Iowa DOT's
policy is to be
sensitive to the environment
and foster and enhance
environmental stewardship.
IOWA DEPARTMENT OF TRANSPORTATION
Presents
ROCKS AND ROADS
Unit 2: What materials
are used in
road construction?
C. Rocks are tested, too?
What tests are run on the materials used to build roads? There are several tests to determine the quality of rocks. Each test tells us something different about the rock. The tests are as follows: Freeze and Thaw Test, Iowa Pore Index Test, Abrasion Test, Specific Gravity and Absorption, and Chemical tests.
Freeze and Thaw Test:
In this test, rocks are frozen and thawed several times, or cycles.
This test tells us how much Iowa’s natural freeze and thaw
cycles will affect the rocks. When the water inside the rock freezes,
it expands and will cause the rock to break or fail. We measure the amount of breakage. The
fewer pieces of rock that break, the longer the rock will last in
good condition. This test ties in directly with the Iowa
Pore Index Test.
Iowa Pore Index Test: The Iowa Pore Index is done to measure the amount of water that can be forced into the pore system of a rock. This measurement directly ties into the rocks’ ability to release water. The quicker the water is released, the less susceptible a rock will be to breaking or failing due to the effects from freezing and thawing.
Iowa pore index apparatus
Iowa pore index control panel with primary
(large pores) and
secondary (capillary pores) readouts.
Abrasion Test: For the abrasion test, rock samples are tumbled in a large drum with steel balls. This test will show how well the rock can withstand the pressures put on it by vehicles as they drive over it on the road. The less rock that is smashed into small bits, the longer the rock will last in good condition as part of a road.
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abrasion tumbler |
Box on the right contains materials before being placed in the abrasion tumbler. Box on the left contains materials after going through the abrasion tumbler. |
Specific Gravity and Absorption
Tests:
These tests are done on rocks that
go into the bases and pavements. They tell us how much liquid a rock
will absorb. It also measures how much a rock weighs. The reason we need
to know this information is so we can get the correct combinations
of materials when making the paving mixtures.
Chemical Tests: These tests tell us how a rock will withstand the deicing salts applied to the roads.
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X-ray Diffraction Machine |
Thermogravimetric Analysis Machine |
X-ray Fluorescence Machine
Pavement Materials: It’s more than black and white.
Now for the top layer of the cake - the pavement materials. These materials include hot mix asphalt (also known as HMA) and portland cement concrete (also known as PCC).
Hot mix asphalt is "the black stuff" used as a pavement material. First, what is asphalt? Asphalt is a tar-like, petroleum material. Asphalt becomes a liquid at high temperatures. When it is at room temperature, it is solid. Asphalt can exist naturally or can be produced as a by-product of petroleum refining.
Just as with rocks, the Iowa Department of Transportation tests the hot mix asphalt. These tests are run to test the in-place characteristics of the asphalt paving. There are two categories of tests that are run: compaction and extraction.
Compaction Tests: These tests simulate the rollers used to compact the asphalt paving as it’s being laid down on the road grade. This test measures the air bubble content of the pavement. Why is it important to know this value? It is important because air voids directly affect to the overall life of the pavement. This is a real-time evaluation of the stability of the pavement.
Extraction Tests: These tests remove the asphalt portion of an asphalt paving sample. In order to remove the asphalt from the paving sample, the sample is heated to temperatures more than 1000° F. This temperature allows all the asphalt to burn off the sample. The difference between the sample weight before and after the heating process shows how much of the sample was asphalt and how much was rock. This way we know if the right amount of asphalt was used.
Where is hot mix asphalt used and how long does it last? Generally, this type of pavement is used on lower volume state highways and county roads. Counties prefer this type of pavement because it usually costs less. The life expectancy of an asphalt pavement is between 15 to 20 years.
Portland cement concrete is the "white stuff" used as pavement material. What is portland cement? Portland cement is made by heating shale, limestone and small amounts of iron-ore to very high temperatures. It is then cooled and ground into a fine powder. Gypsum is added to the powder. The gypsum keeps the powder from hardening instantly when water is added. There are three cement plants located in Iowa.
Portland cement concrete is made by mixing portland cement with water, and coarse- and fine-size rocks. All the materials, including the right amount of air bubbles, must be in the correct proportions in order for the mixture to harden properly. As with hot mix asphalt, a properly mixed concrete has all the rock pieces completely coated all the way around each piece.
The Iowa Department of Transportation also runs tests on portland cement concrete. The tests give us information on how a portland cement concrete pavement will stand up to the pressures exerted by traffic and weather. The two basic categories of tests that are run on new pavements are strength and durability.
Strength Test: This test applies pressure from above on a concrete beam.This pressure is exerted until the beam breaks. This tests the ability of the portland cement concrete pavement to withstand the pressures exerted by vehicles driving on it. If the sample is not able to withstand the simulated pressures of traffic, then it will fail.
Air Content Test: This test measures the air bubble content of a sample of new pavement. The reason this is important is because it directly relates to the durability of the pavement. If there are not enough air bubbles in the pavement, there will be no place for the water to expand into as it freezes. If this happens, it will crack the concrete, causing it to fail. Failing means that it will not last as long as it’s been designed to last and it will need repairs much sooner than it should.
Where is portland cement concrete used and how long does it last? This type of pavement is used on higher volume state and interstate highways. It is also used on county roads, but not as often. It is not used as often because it initially can be more expensive. The life expectancy of portland cement pavement varies. It can last as little as 20 years or more than 40 years.
What can affect the lifetimes of HMA and PCC? Several factors affect pavement life. Some of the factors include: traffic volume, quality of materials used, construction practices, and how well the other layers of the road perform. All of these things function together—and any one of them can add to or detract from the pavement’s lifetime.
As we mentioned earlier, several elements make up the roadway. Collectively they are called the pavement system.
Pavement System
Each layer represents one of the elements of the pavement system. When roads are built higher than the surrounding ground, a structure of compacted earth called an embankment is built. The embankment is built to support the other three layers of the pavement system.
So what are the purposes of these different elements of the pavement system? Well, they all work together to provide a quality durable pavement.
Embankment: The construction of embankments for roadways can take up a large part of the total cost. As you have already learned, the embankment acts to support the pavement system. Embankments can be made from almost any common type of deposit except topsoil. Some types of deposits are easier to compact to form the embankment. Glacial till and sand, for example, compact easier than river silts and clays. River silts and clays must be dried out before
compacting.Subgrade: The subgrade is a select soil material that is carefully compacted to provide uniform support to the pavement. The subgrade lies directly on either the embankment or the native soil. The subgrade is made of either glacial till or clean select sand if these materials are locally available. Otherwise, crushed limestone rock may be used, although this is more expensive.
Base: The base layer provides uniform support to the pavement and allows water that penetrates any joints or cracks in the pavement to move quickly to the subdrain without saturating and softening the subgrade. The base layer lies directly on top of the subgrade and is built of clean sand or rock.
Subdrain: The subdrain collects water from the base and the subgrade and drains that water into the ditch. The subdrain sits alongside the pavement, base and subgrade. The subdrain is basically a trench with a perforated pipe near the bottom, surrounded by clean coarse-size rock which allows rapid transportation of water.
Pavement: The pavement itself resists bending, and distributes vehicle weights over a large area.
