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Background:
A consensus has been
reached among NDOR material engineers, aggregate suppliers,
cement suppliers, suppliers of pozzolanic materials and concrete
producers, that there is a need to quantify the reactivity
levels of the aggregates from the various sources frequently
used in Nebraska. And to find simple means to mitigate the
unwanted expansion and deterioration of concrete due to
alkali-silica reaction (ASR).
ASR takes place
between certain reactive siliceous aggregates (e.g., opal and
chalcedony) and the alkali from Portland cement paste and
external sources. A reaction product gel forms that, in the
presence of moisture, expands and may cause cracking of mortar
and concrete. These surface cracks are aggravated by
freeze-thaw action, leading to shallow delamination, rebar
corrosion, potholes, and other serious problems including
structural failure.
Objectives:
A comprehensive test
matrix along with a program plan is being developed in the Phase
One of the project. The test matrix will consist of each
selected aggregate with varying amounts of cements, fly ashes
(Class C and F), granular blast furnace slag, silica fume and calcined clay. The test matrix and program plan
will take the guesswork out of which tests to conduct in Phase
Two of this project.
Concrete prism tests in accordance with ASTM C1293 will be
conducted to measure the length change due to ASR of concrete.
The duration of this test is fifty-two (52) weeks. The fine or
coarse aggregates in question and 1.25 percent Na2O-equivalent
cement is used for making 3”
´
3” ´
11-1/4” prisms with a water-cement ratio of 0.42 to 0.45.
Measurements are taken at 7, 28, and 56 days, and 3, 6, 9, and
12 months. The tests are conducted under 100 percent relative
humidity and 100oF. Potentially deleterious behavior
is indicated if the one-year expansion is greater than or equal
to 0.04 percent. An accelerated version (100 percent R.H. and
140oF) of this test may also be conducted. If a
particular concrete mix design passes the screening test, other
ASTM strength and durability tests may also be conducted to
further determine the concrete’s acceptability.
Phase Two involves the execution of the test matrix and data
analysis. Based on the findings, specifications for the use of
various aggregates, cements, and pozzolans will be drafted and
circulated for adoption by NDOR. |
