15 h ROAD PAVEMENTS FORUM East London May 2008 HMA Task Group

Resolution #2 From last November That an RPF task group be established under the leadership of Arthur Taute to coordinate activities relating to HMA design, quality assurance, performance and specifications in line with the recommendations of CAPSA’07

HMA Progress

Issue Groups Mix Design –Many activities on research and development Gautrans forensic study APT Rut Challenge –Implementation Lacking

Mix Design Issues Aggregate Packing –Bailey –DASR –(Dominant Aggregate Size Range) – Denneman SATC 2007 Binder Stiffness –Problems with current specification that it can span across several performance grades – Bahia/ Marais/ AsAc Binder Durability –Johan Muller

Mix Design Implementation Performance – need criteria –Rutting Rut testing devices –Cracking Lots of cracking – water sensitive bases Variability –Permeability Water sensitive bases Variability

Performance Investigations Premature Cracking and Rutting as well as Inconsistency between design and actual asphalt laid –Gauteng Forensic Study Inappropriate use (tender mixes at intersections) Gauteng Forensic Study Permeability –Rossmann et al CAPSA07 Rutting –Verhaeghe et al CAPSA07 –APT - Rut Challenge - Gautrans/Sabita - CSIR/Industry

Variability and Risk

Pavement Design Implementation –Pavement designs need to be improved and to take available materials into account –Specifications need revision

Variability and Risk

Issue Groups Quality Management –Components Problems with binder consistency Problems with aggregate consistency –Implementation Quality problems in practice Entire process that can influence quality needs improvement – from design and specification through procurement, mix design and approval, manufacture and construction

Construction Issues Variability within lots Consistency throughout project Uniformity across length and breadth of lanes Diversity of aggregate supply Regularity of supply and surface Homogeneity of materials

Problem Statement We can be as smart as we like on HMA design etc but then we have to be able to implement the design. With new mix paradigm (AC and very heavy loads), variability issues are over- riding any design processes and decisions in respect of the performance of the product.

HMA Task Group Core A Taute D Rossmann H Marais PA Myburgh B Verhaeghe Corresponding JA Grobler, D Pretorius JC vd Walt, E Sadzik, S Oloo J Wise AN Other T Distin, L Sampson E Denneman

How do we improve implementation in the light of the following Design – lack of resources Specification - no new criteria - old Procurement – cheapest price Mix design approval – cumbersome - discourages changes during works Manufacture – commercial interests Construction – focus on doing whatever one needs to, to get acceptance Acceptance testing – statistical methods which assume reasonable uniformity and do often not reject substandard materials

Can Pay Factors Help? USA – common RSA do have some penalties and partial payment but no bonus for good quality work. Clients feel they deserve quality but are not always getting it unless the CE and contractor are “special” May allow for some discounts by those who know they can rely on PF>1 PF should not be considered as a bonus but rather payment for better quality.

Proposed Pay Factors in Sabita Manual and at CAPSA Bonus/penalty for riding quality – some benefits but limited general acceptance Sabita Manual 5 – proposed but not retained – specification issue CAPSA proposals for PF based on consistency and average of –Binder Content –Density –Now Aggregate Gradation

Binder Content Pay Factors – CAPSA07

Density Pay Factors – CAPSA07

Aggregate Gradation Substantial variation between individual samples. Many points on the road that are out of specification. Lot averages generally OK What do to about lots that do not meet specification ? How to encourage conformance?

Problem Statement

Variation occurs through all sieve sizes of source material It is possible to reduce variation by premixing before loading into bins Propose a Pay Factor to accommodate the additional costs

Crusher Dust Variability - Individual Sieves

Grading Pay Factor Need a Pay Factor that provides an indication of the deviation of the average grading of a lot from the target. Consider the sum of the absolute values of the difference between the percentage passing each sieve in a lot and the target grading Problem is that there can be substantial variation within a lot but the average can be OK Therefore need a parameter provides and indication of this variability

Grading Pay Factor Contd. Certain sieves are more critical than others –Eg current spec allows +- 5, +-4, +-3 and +-2 for various sieves with the and being the most critical (+-2) The 2.36 mm sieve is also critical in continuously graded mixes. Therefore the absolute value of the differences needs to be weighted differently for various sieves

Grading Pay Factor Contd. Must try to ensure consistency when applying the Pay Factor across different mixes and gradations Therefore need to standardize on number of sieves being considered and weighting that is applied.

Consider weighted sum of differences Sum the weighted absolute value of differences between actual grading and target grading on selected sieves. Use weightings to correspond to current allowable variation Add extra weight for other important sieves.

Illustrative Weighted Sum of Differences Single source – crusher run

Consider weighted sum of standard deviations Sum the weighted value of standard deviations on selected sieves. Use same weightings as on differences

Example of Sum of weighted std dev per sieve

Proposed Criteria Use weighting of 1 on all sieves and 2 on 2.36mm, 0.150mm and 0.075mm for medium graded asphalt Use a total weighting of 12 Vary the weighting and critical sieves depending on mix design and critical issues. Use values for Sum Weighted Std Dev as follows: <12 – low variability medium variability >17 high variability

Proposed Pay Factor for Grading Consistency

Pay Factor Pay Factor Grading –SStd>17 = 1.06 – 2 x  WD ( max 1.0 min 0.85 ) –SSTd>12 <17 = x  WD ( max 1.05 min 0.85 ) –SStd<12 = 1.20 – 2 *  WD ( max 1.10 min 0.85 ) –WD = Weighted sum of absolute values of differences between actual % passing and target % passing on sieves from 6.37mm to 0.075mm (9 sieves) where weight is 1 for all sieves except 2.36mm, 0.150mm and 0.075mm where it is 2. –SWStd = Sum of Weighted Stdev for the lot on same 9 sieves using the same weighting

Example 1 – Lot with medium variability and low difference

Example 2: Lot with low variability but higher difference

Resulting Pay Factors for 10 lots

Action Plan Mix Design –Need criteria for various rut testing devices – ongoing research rut challenge etc –Need criteria for permeability – problem is mix variability resulting in variable results –Need to start implementing Pay Factors Require trial specification and feedback throughout industry. Have used riding quality Pay Factors with success in the past – therefore should be able to use others. Need to assess how to combine Pay Factors – consider multiplication Need to start the debate and start moving away from what we were doing.