Chiara F. Ferraris

The influence of mineral admixtures on the rheology of cement paste and concrete

Rheological tests on cement paste were used to successfully select the type and dosage of mineral admixtures that improved concrete workability. Among the six different mineral admixtures tested, the ultrafine fly ash (UFFA) was determined to give the best results by reducing the yield stress and viscosity. These improved rheological properties were not achieved by increasing the water demand and/or the high-range water reducer (HRWR) admixtures dosage. Therefore, the addition of UFFA improved the concrete flow without a potential decrease of the hardened properties or an increase in cost. The conclusions reached based on cement paste tests were validated by concrete slump tests. The cement paste rheological data were also compared using two simpler tests, minislump and Marsh cone. The goal was to determine whether the simpler tests could be used to characterize the rheology of cement paste adequately. The conclusions are that these simpler tests are unreliable for measuring workability.

Capillary transport in mortars and concrete

Abstract Results are presented of a study concerning capillary transport of water in concretes and mortars as a function of water/cement ratio, sand size distribution, and curing. Our studies indicate that the capillary sorption (1) of water in concrete exhibits a complex time dependence. At early times, of the order of one hour, the total water uptake increased with the t 1 2 behavior of typical capillary sorption theories. At longer times, of the order of tens of days, a slow crossover regime is seen as the rate of capillary suction decreases. At very long times, of order hundreds of days, a slower uptake which may be driven by the capillary forces in the smaller gel pores is found. Again the t 1 2 behavior is recovered but with a much smaller sorptivity coefficient. We describe an empirical fitting form for data which describes capillary suction at both short and long times. Aspects of the experimental design including sample drying and exposure to air are discussed. The utility of service life predictions from such measurements is discussed.

Measurement of the Rheological Properties of High Performance Concrete: State of the Art Report

The rheological or flow properties of concrete in general and of high performance concrete (HPC) in particular, are important because many factors such as ease of placement, consolidation, durability, and strength depend on the flow properties. Concrete that is not properly consolidated may have defects, such as honeycombs, air voids, and aggregate segregation. Such an important performance attribute has triggered the design of numerous test methods. Generally, the flow behavior of concrete approximates that of a Bingham fluid. Therefore, at least two parameters, yield stress and viscosity, are necessary to characterize the flow. Nevertheless, most methods measure only one parameter. Predictions of the flow properties of concrete from its composition or from the properties of its components are not easy. No general model exists, although some attempts have been made. This paper gives an overview of the flow properties of a fluid or a suspension, followed by a critical review of the most commonly used concrete rheology tests. Particular attention is given to tests that could be used for HPC. Tentative definitions of terms such as workability, consistency, and rheological parameters are provided. An overview of the most promising tests and models for cement paste is given.

Characterization of Metal Powders Used for Additive Manufacturing

Additive manufacturing (AM) techniques1 can produce complex, high-value metal parts, with potential applications as critical parts, such as those found in aerospace components. The production of AM parts with consistent and predictable properties requires input materials (e.g., metal powders) with known and repeatable characteristics, which in turn requires standardized measurement methods for powder properties. First, based on our previous work, we assess the applicability of current standardized methods for powder characterization for metal AM powders. Then we present the results of systematic studies carried out on two different powder materials used for additive manufacturing: stainless steel and cobalt-chrome. The characterization of these powders is important in NIST efforts to develop appropriate measurements and standards for additive materials and to document the property of powders used in a NIST-led additive manufacturing material round robin. An extensive array of characterization techniques was applied to these two powders, in both virgin and recycled states. The physical techniques included laser diffraction particle size analysis, X-ray computed tomography for size and shape analysis, and optical and scanning electron microscopy. Techniques sensitive to structure and chemistry, including X-ray diffraction, energy dispersive analytical X-ray analysis using the X-rays generated during scanning electron microscopy, and X-Ray photoelectron spectroscopy were also employed. The results of these analyses show how virgin powder changes after being exposed to and recycled from one or more Direct Metal Laser Sintering (DMLS) additive manufacturing build cycles. In addition, these findings can give insight into the actual additive manufacturing process.

Using Impedance Spectroscopy to Assess the Viability of the Rapid Chloride Test for Determining Concrete Conductivity.

The suitability of using the initial current from the rapid chloride test (ASTM C 1202) to determine specimen conductivity is tested using impedance spectroscopy with a frequency spectrum of 10 Hz to 1 MHz. The specimen conductivity has an analytical relationship to specimen diffusivity and so is a useful quantity in service life prediction. Measurements made on specimens of different lengths indicate that the total charge passed during the six hour conduction test carried out according to ASTM C 1202 is not a direct measure of specimen conductivity. Further, ohmic heating during the 6 hour test makes it nearly impossible to directly measure any specimen transport property from the results. The total charge passed during the 6 hour conduction test is, therefore, not a reliable quantity for service life prediction. Results indicate that the direct current (dc) measurement of resistance using a voltage of 60 V is sufficient to overwhelm polarization effects, thereby yielding an accurate estimate of the true specimen conductivity. Impedance spectroscopy measurements also indicate that corrosion may form on the brass electrodes, adding bias to a conductivity estimate based upon a dc measurement.

MODIFIED SLUMP TEST TO MEASURE RHEOLOGICAL PARAMETERS OF FRESH CONCRETE

The ease of placement of concrete depends upon at least two physical properties, the yield stress and plastic viscosity. Currently the most common field test is the slump test, and it is related only to the yield stress. Therefore, a simple field test method intended to provide an evaluation of the two Bingham rheological parameters, yield stress and plastic viscosity, was developed. To determine the plastic viscosity the time necessary for the upper surface of the concrete in the standard slump cone to slump 100 mm was measured. The apparatus and test procedure are described. Semi-empirical models are proposed for the yield stress and for the plastic viscosity as function of the final slump and slumping time. The application of the modified slump test for the evaluation of the viscosity is limited to concretes with a slump of 120 to 260 mm. If the validity of this test is confirmed in the future, it could be used as a field quality control test.

Relating Fresh Concrete Viscosity Measurements from Different Rheometers

Concrete rheological properties need to be properly measured and predicted in order to characterize the workability of fresh concrete, including special concretes such as self-consolidating concrete (SCC). It was shown by a round-robin test held in 2000 [1,2] that different rheometer designs gave different values of viscosity for the same concrete. While empirical correlation between different rheometers was possible, for a procedure that is supposed to “scientifically” improve on the empirical slump tests, this situation is unsatisfactory. To remedy this situation, a new interpretation of the data was developed. In this paper, it is shown that all instruments tested could be directly and quantitatively compared in terms of relative plastic viscosity instead of the plastic viscosity alone. This should eventually allow the measurements from various rheometer designs to be directly calibrated against known standards of plastic viscosity, putting concrete rheometry and concrete workability on a sounder materials science basis.

Concrete Mixing Methods and Concrete Mixers: State of the Art

As for all materials, the performance of concrete is determined by its microstructure. Its microstructure is determined by its composition, its curing conditions, and also by the mixing method and mixer conditions used to process the concrete. This paper gives an overview of the various types of mixing methods and concrete mixers commercially available used by the concrete industry. There are two main types of mixers used: batch mixers and continuous mixers. Batch mixers are the most common. To determine the mixing method best suited for a specific application, factors to be considered include: location of the construction site (distance from the batching plant), the amount of concrete needed, the construction schedule (volume of concrete needed per hour), and the cost. Ultimately, the quality of the concrete produced determines its performance after placement. An important measure of the quality is the homogeneity of the material after mixing. This paper will review mixing methods in regards to the quality of the concrete produced. Some procedures used to determine the effectiveness of the mixing will be examined.

Cementitious Paste Setting Using Rheological and Pressure Measurements

For construction schedules, such as placement and formwork removal, to be controlled, how mechanical characteristics of concrete evolve during the setting period needs to be measured. Cement paste and mortar setting time measurement is usually done through the standard Vicat test, which does not determine when concrete pumping or extrusion may occur because of insufficient information (initial and final set time). Hydraulic form pressure and intrinsic material parameter (plastic viscosity and yield stress) variation effects on cement paste setting period are studied. Hydraulic and rheological (shear rate sweep and stress growth) measurements allowed setting monitoring. The authors discuss experiment results and compare them with the cement paste Vicat Test. The study shows that the Vicat test is less sensitive to cement paste setting evolution than the proposed tests, which provide useful information at an early age.

Developing a More Rapid Test to Assess Sulfate Resistance of Hydraulic Cements

External sulfate attack of concrete is a major problem that can appear in regions where concrete is exposed to soil or water containing sulfates, leading to softening and cracking of the concrete. Therefore, it is important that materials selection and proportioning of concrete in susceptible regions be carefully considered to resist sulfate attack. American Society for Testing Materials (ASTM) limits the tricalcium aluminate phase in cements when sulfate exposure is of concern. The hydration products of tricalcium aluminate react with the sulfates resulting in expansion and cracking. While ASTM standard tests are available to determine the susceptibility of cements to sulfate attack, these tests require at least 6 months and often up to a year to perform; a delay that hinders development of new cements. This paper presents a new method for testing cement resistance to sulfate attack that is three to five times faster than the current ASTM tests. Development of the procedure was based upon insights on the degradation process by petrographic examination of sulfate-exposed specimens over time. Also key to the development was the use of smaller samples and tighter environmental control.