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From the surface, it's impossible to see what's new about curing concrete.
But simple changes in the materials and processes result in
increased durability and reduced cracking.
It's worth taking a closer look.
Jason Weiss: "Internal curing is really doing the basics of concrete.
It's really water curing concrete, but instead of supplying the water from the outside,
we're really supplying the water from inside the concrete mix itself.
So, we hide water inside of the concrete to provide internal curing."
[concrete scoop sound]
Internal curing of concrete has been the subject of many laboratory and field investigations
over the last 20 years.
Up to this point, it has been primarily used in bridge deck applications; however,
now, pavement applications such as continuously reinforced concrete,
jointed plain concrete, and pavement repair projects
are being examined for potential applications of internally cured concrete.
One of the agencies leading the way into this paradigm shift: The Illinois Tollway.
Materials Deputy Program Manager, Steve Gillen, says the results so far, are very promising.
Steve Gillen: "Pavement applications, the benefit that we're seeing, or we're hoping to see, is the ability
of the internal curing process to modify the cracking patterns in continuously reinforced
concrete pavements so that cracks can be maintained at a tighter width to prolong the life of
the pavements or reduce the steel content and make them more economical to build."
[loader working]
Here's how the process works.
Manufactured lightweight aggregate such as expanded shale, clay, slate and slag
is used as a substitute for a portion of the normal-weight aggregates
to provide a source of moisture.
Prior to batching, the lightweight aggregate
is pre-wetted in a stockpile with a sprinkler or soaker hose.
Next, a drain-down period reduces surface moisture
and increases uniformity throughout the stockpile.
Water is held in the voids of the lightweight aggregate
and released into the concrete after the paste has hardened.
The mixing and placement of internally cured concrete is identical
to conventionally cured concrete.
Once the concrete sets, the structure and pore network are established.
Any remaining water, curing water, only aids in hydration.
During the curing phase, water in the lightweight aggregate leaves the pores of the aggregate
and continues the hydration process throughout the concrete structure.
The water released later by the lightweight aggregate helps promote more complete hydration.
It covers a greater volume of the concrete structure than water that is applied externally.
Internal curing is particularly beneficial in concrete
with a low water-to-cementitious material ratio.
This is because the water that is absorbed in the lightweight aggregate does not contribute
to the classic definition of the water-to-cementitious material ratio.
[loader reversing]
The water-to-cementitious material ratio is a descriptor of structure of the matrix and
pores that develop in the fluid concrete system,
[paving equipment noise]
whereas the water provided by the lightweight
aggregate during internal curing affects the hardened concrete system.
[loader reversing]
Lightweight aggregate is not the only way to create internally cured concrete.
Jason Weiss: "The other technologies that are available are cellulose fibers.
They can actually absorb water and give it back.
And then the final one is something called the super-absorbent polymer.
The super-absorbent polymer is basically like a baby diaper in reverse.
The basic idea here is it will absorb some water and hold onto that water
until the mix needs it."
To make internally cured concrete with lightweight aggregate the design of the mixture is key.
The mix design is generally identical to that of conventional concrete with similar
air content, water content, and coarse aggregate content.
The exception is that a portion of sand is replaced with pre-wetted lightweight sand.
Internally cured concrete is attainable for many agencies, mainly due to availability
and affordability of lightweight aggregate.
Also, fine, lightweight aggregates already appear on the agency-approved materials lists.
Some of the fine aggregate is replaced with lightweight aggregate
but the volume proportions remain the same.
It's important to note that the water used for pre-wetting the light-weight aggregate
is not part of the water in the mix design.
There is a wide range of tools that can be used to proportion a specific mixture;
A simple spreadsheet is available to convert a conventional
to an internally cured mixture design in a simple step.
The volume of internal curing water needs to equal the volume
of chemical shrinkage water to negate the effects of self-desiccation.
Controlling the aggregate moisture is critical for internally cured concrete
to attain a consistent mixture.
The absorbed and surface moisture in the lightweight aggregate should be carefully measured and
controlled to reduce the amount of variability from batch to batch.
Two methods are commonly used to determine the aggregate absorption and surface moisture:
the paper towel method and the centrifuge method.
[metal clank]
The paper towel method consists of drying the lightweight aggregate with a paper towel
until the towel no longer picks up moisture.
In the centrifuge method, a centrifuge spins the aggregate using centrifugal force to bring
the aggregate to a saturated surface dry state.
This is faster and more reliable than the paper towel method.
Internal curing provides: Increased cement hydration
Densified microstructure of the cement paste matrix
Improved interfacial transition zone between the aggregate and the paste
Reduced permeability
And reduced built-in stress and curing times
These changes can result in many potential benefits including:
Improved durability
Less shrinkage and curling
More efficient use of the cement
Reduced joint damage
Reduced alkali-silica reactivity damage
Improved fatigue resistance
Ultimately, internal curing results in fewer and tighter cracks.
Increased strength and durability results in a longer service life
and lower maintenance costs over the life of the pavement.
Jason Weiss: "So, at this point, there's several different groups who've used internal curing.
The New York DOT has used internal curing on bridges.
The Indiana DOT and some of their local agencies have used it on a series of different bridges.
The Illinois Tollway.
Texas has used it in a lot of pavements and patches.
So, to date there have been over a hundred to a hundred and fifty different bridge decks
that have been cast using internal curing, as well as a wide variety of commercial products."
One example of an internally cured concrete pavement is a test section
on State Highway 121 near Dallas.
This continuously reinforced concrete pavement was constructed in 2006.
Initially the cracks in the pavement had a larger spacing
than that of conventionally cured pavements.
After several years, the crack spacing was similar to that of conventional sections,
but, the cracks remained tighter.
Steve Gillen: "But they learned from that project that it developed different crack patterns,
tighter cracks were developed on those pavements.
Also, to reduce curling and warping strains on the pavements.
That's another reason why we're looking into this as well."
Field experience has shown that, in general, internally cured concrete has similar workability
and strength, with reduced stress development and cracking compared to conventional concrete.
But what about the cost?
Jason Weiss: "The internal curing is going to be slightly more expensive than a conventional concrete.
So, the idea here would be that you're replacing some of the normal sand with a slightly more
expensive expanded sand in this case.
But the benefits can really be substantial."
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