Measurement Science to Assure the Performance of Green Concretes Project

An effective strategy for making a more “green” concrete is to replace the portland cement (the binding component with large energy and CO₂ footprints) with industrial by-products (IBP) such as fly ash, blast furnace slag, and waste glass.  Major questions exist, however, in assuring the performance of these concretes, as evidenced by the fact that the industry-average portland cement replacement in ready-mix concrete was only 15% in 2008, despite the availability of tens of millions of tons of IBP materials that end up in landfills each year.  This project will overcome the measurement science barriers to increasing the percentage of IBP in concrete by providing the industry with the metrics and guidance for the design, construction, and durability of green concrete.  This work, done in partnership with concrete specifiers, producers, contractors, and owners, will provide the knowledge to confidently use a more sustainable form of concrete.  

Objective:  To develop and deliver the measurement science identified by industry for assuring the performance of Industrial-by-Product (IBP) low-carbon footprint concretes with up to 50% by mass of the cement replaced by IBPs by 2015.

What is the new technical idea?  The industry average cement replacement cannot increase significantly without new measurement science to ensure the performance of concretes made with up to 50% IBP materials. This is because the chemistry of an IBP material begins to dominate the chemistry of the portland cement, which can impact the constructability and durability of these blended systems. Because the chemical behavior of IBP materials is not fully understood, variations in the IBP material properties from multiple sources, or even from a single source, can have unanticipated consequences, and correcting for these changes becomes particularly challenging. Furthermore, too little is known about the relative robustness of these blended concrete materials during construction and while in service, and whether the current standard tests accurately reflect the performance of these blended systems.

NIST will design, develop, and operate a Green Concrete Performance Laboratory (GCPL) that is unique in the world. This facility will allow precise measurement and control of external and internal factors (e.g., environment, chemistry) that affect concretes containing up to 50% IBP materials so that standard tests and industry best practice guides can be developed that account for the unique properties of these materials. The precise control of environmental conditions will also be used to develop industry best practice guides for developing suites of performance tests that are tailored to their particular exposure conditions, both during construction and in service.

The GCPL will also be used to develop the additional standard test methods and engineering tools needed to assist end-users in developing new blended cementitious systems and demonstrating that they will have the desired constructability and durability performance. These tools will guide the practitioner in selecting performance improvement strategies that may include changing the proportions, or other additions such as limestone powder, calcium hydroxide, different cement types, non-chloride accelerators, and pre-wetted light-weight aggregate.

What is the research plan?  NIST will use a partnership with industry to help identify and prioritize the measurement science barriers that prevent the development, specification, and acceptance of blended cement concretes with high cement replacement levels. This partnership will exist throughout the project and will include participation from industry, federal agencies, and NGOs representing owners, specifiers, producers, and contractors. A component of the partnership plan is a NIST-led workshop to solicit industry input that will form the basis of a roadmap for developing and delivering the measurement science to assure the performance of concretes made with up to 50% IBP. This roadmap will be a working document that reflects the plan and objectives of the project and acts as a communication vehicle among the partners. It also will be used to publicly convey current and planned NIST activities to the entire industry.

To meet the roadmap goals, NIST will develop a Green Concrete Performance Laboratory (GCPL) to support quality assurance testing for constructability and durability. The facilities will enable quantitative performance tests of concrete and its constituent materials, along with performance tests for its paste and mortar fractions. The laboratory will also include facilities for controlling the exposure environment and the means for monitoring both the material and its environment. In addition to the activities within the GCPL, NIST will seek opportunities to collaborate on complementary work at partnering organizations.

A critical component to assuring success is communicating results to the entire concrete construction industry. NIST will develop tools, such as guides and computer applications, that practicing engineers can use to rapidly develop and evaluate new blended cement concretes. NIST will conduct workshops to help engineers and specifiers achieve assured and adequate performance through both improved standard tests and through the proper selection of the required suite of test methods. NIST will also work with the industrial partners in this project to develop and promulgate new test methods and industry guides.

Two important initial objectives will be to transition the industry away from the current practice of mass-based concrete mixture design methodology to a volumetric one, and to develop standardized test methods for characterizing the temperature sensitivity of concrete setting time. Because the density of IBP materials are typically more than 10% less than portland cement, unanticipated property changes can occur when changing the mass replacement percentage and then rescaling the mix to achieve the desired yield (volume), particularly for larger replacement values. During construction, contractors and specifiers have reluctance to use concretes containing high replacement percentages due to unanticipated long delays in the setting time, which affects construction scheduling. One contributing cause is the low heat of hydration of systems containing relatively low portland cement content and their increased temperature sensitivity, whereby a 5°C drop in temperature can cause a 35% increase in the setting time.

Recent Results:

• Phase I of Green Concrete Laboratory: Identified and initiated procurement requests for the equipment needed to modernize the existing concrete laboratory by building the capability of making and testing mortars and concretes under controlled conditions. The improvements included the capabilities to perform industry standard test methods, modernized mixing, and improved temperature control. Phase II will address the specific needs identified in the roadmap report.
• Green Concrete Performance Workshop: The Green Concrete Performance Workshop was held at NIST on July 20, 2012. Representatives of industry specifiers, owners, producers, and contractors participated in breakout sessions to identify and prioritize barriers to the expanded use of IBP materials in concrete. The workshop summary report will include a plan for meeting the measurement science needs for the most critical barriers.

Standards and Codes:

NIST staff will take leadership roles and ally with the stakeholder partners to promulgate changes or additions to ASTM standards or ACI guides for quantifying or specifying the performance of blended cement concretes. NIST will take a leading role in developing new measurement science, and NIST will partner with others to promote the promulgation of standards and guides, and to develop supporting data. A new guide is planned for developing concrete mixtures on a volumetric basis, instead of the industry standard mass basis.  A new standard test method is planned for quantifying the set time temperature sensitivity of a concrete mixture.