Long Term Performance of Concrete Project

Sustainable concrete construction design for durability requires performance-based knowledge of expected durability and service life prediction.  Implementation of both design for durability and performance-based standards and specifications is limited by:

1. The lack of rapid, science-based but simple test methods for characterizing the transport properties and degradation resistance of concrete,
2. A lack of understanding of the influence of concrete cracking on long term performance, and
3. A lack of innovation in approaches to producing durable concrete.

This project will address these deficiencies in a comprehensive and timely manner by the end of FY2014.

Objective: Facilitate the realization of performance-based standards and design for durability as accepted components of the concrete specification and design process, by meeting the critical measurement science needs to design, predict, and achieve the targeted long term service life performance of concrete by 2014.

What is the new technical idea? The concrete industry initiated a move to performance-based standards and design for durability in 2002 with its Prescriptive to Performance (P2P) initiative, spearheaded by the National Ready Mixed Concrete Association (NRMCA).^[1] Implementation on even a limited scale has not been achieved due to the lack of critical performance metrics for characterizing durability and the nonexistence of a durability design code.^[2] This project will take a three-pronged approach to accelerate the realization of design for durability and performance-based standards and specifications:

1. Promote standardization of needed test methods to assess the transport properties and degradation resistance of concrete and its component materials,
2. Develop a fundamental understanding of the influence of cracking on transport properties and durability that is necessary to support and extend existing service life models, and
3. Demonstrate the NIST VERDiCT technology, an innovative new paradigm for producing durable concrete mixtures with increased service life.

This approach will provide the tools necessary to design and evaluate new and existing concrete structures, helping to lay the foundation for the comprehensive development of performance-based standards. To achieve maximum impact, these efforts will be focused on producing end-user deliverables (standards, specifications, demonstration projects, and guidance documents) through strategic partnerships and opportunistic leveraging of previous and ongoing efforts, which will feed into the overall service life prediction of concrete part of the Sustainable Engineered Materials program.

What is the research plan? The research plan is divided into three major thrusts:

1. Develop science-based, simple, rapid test methods to support performance-based specifications,
2. Quantify the influence of cracking on transport, and
3. Promote a new paradigm for producing concrete mixtures having significantly increased service life.

To perform durability-based design using performance-based standards, simple, rapid, field-ready performance metrics are needed.  Research by NIST and its partners has indicated that concrete resistivity is a likely candidate for providing this metric.  Standardization efforts have been discussed for the past five years; within the past year, a provisional standard for surface resistivity was implemented by the American Association of State Highway and Transportation Officials (AASHTO TP95-11), while the American Society for Testing Materials (ASTM) passed a new standard test method for bulk conductivity of concrete (a related measure). NIST, through its participation in the ASTM subcommittee C09.66, will focus our efforts on drafting and championing a surface resistivity standard test method for ASTM, harmonized if possible with the AASHTO standard.  We are assisted in this effort by Prof. Jason Weiss of Purdue University. To accelerate testing of sulfate resistance, a key concrete durability concern, NIST has developed a new mini-bar cement paste sulfate attack test method that dramatically reduces both the material and time requirements for evaluating the resistance to sulfate attack of (blended) cement pastes. The main measurement science research has been done, but it was deemed necessary by ASTM to first conduct a ruggedness test to optimize specimen geometrical parameters. This ruggedness testing is being completed in FY2012 and will be presented in FY2013 to the ASTM C01.29 subcommittee at its December, 2012 meeting.

Current concrete service life models have no realistic techniques to incorporate the effects of cracks. Without this, such models can dramatically overestimate new or remaining service life, resulting in potential economic and possibly even life-safety consequences.  Current concrete service life models for un-cracked concrete are based on fundamental scientific principles following years of basic research, much of which was performed at NIST. To move forward, a fundamental understanding of the effects that cracks have on the buildup of chlorides at the steel reinforcement must be developed before these service life models can be updated/extended to consider cracking. Our efforts in this area have first focused on the practical problem of transverse cracking directly above the reinforcement, via a combined experimental/computer modeling approach. The program’s x-ray microtomography unit is being employed to characterize the microstructure of 25 mm cubic mortar specimens with and without cast-in cracks. These specimens are then exposed to chloride solutions for prescribed periods of time. Quantitative measurements of chloride ingress are then obtained using x-ray micro fluorescence available in the Materials Measurement laboratory (MML) at NIST to validate model-estimated ingress profiles, and to ultimately establish simple relationships between cracking and transport.

Finally, the NIST-developed Viscosity Enhancers Reducing Diffusion in Concrete Technology (VERDiCT) has demonstrated a new paradigm for producing durable concrete, by focusing on the pore solution as opposed to the matrix. Previous work to validate the VERDiCT technology has successfully demonstrated the value of VERDiCT in improving both early and later age performance of concrete, by reducing both early-age autogenous stresses (reducing early-age cracking) and long term chloride transport (via reductions in both diffusion and sorption). These benefits have been demonstrated in mortar and real world concrete mixtures, the latter prepared in collaboration with the National Ready-Mixed Concrete Association. To further demonstrate the benefits of this technology, the potential ability of VERDiCT to also increase sulfate resistance is being evaluated in standardized mortar bar testing. To address the final step of field testing of the VERDiCT technology, a partnership with the Universidad Autónoma de Nuevo Leon (UANL) in Monterrey, MEXICO has evaluated the performance of VERDiCT in actual concrete field exposures at a plant that produces chemicals that are extremely corrosive to normal concrete and in a series of self-compacting concretes prepared in the laboratory. The NIST/UANL collaboration builds on previous joint research between the two institutions and takes advantage of the severe exposure environment (chlorides, carbonates, and temperature) provided at the chemical plant in Monterrey that is unavailable in the US. In addition to the VERDiCT technology, NIST is conducting research on other paradigms for increasing service life, such as the application of sealants and coatings that is being investigated as part of an ARRA-funded Ph.D. research project being conducted by Scott Jones under the supervision of the principal investigator.

A critical aspect of these three technical thrusts will be engagement of partners and end users, including members of the appropriate ASTM subcommittees and American Concrete Institute (ACI) technical committees to both leverage their research efforts and to provide an exchange of technical ideas, research directions, and outputs. Thus, these committees will serve as both partners and end users. Other end users include state Departments of Transportation, with NIST already having significant contacts and ongoing collaborations with Florida, Virginia, Indiana, and Oregon. Technology transfer is being facilitated by the development of web-based guides and software, such as the concrete electrical conductivity calculator that was developed and launched in FY2012. As a first test case, this software will be employed in the education of Civil Engineering undergraduate students at Purdue University in the fall of 2012.

[1] 

Bickley, J.A., Hooton, R.D., and Hover, K.C., “Preparation of a Performance-Based Specification for Cast-in-Place Concrete,” RMC Research & Education Foundation, Phase 1 Final Report, 2006.

Obla, K.H., and Lobo, C.L., “Experimental Case Study Demonstrating Advantages of Performance Specifications,” RMC Research & Education Foundation, 32 pp, 2006.

Hover, K.C., Bickley, J.A., and Hooton, R.D., “Guide to Specifying Concrete Performance,” RMC Research & Education Foundation, 39 pp, 2008.

[2] ACI ITG-8R-10 “Report on Performance-Based Requirements for Concrete,” American Concrete Institute, Dec. 2010, available at http://www.concrete.org/pubs/newpubs/ITG-8R-10web.pdf.

Recent Results:

Outputs:

• Bentz, D.P., Snyder, K.A., Peltz, M.A., Obla, K., and Kim, H., “Viscosity Modifiers to Enhance Early Age Performance and Long Term Durability of Concrete,” WERB approved and submitted to ACI Materials Journal in FY2012.
• Weiss, W.J., Snyder, K.A., Bullard, J.W., and Bentz, D.P., “Using a Saturation Function to Interpret the Electrical Properties of Partially Saturated Concrete,” WERB approved and submitted to ASCE Journal of Materials in Civil Engineering in FY2012.
• Collaborative exchange with Universidad Autónoma de Nuevo Leon in Monterrey, MEXICO with UANL staff spending one month at NIST in July 2012 and NIST principal investigator visiting UANL in August 2012 to investigate the performance of the VERDiCT technology in the severe exposure environment (chlorides, carbonates, and temperature) provided at the chemical plant in Monterrey that is unavailable in the US.

Outcome:

• Concrete conductivity calculator web site established at http://concrete.nist.gov/Concreteconductivity.html

Standards and Codes:

Impact:

• ASTM C1760-12 Standard Test Method for Bulk Electrical Conductivity of Hardened Concrete (based on previous NIST research) approved in FY2012

Outcomes:

• Draft standard test method “Measurement of Mass Loss versus Time for One-Dimensional Drying of Saturated Concretes” developed in FY2012 (ASTM Work Item #37029 in subcommittee 09.66)
• Changes proposed and balloted for ASTM C1585-11 Standard Test Method for Measurement of Rate of Absorption of Water by Hydraulic-Cement Concretes in FY2012