FALL 2017

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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Page 49 of 63

FHWA Concrete Technology Advancements: New Tests and the Performance Engineered Mixture Specification by Mike Praul and Gina Ahlstrom, Federal Highway Administration For a number of years, the Federal- Aid Highway Program has been moving in a performance-based direction, seeking to link program- and project-level criteria with the results obtained. This was formalized in the MAP-21 (Moving Ahead for Progress in the 21st Century Act) legislation and has continued with the Fixing America's Surface Transportation Act. While this move toward performance criteria for the highway program has been on a programmatic level, concrete materials technology has moved in a similar direction. Recent developments in concrete technology have, for the first time, made it possible for highway concrete to be specified with a performance specification and for meaningful upgrades to take place with quality-control (QC) processes and programs. PEM and What It's About American Association of State Highway and Transportation Officials (AASHTO) recently published provisional specification PP 84-17, Standard Practice for Developing Performance Engineered Concrete Pavement Mixtures, known as the performance engineered mixture (PEM) specification. This specification is the result of a multiyear collaboration among the Federal Highway Administration (FHWA), state departments of transportation, researchers, and industry. The underlying concept behind the PEM specification is to understand what makes concrete last and what failure mechanisms affect concrete durability. Then critical properties are specified and mixtures are designed to meet the required level of performance. Finally, testing is done on those critical properties, both as a QC function by the contractor during production and as an acceptance function by the agency. The following six performance areas are identified as critical to durable in-service concrete: - Strength - Reducing unwanted cracking due to shrinkage - Hardened cement paste freeze-thaw durability - Transport properties (permeability) - Aggregate stability - Workability In the AASHTO PP 84-17 specification, each of these properties has a table intended to guide the user in selecting the appropriate criteria to incorporate into a specification. Specifiers can choose which performance parameters they want to include into their specifications, with the understanding that not all owners may be willing to move to a complete performance specification right away. It is significant that while the title of the specification denotes applicability to concrete roadway paving, the concepts are nearly all directly applicable to structural concrete as well. Durable Concrete Properties The AASHTO PP 84-17 specification is flexible and can be tailored to the needs of the specifier. There are various performance and prescriptive options for many of the critical properties and tests. In addition, guidelines for each test are included, such as whether the test is used as a mixture qualification test, as an acceptance test, or both. Selected highlights for each property are as follow: - Strength: In-service concrete rarely fails due to poor strength. This section of AASHTO PP 84-17 mirrors the common practice of specifying either compressive or flexural strength. - Reducing unwanted cracking due to shrinkage: For specifiers who want to retain a prescriptive approach to shrinkage, the specification suggests a maximum volume of paste. The performance approach lists several options for assessing shrinkage, including a dual ring test that is being developed as an AASHTO standard test. - Hardened cement paste freeze-thaw durability: Freeze-thaw durability is addressed with the prescriptive option of specifying a maximum water-cement ratio. The performance approach has several options, including using the super air meter (SAM) (AASHTO TP 118) in lieu of the traditional Type B pressure pot (AASHTO T 152). The SAM is a new test that offers the significant advantage of providing information (cont. next page)

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