Monday, December 16, 2013

Has road design changed much since the 1950's, when our modern highway system was first being built?

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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