17-06-2013, 01:10 PM
SORPTIVITY-BASED SERVICE LIFE PREDICTIONS FOR CONCRETE PAVEMENTS
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ABSTRACT
The degradation of concrete pavements is often controlled by the transport of a deleterious
species (chloride or sulfate ions, or water in the case of freeze/thaw) into the concrete. With this
in mind, a three-year research project, funded by the Federal Highway Administration, has
culminated in the development of sorptivity-based service life models for concrete pavements
and bridge decks. To develop a service life model, one needs to identify and model the
suspected degradation mechanism, develop laboratory tests to evaluate the critical material
properties, and adequately characterize the exposure environment. For this project, degradation
mechanisms for sulfate attack (ettringite-induced expansion) and freeze/thaw degradation
(critical saturation of the air void system) have been postulated. To evaluate sorptivity, a
laboratory-based testing protocol for conditioning and assessing the sorption properties of field
concrete cores has been developed and submitted to ASTM committee C09 for standardization.
To characterize the exposure environment, a one-dimensional finite difference computer model
which utilizes typical meteorological year weather data supplied by the National Renewable
Energy Laboratory has been developed to predict the concrete pavement surface temperature and
time-of-wetness history for a wide variety of geographical locations throughout the United
States. Finally, these methods and computational tools have been integrated into a computer
software package, CONCLIFE, which provides sorptivity-based service life predictions.
INTRODUCTION
As with all concretes, those for pavements and bridge decks have undergone significant changes
in their mixture proportions during the last decade. The use of pozzolans such as silica fume, fly
ash, and slag is now commonplace, with many state DOTs progressing to ternary blends
(cement, silica fume, and fly ash) to produce optimum-performing mixtures. Much of this
paradigm shift in mixture design is due to increased interest in the durability of field concrete.
No longer is 28 d compressive strength the lone criteria for concrete acceptance nor the
benchmark by which different mixtures are compared. Instead, designers are targeting concrete
mixtures that will perform adequately in the field for 50 years and more. With this change,
obviously, the prediction of concrete service life is moving to the forefront of concrete research.
EXPERIMENTAL
Materials
Cores of field concrete were obtained from the Rhode Island and Missouri Departments of
Transportation during the summer 2000 construction season. The specimens received from the
Rhode Island DOT were cores from small slabs (610 mm x 610 mm x 914 mm deep or 2 ft x 2 ft
by 3 ft). The slabs were prepared by pumping the concrete into the molds. The slabs were then
field cured and the cores were removed after 28 d. The cores were 368 mm (14.5 in) in length
and 100 mm (4 in) in diameter. The specimens received from the Missouri DOT were cores from
actual pavement slabs, not from separately cast specimens. These were taken from Route 65 in
Benton County and from Route 13 in Henry County. The cores were either from the driving lane
or the passing lane and were all from the northbound direction of the road. All specimens were
received wrapped in plastic bags and were immediately stored in limewater upon reception by
NIST until testing time. A 50 mm (2 in) slice was cut from each specimen for testing sorptivity.
Precautions were taken to always use the top surface of the pavement. The compositions of the
three concretes are given in Table 1.
Freeze/Thaw Degradation
The service life model for freeze/thaw degradation is based on the critical air void saturation
concept of Fagerlund7. The basic assumption is that the air voids in field concrete are slowly
filled by liquid water during environmental exposure. This “filling” rate is assumed to be
equivalent to the later age sorption coefficient discussed above. When a critical fraction of these
air voids have become saturated (water-filled), the next freeze/thaw cycle will cause damage to
the concrete. For the purposes of the model developed in this paper, failure is characterized by
the time necessary to achieve the critical saturation. The subsequent cracking developed due to
cyclic freezing and thawing is not considered. Critical parameters are the porosity and air void
content of the concrete and its sorption characteristics.