10. Segregation
10.5 Confirming and Quantifying Segregation
Identifying the presence of physical segregation can too often be subjective.
It is best to recognize segregation as it is occurring and make corrections as soon as possible. Unacceptable areas of segregation should be replaced as part of the project contract during construction.
The severity of segregation falls into three general categories:
- Slight—Area where there is slightly more coarse aggregate than in the surrounding acceptable mix.
- Medium—Area has significantly more coarse aggregate than the surrounding acceptable mat and usually exhibits some lack of surface fines.
- Severe—Area appears very coarse in comparison to the surrounding acceptable mat, with stone against stone and little or no fines present.
Work is typically suspended when the medium or severe level of segregation is identified. This allows the contractor the opportunity to address and take corrective action against any further segregation on the project.
If the contractor and the agency agree that an area is significantly segregated, the repair, or even a remove/replace, often takes place quickly. However, if there is disagreement regarding the existence, severity, and extent of the segregation, a method to quantify segregation is helpful in reaching a resolution.
Physical and thermal segregation will cause the mix to be outside of the tolerances for one or several of the specification requirements. Comparing the segregated area test results with the test average, standard deviation, and range representing the rest of the lot’s test results is a way to quantify segregation.
Tests that can help identify and quantify segregation include the following:
- Infrared temperature readings (by scan).
- Density readings (nuclear or lab).
- Binder content (extraction/ignition oven).
- Aggregate gradation (after binder extraction/ignition oven).
- Permeability testing (field or lab).
- Sand patch test (ASTM E965).
Many of these tests are already performed for the general acceptance of the lots. Comparison can also be made to the tolerances from the approved JMF.
Permeability testing can be performed with either a laboratory falling head permeameter or a field permeameter.
A core taken from a visually segregated coarse area in the mat (also known as a rock pocket) will naturally have a lower binder content relative to an area that is not visually segregated.
A series of nuclear density gauge readings can be used to quantify segregation. Typically, a specified number of readings are taken in a prescribed pattern. If the range between readings exceeds specified limits, the mat is deemed segregated in that area.