Has road design changed much since the 1950's, when our modern highway system was first being built?
Generally speaking the answer is “yes” roadway design has become more sophisticated. We are using computer software to optimize the roadway design. We are implementing higher quality control in the procedures and materials we use. The pavement is thicker now because there are more heavy trucks using the roadways. We are now re-using more of the existing pavement for the roadway base and adding rubber from used tires in asphalt pavement, where appropriate, to conserve the environment. Furthermore, the rubber makes the roadway surface more durable.
Where
it has not changed significantly is the type of material used. We still
use primarily two types of pavements, hot mix asphalt (HMA) pavement which is a
flexible pavement and Portland cement concrete (PCC) pavement which is a rigid
pavement.
When
HMA is selected as the roadway surface, Caltrans considers incorporating rubber
from old used tires into the design. Here in the San Francisco Bay Area, paving
is sometimes allowed only at nights due to heavy traffic during the day and
closing a traffic lane for paving is not practical. Rubberized HMA
pavement does not hold up well to traffic when the paving operation is done at
an air temperature below 55 degrees. Therefore, it might not be practical
to use rubberized HMA when the air temperature cannot be met. A bill was passed
by the California Legislature requires Caltrans, on a statewide average, to use
rubber in at least 20% of its flexible pavement beginning in 2007, 25%
beginning in 2010 and 35% beginning this year. So far, we have been
successful in exceeding the mandated requirements. Also, Caltrans often
places “Open Graded HMA” at locations with roadway surface drainage
problems. The aggregates in the HMA are more uniform in size creating a
more porous surface than regular HMA. The porous surface helps drain
surface water and improves tire contact with the roadway surface in wet weather.
PCC
slabs are typically 12 feet wide and 12 to 15 feet long with steel dowel bars
keeping the slabs together. However in 2011, Continuous Reinforced
Concrete pavement (CRCP) was installed at the new Presidio Parkway tunnel area
on Route 101 in San Francisco. A significant amount of steel reinforcement bars
were placed within the pavement section. PCC was poured over the steel
reinforcement bars into one massively long slab of approximately 1000 feet long
(see attached photos). The pavement ride is smoother since there are no
pavement joints. It is expected this pavement will have less major cracks
over time than conventional PCC pavement, and thus less maintenance.
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