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Risk-Based Construction Inspection: A Guide (2023)

Chapter: Appendix B - Supplemental Guide Regarding Materials RBI

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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
×
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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Suggested Citation:"Appendix B - Supplemental Guide Regarding Materials RBI." National Academies of Sciences, Engineering, and Medicine. 2023. Risk-Based Construction Inspection: A Guide. Washington, DC: The National Academies Press. doi: 10.17226/27099.
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B-1   A P P E N D I X B Supplemental Guide Regarding Materials RBI

B-2 Risk-Based Construction Inspection: A Guide Property-Based Optimization of Materials For materials that are to be accepted by sampling and testing, the optimization process presented in this appendix provides a systematic approach for further evaluating: • Properties and test methods that are more direct indicators of performance • The appropriate level of owner verification testing (if contractor test data is being used in the acceptance decision) Conceptual Framework The property-based assessment framework, as illustrated in Figure B-1, provides a structured decision process for prioritizing material sampling and testing activities based on the risk (i.e., likelihood and consequences) of nonconformance of key material properties. Step 2: Review existing acceptance plan Step 3: Evaluate importance of acceptance properties as indicators of performance Step 5: Identify project-level considerations that may influence sampling and testing effort Project Factors: • Contractor/supplier qualifications • Material quantities • Project criticality or complexity • Project delivery method Acceptance plan considerations: • Properties • Sampling plan • Test methods • Targets/Thresholds • Pay factor adjustments Evaluate: • Importance of property • Cost of acceptance sampling and testing Step 1: Identify materials of interest Step 4: Assess benefit of using alternative or advanced acceptance properties and test methods Step 6: Determine if acceptance/verification testing can be optimized based on material risk and property importance Figure B-1. Property-based materials assessment.

Supplemental Guide Regarding Materials RBI B-3   Framework Steps The framework shown in Figure B-1 includes a six-step process to determine the extent to which acceptance testing can be optimized or reduced based on performance risk. Step 1: Identify Materials of Interest The Level 2 optimization process applies to materials that are accepted on the basis of sampling and testing, a resource-intensive method that is primarily used for higher risk or more critical materials. A key first step to fully defining the optimization problem entails identifying the specific materials of interest. One may wish to assess all material items requiring sampling and testing or focus simply on specific items having the greatest potential for optimization. Step 2: Review existing acceptance plans for materials of interest For each material of interest, the DOT’s current sampling and testing plan should be reviewed, bearing in mind that although such QA practices are likely based on methods that have historically produced satisfactory quality, they may fail to take advantage of possible efficiencies to be gained from: • Recent developments in the understanding of materials behavior, • Advances in nondestructive testing technology, and • Increasing use of performance specifications and alternative delivery methods that shift more responsibility for quality management to the industry. Step 3: Evaluate the importance of acceptance properties Most if not all tested properties routinely used for acceptance have some relationship to product performance. However, DOT acceptance testing of less important properties can result in inefficient allocation of an agency’s QA resources and unnecessary project costs. The resources allocated to the sampling and testing effort for a particular material property should be consistent with: • Criticality of the contract or pay item, and • The ability of the individual property to act as an indicator of performance. When reviewing the existing QA strategy for a given material (or when selecting acceptance properties for a new pay item), consider the following questions to help determine the relative importance of a property as an indicator of performance: • Is the property more strongly or less strongly associated with distresses? • What is the likelihood that property nonconformance would result in reduced or impaired performance? • Are any properties more suited for the contractor’s quality control function (e.g., gradation for HMA [hot mix asphalt]) than for agency acceptance testing?

B-4 Risk-Based Construction Inspection: A Guide • Does the property provide a measure of fundamental material characteristics (e.g., stiffness modulus, fatigue)? • Is the acceptance property used to determine a payment adjustment or to support a remove-and- replace decision? From the perspective of optimization, additional questions to consider may include: • Is the standard frequency of sampling and testing for certain properties relatively higher than others? What test(s) require the most resources? • What test(s) can only be performed in the state laboratory? • What test(s) require special equipment or expertise? • What properties require destructive testing? Answering such questions requires a thorough understanding and evaluation of each property being considered for inclusion in the QA plan. Historical data obtained from the agency’s project quality management records or asset management system may provide a reliable source of information to support the decision process. Based on such an analysis, any given material property can be characterized as being a primary, secondary, or observational indicator of performance, as described in Table B-1. If analysis indicates that a property is strongly associated with common failure modes, then the property could be considered a primary indicator of performance. It is also important to remember that, as a rule, properties tested in the end product are more closely related to end-product performance than when tested during production. Thus, testing of the in-place (in situ) product (or at the point of placement), while not currently practical for some materials, is a desirable goal. Table B-1. Assignment of material property tiers based on property importance. Property Tier Description Suggested Level of QA Primary Property has a strong relationship to performance or has the highest risk related to noncompliance QA methods designed to provide maximum confidence in the quality of the materials provided Secondary Property has a moderate or less direct relationship to performance; risk related to noncompliance is moderate QA methods designed to provide an adequate level of confidence in the quality of the materials provided. For optimization purposes, consider: • Reducing the level of acceptance testing after start-up for material properties demonstrated to be under control, or • Using alternative nondestructive sampling and testing strategies to expedite testing and increase the sample size.

Supplemental Guide Regarding Materials RBI B-5   Property Tier Description Suggested Level of QA Observational Property has an indirect relationship to performance and risk related to noncompliance is low QA methods entailing observational verification of contractor/producer tests combined with intermittent to random inspection, sampling, and testing of in-progress work Step 4: Assess benefit of using alternative or advanced acceptance properties Current sampling and testing protocols for material acceptance often rely on destructive testing of the in- place product. Although such testing generally provides more accurate and reliable results, it tends to be costly and time intensive. For example, obtaining core samples HMA can be considerably more resource- intensive than nondestructive test (NDT) methods. NDT methods, though often less precise, yield a higher volume of samples (or continuous sampling), which can offset this disadvantage. Examples include Ground Penetrating Radar (GPR) as a measure of durability (in-place air voids, thickness), and Intelligent Compaction to characterize material quality and variability more rapidly. For less critical material items or as a secondary measure of QA (e.g., as a QC tool for screening purposes), the use of rapid NDT methods may provide a cost-effective alternative to standard tests. Use of advanced performance tests (i.e., performance-based mixture design of mechanistic properties) may also be appropriate for acceptance purposes if they can potentially provide stronger indicators of performance for high-risk projects or correlate well with specific performance objectives (e.g., durability, fatigue, or rutting resistance). Step 5: Identify project-level considerations that may influence sampling and testing Consideration of project-related factors can also help with the selection of effective sampling and testing strategies. In effect, a DOT can use a lower frequency of sampling and testing when a material item is supplied by an extremely reliable, qualified supplier, or the material is for a low-volume or less critical/complex project. Step 6: Determine the extent to which acceptance testing can be reduced or optimized based on material risk and property importance The last step in the property-based materials optimization entails deciding the extent to which acceptance testing can be reduced or optimized for a given material based on material risk and property significance. Materials can be generally characterized as high, moderate, and low-risk items. Material property importance can be characterized by primary, secondary, or observational indicators. The QA sampling frequency of tested properties can be reduced for less critical materials and less important properties. DOT-Performed Acceptance Testing Table B-2 provides an example of how a sampling and testing acceptance plan could be optimized based on material risk and property importance when the DOT performs all acceptance testing (i.e., contractor test data is not used in the acceptance decision). In general, the DOT may choose to modify the normal inspection or testing procedures for lower-risk projects or project elements. For primary properties, the sampling frequency may be reduced for tests that are under control (e.g., after ten consecutive samples taken at the normal testing frequency indicate full

B-6 Risk-Based Construction Inspection: A Guide conformance with the specifications). The sampling and testing frequency should revert back to the default frequency if there are failing tests. Table B-2. Example property-based optimization when DOT performs acceptance testing. Property Importance Material Risk Example Properties High Risk (e.g., structural or safety- critical elements, high user impacts, large quantities) Moderate Risk (e.g., structural elements with moderate safety or user impacts) Low Risk (e.g., nonstructural elements, small quantities) Primary indicator • Use default frequencies • If the process is determined to be under control, reduce to 75% of default frequency • Use 75% of default frequency after the process is determined to be under control • Use 50% of default frequency (double lot size) after production start-up • Waive acceptance testing at Engineer’s discretion • Concrete Strength • Concrete air content • HMA In-place air voids • Key mixture properties (e.g., AC) • Performance tests* Secondary indicator • Use 75% of default frequency after the process is determined to be under control • Use 50% of default frequency (double lot size) after production start-up • Observational verification of QC tests with audits of certifications and random verification tests • Slump • Gradation • Secondary mixture properties • NDT* Observational indicator • Observational verification of QC tests with audits of certifications and random verification tests • Observational verification of QC tests with audits of certifications • Random verification of QC records for compliance with specifications • Segregation profile • Temperature • Workmanship indicators • NDT* * Note: Testing of more advanced material properties (i.e., stiffness, dynamic modulus, permeability) or advanced rapid NDT test methods can provide cost-effective strategies to reduce risk or to meet specific performance goals (e.g., durability or long life) when used at lower frequencies or correlated with standard tests. Contractor QC Test Results Used in Acceptance Decision For the case where contractor quality control (QC) test results are used in the DOT’s acceptance decision, the DOT must still perform independent verification sampling and testing in accordance with 23 CFR Part 637. The optimal verification sampling and testing plan would be driven by material criticality and the ability of the property to act as an indicator of performance. A case study example is provided at the end of this appendix (Box B.2) describing how Texas DOT has implemented a risk-based protocol that assigns higher levels of QA analysis to properties that have been

Supplemental Guide Regarding Materials RBI B-7   a. Primary Properties. For primary properties, verification testing frequency is typically a minimum of 10-25% of the QA testing frequency. F- and t-tests are performed on these key material properties on a continuous basis with the addition of each verification test result. The p-values (from the F- and t-tests) are reported for each analysis and tracked over time compared to a specified level of significance for the material (e.g., α = 0.025 for structural concrete and 0.01 for nonstructural concrete). The levels of significance refer to the probability of rejecting the null hypothesis assumption that the DOT and contractor populations are equal. AASHTO R 9 (2005) provides suggested values of αcritical used in the highway construction industry. An example of material categories and αcritical used for statistical analyses is shown in Table B-3. Table B-3. Example level of significance applied to materials. Material Category Level of Significance (αcritical) Embankment, Subgrades, Backfill, and Base Courses 0.01 Asphalt Stabilized Base (Plant Mix) 0.01 Hydraulic Cement Concrete – Structural 0.025 Hydraulic Cement Concrete – Nonstructural 0.01 Hydraulic Cement Concrete Pavements 0.025 Asphalt Concrete Pavement 0.025 This approach enables the DOT to monitor the validation status of each property daily and allows for corrective action to address nonvalidating test results. For a sampling and testing process determined to be under control, a continuous-cumulative lot or a chain lot strategy, as described in Box B.1, has been used to increase the sample size for the F- test and t-test where contractor test results are used in the acceptance decision. historically shown to have greater residual risks related to long-term performance. A general summary of such a risk-based process follows.

B-8 Risk-Based Construction Inspection: A Guide b. Secondary Property. A secondary property provides independent verification for those materials and test methods that are secondary indicators of performance. An example is a slump or gradation test for hydraulic cement concrete. For such low-risk materials and properties, the DOT verification testing frequency could be scaled back and not involve statistical validation (i.e., F and t- testing). c. Observational Property. An observational property can entail observation verification for those materials that require only a few QA tests for compliance with the standard guide schedule or materials having a low risk of failure that will not affect long-term performance. Under the Box B.1: Continuous-Cumulative and Chain Lot Methods The continuous-cumulative method shown in the figure below consists of accumulating incrementally test results from sequential lots (i.e., results from lots 1 and 2; results from lots 1, 2, and 3; results from lots 1, 2, 3, and 4). Lot accumulation starts once one lot is found to be conforming (i.e., favorable t-test or other DOT application). Then, as long as the accumulated lots yield a favorable t-test, the accumulation of lots continues until a failing t-test occurs for consecutive sets of lots. Similarly, in the chain-lot method shown below, a fixed number of lots (e.g., 2 ≤ i ≤ 5) are individually tested. After a specified number of lots i are found conforming, the accumulation of lot results begins. The chain-lot method considers a constant set size of i + 1 lots for assessing the acceptance properties (Arambula and Gharaibeh 2014).

Supplemental Guide Regarding Materials RBI B-9   observational approach, the DOT does not directly perform tests but instead observes the contractor’s QC testing for equipment and procedural compliance with the test standard. Table B-4 provides an example of how a sampling and testing acceptance plan could be optimized based on material risk and property importance when the DOT uses contractor QC data for acceptance purposes. Table B-4. Example property-based optimization when contractor QC data is used in acceptance decision. Property Importance Material Risk Example Properties High Risk (e.g., structural or safety- critical elements, high user impacts, large quantities) Moderate Risk (e.g., structural elements with moderate safety or user impacts) Low Risk (e.g., nonstructural elements, small quantities) Primary indicator • Verification testing at 25% QC frequency with continuous F & t analysis. • If process under control, use chain lot or cumulative sampling. • Verification at 25% QC frequency. • If process is under control, reduce verification by 50% and use chain lot or cumulative sampling. • Acceptance tests at 50% of default frequencies. • Verification at once per quarter w/ no statistical validation. • Concrete Strength • Concrete air content • Cover depth • HMA in-place air voids • Primary mix properties (e.g., AC) • Performance tests* Secondary indicator • Verification at 25% of QC frequency. • If process under control, reduce verification by 50% and use chain lot or cumulative sampling. • Acceptance tests at 50% of default frequencies after start-up. • Verification at once per quarter w/ no statistical validation. • Observational verification of QC tests with audits of certifications. • Slump • Gradation • Secondary mix properties • NDT* Observational indicator • Observational verification of QC tests with audits of certifications and random verification tests. • Observational verification of QC tests with audits of certifications. • Random verification of QC records and compliance with specifications. • Segregation profile • Cracking • Joint consolidation • Workmanship indicators • NDT* * Note: Testing of more advanced material properties (i.e., stiffness, dynamic modulus, permeability) or advanced rapid NDT test methods can provide cost-effective strategies to reduce risk or to meet specific performance goals (e.g., durability or long life) when used at lower frequencies or a once-per-project basis correlated with standard tests.

B-10 Risk-Based Construction Inspection: A Guide Box B.2: TxDOT Risk-Based Approach to Verification Testing Texas DOT (TxDOT), in its Design Build Quality Assurance Program (QAP) Implementation Guide, evaluated the ability of individual material properties to act as indicators of performance. This analysis of material properties was then used as a basis for determining how much owner verification testing should be performed to validate contractor test results used in the acceptance decision (TxDOT 2011). For design-build projects with 15-year capital maintenance agreements, TxDOT’s guide applies three-tiers of owner verification (OV) testing to specific materials and properties, which are based on the TxDOT’s perceived residual risk after the contractor has completed construction and fulfilled its maintenance obligations. As explained in the guide, these levels are as follows: • Level 1 provides continuous analysis for those analysis categories that are strong indicators of performance. Examples include compressive strength for hydraulic cement concrete, percent soil compaction for embankment, and percent asphalt content for hot-mix asphalt concrete. The QA testing frequency is in compliance with the Guide Schedule, and the OV testing frequency should be a minimum of 10 percent of the QA testing frequency. F- and t- tests are performed on these material categories on a continuous basis with the addition of each OV test result. • Level 2 provides independent verification for those materials that are secondary indicators of performance. An example is the slump test for hydraulic cement concrete. The QA testing frequency is required to be in compliance with the Guide Schedule and the OV testing frequency should be a minimum of once per quarter. • Level 3 provides observation verification for those materials that only require very few QA tests for compliance with the Guide Schedule or tests on materials whose risk of failure does not affect the long-term performance of the facility past the contractual maintenance obligations. An example is the entrained air test (Tex-416-A) for non-structural (miscellaneous) concrete riprap where risk of failure does not affect the long-term performance of the facility past the contractual maintenance obligations. Under the Level 3 approach, OV does not perform tests but observes the QA test performance for equipment and procedural compliance with the test procedure. The figure below provides an example of how TxDOT’s guide applies these analysis categories to specific materials and properties.

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Due to budget cuts and reduced experience levels of inspectors and engineers, state departments of transportation (DOTs) have implemented risk-based strategies to achieve greater efficiency in construction inspection. These strategies include prioritizing inspection based on inherent risks related to construction operations, using emerging technology applications to save time, and accepting certification and contractors' test results to offset shortages of experienced inspection resources.

NCHRP Research Report 1039: Risk-Based Construction Inspection: A Guide, from TRB's National Cooperative Highway Research Program, discusses the importance of construction inspection and aims to assist state DOTs and the U.S. Federal Highway Administration in meeting quality standards.

Supplemental to the report are NCHRP Web-Only Document 344: Risk-Based Construction Inspection: Conduct of Research Report and an Inspection Risk Assessment Questionnaire.

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