Roller-Compacted Concrete Research
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Development of Optimal
Mix Design Procedures for RCC Pavements (Click
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Effect of Admixtures on Roller-Compacted Concrete
Mixes
Principal Investigator: Halil Ceylan
Background
Very little documentation exists today on the effect of concrete
admixtures on the behavior of Roller-Compacted Concrete (RCC) mixes.
Because of the low volume of paste in RCC mixtures, the behavior
of admixtures in RCC may not be the same as with conventional concrete.
For example, very little success has been found with the use of
air entraining agents in RCC.
Anecdotal evidence exists on the use of water reducers and set
retarders with RCC mixes, but no documentation is available to guide
the designer in choosing the appropriate use of these admixtures.
For example, water reducers have been used to facilitate the uniform
distribution of cement throughout the RCC mixture. Super-plasticizers
have been used to overcome problems with mixing and discharging
RCC in transit mixers. Retarders have been used to allow a longer
time before set, but the effect of water evaporation over a longer
set time is not understood. Evaporation retarders have been used
on exposed surfaces to reduce the drying on hot, windy days, but
the effectiveness of this application has not been quantified.
RCC mixes are zero slump concrete mixtures that can be stiff, harsh
and difficult to work with. An improvement in the workability of
RCC mixes has been observed with the use water retarders, but again
the effect is not well understood. The measurement of RCC workability
has been expressed through the Vebe test, but other tests that could
measure the rheology of fresh RCC mixtures have not been evaluated.
Objective
The objective of this project will be to develop an understanding
of the effect of concrete admixtures on RCC mixtures, and to prepare
recommendations for their appropriate use. The project emphasis
will be on water reducers, set retarders, and evaporation retarders,
and will not include the study of air entraining agents.
Market Significance of Project
When problems occur with RCC pavement projects, some of the most
common causes are the poor production of the concrete mix, and difficulty
with concrete placement and compaction. Admixtures have been shown
to improve the workability and constructability of RCC mixes, and
understanding the proper use of these admixtures will result in
better project outcomes.
It is expected that RCC construction in the future will see considerable
market growth with smaller projects that may be mixed in ready-mix
plants and placed with conventional asphalt pavers. It is important
to provide guidance to small project producers and contractors in
the use of admixtures so that the best possible product can be obtained.
Utilization of Results
The results from this project will be utilized as guidelines for
producers and contractors in the use of admixtures for RCC mixes.
Project Description
- Review industry experience and literature with regard to use
of concrete admixtures with RCC and other zero slump mixes.
- Evaluate test procedures to characterize the workability, rheology,
and freshness of RCC mixes.
- Perform laboratory investigation of the most promising admixtures
to improve construction and performance of RCC mixes.
- Develop recommendations for producer and contractor guidelines
for use of admixtures.
- Assist with trial evaluation of recommendations by producer
and contractor on an RCC pavement project.
- Prepare final report documenting the work of this project and
the recommended guidelines.
Delivery Date
The project will be completed over a 2-year period, beginning in
July 2007. Thefinal project deliverables and documentation will
be completed by June 30, 2009.
Development of Optimal Mix
Design Procedures for RCC Pavements
Principal Investigator: Dr. Halil Ceylan
Objectives
The main objective of this research is utilize imaging and computer
simulations tools to quantify the internal aggregate structure of
RCC produced through theoretical particle packing simulations modeling
and propose optimal mix design procedures for RCC pavements based
on the research findings. This project will take significant advantage
of the recent work carried out at ISU related to RCC F/T durability
(PCA Project Index No. 06-14). Recent research has shown that particle
packing simulations could be successfully employed to achieve optimal
aggregate structure in RCC mix design. This research will validate
the optimal RCC aggregate structure achieved through the application
of packing models using three-dimensional non-destructive imaging.
If possible, the internal aggregate structure parameters of RCC
mixes obtained through imaging will be related to mechanical performance
and F/T durability. One of the deliverable products would be a computer
software program for designing RCC pavement mixtures.
Significance of Project
RCC pavements are beginning to make serious progress in the North
American market. Increases in asphalt prices have made RCC competitive
with asphalt on a first-cost basis. Many large projects are being
built for commercial and industrial applications (such as port facilities
in Houston and Norfolk, and several auto plants in Alabama). Expectation
for growth is high for the future, especially for port and intermodal
facilities. State and local highway agencies are just beginning
to make use of RCC pavements, but the projects built to date (such
as city streets in Columbus, OH and freeway shoulders in Atlanta,
GA) have been very successful and more growth is expected.
In order to spur the growth of this market, better procedures are
needed for contractors and engineers to develop proper RCC mixes.
Poor mix designs can lead to project failures, and current mix design
procedures are basically trial-and-error methods that require experience
and knowledge of RCC materials. Better procedures are needed to
provide easier and more accurate mix designs, which will assist
in the completion of successful RCC pavement projects.
Utilization of Results
The results from this research project will be directly implemented
by engineers and contractors who are preparing RCC mix designs for
construction projects. Software and instructional materials will
be developed as part of this project and used by designers.
Project Description
With recent advances in computational simulation and non-destructive
three-dimensional imaging technology, there is an opportunity to
study and quantify the internal structure of RCC mixes produced
using different mix design methods and relate it to performance
in a scientific manner. X-ray Computed Tomography (CT) imaging has
gained increasing applications in civil engineering materials research
in recent years.
Several investigators have illustrated the use of CT scans for
the non-destructive evaluation of soils and have shown promising
applications in characterization, modeling and computational simulation
to optimize pavement mix design, predict performance and conduct
investigative forensic studies. Specifically, X-ray CT volumetric
images have been successfully used to study the various aspects
of asphalt concrete such as the internal structure, the orientation
of particles, any lack of homogeneity (segregation) in aggregate
sizes, distribution of air-voids, presence of cracks, the distribution
of asphalt, etc (Masad et al., 1999; Masad and Button, 2004; Wang
et al., 2004; Gopalakrishnan et al., 2006). Similarly, X-ray High
Resolution CT (HRCT) has been successfully used to examine properties
of cement concrete in recent years (Hall et al., 2000; Stock et
al., 2002).
An on-going PCA project at Iowa State University (ISU), entitled
“Characterizing Voids and Sorptivity of Roller Compacted Concrete
to Access the Frost Durability” (Project Index No. 06-14)
seeks to explore and/or develop a rational technique that characterizes
the compaction void structure in RCC and investigate the relationship
between the compaction void structure and freeze/thaw (F/T) resistance
of RCC. Using knowledge gained from this research, the aggregate
structure of RCC can be optimized, improving performance while reducing
paste requirements, and thus costs.
Work Program Tasks (Including Deliverables)
The proposed study will contain the following major tasks:
Task 1.0: Conduct a comprehensive literature review on RCC mix
design procedures
The objective of this task is to conduct a detailed literature
survey on the historical development of RCC mix design procedures
both in the US and in other parts of the world. Several case studies
have been reported in the literature on the long-term performance
and durability of RCC pavements based on different mix design approaches
(empirical, semi-empirical, and theoretical). This task will evaluate
the state-of-the-practice and state-of-the-art in RCC mix design
and synthesize national and international activities in the areas
of RCC mix design development and application.
Task 2.0: Design experimental plan
The experimental plan will identify the number of RCC specimens
required for testing and the test factorial. As mentioned previously,
a large number of RCC specimens from the on-going PCA project at
ISU will be utilized to characterize the compaction air void structure
of RCC specimens. The test factorial will also include preparation
of specimens for verifying the optimal RCC aggregate skeleton produced
through the theoretical CPM model and for studying the effect of
compaction on compressive strength and tensile strength of RCC.
Task 3.0: Proportion Concrete Aggregates for Minimum Voids Ratio
In order to develop methodology for optimizing concrete mixes by
void minimization, it is considered vital to select a suitable particle
packing model which can estimate the packing density/voids ratio
of the concrete particle mix system as accurately as possible. A
number of particle packing models have been developed over the past
70 years, such as the Furnas, Aim, Toufar, Stovall, Dewar and De
Larrard models. Two models have been developed by De Larrard: the
Linear Packing Model (LPM) and Compressible Packing Model (CPM).
Both models require the particle size distribution (PSD) to be measured
for all constituent materials. Laboratory studies and limited field
investigations have shown that the CPM method has proven reliable
in designing optimum RCC mixes. Therefore, the CPM method will be
used as a starting point for the development of optimum RCC mix
design software.
The effects of the proportions of aggregates on the voids ratio
will be determined for a range of laboratory combinations of the
coarse aggregate and coarse aggregate and fine aggregate combinations
in the laboratory. The packing model will be tested using packing
density data and porosity data reported in the literature for mono-sized,
binary, and ternary mixtures. Different cement contents and aggregate
proportions will be used and tested to verify the theoretical findings.
RCC specimens will be prepared with constituent proportions suggested
by the optimized packing simulations for verification through three-dimensional
X-ray CT imaging. The compaction procedure will be varied (use of
impact hammer, gyratory compactor, etc.) to quantify the effect
of compaction on RCC internal structure which in turn will be related
to packing characteristics. The ISU research team has significant
experience related to particle packing simulations which will be
useful in this research (Shashidhar and Gopalakrishnan, 2006).
Task 4.0: Analyze the air void structure and internal aggregate
structure of RCC samples with X-ray Computed Tomography
The Center for Non-Destructive Evaluation (CNDE) at ISU has an
in-house X-ray CT system as well as a high-resolution CT with customized
software for data acquisition, volumetric file reconstruction, and
visualization. The advanced imaging facilities available at the
CNDE can help in providing valuable insight into understanding the
relationship between RCC internal aggregate structure and performance
leading to the development of optimum RCC mix design procedures.
Of the main imaging techniques available, CT has the advantage of
imaging a 150-mm diameter core of RCC with sufficient resolution
and clarity for quantitative analysis. There are four parameters
that affect the quality of a tomographic image, i.e., spatial resolution,
contrast resolution, noise, and artifacts. Finer resolution can
be obtained with an X-ray high resolution tomography system using
a 160-kV microfocal X-ray source. Resolutions down to 10 µm
can be obtained with this technique.
In both projection radiography and computed tomography implementations,
what determines the quality of the outcome is the contrast between
the properties of the sought out features and the matrix. The hardened
concrete, that will form the target material in this work, is composed
of three different components, namely, aggregates, cement and air
voids. Although cement paste and aggregates are not too different
from each other chemically, aggregate density is significantly higher
making the aggregate radiographically quite distinct compared to
the cement. Preliminary studies conducted at ISU’s CNDE showed
the potential application of both these techniques in characterizing
the air void system parameters in hardened concrete. The 2D X-ray
CT image slices will be used to characterize the distribution and
orientation of coarse aggregates. Mechanical testing will be undertaken
to establish whether the differences observed in the RCC aggregate
structure has a bearing on the mechanical performance of the specimens.
Task 5.0: Verify the RCC optimal aggregate skeleton achieved through
particle packing models
The Compressible Packing Model has proven reliable in designing
optimum RCC mixes. This model makes it possible to design mixes
having optimal compactness and workability regardless of type of
application. Generally, only a single trial laboratory batch is
required. However, until now, this has not been physically verified
and information regarding the internal aggregate structure of mixes
produced using this method is lacking.
To verify this methodology, the RCC specimens prepared in Task
3 will be utilized. For each of the specimens, the internal aggregate
structure will be characterized using appropriate imaging indices.
The RCC specimens that have already been casted as part of the current
RCC F/T durability study at ISU using the empirical soils approach
will also be studied using imaging methodology for comparison. Through
examination of the internal aggregate structure using techniques
like X-ray CT, aggregate matrices that exhibit enhanced RCC performance
can be identified and steps can be taken to develop compaction techniques
that can make this structure.
Task 6.0: Develop a software tool for optimal RCC mix design
The findings of this research study will be incorporated into a
user-friendly software tool kit for optimum RCC mix design which
could be used by engineers and contractors who are preparing RCC
mix designs for construction projects. Instructional materials related
to documentation and training will also be developed as part of
this project.
Task 7.0: Final report
A draft final report will be prepared documenting all activities
conducted in this project, including the results of the packing
model analysis and verification, three-dimensional image analysis,
and documentation of efforts related to the development and validation
of user-friendly software toolkit (spreadsheet based tool) for optimum
RCC mix design. The report will be finalized based on review comments
received from PCA.
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