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The air void system purposely introduced by an air-entraining admixture (AEA) is of great significance for the protection of concrete from freeze–thaw damage. Fly ash has been globally used in concrete, while the unburnt carbon in fly ash can adsorb AEA molecules and, thus, increase the AEA demand. Previous studies primarily focused on the air content of fresh fly ash concrete. This paper aimed to explore the stability and distribution of air voids in fly ash concrete at the fresh state. To achieve this goal, eleven different fresh fly ash concrete mixtures with an initial air content of 6 ± 1% were prepared in the laboratory. Samples were taken at various times within 75 min after initial mixing to investigate the air content and air void distribution in fly ash concrete at the fresh state using a super air meter (SAM). The results indicated that there was no significant correlation between loss on ignition (LOI) of fly ash and AEA demand to achieve the initial air content of 6 ± 1%. Class C fly ash concrete tended to have a better air content retention than Class F fly ash concrete. Compared with LOI, AEA demand had a stronger correlation with air content retention. Most of the fly ash concrete mixtures had a satisfactory air void system immediately after mixing, but the SAM number showed an increasing trend over time, suggesting the coarsening of the air void system with time.

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A composite separator layer in Portland cement concrete pavement, formed by a lower bearing layer and micro surfacing, was seldom used in airport pavement, but it has great application potential. This paper studied the properties of the composite separator layer and its benefits for pavement. The basic properties investigated in the study were resilient modulus and interlayer property between the separator layer and concrete. The resilient modulus of the micro surfacing was tested to calculate the modulus of the whole separation layer. The interlayer shearing test was done on core specimens using a self-developed facility that can apply load in the normal direction. Shearing test results show that temperature affects the shape of the shearing curve and normal stress linearly affects the stable shear force. Additionally, an in-situ erosion test was carried out to prove the anti-scouring property of the separator layer. Furthermore, a finite element model (FEM) model was established to study the influence of the composite separator layer. The FEM employed the modulus test results and the measured interlayer characteristic. The interlayer characteristic was simplified into a two-stage constitutional model. Monitored data on the mechanical response of the pavement structure in an airport validated the FEM model. Results show that the separator layer reduces the chance of contraction crack under the sudden temperature drop in the concrete slab before sawing. FEM results show that though the separator layer slightly increases the tensile stress of concrete, the tensile stress is greatly reduced if the slab develops voids in the future. The most significant benefit of the separation layer is it reduces the chance of void occurrence so that the lifespan of pavement with a separator layer is improved compared to regular pavements which frequently have voids beneath the slab. With the finite element model results and cumulative fatigue life equations, the separator layer was estimated to elongate the pavement service life for years, depending on the time voids appear.