Recycled Binder Availability: Testing, Design, and Specification
Last updated August 31, 2026
Some of the binder in reclaimed asphalt pavement stays trapped inside its particles and never blends into the new mixture. Standard volumetric design assumes all of it does. Measured availability runs anywhere from about 40 to 100 percent, while agencies that apply any discount mostly specify 75 percent or higher, so high recycled content mixtures routinely carry less effective binder than their designs claim. The course covers how availability is defined and measured, including the glass beads tracer method from NCHRP Project 09-68 and the sieve analysis method behind North Carolina's availability-adjusted design procedure. It then works through the adjusted binder content calculation, the volumetric failures the added binder creates and the three specification routes for reconciling them, the performance trade between cracking and rutting resistance, and how Georgia and Florida arrived at the values they specify. It closes with stockpile variability in production, the cost and break-even picture, and the gaps that remain in the field evidence.
Learning Objectives
Explain the blending mechanism that keeps part of the recycled binder in a mixture from combining with the virgin binder.
Compute an availability-adjusted binder content from measured stockpile inputs on the correct recycled content basis.
Anticipate the volumetric effects of an availability adjustment on air voids, VMA, and VFA in a finished design.
Evaluate a specified availability value against the measurement method and production temperature it was derived from.
BasePDH | Course 019 | 4 Professional Development Hours
Recycled Binder Availability: Testing, Design, and Specification
1. Availability as a Design Variable
Superpave volumetric mix design treats the binder in reclaimed asphalt pavement as interchangeable with virgin binder. Every gram counted in the recycled material's measured binder content is credited toward the mixture's effective binder content, and the virgin binder is reduced accordingly. That accounting is what makes a high recycled content mixture cheap. It is also what makes it dry, because a fraction of the recycled binder never leaves the particle it arrived on, and the design has already spent it.
NCHRP Project 09-68 defines recycled binder availability (RBA) as the amount of recycled binder from reclaimed asphalt pavement, and from recycled asphalt shingles where used, that activates and contributes to the total effective binder content in an asphalt mixture, expressed from 0 to 100 percent [1]. The definition is attributed upstream to Epps Martin et al. (2021). Two features of it govern everything downstream. It is a property of the mixture, not of the recycled material sitting in the stockpile, so the same reclaimed pavement carries a different availability in two different mix designs. And it is a quantity parameter: it says how much of the recycled binder participates, and says nothing about what condition that binder is in.
1.1 Quantity and quality are separate problems with separate remedies
The distinction between binder quantity and binder quality is the one most often collapsed in practice, and collapsing it produces the wrong fix. Recycled binder is aged, which is a quality problem: it is stiff, its relaxation capacity is poor, and it drives fatigue and thermal cracking. Recycled binder that never activates is a quantity problem: the mixture is short of the effective binder needed for coating and flexibility. A softer virgin binder grade or a recycling agent addresses quality. Neither addresses quantity, because neither adds any binder to the mixture [1].
Availability is the quantity parameter, and the mechanism that limits it is physical rather than rheological. Reclaimed pavement arrives from milling as agglomerations, clusters of aggregate particles held together by aged mastic. Mastic on the outside of a cluster is in contact with virgin binder and available to blend. Mastic bound inside the cluster is not, and virgin binder does not diffuse into it on the timescale of plant mixing [3]. NCDOT RP 2021-06 states the consequence for measurement directly: agglomerations reduce recycled binder surface area and block diffusion into the cluster, a finding it traces across Al-Qadi et al. (2009), Yan et al. (2017), Park et al. (2020), Stimilli et al. (2015), and Bressi et al. (2015) [3].
1.2 The vocabulary is not settled, and the differences are load bearing
Four terminologies are in circulation for what is nearly the same idea, and a specification that borrows a value across them will import the wrong number.
The RILEM TC264 TG5 vocabulary, summarized in NCHRP 09-68 Volume II, splits the concept three ways [1]:
| Term | Definition | Property of |
|---|---|---|
| Degree of Activity (DoA) | The minimum amount of recycled binder that is active without additives | the recycled material |
| Degree of Availability (DoAv) | The maximum amount available for blending with virgin binder and any additives, including recycling agents and warm mix products | the recycled material |
| Degree of Blending (DoB) | The extent to which the recycled binder contributes to the properties of the mixture made of virgin materials, recycled material, and additives | the mixture |
Degree of Blending is the concept equivalent to RBA as NCHRP 09-68 defines it; Degree of Activity and Degree of Availability describe the recycled material alone [1]. Sources given are Lo Presti et al. (2020), Menegusso Pires et al. (2021), and Sobieski et al. (2022).
NCDOT RP 2021-06 draws its own two-term split, and it is the cleaner one for design work. Recycled binder availability there is an inherent property of a recycled material source: the fraction of its total recycled binder available to blend with virgin binder. Recycled binder contribution (RBC) is a property of a mixture: the fraction of total recycled binder that ends up inside the virgin binder matrix, set by availability plus the interaction between recycled and virgin materials, the additives used, and production conditions. The report states plainly that the two terms are not interchangeable [3].
NCHRP 09-68 is not internally consistent on its own wording either. Volume II Appendix C defines availability as binder that "becomes activated during the asphalt mixture production process"; Appendix D says "becomes available during"; Appendices F and G say "activates and contributes to the total effective binder content" [1]. The concept is the same in all three. The drift matters only because a specification that adopts one phrasing and cites a study that used another has quietly changed what is being measured. When reading a reported availability value, the first question is whose definition produced it and whether that definition describes the stockpile or the mixture.