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Adjusting Workability for Successful 3-D Concrete Printing, Part 1 of 2 (ACI Spring 2021 Convention, Virtual Sessions) Ability to extrude and to achieve shape stability of layer-wise additively manufactured cement-based elements depends upon the early-age rheological properties (shear moduli, yield stress, viscosity) of the deposited materials. Upon successful extrusion, buildability challenges can originate from two common failure mechanisms: yielding of the material in lower layers and buckling of the element. However, it is yet unclear which among various rheological properties controls the early-age materials’ deformation during printing processes and thus contributes to the resulting buildability of the elements. This presentation focuses on how buildability is dependent upon rheological properties as well on predicting the buildability using a buckling theoretical framework. Specifically, the relationship between early age rheological properties of various cement pastes and the buildability of hollow cylinders dominated by buckling failure mechanism was investigated. It was found that certain shear moduli of the fresh pastes (G, G*, and G’) obtained from oscillatory shear stress sweep tests performed within the first 30 minutes after combining water and cement showed better correlation with buildability of hollow elements than some other rheological properties (i.e., loss modulus G”, yield stress s yield , yield strain yield, and complex viscosity ?*). Measured shear modulus (G) was used to calculate elastic modulus (E) of the pastes using the assumed value of the 0.5 for Poisson’s ratio () for fresh cement paste. Euler’s buckling theory was used to predict buildability (i.e., ultimate height) of hollow cylinders. It was found that Euler’s theory overestimates the buildability from 93% to 194%, mainly due to assumption of ideal geometry (i.e., no initial or printing imperfections) and linear elasticity. An isosceles triangle was also developed and used as a support structure for assessment of early-age deformation of individual cement paste filaments over varied.

Upcoming Presentation

June 21 - 27

Evaluation of a Modified ASTM C157 for Early-Age Volume Change in Concrete
by Matthew O'Reilly, University of Kansas

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Technologies to Reduce Shrinkage and Cracking (ACI Fall 2020 Convention, Virtual Sessions) A potential shortcoming of ASTM C157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, is the inability to observe behavior between the time of final set of concrete and the first 24 hours after casting. A modified version of ASTM C157, where length-change measurements begin approximately 5 to 5½ hours after casting was developed by University of Kansas Researchers to evaluate the effects of expansive admixtures on free shrinkage, where much of the early-age expansion would otherwise be missed. Results show that combinations of supplementary cementitious materials, internal curing via pre-wetted lightweight aggregates, shrinkage-reducing admixtures, and/or shrinkage-compensating admixtures in concrete produce significantly more swelling within the first 24 hours after casting than ordinary portland cement mixtures, a majority of which is not observed when using the standard test method. Furthermore, the test results for mixtures tested in accordance with both procedures show that the amount of drying shrinkage (length change from the end of curing through one year of drying) is virtually unaffected by the difference in test procedures. Apart from the reduction in shrinkage provided by the technologies listed above, the additional swelling further mitigates the potential for cracking in concrete.

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