Tensile Strength Ratio (TSR): AASHTO T 283 Explained

The tensile strength ratio, or TSR, is the number most agencies use to decide whether an asphalt mix is likely to strip when it gets wet. The idea is simple: make two matched sets of compacted specimens, soak and stress one set with water, then crush both and compare how strong they are. If the wet set keeps most of its strength, the binder is holding onto the stone. If it loses a lot, water is getting between them.

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The short version

Formula: TSR = average conditioned strength ÷ average unconditioned strength.
Test method: AASHTO T 283 (modified Lottman). ASTM D4867 is similar.
Common minimum: 0.80 under Superpave. Many agencies have used 0.70 to 0.80.
Typical results: often 0.70 to 0.90, with some mixes below or above that band.

Why moisture damage matters

Stripping happens when water breaks the bond between asphalt binder and aggregate. Traffic squeezes water in the pores of the mix, building pressure and scouring the binder away from the stone. The pavement may look fine for a while and then ravel, pothole or rut once enough binder has let go. Our page on stripping and moisture damage covers the field signs. TSR is the lab screen meant to catch a weak mix before it is placed.

How the T 283 test is set up

This is an outline, not the full procedure.

Mix and cure. Loose mix cools for about 2 hours, then spends 16 hours curing in a 140°F (60°C) oven, then is reheated at 275°F (135°C) for 2 hours before compaction. The curing step lets absorptive aggregate soak up binder the way it would in a plant and truck.
Compact six specimens, usually about 6 inches across and close to 4 inches tall, to roughly 7 percent air voids or the voids expected in the field. A gyratory compactor is typical.
Measure voids. Each specimen's bulk specific gravity and the mix's maximum specific gravity give its air voids. Our pages on bulk specific gravity and theoretical maximum specific gravity explain those numbers.
Split into two sets of three with similar average voids. One set stays dry. The other is conditioned.
Saturate the conditioned set. Specimens sit under water in a vacuum container until 55 to 80 percent of their air voids are filled. Above 80 percent the specimen is considered damaged and is thrown out.
Optional freeze. Each saturated specimen goes into a sealed bag with about 10 mL of water and spends at least 16 hours at 0°F (−18°C).
Hot soak. The conditioned set sits in a 140°F (60°C) water bath for 24 hours.
Bring both sets to 77°F (25°C) for at least 2 hours.
Split them. Each specimen is loaded across its diameter at 2 inches per minute until it cracks in two. The peak load is recorded.

From mixing to the last break, the full test can take close to a week, so it is usually done during mix design rather than daily production.

The math

Indirect tensile strength: St = 2P ÷ (π × t × D)

Tensile strength ratio: TSR = S2 ÷ S1, where S1 is the average dry strength and S2 is the average conditioned strength.

Example (made-up numbers): specimens are 6.0 inches across and 3.75 inches thick. The dry set averages a peak load of 2,400 lb. St = (2 × 2,400) ÷ (3.1416 × 3.75 × 6.0) = 4,800 ÷ 70.69 = about 67.9 psi. The conditioned set averages 2,000 lb, which works out to about 56.6 psi. TSR = 56.6 ÷ 67.9 = 0.83, so this mix would clear a 0.80 minimum by a small margin.

Labs also look at the broken faces. Bare, uncoated stone on the conditioned specimens is a warning sign even when the ratio passes.

Why air voids can swing the result

Water gets into a mix through its connected air voids, so the void level of each specimen changes how much water it takes on and how much strength it loses. Small differences in voids between the two sets can move the TSR noticeably. That is why the standard has the lab match the average voids of the two sets and control the degree of saturation.

Limits of the test

It gives a pass or fail signal, not a prediction of how many years a pavement will last.
Repeatability between labs has long been a concern, and mixes that look stripped can still post decent ratios in some cases.
A ratio alone hides how strong the mix is to begin with. A weak mix can post a decent ratio simply because there was little strength to lose, which is why some agencies also look at the raw strength values.

Because of those gaps, some agencies pair TSR with the Hamburg wheel tracking test, which rolls a loaded wheel over specimens under hot water and looks for a sudden change in rutting rate.

Fixing a low TSR

Hydrated lime, usually about 1 to 1.5 percent of the aggregate weight, added as a slurry or to damp aggregate.
Liquid anti-strip, usually amine-based, at roughly 0.1 to 1 percent of the binder weight.
Cleaner aggregate, with less clay coating and dust.
A different rock source, since some mineral types simply bond better with asphalt.

For a homeowner, TSR shows up as a line on a mix design sheet, but it is part of why a good plant mix holds together through wet springs. Once the mix is chosen, the asphalt calculator turns area and thickness into tons.

Questions people ask

What is tensile strength ratio in asphalt?

It is the average indirect tensile strength of moisture-conditioned specimens divided by the average strength of matching dry specimens. It shows how much strength a mix keeps after water exposure. A ratio of 0.85 means the wet set kept 85 percent of the dry set's strength.

What TSR value is required?

Superpave mix design calls for a TSR of at least 0.80. Many agencies have used minimums between 0.70 and 0.80, so check the spec on your project.

Which test method gives TSR?

AASHTO T 283, often called the modified Lottman test. ASTM D4867 is a related method. Both split compacted specimens across their diameter after one set has been conditioned with water.

How is indirect tensile strength calculated?

St = 2P ÷ (π × t × D), where P is the peak load in pounds, t is the specimen thickness and D is the diameter, both in inches. The answer is in psi.

Why are specimens compacted to about 7 percent air voids?

The test aims to mimic a newly built pavement, which usually has more air voids than a lab design specimen. Water needs those connected voids to get into the mix, so very dense specimens would hide a moisture problem.

What if a mix fails the TSR test?

Common fixes are adding hydrated lime or a liquid anti-strip agent, changing the aggregate source or cleaning up dusty fines, then retesting. Some agencies also run a Hamburg wheel tracking test for a second opinion.

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