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[traffic sound]
Weather, chemical exposure, and normal traffic
cause wear and tear on concrete day-after-day and year-after-year on American highways.
Hydraulic cement concrete is composed of aggregates bound together by a hydrated cement paste,
resulting in hardened concrete that is strong and stiff, but that does not necessarily mean
that the concrete will be durable, and that the structure will last the intended design life.
External elements, roadway conditions, and composition of constituent materials
vary greatly from coast to coast, so there is no simple one-size-fits-all solution for ensuring
durability of a concrete mixture.
Durability is not an intrinsic material property of concrete.
Concrete mixtures need to be designed to control or mitigate material-related distresses,
for the exposure conditions.
The most common material-related distress mechanisms that affect pavements include
freeze-thaw deterioration of the hydrated cement paste or of the aggregate,
deicer scaling of the concrete surface, alkali-silica reactivity, sulfate attack, and chemical deicer attack.
These distresses can be mitigated by proper mixture design and ensuring that the concrete
placed in the field meets the mixture specifications.
No test, or combination of tests, directly measures the durability of concrete.
Instead, multiple tests are employed on both the constituent materials and on the concrete
mixture during the mixture design process, as well as during the construction of the pavement,
to assess how durable the hardened concrete will be in a specific environment.
Durability tests for paste freeze-thaw deterioration include:
Microscopic analysis of hardened air void system using ASTM C457;
Cyclic freeze-thaw tests of hardened concrete beams using AASHTO T 161;
Air content tests of fresh concrete using AASHTO T 152, AASHTO T 196, and AASHTO T 121;
and super air meter or SAM test of air in fresh concrete using AASHTO TP 118.
The most rigorous methodology to evaluate the air-void system in concrete is by viewing
a polished concrete specimen under the microscope.
It requires extensive sample preparation and a trained technologist who must distinguish
among the various concrete constituents and record measurements.
Currently, there are also automated methods based on analysis of digital images.
Because of the time involved in preparing and testing the samples, microscopic analysis
of the hardened air void system is more suitable for evaluation during the design phase and
forensic studies rather than quality assurance testing.
The most common test for evaluating resistance of aggregate to freeze-thaw deterioration
is AASHTO T 161 with samples prepared using ASTM C 1646.
This test also evaluates resistance of the concrete paste to freeze thaw deterioration.
In this test method, concrete beams are subjected to rapid freezing and thawing cycles.
In Procedure A, the specimens are frozen and thawed in water, whereas in Procedure B,
freezing occurs in air while thawing is done in water.
The potential for deterioration is most often assessed through the linear expansion of the
specimen or through the reduction in the dynamic modulus of elasticity reported as the durability factor.
The test is more severe than field conditions because the concrete is subjected to rapid
freezing and thawing in a saturated state, therefore aggregates that pass this test are
generally going to perform well in the field.
Typical acceptance threshold values include a durability factor greater than 60 to 80 percent
and length change less than 0.035 percent at 300 to 350 freeze thaw cycles.
The pressure method, AASHTO T 152, is the most common test to assess the total air content
of paving grade concrete made with normal weight aggregates.
The total air content present is read directly from the gauge of a calibrated pressure meter.
Air content tests have traditionally been used as a quality assurance tool because it
can be conducted on fresh concrete during paving operations.
Michael Praul: "Some of the newer technologies that have come about are tests like the super air meter,
which is based off the existing air test that's been around since the late 40s, but the advantage
of the new test is, it better characterizes the air system in concrete so it gives us
a much better indicator of the freeze-thaw durability."
Some aggregates will crack or expand when saturated and subjected to repeated freezing
and thawing cycles.
This deteriorates the concrete and impacts durability.
The Super Air Meter test measures the air void system in fresh concrete under a series
of high pressures and provides the SAM number, which is a measure of the air system quality
and freeze-thaw performance expectations.
Good SAM number results indicate an air system that will accommodate the water expansion
and help prevent the concrete from cracking.
A traditional air meter only provides the percentage of air in the concrete mixture.
Knowing the percentage of air in the mixture only tells part of the story.
These two examples have approximately the same amount of air, but vary in the way the
air is distributed.
The illustration on the left has tiny bubbles spaced evenly throughout the concrete,
which is preferred from a freeze-thaw durability standpoint.
Jagan Gudimettla: "The one on the right is an exaggerated situation but I think, you know,
it does explain the picture where you see pockets of big air bubbles,
and this particular case is not really freeze-thaw durable."
To run the test, start by filling the bottom bowl and consolidate as if a typical
AASHTO T 152 test is being run,
[hammer hitting bowl]
and secure the lid.
[scraping sound]
As opposed to a conventional air meter, with the SAM meter, the test is repeated multiple
times without releasing the pressure in the bottom bowl.
Pressurize the top chamber to 14.5 psi, then release the pressure from the top chamber
to the bottom chamber.
Next, pressurize the top chamber to 30 psi, and release pressure to the bottom chamber.
Repeat the steps one more time, at 45 psi.
Release the pressure and run the entire test a second time.
The differences between the first and second pressure steps are used by the super air meter
to calculate the SAM number.
The SAM number has been correlated to the average size and spacing between air voids
in the concrete mixture also known as the spacing factor.
A higher SAM value indicates increased susceptibility to freeze-thaw deterioration.
A SAM value of 0.20 indicates 90% certainty that the spacing meets ACI durability recommendations.
Test methods used to assess transport properties in paving concrete include:
Water absorption test using ASTM C1585; Rapid chloride penetration test using AASHTO T 277;
and surface resistivity using AASHTO T 358.
The transport of fluids and gases into concrete impacts durability as all material related
distresses involve the transport of moisture and other substances such as chloride ions
or sulfate ions into the concrete.
The water absorption test is a relatively simple test in which the degree and rate of
water absorption is measured into a conditioned concrete sample at specified intervals for
a minimum of 8 days.
This test can be used to measure the initial absorption and the secondary absorption,
which can be then be used to compute the degree of saturation.
The rapid chloride permeability test includes measurement of the total charge passed by
a specified voltage in 6 hours across a concrete specimen placed between sodium hydroxide and
sodium chloride solutions of specified concentrations.
The test measures the conductivity of the saturated concrete including the effects of
all dissolved ions.
The result of the test in coulombs is used to make a general assessment of the chloride
ion penetrability based on a qualitative rating.
"Another newer technology that we have is the surface resistivity test, which is a more
modern replacement for the rapid chloride permeability test.
It gives an indication of the concrete's ability to resist chloride and other contaminants
getting into the concrete and deteriorating the concrete."
Surface resistivity is a simple test used to evaluate the electrical resistivity of
water-saturated concrete to provide an indication of the concrete's permeability.
The test is performed on cast concrete cylinders as described in AASHTO TP 95.
"The surface resistivity meter basically has four probes on it.
The outer probes apply voltage and the inner probes measure current.
Using Ohm's Law, we could measure resistance, and from resistance we can calculate resistivity.
There are no chemicals involved and the other big advantage with this is, it's nondestructive."
A pre-established relationship between surface resistivity and the rapid chloride permeability test
is used to determine permeability.
Traditional permeability tests can take more than two days to complete, but the surface
resistivity test takes less than 5 minutes to take readings, which saves time, and money
for agencies and contractors.
"You can run this particular test on everyday sample to just check and see how consistent a mix is.
With the rapid chloride test, because it's so labor-intensive, we were never able to do that."
In addition to the tests for concrete mixtures, there are several tests to assess durability
of the concrete constituent materials.
Constituent materials tests include: Petrographic evaluation of aggregate using
ASTM C295 for a variety of durability distresses associated with concrete constituent components;
Determination of chemical composition of carbonate aggregates for alkali-carbonate reactivity;
Alkali-carbonate reactivity concrete prism test using ASTM C1105;
Expansion testing of mortar bars using AASHTO T 303 to evaluate alkali-silica reactivity;
Expansion testing of concrete prisms using ASTM C1293 to evaluate alkali-silica reactivity;
and expansion testing of mortar bars using ASTM C1012
to evaluate resistance to sulfate attack.
Durability reflects how concrete pavement maintains its integrity in the service environment
over its entire life.
Moderate, severe, and very severe exposure conditions typically require 6 months,
12 months, and 18 months of immersion to test durability.
Tests such as the super air meter and surface resistivity can deliver a quick assessment
as a screening tool for field applications to determine if a concrete mixture meets durability criteria.
For more information on ensuring durability of concrete, visit our website at fhwa.dot.gov/pavement
or follow FHWA on social media.
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