Materials: Aggregates
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Aggregates Recycled Aggregates
Construction
materials are increasingly judged by their ecological characteristics.
Concrete recycling gains importance because it protects natural
resources and eliminates the need for disposal by using the readily
available concrete as an aggregate source for new concrete or other
applications. According to a 2004
FHWA study, 38 states recycle concrete as an aggregate base;
11 recycle it into new portland cement concrete. The states that
do use recycled concrete aggregate (RCA) in new concrete report
that concrete with RCA performs equal to concrete with natural aggregates.
Most agencies specify using the material directly in the project
that is being reconstructed. For a summary of the findings, click
here.
Recycling of concrete is a relatively simple process. It involves
breaking, removing, and crushing existing concrete into a material
with a specified size and quality. See ACI 555 (2001) for more information
on processing old concrete into recycled concrete aggregates. The
quality of concrete with RCA is very dependent on the quality of
the recycled material used. Reinforcing steel and other embedded
items, if any, must be removed, and care must be taken to prevent
contamination by other materials that can be troublesome, such as
asphalt, soil and clay balls, chlorides, glass, gypsum board, sealants,
paper, plaster, wood, and roofing materials.
Applications
In general, applications without any processing include:
- many types of general bulk fills
- bank protection
- base or fill for drainage structures
- road construction
- noise barriers and embankments
Most of the unprocessed crushed concrete aggregate is sold as 37.5
mm (1½ in.) or 50 mm (2 in.) fraction for pavement subbases.
After removal of contaminants through selective demolition, screening,
and /or air separation and size reduction in a crusher to aggregate
sizes, crushed concrete can be used as:
- new concrete for pavements, shoulders, median barriers, sidewalks,
curbs and gutters, and bridge foundations
- structural grade concrete
- soil-cement pavement bases
- lean-concrete or econo-crete bases
and
- bituminous concrete
Recycled Aggregate Characteristics
The
crushing characteristics of hardened concrete are similar to those
of natural rock and are not significantly affected by the grade
or quality of the original concrete. Recycled concrete aggregates
produced from all but the poorest quality original concrete can
be expected to pass the same tests required of conventional aggregates.
Recycled concrete aggregates contain not only the original aggregates,
but also hydrated cement paste. This paste reduces the specific
gravity and increases the porosity compared to similar virgin aggregates.
Higher porosity of RCA leads to a higher absorption. Click
here for information on differences in the behavior and properties
of RCA compared to concrete made with virgin aggregates.
Mix Design
It is generally accepted that when natural sand is used, up to
30% of natural crushed coarse aggregate can be replaced with coarse
recycled aggregate without significantly affecting any of the mechanical
properties of the concrete. As replacement amounts increase drying
shrinkage and creep will increase and tensile strength and modulus
of elasticity will decrease, however compressive strength and freeze-thaw
resistance are not significantly affected. For more information,
click here.
It is recommended that RCA be batched in a prewetted and close
to a saturated surface dry condition, like lightweight aggregates.
To achieve the same workability, slump, and water-cement ratio as
in conventional concrete, the paste content or amount of water reducer
generally have to be increased.
Concrete with RCA can be transported, placed, and compacted in the
same manner as conventional concrete. Special care is necessary
when using fine RCA. Only up to 10% to 20% fine RCA is beneficial.
The aggregate should be tested at several substitution rates to
determine the optimal rate.
Often recycled aggregate is combined with virgin aggregate when
used in new concrete. An example of a mix design using recycled
aggregates in a pavement application is shown following table.
| Example mix
designs for recycled concrete pavements |
| Concrete Ingredients |
Minnesota DOT lb per cu yd |
Wisconsin DOT
lb per cu yd |
Grand Forks, ND Int'l Airport
lb per cu yd |
Wyoming DOT
lb per cu yd |
| Cement (Type I) |
472 |
480 |
400 |
488 |
| Fly Ash (Type C) |
83 |
110 |
130 |
133 |
| Water |
255 |
265 |
230 |
258 |
| Recycled CA |
1630 |
1815 |
1650 |
1349 |
| Natural CA |
— |
— |
— |
601 |
| Recycled FA |
— |
— |
— |
253 |
| Natural FA |
1200 |
1315 |
1260 |
882 |
Admixtures:
Air entrained
Water reducer |
— |
— |
— |
— |
yes |
yes |
yes |
yes |
no |
no |
yes |
yes |
| Source: ECCO,
Recycling Concrete and Masonry, 1999 |
Sustainability Recycling concrete provides
sustainability several different ways. The simple act of recycling
the concrete reduces the amount of material that must be landfilled.
The concrete itself becomes aggregate and any embedded metals can
be removed and recycled as well. As space for landfills becomes
premium, this not only helps reduce the need for landfills, but
also reduces the economic impact of the project. Moreover, using
recycled concrete aggregates reduces the need for virgin aggregates.
This in turn reduces the environmental impact of the aggregate extraction
process. By removing both the waste disposal and new material production
needs, transportation requirements for the project are significantly
reduced.
In addition to the resource management aspect, recycled concrete
aggregates absorb a large amount of carbon dioxide from the surrounding
environment. The natural process of carbonation occurs in all concrete
from the surface inward. In the process of crushing concrete to
create recycled concrete aggregates, areas of the concrete that
have not carbonated are exposed to atmospheric carbon dioxide.
The LEED® Green Building Rating System recognizes
recycled concrete in its point system. Credit 4 (Materials and Resources)
states, “specify a minimum of 25% of building materials that
contain in aggregate a minimum weighted average of 20% post-consumer
recycled content material, OR, a minimum weighted average of 40%
post-industrial recycled content material.” Using recycled
aggregates instead of extracted aggregates would qualify as post-consumer.
Because concrete is an assembly, its recycled content should be
calculated as a percentage of recycled material on a mass basis.
Credit can also be obtained for Construction Waste Management.
It is awarded based on diverting at least 50% by mass of construction,
demolition, and land clearing waste from landfill disposal. Concrete
is a relatively heavy construction material and is frequently recycled
into aggregate for road bases or construction fill. See Building
Green with Concrete: Points for Concrete in LEED®
for more information on the LEED® rating system.
Case Study: O’Hare Modernization Project
Recycled
concrete aggregates are being considered for use in the O’Hare
Modernization Project (OMP). Laboratory testing using a two-stage
mixing method showed that using RCA from Chicago O’Hare International
Airport for the coarse aggregate reduces bleeding and segregation
and produces similar workability, compressive strength, and shrinkage
as virgin aggregates.
In October of 2009, a field test of RCA was initiated in two lanes
at Gate F7B. In a side-by-side comparison, a lane of concrete using
virgin aggregates was placed next to a lane of concrete using RCA.
The RCA was produced on-site using concrete removed at the airport.
The ready mixed concrete supplier treated the RCA like lightweight
aggregates and was able to produce concrete in a single-stage mixing
process. Contractors placing the RCA concrete said the workability
was similar to that of the virgin aggregate concrete and the placement
had good finishability. Within four days, the placement was in full
service.
Sensors were placed in the concrete at the time of placement to
measure the internal relative humidity, temperature, and the lift-off
of the slab from the cement-treated permeable base. Other properties
were regularly monitored, such as surface appearance and joint width.
After five months of monitoring, the data between the two concrete
lanes are statistically the same, showing no difference in behavior
between the RCA concrete and virgin aggregate concrete.
For more information, contact the Center
of Excellence for Airport Technology at the University of Illinois.
References:
ACI Committee 555, Removal and Reuse of Hardened Concrete,
ACI 555R-01, ACI Committee 555 Report, American Concrete Institute,
Farmington Hills, Michigan, 2001, 26 pages.
FHWA, Transportation Applications of Recycled Concrete Aggregate,
Federal Highway Administration, Washington, D.C., September 2004,
47 pages.
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