PCC carries weight in Illinois, study says A study by the
University of Illinois and the Illinois DOT shows that concrete
pavements in Illinois have performed very well. The report,
Performance of Original and Resurfaced Pavements on the Illinois
Freeway System, Research Report 540-1, is dated February 1997.
Conducted in cooperation with the FHWA, the study included
about 2,000 centerline miles of heavily trafficked, interstate
concrete pavements constructed between 1957 and 1994. About
two-thirds were constructed as continuously reinforced pavements
(CRCP) ranging in thickness from 7 to 13 in. About one-third
were 10-in., 100-ft jointed reinforced pavements (JRCP). The
performance was evaluated by survivor analysis, which is a
statistical method for determining the life expectancy of
subjects in an experiment. The technique is more appropriate
than simply computing the average life of failed (overlaid)
sections when not all sections in the experiment have yet
reached the end of their life. The report concludes that
"bare concrete pavements in Illinois performed very well,
carrying far more traffic to failure (first overlay) than the
amounts for which they were originally designed." Depending on
pavement thickness and type (CRCP and JRCP) the age before
overlaying varied from 22 to 29 years, ESALs carried from 18 to
39 million. The ratio of ESALs carried to design ESALs varied
with concrete pavement type and thickness from 2 to 14. This
was for pavements without D-cracking. Pavements with D-cracking
were analyzed separately. The average actual/design ESAL ratio
was less (about 3.0) as would be expected. This shows that
efforts to screen D-cracking aggregates is a cost-effective way
to increase pavement life. An interesting comparison is made
between CRCP and JRCP. The report states, "A 10-in. JRCP carries
approximately the same ESALs as an 8-in. CRCP. A 10-in. CRCP
carries far more ESALs (2.1 times) than a 10-in. JRCP on
average. The performance of JRCP and CRCP are clearly different,
which is not in agreement with the AASHTO design guide. The
guide is clearly over designing CRCP." An editorial note from
the reviewer states that JRCP would have performed more
similarly to CRCP had 30- to 40-ft joints been used as is the
modern practice. Packard is director of engineering-design
for the American Concrete Pavement Association. You may write
him in care of the editor.