|
Frequently Asked Questions
Water Resources Home
> RCC >
FAQs > Proportioning
using the soils approach
Q: How are RCC mixtures for water resource
applications proportioned using the soils spproach?
A: As roller-compacted concrete (RCC)
increases in popularity, many professionals from diversified fields
are designing RCC mixtures. Because RCC is a dry mixture that is
compacted like controlled granular fill and because after curing
RCC behaves like concrete, there are two approaches to proportioning
RCC mixtures: soils approach and concrete approach. Generally, professionals
with geotechnical background tend to follow the soils approach and
professionals familiar with proportioning concrete mixes tend to
favor the concrete approach.
The soils approach is outlined in the following steps:
- Selection of aggregate
- Estimating a cementitious content range for trial mixes
- Performing a modified Proctor test on one RCC mixture containing
mid-range cementitious content and determining the maximum dry density
and optimum moisture content
- Molding and testing cylindrical specimens
- Determining cementitious content based on test results and project
requirements
First, a suitable aggregate should be selected. Because aggregate
delivered to the jobsite is the most expensive ingredient in RCC,
it is always recommended to check for possible sources near the job
site. If justified based on project size and site geology, on-site
mining and processing should be considered. For most projects, designers
select durable aggregate meeting ASTM C33 requirements for concrete
aggregates. Well-graded aggregates are desired to reduce segregation
and achieve high density and strength.
The next step is to determine the amount of cement and pozzolan (if
pozzolan is used). On most projects, the cement is portland cement
Type I/II and the pozzolan is fly ash Class F or C. In the soils approach,
the cementitious materials are expressed as percentage of the dry
weight of aggregate. A series of trial mixes using different cementitious
contents are performed in a laboratory. Based on experience and knowledge
of the available aggregate, RCC strength, durability, and thermal
requirements, an experienced professional can estimate a range of
cementitious content for the trial mixes. A modified Proctor compaction
curve (ASTM D1557) is then developed using one RCC mix at mid-range
cementitious content. From this curve, the mixture optimum moisture
content and maximum dry density are determined.
 |
| Molding RCC cylinder using a vibrating hammer. |
Higher RCC wet density can be achieved when the mixture is compacted
at slightly above the optimum moisture content. Also to allow for
evaporation in the field, a design moisture content of between 0.5
to 1.0 percent above optimum moisture content is generally selected
for the RCC mix design.
Next, trial batches are mixed and a series of test cylinders are made
at different cementitious contents. At least three cementitious contents
should be used: a mid-range and 10 to 15 percent above and below mid-range.
At least two cylinders should be molded for each cementitious content
and for each test age.
Several methods can be used to manufacture the specimens. These include
the vibrating hammer (ASTM C1435), modified Proctor (ASTM D1557),
or pneumatic tamper. Generally, the vibrating hammer is the most commonly
used method for the soils approach mix proportioning. All methods
are discussed in detail in the references listed below.
After the specified curing period, the molded cylinders are tested
for compressive strength (ASTMC 39) and/or split tensile strength
(ASTM C496). The cylinders can also be used to determine the density
of cured RCC. Strength tests are performed at varying ages, depending
on project requirements. The test results are then plotted to develop
age versus strength curves and the cementitious content meeting the
project requirements is selected from the curves.
References:
1. Design Manual
for RCC Spillways and Overtopping Protection, 2002, Portland
Cement Association Publication EB218, Chapter 7
2. Y. K. Choi and J. L. Groom, “RCC Mix Design-Soils Approach.”
Journal of Materials in Civil Engineering, January/February
2001, pp. 71-76
3. Roller-Compacted
Concrete Density: Principles and Practices, 2004, Portland
Cement Association Publication IS541
4. Roller-Compacted
Concrete Quality Control Manual, 2000, Portland Cement
Association Publication EB215.02, pp. 7-18
5. Design Manual
for Small RCC Dams, 2003, Portland Cement Association Publication
EB225.01, Chapter 8
|
 |
More on RCC:
Estimating RCC Costs for Dams
Performance
of RCC
Recent Projects
FAQs
RCC
Database
RCC/Soil Cement Contractor Directory
|