aggregate & about 10% of brick content. It is relatively high-quality material which has high strength, toughness and low amount of impurities. Its source consisted of crushed concrete from demolished apron deleterious debris (e.g. wood chips, styrofoam, nails, metal ties, glass etc.). RCA-2 has such surface textures which are grainier resulting from more loose-rounded adhered mortar particles on the surface. It has the highest amount of adhered mortar (about 56% by weight).

C). Class RCA-3 :-

The origin of RCA-2 is crushed concrete as well as brick masonry. It contains about 50% of crushed concrete and natural aggregate & other remain of brick contain. It is a mixed material which has almost the same level of impurities as RCA-1, but the potential usage of this aggregate is extensive compared to RCA-1. The source of RCA-3 is produced from the crushing of returned concrete at a local ready- mixed concrete plant. RCA-3 has such surface textures which are grainier resulting from more loose-rounded adhered mortar particles on the surface. It has average amount of adhered mortar (about 50% by weight).

PROPERTIES OF RECYCLED CONCRETE AGGREGATES (RCA):- 

1). Adhered Mortar content :-

RCA contains residual adhered mortar particles on the surface. The adhered mortar portion of the RCA consists of both hydrated and unhydrated cement particles and the original fine aggregate. The adhered mortar content can have a significant effect on the aggregate properties, including absorption, density and abrasion resistance.

2). Density :-

The density of RCA is dependent on the adhered mortar content, the density of the adhered mortar itself and the density of original aggregate. Due to adhered mortar the density of RCA is normally lower than the density of natural aggregate. The main reason for the lower density is that the adhered mortar is lighter in weight compared to the same volume of the natural aggregate, which decreases the density.

3). Porosity & Water Absorption :-

The crushed concrete aggregate has greater water absorption capacity due to high porosity; which is because of adhered mortar. On the other side, the source of natural aggregate is rock, which tends to have low porosity and low water absorption. RCA-3 shows the highest contribution of the adhered mortar to absorption. This suggests that the adhered mortar on the RCA-3 has a higher porosity and thus, a lower density. So RCA-3 has lowest density.

4). Specific Gravity :-

As described above, the origin of the natural aggregate is the hard rock with old adhered mortar. So, the specific gravity of the crushed concrete aggregates is lower as compared to the natural aggregate.

5). Shape and Gradation :-

The shape and gradation of recycled concrete aggregates are dependent on the production method and type of crusher used for the manufacturing of RCA. The crushed concrete aggregates have a rounded and spherical shape and can be grainy in texture.

6). Strength :-

Recycled concrete as aggregates are relatively weaker in strength as compared to the natural aggregates. Crushing and abrasion value of RCA is also lower then natural aggregates. The NA (natural aggregate) has an ACV (aggregate crushing value) that was about 27% higher than RCA-1, 43% higher than RCA-2 and 57% higher than RCA-3.

You can check the various strength of aggregates by performing the different tests of aggregates such as  crushing value test;  Los Angeles abrasion test;  impact value test;  compressive strength test using rebound hammer etc. For understanding the technical aspect of it in a better way.

NOTE:- Compressive strength & strength are two different things.

Experimental program:-

Aggregate properties tested included: particle size distribution (gradation), bulk and relative density, adhered mortar content (RCAs only), absorption, adhered surface moisture, abrasion resistance, and aggregate crushing strength. Concrete properties tested included: slump, hardened density (γc), compressive strength (fc0), splitting tensile strength (fc0), and modulus of elasticity (Ec). Concrete mixture proportions were developed to reach target compressive strengths associated with structural concrete (i.e. 30–60 MPa). In the Phase 1 mixtures (30 and 50 MPa), only the slump and compressive strength were tested. In Phase 2 mixtures (40 and 60 MPa), slump, compressive strength, density, splitting tensile strength, and modulus of elasticity were tested. Results for both phases are presented and discussed in the same sections. (See Figure given below).

Experimental program overview:-

Particle size distribution of coarse aggregates

 Table:- Coarse aggregate physical properties

Coarse aggregate physical properties

Note:-

n/a = not applicable, NAs contain no adhered mortar.

a.Based on the thermal expansion method.

b.Time required for the aggregate to reach its SSD condition.

c.MC adhered = MC moisture 24 h – Absorption % (SSD condition).

d.Abrasion loss was determined using the Micro-Deval method (percent mass loss).

USES OF RECYCLED CONCRETE AGGREGATES (ALSO CONCLUSION USES) :-

Strength of concrete block made of recycled concrete aggregate is less than that of natural aggregate. So the choice of using recycled aggregate for concrete block is less preferable but using it for filling, subgrade layers of pavement  and more applications (which are given below) could be good choices.

1. In the road construction like pavements, shoulders, median barriers, sidewalks, freeways etc.
2. In the construction of bank protection such as embankment.
3. In the construction foundations & decks of building, bridge airport runways etc.
4. In the construction of curbs, gutters, noise barriers.
5. For the bases of soil-cement pavement.
6. For the lean concrete, bituminous concrete.
7. For filling rabbet (landfilling) in any construction.

CONCLUSIONS :-

A comprehensive research program was conducted that included 14 concrete mixtures incorporating one NA and three RCA sources. The RCA concrete mixtures had the same mixture proportions (i.e. cement content, water–cement ratio, aggregate volume, etc.) as the NA concrete mixtures, but replaced 100% of the natural coarse aggregate with RCA. A number of different aggregate and concrete properties were measured, and key relationships between aggregate and concrete properties were evaluated. All aggregates were pre-soaked for 24 h prior to batching. The study systematically investigated the failure mechanisms associated with RCA concrete and evaluated how fundamental aggregate properties can influence these mechanisms. Efforts were made to quantify statistically significant effects of replacing NA with various RCA types on the properties of the resulting concrete.

As per Greece researcher ‘Oikonomou N.D.(2015)’ (Published in Recycled Concrete Aggregates), all over the world the construction industry takes 50% of raw materials from nature, consumes 40% of total energy and produces 50% of total waste.

Don’t waste concrete, use RCA, #Clean India.

The use of recycled concrete aggregates from the construction demolition waste looks like a potential application in the construction of buildings or houses as a sustainable alternative to natural aggregates. Use of recycled concrete aggregates reduces the amount of virgin aggregates to be created, hence less exploitation of natural resources. It also helps reducing the space & energy required for the landfill disposal as well.

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