Concrete Laboratory Overview
The primary purpose of the Concrete Laboratory is to conduct research to develop a better, more durable, cost-effective, and sustainable concrete infrastructure by:
- Investigating and characterizing concrete component materials used for highways including cement, aggregate, supplementary and alternative cementitious materials, and admixtures.
- Assessing and improving durability aspects of concrete.
- Developing and evaluating new test methods or enhance existing testing procedures.
- Collaborating with academia and industry in advancing new emerging technologies.
- Performing forensic investigations requested within the Agency, by State departments of transportation (DOTs), and other governmental agencies.
- Supporting Performance Engineered Mixtures (PEM) initiative by assessing and validating methodologies and criteria.
The Concrete Laboratory conducts research in many areas related to concrete materials, such as fly ash, slag cement, and alternative cementitious materials with little or no hydraulic cement. The laboratory collaborates with academia, other government agencies, and industry, leveraging expertise in conducting research to address issues of national significance. The Concrete Laboratory is inspected by the Cement and Concrete Reference Laboratory (CCRL) and accredited by the American Association of State Highway and Transportation Officials (AASHTO) Materials Reference Laboratory.
Recent Accomplishments and Contributions
ASSESSMENT AND VALIDATION OF CONCRETE DURABILITY TESTING PROCEDURES IN SUPPORT OF AASHTO PP84-17
Summary: As part of an effort to promote more durable, longer-lasting, and cost-effective concrete infrastructures, Federal Highway Administration (FHWA), in partnership with State DOTs, academia and the industry, has been working to establish a new mindset for concrete mixtures design and quality assurance procedures, moving from a prescriptive to a performance approach. The impetus behind the FHWA’s PEM program is to help State DOTs shift from prescriptive to performance specifications, while giving them the freedom to use prescriptive specifications in the interim.
A suite of new and modified testing procedures was proposed under the PEM initiative, and incorporated into the AASHTO practice, PP-84-17. These procedures cover several categories, each consisting of several testing procedures that require assessment, refinement, and validation before they can be implemented. The Concrete Laboratory at Turner-Fairfbank Highway Research Center (TFHRC) was charged with that task as well as with verifying the criteria proposed in PP84. As these procedures are balloted and approved by AASHTO, they will be integrated into a performance-related specifications (PRS) program.
The primary objectives of the TFHRC study are:
- Assess, refine, and validate durability-related test methods as depicted in the AASHTO, PP-84- 17, and when necessary, recommend changes to these test methods.
- Create a pore solution database of mixtures used by State DOTs.
- Create a sorptivity database of mixtures used by State DOTs.
- Conduct ruggedness evaluation.
- Determine the test precision.
Assessment and Validation of New Rapid ASR Tests
Although Alkali-Silica Resistivity (ASR) has been the focus of extensive research since the late 1930s, there is still no reliable test method to evaluate its prevention and mitigation measures. Many of the exposure blocks in North America, including ones in Texas and Canada, are failing and exhibiting signs of ASR despite passing the 2-year American Society for Testing and Materials (ASTM) C1293. It is the consensus of the concrete community that ASTM C1293 can properly detect ASR in reactive aggregate; however, when with the mixtures that contain supplementary cementitious materials (SCMs) as a mitigation or prevention treatment, its reliability has been inconsistent. The main issue with the test is said to be the leaching of alkalis.
Two new test methods, AASHTO T380—Miniature Concrete Prism Test (MCPT), and Concrete Cylinder Test (CCT)—have been identified as testing procedures that potentially address the shortcomings of the ASTM C1293, such as alkali leaching and long testing duration, and assess the efficiency of the ASR mitigation and prevention measures. The MCPT takes only eight weeks to complete, while the CCT takes up to nine months.
This research, in collaboration with Oregon State University and University of Texas Austin, focuses on examining the reliability of these two test methods—the MCPT and the CCT—in assessing ASR-mitigation measures. The primary objective of this research is twofold:
- Examine the use of CCT and MCPT as accurate and accelerated test methods in evaluating ASR-mitigation measures and compare the laboratory test results with the data at various exposure sites in North America.
- Determine the proper test duration and expansion limits for the proposed test methods.
Reducing the Specimen Size of Concrete Flexural Strength Test for Safety and Ease of Handling
Summary: This project evaluated the feasibility of using smaller specimen sizes. A total of 22 concrete mixtures were prepared with varying water to cementitious ratios (w/cm), coarse aggregate types, and maximum nominal sizes. In addition, an interlaboratory study (ILS) for the determination of the precision of the test procedure was carried out in collaboration with ASTM and 22 laboratories.
- AASHTO T23 and AASHTO T97 have been revised to allow for the use of the smaller-size beams and to include the precision statement in AASHTO T97.
- Several stakeholders have already started using the smaller-size beams.
Impact of Deicing Salts on Transport Properties of Concrete
This study aimed to evaluate the combined effect of diffusion and absorption on transport properties of concrete samples exposed to deicing salts. Plain concrete, 30 percent fly ash F and 50 percent slag cement concrete mixtures, with w/cm of 0.42 or 0.50 were exposed to NaCl, CaCl2, and MgCl2 continuously or in wet and dry cycles for up to a year. The rate of absorption and the apparent diffusion coefficient were found to depend on the mixture design, exposure conditions, and cations of the salts in solution. Results showed the importance of careful interpretation; transport testing results depend on the exposure history and testing protocols.
Relying solely on test results without understanding concrete’s exposure history and the factors that affect individual tests can be misleading.
Influence of Aggregate Characteristics on Concrete Performance
Summary: This was a collaborative project between TFHRC and the National Institute of Standards and Technology (NIST), and evaluated and quantified the effect of aggregate characteristics that are not normally considered on concrete mechanical performance. The results have demonstrated that for similar mixture proportions, the selection of coarse aggregates can have a measurable influence on concrete performance for both mechanical and transport properties. The incompatibility of certain paste and aggregate properties likely promote the development of interfacial stresses, potentially causing microcracking, weakening the bond between the two phases, and lowering the measured concrete strength. The results also demonstrated that selection of an optimum aggregate for a specific concrete application will require knowledge of the binder used; some aggregates performed better with ordinary concrete than they did with ternary blends and vice versa. The bond between aggregates and paste/mortar greatly influences mechanical properties of the produced concrete.
Held the “4th TFHRC Workshop on Emerging Developments Related to Concrete Pavement Durability” on November 12–14, 2019.
The Concrete Laboratory’s capabilities include mixing, curing, and conducting tests on cementitious paste, mortar, and concrete. The Laboratory is equipped with facilities for evaluating early-age properties (e.g., rheological properties, setting, and calorimetry), mechanical properties (including strength and modulus of elasticity), volume changes, and concrete durability including formation factor, freezing and thawing, permeability, ions penetrability, and alkali-aggregate reaction.
Services are focused on research and investigations at TFHRC or in cooperation with other governmental agencies, as well as academia and industry. Services also include performing forensic investigations requested within the agency and by State DOTs and other governmental agencies.
Mixture Preparation and Curing and Conditioning of Specimens
The Concrete Laboratory has the capability to mix pastes with a high-shear mixer (ASTM C1738), a vacuum mixer, or a small planetary mixer (ASTM C305). Mortars are prepared with three different sizes of planetary mixers. Concrete is prepared with a 2 ft3 pan planetary mixer, a 0.75 ft3 or 6 ft3 drum mixer, and a high-shear mixer.
Curing and conditioning is done in the curing room, in one of the three temperature-controlled curing tanks which automatically maintain constant water level and temperature, or in a walk-in environmental chamber (Figure 1). Three smaller environmental chambers (Figure 1), located in a separate room, are used for conditioning or to maintain specimens at specific temperature and R.H. condition during testing.
The shrinkage room is used for conditioning or testing specimens under standard or other specific controlled conditions.
The Concrete Laboratory can monitor hydration reactions over time using an isothermal calorimeter (Figure 2), or a semi-adiabatic calorimeter and an automated setting time apparatus. Workability is assessed with a flow table, a vebe consistometer, and a dynamic shear rheometer (Figure 3). A super air meter (SAM) is used to measure the air-void system of fresh concrete.
Mechanical Properties Evaluation
Compressive, splitting tensile, and elastic modulus are determined in one of the two universal testing machines (1 million lbf and 500 klbf) (Figure 4). For modulus of elasticity, a compressometer/extensometer with linear variable differential transformers (LVDTs) is used. Flexural strength is obtained in a beam tester (Figure 4). Four creep frames are also available.
Volume Changes Evaluation
The Concrete Laboratory possesses equipment and facilities for assessing free and autogenous shrinkage. In addition, a dual ring is used to evaluate stress development and cracking potential on concrete due to restrained volume change (AASHTO T363). The effect of temperature on volume changes is determined using the coefficient of thermal expansion (CTE) test apparatus (Figure 5).
The Concrete Laboratory includes facilities for investigating the effects of chemical and environmental exposure on concrete, including two automated freeze-thaw chambers (Figure 6) with the capacity for 17 specimens, a resonant frequency tester, computer-controlled chloride penetration test equipment, chloride profiling and titration (Figure 7), and the formation of calcium oxychloride using a low-temperature differential-scanning calorimeter (LTDSC) (Figure 8).
Pore solution is expressed from paste, mortar, or concrete to be used in the formation factor determination or to monitor chemical reactions (Figure 9). The chemical analysis of the pore solution or the chemical materials characterization is carried out with an x-ray fluorescence spectrometer (XRF) (Figure 10), while the pore solution resistivity is obtained with the cell resistivity (Figure 11).
The Concrete Laboratory is also involved in assessing distress mechanisms such as alkali-aggregate reaction.
Cementitious materials are chemically characterized using an XRF spectrometer (Figure 10) and aggregates are physically characterized regarding their specific gravity, absorption, and unit weight. In addition, they can have their shape, angularity, and texture determined by using the Aggregate Image Measurement System (AIMS) (Figure 14).