Crushed Edwards Limestone Road Base

More than fifteen million tons of Crushed Edwards Limestone Flexible Base is produced annually from a geological limestone formation named the Edwards Plateau.^[1] The Edwards Plateau is located in Southwest Texas in the United States of America. Limestone from the Edwards formation has been quarried extensively because it has several characteristics that make it easy to quarry and uniquely useful as a road base material..

Edwards Limestone Road Base

In most cases, this stone can be found within inches of the surface. This is beneficial because the lack of overburden means that only a small amount of waste material must be removed before productive quarrying can began. The Edwards limestone is composed primarily of Calcium Carbonate and Magnesium Carbonate.^[2] Since the stone contains very little combined silica oxide the stone is minimally abrasive to crushing and screening equipment used in the quarrying process

Many road builders and engineers consider the use of high quality road base to be a vital factor in building long lasting roads. Because crushed Edwards Limestone Road Base has been used for over 60 years, its service record is well known.

History

The Edwards Plateau Limestone was formed 120 million years ago when much of Texas was covered by a shallow sea.

Crushed Edwards Limestone Base has been used extensively in the Austin, Texas, USA area since the mid 1940’s. Prior to this the primary road building material being used in the Austin area was pit run gravel stabilized with emulsified asphalt or caliche. Both pit run gravel and caliche are naturally occurring deposits and are inconsistent in quality. Roads built of crushed limestone base have proven to hold up better than caliche or pit run gravel in wet weather over a long period of time.^[3]

With the introduction of Crushed Edwards Limestone Base into the Austin Texas Market in the 1940s, road builders soon began to appreciate the unique properties of this excellent road base. Crushed Edwards Limestone Base was inexpensive, easily placed, and resulted in long lasting, smooth riding roads. The only equipment required to lay crushed stone base is a motor grader, water truck and compaction roller. By the mid 1950’s this crushed base material was the standard road building base material used in the Austin area.^[4]

Because excellent results had been achieved on many projects, crushed Edwards Limestone Base was chosen for use on the interstate highway as it was built between Temple and New Braunfels, Texas. Crushed limestone base was chosen in this area in spite of the fact that concrete paving was being used on much of the interstate system. The decision to build Interstate Highway 35 out of crushed limestone base with a relatively thin asphalt surface turned out to be a success. While much of the interstate system has been rebuilt numerous times, the sections built using Crushed Edwards Limestone have only required periodic resurfacing with hot mix asphalt. Other areas of the country are not blessed with a local material with the properties of the Edwards limestone.

Road Base Quality Factors

Limestone Road Base Holding Up 200 Tons

Limestone Sticking Together

A good road base should perform well in both dry and wet weather. Since limestone base is made up of broken particles of solid rock, it performs well even in wet weather. A road base’s function is two fold. First, it must provide a roof over the underlying soil. Soils are strong when they are dry and weak when they are wet. Edwards Limestone has a characteristic of adhesion to itself. This stickiness and the fact that there is a crushed rock to fill every void, allows the crushed limestone base to form a dense impervious mat that sheds water. Second, a road base must spread the concentrated load of vehicles using the road to a large area of the weaker soil below. Once properly packed into place, the limestone base is itself strong enough to support the largest rubber tired wheel load. The limestone base layer then spreads the load over a large area of soil below.

Road Base Component Sizes

Pavement Design Considerations

Depending on the strength of the underlying sub grade and the weight and density of the expected traffic, the depth of flexible base will vary between six inches and thirty inches. Crushed Edwards Limestone Flexible Base had an average sales price of $4.88 in Texas in 2007.^[1] If the design proves to be too thin it is easy to add depth to an existing base layer. Limestone base is typically placed in two or more courses of four to six inches in depth.

Since Crushed Edwards Limestone Base is not usually placed with cement, asphalt, or lime additives, there is not a time restriction on completing placement of the base course. Limestone Base can be easily picked up after placement and be reused in another location.

Carbon Dioxide Footprint

Crushed Limestone base has a low carbon dioxide footprint compared to hot mix asphalt and reinforced Portland cement concrete. To quarry and process one ton of limestone base requires 0.17 gallons of diesel, 1.05 pounds of explosive, and 0.95 Kilowatt hours of electricity. The carbon dioxide output from these energy inputs is 9.01 pounds of carbon dioxide per ton of limestone base. By way of comparison, a ton of concrete (88% sand and aggregate – 12% cement) has a carbon dioxide footprint of 309.16 pounds of carbon dioxide.^[5] Hot mix asphalt (94% sand and aggregate – 6% liquid asphalt) has a carbon dioxide footprint of 93.56 pounds of carbon dioxide per ton.

Placing Limestone Flexible Base

Crushed Edwards Limestone Base is easy to place, but the sub grade must be properly prepared. The steps to preparing a sub grade are to first insure that the roadway is well drained.^[6] Next, the sub grade must be dried or wet to optimum moisture level and compacted. When the sub grade is ready, then the base can be dumped on the sub grade, spread, and compacted.Both the sub grade and limestone base should be compacted at optimum moisture. Optimum moisture is the amount of moisture that will allow a material to be compacted to maximum density. This number is determined in the laboratory. Experienced construction personnel can usually get very close to optimum moisture, simply by observing the appearance of the materials. Many people, experienced in placing limestone base, will intentionally get the base a little too wet and then let it dry back as they compact the base.

Location

To be economical, a construction material must be quarried close to the point of use. This is because it costs as much to haul crushed stone 25 miles as it does to quarry and process the material. Millions of tons of limestone base have been hauled into South East Texas and Southern Louisiana because there is a minimal amount of locally produced aggregate and basically no limestone.

In Central Texas, just west of Interstate Highway 35, lies a band of Edwards Limestone that has supplied the surrounding communities with quality base materials for decades. The main body of the Edward’s Plateau, however, covers a large area of South West Texas.^[7]

References

1. ^ ^{a b} U.S. Department of the Interior, U.S. Geological Survey, November, 2010 http://minerals.usgs.gov/minerals/pubs/state/2007/myb2-2007-tx.pdf
2. ^ Bliss, James D., Timothy S. Hayes, and Greta J. Orris. USGS 2008-3089 http://geology.com/usgs/limestone/
3. ^ http://en.wikipedia.org/wiki/Caliche
4. ^ Colletti, Shannon, June 13, 2008 http://impactnews.com/georgetown-hutto-taylor/local-news/143-news/1203-georgetowns-largest-quarry-turns-50
5. ^ http://en.wikipedia.org/wiki/Portland_cement
6. ^ Summers, C.J., 2000-2010 http://www.highwaysmaintenance.com/drainage.htm
7. ^ http://en.wikipedia.org/wiki/Edwards_Plateau

See also

• Limestone
• Calcium carbonate
• Edwards Plateau