Research in Trench Reinstatement Materials. Phil Sabin National Technical Manager Aggregate Industries

Transcription

1 Research in Trench Reinstatement Materials Phil Sabin National Technical Manager Aggregate Industries

2 Agenda Air voids in asphalt mixtures What are they? Why do we measure them? What role do they play, good or bad? Can we accurately measure them? Can we control them? Are our current specifications for air voids sensible? 2

3 What are air voids? Definition: The amount of air in a compacted asphalt mixture, expressed as a percentage of the total volume of the mixture In situ air voids are partly a function of the design and manufacture of the asphalt mixture but more particularly an outcome of the compaction achieved during placing.. Air Bitumen Aggregate Total Volume Air Void 3

4 Why do we measure them? Used as a surrogate measure of durability, air voids in a mixture can have a major influence on in service performance Air and/or water ingress will cause gradual hardening of the asphalt binder leading, eventually, to fretting due to the binder having insufficient cohesion to hold the aggregate particles together. The compacted density will effect the following: Structural strength Resistance to deformation Permeability / Durability 4

5 What role do they play, good or bad? At around 15-20% air voids can be connected and the asphalt mix is bordering on becoming permeable. This gives rise to: Mix instability Secondary compaction Water damage to underlying layers Accelerated ageing of the asphalt mix/poor durability At 5-10% air voids the aggregate particles come into contact with each other and void spaces are largely un-connected and moisture/air penetration is low. At low air voids, typically <3%, the binder almost totally fills the void space between the aggregate particles, so that the mix acts as a fluid and is therefore prone to deformation. 5

6 Can we reliably predict or control air voids? Known volume Length Known mix density Depth Width Target density to achieve maximum void content of X% Target density to achieve minimum void content of Y% 6

7 Can we reliably predict or control air voids? AC 6 dense surf footways Determination of surcharge to give x% air voids at: Min voids Max voids Mix Max density / (kg/m3) 2500 Target Air Voids / % 2.0% 13.0% Compacted density at target air voids / (kg/m3) Mix loose density / (kg/m3) 1800 Compacted layer height / mm 25 Layer height including surcharge / mm Surcharge required / mm Control required +/- 1.9mm 7

8 Can we reliably predict or control air voids? AC 10 close surf Carriageways Determination of surcharge to give x% air voids at: Min voids Max voids Mix Max density / (kg/m3) 2500 Target Air Voids / % 2.0% 11.0% Compacted density at target air voids / (kg/m3) Mix loose density / (kg/m3) 1800 Compacted layer height / mm 35 Layer height plus surcharge / mm Surcharge / mm HRA 30/14 F surf Carriageways Determination of surcharge to give x% air voids at: Min voids Max voids Mix Max density / (kg/m3) 2450 Target Air Voids / % 2.0% 7.0% Compacted density at target air voids / (kg/m3) Mix loose density / (kg/m3) 1800 Compacted layer height / mm 40 Layer height plus surcharge / mm Surcharge / mm

9 Can we accurately measure them? Total air voids content can be reasonably accurately measured from a calculation using the Theo Max Density of the mix and the bulk density of a core. Inaccuracies in the measurement begins to occur on thin cores and cores with rough surface textures even when using the EN Procedure C sealed specimen method. Air void content of a core is currently determined as an average for the entire core thickness. 9

10 Routine Laboratory Air Void Calculation 10

11 Recent research by Aggregate Industries X-ray tomography to look at the void distribution throughout the core Air void distribution changes along the thickness Air void inter connectivity 11

12 Can we reliably predict or control air voids? 12

13 Can we reliably predict or control air voids? Top slice 6.9% AV Middle slice 11.8% AV Bottom slice 23.5% AV 13

14 Can we reliably predict or control air voids? 14

15 Can we reliably predict or control air voids? 15

16 Are our current specifications sensible? Current void content specifications are derived from machine laid mixtures, pre-compacted by a vibrating screed and rolled by highly efficient compaction equipment and are therefore inappropriate values for the installation methods and equipment used in trench re-instatements. Control of compaction by the application of a surcharge is bordering on the impossible. Achievement of void contents by a compaction method statement is a hit or miss affair. Current air voids test procedures are crude measures of asphalt layer permeability and subsequent durability. Better direct measurement methods of permeability exist. 16

17 On going work We continue to engage and provide expertise to Specification makers in HAUC at local and National level AI is current engaged in a number of R&D projects and live trials with our customers to improve the likelihood of compliance with void requirements in trench reinstatements Through its Asphalt Academy, Aggregate Industries is providing free technical training in reinstatement in asphalts to its customers. If we can help you contact: Phil Sabin Philip.Sabin@aggregate.com 17

18 Air voids (%) Results of recent trials Core/layer thickness (mm) 18