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Concrete is a key building block for infrastructure such as roadways, bridges,
buildings, and other landmarks.
It has been used for over one hundred years as a durable, long-lasting material for paving
applications ranging from residential streets to interstate highways.
Concrete is made of paste and aggregates, or rocks.
The paste that binds aggregates is made of cement and water.
When first placed, the mixture is plastic, but soon a chemical process known as
hydration begins, causing the paste to harden, and transforming
the mixture into a hard, durable, rock-like material known as concrete.
Aggregates, such as sand, gravel, or crushed stone typically make up 60 to 75 percent of
concrete by volume and 70 to 85 percent of concrete by weight.
Aggregates are more than just filler and are essential ingredients that strongly affect
the characteristics and performance of concrete.
Aggregates used in concrete can be divided into two broad categories, fine and coarse.
Fine aggregates consist of natural sand or crushed stone passing through the
3/8-inch sieve but mostly less than 0.2 inches and retained on the #200 sieve.
Coarse aggregates consist of gravel or crushed stone generally greater than 0.2 inches,
and typically range from 3/8 inch to one and half inches.
Aggregate characteristics that affect concrete properties include: gradation and voids;
durability and soundness; shape and surface texture; abrasion and skid resistance;
absorption and surface moisture.
[rocks falling]
The majority of aggregate used in concrete comes from gravel and crushed stone.
Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed.
Crushed stone is produced by crushing quarry rock, boulders, cobbles, or large gravel.
David Lange: "Traditionally there was quite a bit of river gravel
also used in the production of concrete.
But there becomes a scarcity of these natural resources over time, and I think the tendency
in the marketplace is that the crushed limestone is emerging as the more common coarse aggregate."
Aggregates should meet the requirements of AASHTO M6 and M80.
These specifications define the requirements for gradation and quality of fine and coarse
aggregate for use in concrete by limiting the permissible amounts of deleterious substances.
Aggregates must be clean, hard, and strong.
They must be free of adsorbed chemicals, coatings of clay or other fine materials in amounts
that could affect bonding or hydration of the cement paste.
[rocks falling]
After harvesting, aggregates are processed by crushing, screening, and washing
to obtain proper cleanliness and gradation.
Care must be taken in the handling and storing of aggregates to minimize
segregation, degradation, and contamination.
It is not sufficient to describe an aggregate by its maximum and minimum size because the
particle size distribution affects the concrete's characteristics.
Therefore, gradation limits are specified by agencies for a range of sieve sizes.
Chris Pronoitis: "Well first of all, we check the size of the stone in our QC lab.
We check the different quantities of the size aggregate that are
required to be in the stone."
[sorting gravel]
Gradation is the particle size distribution of an aggregate as determined by a sieve analysis
performed using wire mesh sieves with square openings.
The procedure for sieve analysis is detailed in AASHTO T27, with sieve opening sizes listed
in AASHTO M6, M80, and M43.
Tolerances for dimensions of sieve openings are listed in AASHTO M92.
The gradation and gradation limits are expressed
as the percentage of material passing each sieve.
Historically, in the U.S., coarse aggregate gradation and fine aggregate gradation were
typically specified separately, without addressing the gradation of the overall system.
While this is useful for managing stockpiles of coarse and fine aggregate, it is the combined
gradation that is critical for performance.
In the 1990s, pioneering work by Shilstone, led to the development of the
workability factor chart for the combined gradation.
The chart was based on field experiments and experience with a focus on
reducing segregation in mixtures.
The combined gradation of the aggregate affects the void content of the concrete mixture.
Denser gradation results in decreased void content by improving packing efficiency.
The combined gradation, maximum aggregate size, and void content, affect relative aggregate
proportions and also cement and water requirements.
These proportions affect uniformity, workability, stability, pumpability, economy, porosity,
and shrinkage, of the concrete mixture, which in turn affects
durability, long-term performance, and smoothness.
For paving concrete, an optimized gradation, as represented by the "Tarantula curve,"
is an effective way of combining aggregate particles to achieve the desired mixture properties.
The Tarantula curve establishes the boundaries for the percent of aggregate particles retained
on various sieve sizes to minimize segregation and ensure good workability.
Physical durability problems can occur with aggregates susceptible to repeated cycles
of freezing and thawing, or wetting and drying, and associated volume changes.
Aggregates are considered unsound if these volume changes lead
to the deterioration of concrete.
Volume changes associated with wetting and drying are very rare; as such, soundness primarily
pertains to freeze-thaw resistance, as tested using AASHTO T161 and T103.
Many agencies use AASHTO T104 to test for aggregate soundness
and resistance to disintegration by sulfates.
This test simulates weathering due to freezing and thawing.
Chemical durability problems can occur due to reactions between the aggregate and other
materials such as some of the components with cement.
The reactivity is potentially harmful only when it produces
significant expansion and related cracking.
A majority of chemical durability problems result from a reaction between reactive silica
contained in the aggregates and alkalis in the cement.
An aggregate's susceptibility to this durability problem called alkali-silica reaction, ASR,
can be evaluated using AASHTO T303 and other ASTM standards.
Some aggregates, such as fine grained dolomitic limestones with large amounts of calcite,
clay, or silt, can be susceptible to expansive alkali-carbonate reactivity.
The shape and surface texture of the aggregate primarily influences
the workability of the fresh concrete.
Rough-textured, angular, elongated particles increase the void content between particles,
requiring more water to produce workable concrete as compared to smooth, rounded, compact aggregates.
Thus, they require more cementitious content to maintain
the same water to cementitious materials ratio.
Aggregate should be relatively free of flat and elongated particles where the ratio of
length to thickness exceeds a specified value.
An indirect method of establishing a particle index as an overall measure of particle shape
or texture is provided in ASTM D3398.
Aggregates play an important role in determining the resistance of concrete
to surface abrasion and wear.
A good aggregate is hard, dense, and strong, and free of soft, porous, or friable particles.
Aggregate resistance to abrasion and degradation can be tested by the
Los Angeles abrasion test, AASHTO T96.
Harder aggregate should be selected in highly abrasive conditions to minimize wear.
Absorption and surface moisture of aggregate are determined using AASHTO T255.
As the internal structure of aggregate is made up of solid material and voids that may
or may not contain water, the amount of water in the concrete mixture has to be adjusted
to include the moisture conditions of the aggregate.
Manufactured aggregate is derived from the by-products of other manufacturing processes.
The production of manufactured aggregate requires varying degrees of processing depending on
the materials being used.
The properties of manufactured aggregates can also vary considerably
depending on the materials.
Expanded shale, expanded clay and expanded slate lightweight aggregate is prepared by
expanding select minerals in a rotary kiln at temperatures over 1800 degrees Fahrenheit.
Manufactured lightweight aggregate has a lot of voids in the aggregate particle
such that it allows moisture to be retained.
Andrea Breen: "It puffs up like a popcorn-like material and has a lot of air voids in it,
which gives it its lightweight quality."
Internally cured concrete, pre-wetted expanded shale, clay, or slate lightweight aggregate
is used to replace some of the conventional sand
to supply additional curing water throughout the concrete mixture.
[hydraulic breaker splitting rocks]
Recycled concrete aggregate, also known as RCA, is made up of old concrete from sidewalks,
pavements, curbing and building slabs.
After removal the concrete is processed by crushing the concrete into smaller pieces
and then screened.
The screening process sizes the crushed concrete.
Any leftover steel pieces are removed before being used as aggregate.
Aggregates can also be recycled or reclaimed from other sources such as
recycled asphalt pavement, also known as RAP.
David Lange: "So we can use recycled aggregates,
or reclaimed aggregates in concrete. We can also use it as base material
and if we grind it up fine we can even use it as replacement for fine aggregates.
So this trend of using recycled aggregates is a very strong trend
that supports all of our sustainability goals."
RCA generally has a higher absorption and
a lower specific gravity than conventional aggregate.
RCA may also contain chlorides from roadway deicing salt application.
High chloride levels can cause corrosion of the steel within the concrete.
However, the quantity of chloride typically found in old concrete pavement is low
and not a significant concern.
Studies have shown that all things being equal, the modulus of elasticity
and flexural strengths of concrete using RCA and RAP are typically
lower than those for concrete containing virgin aggregate.
As with any new aggregate source, recycled and reclaimed aggregates should be tested
for durability, gradation, and other properties.
Aggregates comprise much of the bulk and as such are an essential component of concrete.
Choosing the proper aggregates combined into the proper gradation is crucial to producing
a workable and consistent concrete mixture that is economical and durable.
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