A PARADIGM SHIFT ON PERMEABILITY OF THIN ASPHALT SURFACINGSPresenter: Dr Emile Horak
co-authors Wim Hofsink, Herman Marais, Joanne Muller and Ane’ Cromhout
Paradigm: The concept and meaning
Thinking patterns, rules to look at the world, data, information, skewed by confirmation and hindsight biases
Kuhn (1962)- biases in scientific research
Pervasive nature of paradigms
Rules, standards, guidelines, culture, religion, how we look at information
Rules can make us blind to all available information
The thinking rule : “If it is not square,… it is not there! …” It is round, does
not fit the rule. Result: It is
subconsciously ignored.
We literally become ignorant
to this
Paradigm shift is often to look at all the information or from a different perspective : It exposes our ignorance!
Our ignorance are forced to be challenged- versus previously just confirmation what we already know!
Agnotology: The study field of ignoranceor doubt, particularly the publication of inaccurate or misleading scientific data.
The study of culturally induced ignorance or doubt.Robert N Proctor and Londa Schiebinger
Agnotology is in the other direction of “knowledge”…Agnotology, the study of ignorance. Agnotology is seldom discussed, because studying the absence of something — in this case knowledge — is incredibly difficult.
Epistemology, or the study of knowledge. This field helps define what we know and why we know it. (Confirmation and hindsight biases)
The difficulty of studying something we don’t know
ResearchSearch
Ignorance is bliss! Modern effect of the internet! Everyone is an instant expert!
The ten thousand hour rule applies: Gladwell
Do not express this image in the Afrikaans language!
Broad based information gathering• As-built information• Quality control and quality assurance information• Surveys (eg visual, rutting, Falling Weight
Deflectometer (FWD), riding quality, macro texture, Dynamic Cone Penetrometer (DCP)
• Ground Penetration Radar (GPR) • Test pits and material sampling• Laboratory tests
Climbing the slope of enlightenment: FORENSIC INVESTIGATIONS
More detailed and specialised investigation phase• X-Ray Diffraction (XRD), or X-Ray Florescence (XRF), • Bending Beam Rheometer (BBR), • Multiple Stress Creep Recovery (MSCR) Test, • Dynamic Shear Rheometer (DSR), • Multiple-Stress Creep-Recovery (MSCR) test, • Asphalt Mix Performance Test (AMPT), • Computed Thomography (CT) scans and modelling, etc
Climbing the slope of enlightenment: FORENSIC INVESTIGATIONS
Reason’s “Swiss cheese” model -distress development –multiple factors contribution on a random basis
Other holes due tolatent conditions
Some holes dueto normal variation
Distress Development in a road is
never just a single factor. The complexity can lead to confusion! Fact and fiction must be separated
Layers of cheese slices - random outlier type defects - the holes through which effects can permeate to the next ‘cheese’ layer
Water from the top as main cause of distress
Distress observation on surface therefore misleading
Open surface allowing water into asphalt and lower layers
Horizontal permeability being higher than vertical
Water entering base at weak spot away from original surface entrance
Distress development in base and away from original water entrance
Distress development – often due to the confusing contribution of the whole pavement system
The density precipice
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Dens
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ter p
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89 90 91 92 93 94 95 96 970
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Density %
Water permeability (l/hr/m2)
The proverbial edge of the cliff
Typical correlation of density of asphalt versus measured permeability
PERMEABILITY OF THIN ASPHALT LAYERS
McDaniel R (2019) Through thick and thin. Asphalt Pavement Magazine Volume 24,
PERMEABILITY OF THIN ASPHALT LAYERS
Typical HMA zone where
permeability may increase exponentially
Permeability is the flow of water through interconnected voids
Variability due to: Influence of stone size on voids and therefore permeability
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2.5 3.5 4.5 5.5 6.5 7.5Voids in the Mix (%)
Probability Density FunctionsVoids in the Mix
90th percentile Project B6.65
90th percentile Project A4.86
Median Project A4.33
Median Project B5.53
Voids specification 3% to 6%
Cromhout et al CAPSA 2019
Variability in the air voids demonstrated with Probability Density Functions (PDFn)
PERMEABILITY OF THIN ASPHALT LAYERS
Evaluation by means of
averages, mean or median can be totally misleading regarding actual
variability
a) Micro cracks during paving operation
WAYS THAT WATER ENTER THE ASPHALT SURFACING
Shear cracks - compaction rolling . Become visible only after rain. Fines pumped to line the lips of the transverse fine cracks
b) Rolled in chips (RIC) provides openings in asphalt layers
WAYS THAT WATER ENTER THE ASPHALT SURFACING
Side view Top view
Cracked or crushed RiC can cause a dramatic increase in surface permeability
WAYS THAT WATER ENTER THE ASPHALT SURFACING
(Grobbelaar, 2014)
Once the water is inside it will go sideways!Water appearing away from Marvil
indicating horizontal flow in the asphalt layer.
The Marvil has several short comings and should be improved to allow better
permeability measurement of either vertical or horizontal flow
Maree and Viljoen (1983) MARVIL criteria for bases and surfacings
Layer Percentile permeability limits (l/hr)
Mod-dry regions
Wet regions
Surfacings <0.05 <0.01
G1 <0.05 <0.02
G2 & G3 <3.0 <1.5
Horizontal =up to 10 times vertical permeability
The Marvil test in ‘no man’s land’ Therefore more prone to measure horizontal permeability !
Diameter of Marvilis 175mm
Work done by Harris: Based on approximately 180mm thickness of asphalt. Indications that thinner asphalt layers may be more permeable horizontally than vertically
Horizontal flow
Vertical flow
Permeability measurement
Vertical permeability - not done regularly, horizontal virtually never
Permeability predictions using permeability equations and Rational Bailey Method Ratios can help benchmark asphalt mixes permeability propensity
𝒌𝒌 = 𝟒𝟒𝟒𝟒𝟏𝟏𝟏𝟏𝟏𝟏
𝟐𝟐𝟑𝟑∗ 𝑨𝑨𝑨𝑨𝟏𝟏𝟏𝟏𝟏𝟏
∗ 𝐷𝐷753.70
k= coefficient of permeability(mm/s)AV = total air voids (%)D75= Sieve size through which 75% of the aggregate is passing (mm)
Improved air void and flow path simulation method , Vardanega and Waters (2011)
Summary and conclusions
• Collective experience regarding permeability have characteristics of a paradigm shift.Implying a new look at existing information
• It is recognized as significant as experience shows that asphalt surfacings fail due to the outliers and not the average values
Summary and conclusions (Cont)• Statistical procedures should therefore be re-
examined to reflect the probability density function of as built results to give a better appreciation of variability outside the specified ranges.
• Permeability is a measure of the interconnectedness of voids. Therefore the need for accurate determination of voids in the laboratory is recognized. –CoreLok method
Summary and conclusions (Cont)
• Construction and mix design factors may contribute significantly towards vertical permeability of asphalt layers. Micro-cracks (rolling technique or cold compaction) and the continued use of Rolled in Chips (RIC)
• It is recommended that various obvious alternatives to RIC be considered and evaluated.
Summary and conclusions (Cont)
• Recognition that horizontal permeability in asphalt layers is between 3 to 10 times higher than vertical permeability
• Vertical and horizontal permeability are seldom measured separately in the laboratory.
Summary and conclusions (Cont)
• The Marvil field permeameter- prone to tester capability - may actually measure horizontal permeability more than vertical permeability (diameter size)
• Improvements needed for better field permeameter as well as laboratory permeameters to measure both vertical as horizontal permeability
Summary and conclusions (Cont)
• Current asphalt density specifications (egminimum density of 93% (or lower) implies voids in the field mix is already 7% (or higher) with associated random higher voids in the mix.
• It is like playing on the edge of the precipice! It becomes a dangerous flirtation with high permeability
Summary and conclusions (Cont)
• Permeability potential can be benchmarked via field and laboratory measurements and should be formalized in the new TG35 or update
• Permeability potential can be monitored via binary aggregate principles correlated with rational Bailey ratios and porosity calculations from the grading and aggregate packing
Let us get climb the slope of enlightenment by investigating our ignorance
Let us admit: We are still ignorant about void connectivity and permeability!
Rules can make us blind to all available information
The thinking rule : “If it is not square,… it is not there! …”
It is round, do not fit the rule.
Result: It is subconsciously
ignored. We literally
become ignorant to this
Do not translate this image to the Afrikaans language!
Most SA Hot Mix Asphalt (HMA) tend to lean towards the lean side-therefore higher up the air voids curve
b) The slippery precipice below specified density levels
PERMEABILITY OF THIN ASPHALT LAYERS
Density reduction or binder content reduction both can cause a significant increase in voids in the mix
PERMEABILITY OF THIN ASPHALT LAYERSb) The slippery precipice below specified density levels
The Vexing Problem of Variability
Redivo S. 2012 ‘Correlation of Asphalt Surfacing Permeability of Laboratory and Field Prepared Specimens’.
PERMEABILITY OF THIN ASPHALT LAYERS
Water oozing from only certain areas of the asphalt = Variable permeability
Not all information are equal: Sources and quality
Data sets such as “as-built” information can at best be dubious
Personal memories of what happened on site can be directly linked to “fruits and nuts” like ignorance
I love Wikileaks!
