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Q: What is the proper joint spacing for RCC pavements? (Click for answer)
Q: Why
is compaction of RCC so important? (Click for answer)
Q:
What types of aggregates can be used to create a quality RCC mix
for pavements? (Click for answer)
Q:I
know that roller-compacted concrete (RCC) uses the same basic ingredients
as conventional concrete – aggregates, water, and cement –
but are they mixed together in the same proportions? (Click for
answer)
Q:
Do you need to cure RCC? (Click for answer)
Q: How does hot weather affect the construction
of RCC pavements?
There are two factors that should be considered when
evaluating hot-weather construction of RCC pavements: ultimate strength
and workability.
Ultimate Strength. The optimal curing temperature
for concrete is from 50 degrees F to 70 degrees F. When concrete
is cured at temperatures above 80 degrees F the early strengths
(1, 3, 7 days) are higher than concrete cured at normal temperatures.
However, ultimate strength is reduced. Concrete cured at 90 to 105
degrees F will see 28 day strengths reduced 5 to 15 percent, respectively
(Refs 1,2), compared to curing at 73 degrees F.
These strength reductions are related to the temperature of curing,
not the temperature at placement. With RCC pavements there is a
large surface area compared to the concrete thickness, so heat of
hydration is not a significant concern. However, the higher placement
temperatures will increase evaporative losses, and with the very
dry consistency of RCC rapid surface drying and subsequent surface
dusting can be an issue during hot weather placement. The use of
water curing to keep the RCC surface moist will help to reduce evaporative
losses and ensure a strong, durable surface, in addition to reducing
the curing temperature.
Construction specifications for RCC dams often require that the
concrete mix temperature not exceed 80 degrees F (Ref 3). This is
to reduce the chance that cracking might occur because of the difference
in temperature between the concrete and the ambient air during curing.
Methods for reducing the concrete temperature for mass concrete
placement include using chilled water, ice chips, cooled aggregate,
night placement and liquid nitrogen in extreme cases. The problem
with relying on chilled water to cool the RCC is that, unlike conventional
concrete, there is generally insufficient water in the mix to make
a significant impact on lowering the concrete temperature.
Since heat of hydration is not a concern with RCC pavements, a
better approach to reduce the temperature of the concrete mix is
to cool the coarse aggregate either by shading the aggregate piles
or sprinkling the piles with water. The water sprinkling approach
also aids in the mixing operation by providing moist aggregate which
helps assure a more uniform, consistent mixture.
Workability. Hot temperatures will make the concrete
less workable and more difficult to place and compact, resulting
in a poorer quality final product. High temperatures lead to higher
rates of moisture evaporation, which is very important to monitor
with RCC because there is so little moisture in the concrete. As
temperatures increase from 70 degrees F to 90 degrees F, the time
to initial set and final set are reduced by 20 to 30 percent (Ref
4).
When placing RCC during hot weather, it will be to the contractor’s
advantage to keep the concrete as cool as possible during placement
and compaction. As ambient air temperature increases beyond 90 degrees
F, the time allowed from time of mixing to completion of compaction
should be reduced accordingly (for example, from 60 minutes to 30
to 45 minutes). To compensate for moisture loss during hauling and
placement, additional mix water can be added at the plant. For long
haul times, consideration should be given to the use of hydration-stabilizing
admixtures to provide more workability time.
References
- Klieger, Paul, Effect
of Mixing and Curing Temperature of Concrete Strength,
Research Department Bulletin RX103, Portland Cement Association,
1958
- Verbeck, George J., and Helmuth, R. A., “Structures and
Physical Properties of Cement Pastes,” Proceedings,
Fifth International Symposium on the Chemistry of Cement,
Vol III, The Cement Association of Japan, Tokyo, 1968.
- Guide for
Developing RCC Specifications and Commentary: Roller-Compacted
Concrete for Embankment Armoring and Spillway Projects,
Portland Cement Association Publication EB214, 2000.
- Burg, Ronald G., The
Influence of Casting and Curing Temperature on the Properties
of Fresh and Hardened Concrete, Research and Development
Bulletin RD113T, Portland Cement Association, 1996.
Return to top.
Q:
Do you need to cure RCC?
As with conventional concrete, curing is very important for RCC.
However, RCC has no bleed water, so the main concern is drying.
At least three negative things will happen if RCC is allowed to
dry: 1) the concrete will experience drying shrinkage which will
lead to cracking, 2) the cement will not continue to hydrate which
will result in lower strengths and less durability, especially at
the surface, and 3) dusting of the surface is more prevalent.
To keep RCC from drying the surface should be kept moist for 7
days, or until a curing compound is applied. The surface should
be gently moistened with water from the time compaction is complete.
Curing compounds conforming to ASTM C 309 which are used for conventional
concrete can be used for RCC. However, because RCC has a more open
texture surface than conventional concrete, the curing compound
application rates are 1.5 to 2 times the application rates used
for conventional concrete. (See figure) It is good practice to apply
the curing compound in two coats with the second coat applied perpendicular
to the first.
Other curing techniques such as plastic sheeting and wet burlap
are not commonly used for RCC pavements because of the large coverage
area; however, for small areas these methods have proved successful.
If the RCC is going to be surfaced with asphalt, a bituminous prime
coat will also serve as a good curing compound to seal in the moisture.
Before placing the RCC it is also important to moisten the base
or subgrade material immediately beneath the concrete so that moisture
from the concrete is not drawn into the subgrade.
Return to top.
Q:I
know that roller-compacted concrete (RCC) uses the same basic ingredients
as conventional concrete – aggregates, water, and cement –
but are they mixed together in the same proportions?
A: The correct proportioning of the raw materials
is critical to the production of quality RCC mixes. The mix design
process should not be approached as one of trial and error, but
rather a systematic procedure based on the aggregates, water, and
cementitious materials used in the mix. This knowledge of the ingredients
is coupled with the construction requirements and specifications
for the intended project in order to ensure a RCC mix that meets
the design and performance objectives.
There currently exists several methods for proportioning RCC mixes
for pavements; however, there is not one commonly accepted method.
The main RCC proportioning methods include those based on concrete
consistency testing, the solid suspension model, the optimal paste
volume method, and soil compaction testing. Whichever method is
employed, the goal is to produce an RCC mixture that has sufficient
paste volume to coat the aggregates in the mix and to fill in the
voids between them.
Regardless of which proportioning method is used, it is important
that an RCC mixture meet the following requirements:
- the fine and coarse aggregates should be chosen to achieve
the required density and to provide for a smooth, tight surface
- the moisture content should be such that the mix is dry
enough to support the weight of a vibratory roller yet wet enough
to ensure an even distribution of the cement paste
- the cementitious materials used should meet the required
design strength requirements at minimal cost
Return to top.
Q:What
types of aggregates can be used to create a quality RCC mix for
pavements?
A: Because Roller-Compacted Concrete (RCC) uses
aggregate sizes often found in conventional concrete, a Ready Mixed
Concrete (RMC) producer will probably discover the necessary coarse
and fine aggregates for RCC already stored in existing bins or stockpiles.
However, the blending of aggregates will be different than what
the producer is used to with conventional concrete.
Coarse aggregates consist of crushed or uncrushed gravel or crushed
stone while the fine aggregates consist of natural sand, manufactured
sand, or a combination of the two. Crushed aggregates typically
work better in RCC mixes due to the sharp interlocking edges of
the particles, which help to reduce segregation, provide higher
strengths, and better aggregate interlock at joints and cracks.
Because approximately 80 percent of the volume of a high-quality
RCC mix is comprised of coarse and fine aggregates, they should
be evaluated as to their durability through standard physical property
testing such as those outlined in ASTM C 33.
The American Concrete Institute (ACI) has established aggregate
gradation limits that have produced quality RCC pavement mixtures.
These ACI gradation limits effectively allow the use of blends of
standard size stone, most commonly #67’s, #7’s, #8’s,
and #89’s, along with sand, to be used in RCC pavement mixes.
Sieve Size |
Percent Passing |
Inch |
Millimeter |
Minimum |
Maximum |
3/4" |
19.000 |
100 |
100 |
1/2" |
12.500 |
70 |
90 |
3/8” |
9.500 |
60 |
85 |
#4 |
4.750 |
40 |
60 |
#8 |
2.360 |
35 |
55 |
#16 |
1.180 |
20 |
40 |
#30 |
0.600 |
15 |
35 |
#50 |
0.300 |
8 |
20 |
#100 |
0.150 |
6 |
18 |
#200 |
0.075 |
2 |
8 |
Both ACI and the Portland Cement Association (PCA) recommend the
use of dense, well-graded blends with a nominal maximum size aggregate
(NMSA) not to exceed ¾-inch (19 mm) in order to help minimize
segregation and produce a smooth finished surface. Gap-graded mixes
that are dominated by two or three aggregate sizes are not desirable
for RCC. Additionally, the recommended gradation calls for a content
of fine particles (2% to 8% passing the #200 (75 µm) sieve)
that is typically higher than that of conventional concrete. This
eliminates the need for washed aggregates in many cases and produces
a mix that is stable during rolling.
In cases where washed aggregates are being used, it may be difficult
to meet the specification for 2% - 8% fine particles. In cases like
this, fly ash can be added to the mix to provide the desired fines
content. These fines provide lubrication that helps to distribute
the paste throughout the mix. However, these fines need to be non-plastic
with their Plasticity Index (PI) not to exceed 4.
In many cases, aggregates used in typical highway construction
will also meet the RCC gradation requirements mentioned above. Graded
aggregate base material, crusher run material, and aggregates for
Hot-Mix Asphalt (HMA) paving mixes can be used with little or no
modification in RCC mixes.
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