Aggregates for Chip and Seal Pavement Andrew Gauck

Aggregates for Chip and Seal Pavement Andrew Gauck Harrison Sand Gravel Co.

Aggregate Quality that affects Performance • Gradation • Absorption • Durability

Gradation

Aggregate gradation plays an important role in seal coat design, construction and performance. Gradation refers to the distribution of the various sized stones that make-up the aggregate matrix. Aggregates used in seal coat construction are normally classified as either one size or graded.

One sized Aggregate

INDOT Seal Coat Gradation

Decantation - A test utilizing water to determine the amount of material that is passing the No. 200 sieve. The decantation test is conducted on both fine and coarse aggregate and is usually done in conjunction with the sieve analysis test.

Why is decant important to me?

Dusty Aggregates containing dust should not be used for seal coating unless certain precautions are taken. To avoid dusty aggregate, the specified aggregate gradation should have 1. 5 percent or less passing the #200 sieve (75 cm). Dust will coat the outside of the aggregate particles and prevent them from bonding with the bituminous binder. Consequently, extensive chip loss will result.

Specification Difference

Loose Aggregate on the road is included in the gradation

Absorption

• Absorption: The increase in mass due to water in the pores of the material.

ABSORPTION The absorption quality requirement applies only to coarse aggregates, but this data is necessary on fine aggregate for other purposes, such as mix design and water/cementations ratios. All aggregates are porous, but some are more porous than others. How porous an aggregate is determines how much liquid may be absorbed when soaked in water. AASHTO T 85 defines absorption as the increase in the weight of aggregate because of water in the pores of the material, but not including water adhering to the outside surface of the particles. Absorption is expressed as a percentage of the dry weight. Absorption requirements are of concern only regarding aggregates used in hot mix asphalt and portland cement concrete. The intent is to avoid using highly porous, absorptive aggregates because extra water and cement or asphalt is needed to make a good mix.

AGGREGATE VOIDS There aggregate particle voids, and there are voids between aggregate particles. As solid as aggregate may be to the naked eye, most aggregate particles have voids, which are natural pores that are filled with air or water. These voids or pores influence the specific gravity and absorption of the aggregate materials. Figure 3 -1. Aggregate Specific Gravities. The voids

Durability

Test that determine Durability LA Abrasion Freeze Thaw

• Los Angeles Abrasion Test • A common test used to characterize toughness and abrasion resistance is the Los Angeles (L. A. ) abrasion test. For the L. A. abrasion test, the portion of an aggregate sample retained on the 1. 70 mm (No. 12) sieve is placed in a large rotating drum that contains a shelf plate attached to the outer wall (the Los Angeles machine – see Figure 3. 9). A specified number of steel spheres are then placed in the machine and the drum is rotated for 500 revolutions at a speed of 30 33 revolutions per minute (RPM). The material is then extracted and separated into material passing the 1. 70 mm (No. 12) sieve and material retained on the 1. 70 mm (No. 12) sieve. The retained material (larger particles) is then weighed and compared to the original sample weight. The difference in weight is reported as a percent of the original weight and called the "percent loss".

LA Machine

Abrasion Resistance Abrasion resistance applies only to coarse aggregates. Aggregates vary in their resistance to fracturing under impact (toughness) and breaking down into smaller pieces from abrasive action (hardness). The acceptable limits are set by the Los Angeles Abrasion Test in the test method AASHTO T 96. The limits vary from 30. 0 to 50. 0 percent, depending on the classification of the aggregate. The percentage is a measure of the degradation or loss of material as a result of impact and abrasive actions. Section 904. 03 details the requirements. Abrasion requirements do not apply to blast furnace slag.

Soundness The quality of soundness applies to both fine and coarse aggregates. The durability of aggregates or their resistance to the forces of weathering is undoubtedly one of the most important considerations in the selection of a material for highway construction. The primary exposure that INDOT is concerned with is alternate freezing and thawing. INDOT uses three different test methods to evaluate soundness: 1) The sodium sulfate test in test method AASHTO T 104; 2) The freezing and thawing test in test method AASHTO T 103; and 3) The brine freeze/thaw in ITM 209.

The sodium sulfate test requires immersing an aggregate sample in a sodium sulfate solution for a period of time and then determining the weight loss of particles on a given set of sieves. The brine freeze/thaw requires the aggregate to be enclosed in a bag containing a 3 percent sodium chloride solution and subjected to 25 cycles of freeze and thaw. The freezing and thawing test requires subjecting a sample of aggregates sealed and totally immersed in water to freezing and thawing of 50 cycles before determining the mass loss. The freezing and thawing in water test is the method that most accurately simulates actual field conditions, but the test requires a long period of time to conduct. The “quick” checks for soundness of the aggregate are the brine freeze/thaw and sodium sulfate test. If the aggregate fails either the brine freeze/thaw or the sodium sulfate test, the material is tested using the freeze and thaw method. An aggregate that reasonably fails the brine freeze/thaw and/or sodium sulfate test but then passes the freeze and thaw in water method is an acceptable material for use on INDOT contracts.

WHERE TO FIND YOUR IMPORTANT INFORMATION

INDOT

Your local Producer

INDOT Example SUM Qual Form

INDOT Example Sum Qual Form

30 point statistical data

Most Important Ask for Information INDOT requires producers to test and keep records of Gradation and Quality. Most all contracts require disclosure of these records! Shouldn’t you !!!!

Aggregate Quality means nothing if Best Practices are not followed for placement of the Surface.

Chip and Seal will not remedy conditions due to a poor sub-base. It can keep good pavement in good condition by sealing out water, but provides no structural strength and will only repair minor cracks.

Some Best Practices • the road surface needs to be properly cleaned of debris and any holes patched. • The asphalt must be fluid so the rock will be embedded by the displacement of the asphalt. • a rubber-tire roller is used to set the rock into the liquid asphalt. Rolling orients the flat sides of the rock down and produces a tighter chip seal. • It takes two to four passes of the roller to set the rock. • Sweeping is done at the completion of the chip seal process to remove surplus rock from the surface.

? ? ? Questions ? ? ?