1 Principles of Resolving Test Result Differences Presented by Cindy Rutkoski Rocky Mountain Asphalt Education Center Instructor
2 Prevent test result differences before they happen What can contribute to causing test result differences Having a procedure in place to resolve test result differences Summary of Presentation
3 Take steps to prevent test result difference should be taken. More time, confusion and money is involved later, if prevention steps not taken seriously to begin with. Not doing so, can affect pavement quality & cause additional cost to contractor and/or owner.
4 Measures that can be taken to help prevent test result differences before they happen. Calibrate & maintain equipment. Adequate training & certified technicians. Use proper sampling procedures, resulting in similar field samples between labs. Use same splitting procedure between labs, especialy if problems persist. Follow standard test procedures.
5 Check testing purpose is to compare testing equipment & personnel & test procedures before project begins to indentify & correct potential problems. Process can be used any time confidence in test result differences occur. Dispute resolution Established process to address questions over acceptance test result differences in the properties & pay for asphalt mixtures and placement. Understanding by all parties at the onset of the project is paramount.
6 Basic Principles of Asphalt Testing Representative Samples Consistency in processes Accurate test results completed in a reasonable amount of time Good Communication
7 Representative Samples Field samples taken properly to represent material being produced and placed. Good split samples (similar in gradation) for testing between labs. Failing to accomplish this will result in differing test results
8 Avoiding Test Result Differences Follow established sampling procedures and in approved locations. Sample in such a way and in a location where aggregates or asphalt mixtures are in a well mixed state. (not from the back of a truck or the paver hopper). Fill containers simultaneously, (ex. one scoop per container back and forth till full) Use Quarter Master properly, to mix one or more field samples together, to then distribute between labs. Obtain large enough samples, as per NMP
13 Avoiding Test Result Differences Many test result differences are caused by improper splitting. Proper splitting is important, the resulting test sized specimen should result in a sample that is representative of the larger mass of material being produced or placed. Follow established procedures, using proper equipment correctly. Splitting method determined at Pre Pave meeting. Sample must be kept well mixed during splitting process.
14 Typical Splitting Methods Method A-Selection by scoop Method B-Quartering Method C- Riffle type splitter Method D-Selection by cross section
15 Win Win Situation Accuracy Test results that are accurate, result in comparable test results between labs. Result in a Quality Product. Supplier/Contractor receives deserved Incentives Owner/Agency receives quality product Can t get accurate results from bad samples or un-calibrated equipment, you will end up with test result differences.
16 COMMUNICATION Perform tests & Communicate test results as soon as possible The earlier problems or test result differences can be identified the easier it is to resolve them. Changes that might need to be made at the plant or on the road can be made. The whole idea is about quality and fair payment! Results being withheld or no urgency with completing tests and reporting results is counter productive in test result disputes and resolution.
17 Resolution Process & steps to be taken should be discussed & understood at the Pre-Con &/or Pre Pave meetings. All personnel including the Project Engineer must be familiar with procedures and when to use them. Project Engineer must be informed in writing of issues as they come up. Proper steps must be followed in order to employ procedures.
18 Resolution when there are differences between test results Differences should be resolved at the lowest level possible. Best when addressed on the project by project people involved in the project. Inform those concerned. CDOT procedure as example
19 Issues settled at lowest level as possible. Issues settled on the project level are most effective. 1. Initial check testing performed satisfactorily must be complete. 2. No challenges can are allowed unless: Check testing is complete. Differences must be larger than allowable differences. Documented split sample submitted by the Project Engineer. 3. Levels of resolution must followed, dates & personnel involved documented and completed within stated time constraints. 4. Last level blind samples submitted confidentially. These results become the results used in the pay factor calculation in place of the results that are in question. (No picking and choosing) CDOT Procedure - example
20 Basic asphalt tests where differences may occur & then cause other test result differences as a result. Binder Content/Moisture Aggregate Gradation/Specific Gravity & Absorption Theoretical Maximum Specific Gravity commonly called the Rice Test Compaction/Density/Air Voids Final step 1. Nuclear Method / Correction Factor 2. Bulk Specific Gravity of Cores Air Void & VMA analysis
21 Asphalt Content Determination Two Methods AC Content Nuclear Gauge - Quick results (once calibrated) - Quick way to check on aggregate source changes - New calibrations with any source changes Ignition Oven / Aggregate gradation - Direct correlation between AC content & gradation is good information - No need for belt cut - Must use accurate Correction Factors Both methods require moisture correction determinations to be run for accurate results.
22 Possible Causes for Test Result differences with % binder determination Nuclear AC content gauge: Unrepresentative samples (gradation) (splitting) Inaccurate or no moisture determination. Pan packed when sample is to cool. Switch pans if same base weight is used, way to check. Ignition Oven Method: Not applying CF or moisture determination. CF(correction factor) not accurate. Using wrong CF for mix. CF are not inter-changeable between ovens. CF does not go with a particular mix. Unrepresentative samples (gradation) (splitting)
23 Nuclear Content Gauge NCAT Ignition Oven Infrared Oven
24 Moisture Corrections Must be done, either method Moisture issues may be more prevalent when: 1. High RAP/RAS mixtures (they are prone to moisture retention) 2. Rainy weather 3. Using Warm Mix (lower plant) 4. If there are fluctuating volumetrics or AC contents, could be due to moisture in mixture Not applying a moisture correction results in an in-accurate binder content.
25 Asphalt Content Determination Both Methods are affected by aggregate gradation of sample if issues arise, could be result of poor splitting techniques. How improper sampling & splitting can affect results Gradation of sample must be representative. - Too fine = higher AC Content - Too coarse = lower AC Content Advantage of using the Ignition Method direct correlation
26 Test Result Differences between plant produced material (field samples) and mix design If an aggregate source has changed several things can happen. New mix design needed New calibration for Nuc Gauge Correction Factors no longer accurate for Ignition Oven Volumetric calculations will be off for air voids & VMA Reason is that the Bulk Specific Gravity/Absorption of aggregates may have significantly changed. Need for consistency in materials
27 Aggregate Gradation A Mix Design is based on the continued use of the proposed aggregates sources in the percentages determined during the mix design process, within the allowable deviations. (although adjustments from the mix design often take place once the mix is being produced in the plant) Percent AC is based on using this recipe to obtain specified % air voids and meet the specified minimum VMA requirement. Changes in gradation can cause compaction problems, result in under-coated or over-coated aggregate. Rice value is directly correlated to the % AC and gradation of mix being placed.
28 Things that can affect the Maximum Specific Gravity Test Short term aging of the samples before running the RICE test is very important. The absorption process of the binder must be allowed to take place. Density test results will vary & be in-accurate when using unrepresentative rice values. Errors in this test procedure arise from improper splitting techniques. Calibration weights on flasks/lids need to be accurate. Pressure gauge pulling correct vacuum. Water temperature monitered.
29 Rice test
30 Determining %Compaction & % Air Voids Two methods commonly used: Moisture Density Nuclear Gauge 1. Quick, non destructive. 2. Gauge set on correct mode (MA). 3. In-put correct numbers in gauge (max. density, corr. factor). 4. Calibrated & maintained. 5. Correlate gauge to pavement using Correction Factors. Bulk Specific Gravity on cores 1. Smaller quantity projects. 2. Correction Factors for MD gauge. 3. Check on gauge readings when in question. 4. Used for lab compacted specimens as well as roadway cores.
31 Preventing & Resolving test result differences in the determination of roadway densities Nuclear Gauge Method -Gauge set in right mode. -Correct Maximum Density for mix being placed. -Correct CF used for mix type, lift and gauge. -Gauge calibrated and maintained. Bulk Specific Gravity using SSD Method of Roadway Cores -Water temperature correct. -Towel not to wet or dry for SSD measurement. -Not shaking or rolling specimen before SSD determination. CoreDry Apparatus newer method CDOT approved Use CorLok Device newer method - FHWA
32 Nuclear Gauges
33 Bulk Specific Gravity on Cores
34 CoreLok / CoreDry
35 No one test result stands alone One test result will affect others Once again, to achieve proper compaction, we need accurate rice values. To achieve accurate rice values we need to have samples that are representative of the mix being placed. We also need to have an accurate determination of the asphalt cement (binder).
36 Rice value needs to be Accurate Using an incorrect rice value that is too high will result in a calculated density that is actually lower than what the density of the roadway really is. This could result in over compaction of the mat. Using an incorrect rice that is too low will result in a calculated density that is actually higher than what the density of the roadway really is. This could result in under compaction of the mat.
37 Correction Factors (CF) Correction factors correlate the gauges to the mat, allowing for calculated density to be more accurate for the mix being placed. Cores are taken - cores measure the actual density Correction Factor (CF) is calculated off the difference between the average of the cores and the average of gauge wet densities. CF is applied in the calculation of the density from the gauge readings verify correct CF is being used for mix, lift and gauge.
38 Relationship between Rice & %AC Relationship that should be seen if values are accurate Trend that should be seen As the AC Content increases, the Rice Value should decrease As the Rice Value increases, the AC Content should decrease. Helps in test result analysis to determine if results are accurate.
40 Another Trend that should be seen: There is also a direct relationship between the binder content(%ac) and % Air Voids. High AC Low AC Lower Rice Lower Voids Higher Rice Higher Voids If this trend is not seen, check % binder determination test process, samples at compaction temperature long enough, to long, gyratory angle set properly.
41 Air Void Analysis & VMA What is VMA & why is it important? VMA = Voids in the Mineral Aggregate The space between the uncoated aggregate, that allows room for the binder to coat the aggregate. When mix is compacted, VMA is made up of the effective binder (portion left as a coating on the aggregates) and air voids. Desirable for mix to meet minimum VMA specs. Voids are affected by the gradation of mix & % binder. It is desirable if the aggregate sources remain fairly consistent with aggregate proposed for the mix design.
42 % Air Voids & VMA accuracy Values used in determination of % air voids & VMA in compacted mixture: Gmm Maximum Specific Gravity (Rice) Gmb Bulk Specific Gravity of Compacted Asphalt. Values needed for calculating VMA are: Gmb bulk specific gravity of compacted specimen. Gsb bulk specific gravity of total aggregate (no significant agg source changes off mix design). Pb % binder. Ps aggregate, % by total weight of mixture Pa - air voids in Compacted Mixture, % of total volume Accurate measurements are needed for the calculation of % air voids and VMA. * % Pa = 100 (Gmm-Gmb / Gmm) * VMA = 100- (Gmb x Ps/ Gsb)
43 Temperature Another important part of preventing test result differences, is maintaining and calibrating all testing equipment, including thermometers and ovens. Another Important Element
44 Resolving Test Result Differences in Air Voids The Temperatures used in lab testing must be accurate. Ovens must be checked for temperature accuracy. - Each shelf and location on that shelf must be checked to determine if samples are getting to compaction temperature throughout the specimen. - Specimens must be at compaction temperature for at least 15 minutes prior to compaction and no longer than 4 hours. - Bulk tank temperatures also need to be monitored for the Gmb
45 Resolving Test Result Differences Air Voids Gyratory angles Another important factor that can affect the percent of air voids, is the accuracy of the angle that the gyratory is set at. The angle must be checked periodically and anytime there are test result differences that seem to persist. Check testing programs performed before production starts can indentify such problems before they happen.
46 Representative samples & test specimens along with standard test procedures performed consistently & accurately, will contribute to producing & placing a quality pavement. Accuracy & good communication help monitor the mix production & placement. No one test result stands alone, all mix properties work together & are affected by one another, hence, the importance of accurately & continually monitoring mix properties and compaction throughout production. Summary
47 Summary of Preventing & Resolving test result relationships -Follow standard test procedures accurately. Pay attention to test results and trends, do they make sense. -Qualified technicians. - When issues arise, an open door policy with the involved labs to verify procedures being used, equipment & agreeing on same method of splitting. * TEMPERATURES Lottman, rice, oven temp. of samples at time of compaction on gyratory. -Check testing program identify potential problems up front. - Lab equipment maintenance & calibrations kept up. - Establish a Dispute Resolution Process, be familiar with the steps to follow. - Address problems quickly at the lowest level and keep project people informed.
48 Gyratory Angle, sample temperatures, representative samples. Rice tests Calibration weights, water temperature, careful splitting, shake times, vibrating tables calibrated. Ignition Oven Correction Factors correct and applied, moisture corrections, aggregate break down accounted for if applicable, samples representative, gradation effects results, RAP (other additives) included in samples. AC Content gauge Corrected for moisture, representative samples, gradation effects results. Air void analysis Bulk Specific Gravity run properly, use Coredry or CoreLok apparatus. Density gauge testing using gauge properly, right settings, accurate max density, accurate CF. Common problems that can occur
49 Remember no test result stands alone Questions?