19-11-2012, 06:26 PM
MECHANISTIC-EMPIRICAL DESIGN CONCEPTS FOR CONTINUOUSLY REINFORCED CONCRETE PAVEMENTS IN ILLINOIS
MECHANISTIC-EMPIRICAL DESIGN.pdf (Size: 651.86 KB / Downloads: 54)
INTRODUCTION
A continuously reinforced concrete pavement (CRCP) is constructed without manmade
transverse contraction or expansion joints and contains spliced longitudinal reinforcing
steel bars. This type of pavement is characterized by the development of transverse cracks
spaced roughly 2 to 6 ft apart. The steel reinforcement is designed to promote regularly
spaced cracks and to hold these transverse cracks tightly together. Illinois has extensive
experience with CRCP, as this type of pavement has been widely used in the state since the
mid-1950s (Gharaibeh et al., 1999).
Punchouts have been the most serious structural performance problems for CRCP in
Illinois (Zollinger and Barenberg, 1990). A punchout is an isolated piece of concrete that
settles into a depression or void at the edge of the concrete slab as shown in Figure 1. This
type of distress is a structural failure and develops at a location bounded by two transverse
cracks, a longitudinal fatigue crack, and the edge of the pavement. They can also occur at
the intersection of Y cracks. Erosion of the subbase and subsequent loss of support under
the slab has been identified as a primary cause of punchout formation (Zollinger and
Barenberg, 1990).
CRCP DESIGN PROCEDURES
A number of CRCP design procedures have been developed over the years to
determine thickness and/or longitudinal steel requirements. The Portland Cement
Association method (PCA, 1984) determined from a finite element study with JSLAB that
CRCP bending stresses were smaller with short crack spacing, but had higher corner
deflections. Based on these analyses, the PCA design method recommended using the
same thickness for CRCP as calculated from their jointed plain concrete pavement (JPCP)
design method. The 1993 AASHTO design method provides for CRCP thickness and
longitudinal steel design. The AASHTO CRCP thickness requirement is based on the
AASHTO thickness design equations for jointed concrete pavements, with the use of slightly
different load transfer coefficients. The AASHTO (1993) design method is based on
empirical equations derived from testing of doweled-jointed plain and reinforced concrete
pavements sections at the AASHO Road Test. The AASHTO procedure for CRCP also
determines the longitudinal reinforcing steel content to limit crack spacing, crack width, and
allowable steel stress (Huang, 2004). Neither the PCA nor the AASHTO design methods
design the slab thickness to resist the development of punchouts in CRCP.
IMPROVED CRCP DESIGN PROCEDURE IN ILLINOIS
The objective of this study was to develop and implement an M-E design procedure
that IDOT can use for routine CRCP design. When considering improvements to the CRCP
design procedure in Illinois, two options were considered. The first was to use the M-E PDG
(NCHRP, 2007) and calibrate its performance prediction models against a range of Illinois
design inputs. The second was to develop a new mechanistic-empirical design procedure
for Illinois based on the M-E PDG performance prediction models. The second option was
selected because it allows for the modification or exclusion of specific models and inputs
now and in the future. This report proposes CRCP design concepts for Illinois based on
mechanistic-empirical design principles taken largely from the models contained in the M-E
PDG.
CRCP DESIGN CONCEPTS
This chapter lists equations and concepts used in the proposed CRCP design
procedure relating to climate, concrete properties, traffic, transverse crack spacing, crack
width, load transfer, tensile stresses, fatigue damage, and punchout prediction. The
majority of these parameters change continuously during the design life, and a seasonal
approach is used to describe this time-dependent behavior.
CLIMATE
The temperature differential between the top and bottom of a concrete slab ( ΔT ) is
critical to the calculation of curling stresses and subsequent pavement damage. Ambient
(air) temperatures and the temperature at the depth of reinforcing steel are also used in the
calculation of mean crack spacing and average crack width. The Enhanced Integrated
Climatic Model (EICM) version 3.4 (Larson and Dempsey, 2008) was run to obtain slab
temperature differential frequencies, as well as the temperature at the depth of steel for
Illinois.
The model was run for four concrete thicknesses (8, 10, 12, and 14 in.) located in
Champaign, Illinois. This location was chosen because past work with the EICM has found
that Champaign provides a representative climate for Illinois (Roesler et al., 2008). The
concrete pavements were assumed to have a 4-in. asphalt concrete base and a concrete
short-wave absorptivity value of 0.65. This value can vary depending on the color of the
concrete and may be as high as 0.85. Temperature differentials through the concrete and at
the depth of steel temperatures were obtained for every hour over a seven-year period
(December 1997 to November 2003) for each of the four concrete thicknesses. The climatic
data were also used to determine the minimum and average seasonal ambient
temperatures that were needed for the crack spacing calculation.
Temperature at the Depth of Steel
Depth to steel (ζ ) is defined as the depth from the surface of the concrete slab to
the top of the reinforcing steel. Current IDOT standards put the depth of the longitudinal
reinforcing steel at 3 in. when the pavement thickness is less than or equal to 8 in., and at
3.5 in. when the pavement thickness is greater than 8 in. The new proposed depth to steel
as a function of slab thickness is presented in Table 2, which is approximately one-third of
the concrete slab thickness. For the purposes of this climatic analysis, the steel depth was
taken as 4 in., regardless of design life. The overall thickness of the concrete slab had a
negligible effect on the temperature at the depth of steel. The average seasonal
temperatures at the steel depth for Champaign are shown in Table 3. These values were
calculated using hourly temperature data from the seven-year period examined with the
EICM.