3D Printing with Concrete - Development of a Vibrating Nozzle for Material Extrusion
Presented By: Karthik Sooryanarayana
Affiliation: University of Illinois At Urbana Champaign
Description: The rheological requirements for 3D printing of concrete are two-fold. During pumping and extrusion, the concrete needs to behave like a fluid, and solid like after extrusion to resist the self-weight of the layer as well as the weight of subsequently printed layers. Vibration can be used to alter the yield stress of fresh concrete to control its fluid/solid state. Vibration causes an immediate and reversible reduction in the yield stress of granular suspensions such as concrete. A modified concrete rheometer was used for the rheological characterization of concrete mixtures during vibration. The effect of changing mixture design and vibration parameters of the rheology of concrete was studied. A prototype vibrating nozzle is being developed to understand the relation between rheology and the printing process (see attached figure). Material deposition by the vibrating nozzle can potentially lead to greater layer height and faster 3D printing. Vibration allows for the printing of cementitious mixtures with coarse aggregates and has the potential to make 3D printing of concrete more economical, durable, and sustainable. This study evaluates the implication of using vibration to deposit concrete using the vibrating nozzle prototype. Rheological limits are being established to define the printability of the concrete mixtures.
Shear Behavior of Macro-Synthetic Fiber-Reinforced Concrete
Presented By: John Paul Gaston
Affiliation: University of Washington Seattle
Description: Macro-synthetic fibers are often used as secondary reinforcement in concrete members, designed to control shrinkage and temperature cracks. While a large body of research has investigated the shear behavior of steel fiber-reinforced concrete beams, few studies have investigated the contribution of macro-synthetic fibers to the shear strength of structural members. This ongoing research, funded by the ACI Concrete Research Council, investigates the combined use of macro-synthetic fibers and conventional deformed bars to resist shear forces, quantifying the potentially beneficial interaction between the two types of reinforcement. Twelve panel elements (36 in x 36 in x 3 in), representing rectangular portions of reinforced concrete beams or walls, will be subjected to in-plane stresses using the University of Washington's Panel Element Tester, one of the only several similar devices in the world. The variables of interest within the experimental program include the fiber volume fraction and transverse reinforcement ratio. Data from the panel tests and the existing literature will be used to develop design equations for the shear strength of macro-synthetic fiber-reinforced concrete members that also contain conventional transverse deformed bar reinforcement. The design equations will be developed to be consistent with the approach used in ACI 318 and will be based on a rational shear behavior model developed as part of this research effort. This presentation will discuss the results of the experimental campaign to date and present preliminary findings of the research program.
Alkali-Silica Reactivity of Belitic Calcium Sulfoaluminate (BCSA) Cement Concrete
Presented By: Tayyab Adnan Habibur Rahman
Affiliation: Clarkson University
Description: Belitic calcium sulfoaluminate (BCSA) cement is an alternative cement known for its rapid strength development and volume stability. This has long supported its use for accelerated pavement and bridge deck repair. More recently, researchers have investigated its use in structural concrete, where its low embodied energy and CO2 emissions contribute toward infrastructure decarbonization. However, insufficient knowledge on its fundamental material properties limits practical adoption. Specifically, more information is needed on the long-term durability of BCSA composites. To that end, this research studies alkali-silica reaction (ASR) in BCSA concrete. ASR is a deleterious reaction between alkaline cements and certain reactive aggregates that causes premature degradation and failure of concrete structures. Some researchers suggest BCSA cement is immune to ASR, but these claims have not been validated and the mechanisms of mitigation are not well understood. In this study, we evaluate the alkali-silica reactivity of BCSA concretes made with non-reactive limestone and Spratt siliceous limestone aggregates using accelerated and long-term expansion tests. Expansion results are similar between BCSA concrete with non-reactive and reactive aggregates, suggesting ASR did not occur. However, petrographic analysis with scanning electron microscopy and energy dispersive spectroscopy reveal abundant gel-filled through-aggregate cracking and significant mobility of iconic species between the aggregate and paste phases. These are all clear indicators of ASR. Although Spratt limestone is an extreme case, these results suggest ASR can occur in BCSA concrete under certain conditions but may not be evident from standard expansion tests.
Highly Dispersed CNT Reinforced Metakaolin-Cement Mortars with Enhanced Load-Bearing Capacity and Energy Absorption Capability
Presented By: Rohitashva Singh
Affiliation: University of Texas Arlington
Description: The use of pozzolanic materials with a low energy-consumption footprint, such as metakaolin (MK), is a promising way for developing sustainable eco-efficient concrete with enhanced mechanical and fracture properties. The effective incorporation of nanoscale fibers, such as carbon nanotubes (CNTs), into the relatively brittle Metakaolin-Ordinary Portland Cement (MK-OPC) matrix holds great potential in enhancing the composite's energy absorption capacity. In this study, the development of MK-OPC mortars reinforced with low amounts of highly dispersed CNTs, ~0.1 wt%, is carried out. The mechanical and fracture properties were evaluated through fracture mechanics tests on notched beam specimens following the Linear Elastic Fracture Mechanics (LEFM) and Two-Parameter Fracture Model (TPFM). The results indicate that the synergistic effort of MK with high surface area and CNTs with high aspect ratio lead to substantial improvements in load-bearing capacity, modulus of elasticity, +70%, first-crack strength, +60%, and fracture toughness +80%; and energy absorption capability, first-crack and flexural toughness, +80%.
Characterizing the Durability Properties of High Early Strength Concrete
Presented By: Ragini Krishna Nikumbh
Affiliation: Kansas State University
Description: High early strength concrete (HESC) has numerous applications, where several methods are available to achieve its high early strength. Generally concrete mixtures are designed with high content of ASTM C1600 cement, low water to cement ratio, and a high dosage of superplasticizers. The strength properties of HESC have been widely studied, but limited documentation is made of its durability characteristics. With that objective, this study focuses on understanding its freeze-thaw resistance, surface scaling, autogenous & drying shrinkage; by examining the effect of using: (i) Low (LCCM), standard (SCCM), and high (HCCM) cement content of ASTM Type III and CSA/CSA blend cement; (ii) Different types and combinations of admixtures; (iii) Internal curing (IC) using light-weight aggregates (LWA). It has been observed that irrespective of cement content, both type III and CSA/CSA blend cement, achieved a minimum compressive strength of 1800 psi or flexural strength of 380 psi within 6 hours of mixing. Concrete mixtures designed with type III cement required using an accelerating admixture to achieve strength thresholds. HCCM exhibited concerns such as excessive shrinkage, higher heat of hydration, poor aggregate-past bond, and reduced permeability, while LCCM performed most satisfactorily. Comparing mixtures prepared with and without IC, LWA was very effective in mitigating excessive shrinkage. A well-entrained air-void distribution is known to be most effective in ensuring frost durability, but the complex chemistry between superplasticizers and higher heat of hydration associated with HESC makes it challenging to deliver a well-distributed air-void system.
Mix-Proportioning Optimization of Binders and Mortars Using Accelerating Mineral Admixtures for High-Early-Strength and Durability
Presented By: Micah Stark
Affiliation: Texas A&M University College Station
Description: Rapid-setting and high-early-strength concrete are desired in additive manufacturing (3D printing), repair and retrofit of existing structures, and for accelerated construction. Calcium sulfoaluminate cement (CSA) and calcium aluminate-based admixtures have been used to achieve both rapid-setting and high-early strengths. Compared to ordinary Portland cement (OPC), CSA-based materials also have less shrinkage, and are calcined under lower temperatures, emitting up to 40% less CO2 during manufacturing. Nevertheless, long-term durability could be compromised when using these materials. In this study, the effects of replacing OPC with two accelerating mineral admixtures, CSA and a calcium aluminate-based Portland cement accelerator, are investigated. Multi-objective mix proportioning optimization is performed using machine-learning based tools to couple satisfactory early-age and long-term performance levels. Preliminary results from 7/8-inch diameter cement-paste cylinders have shown that compressive strengths up to 7 ksi (50 MPa) can be attained within the first hour of casting when partially replacing OPC with a calcium-aluminate based mineral accelerator. Experimental work is underway to further understand the effects of mix proportions on multi-objective performance outcomes related to durability and strength.
Seismic Fragility Analysis of Nuclear Reactor Concrete Containment Considering Alkali-Silica Reaction
Presented By: Chanyoung Kim
Affiliation: Ulsan National Institute of Science and Technology
Description: This study aims to investigate the seismic fragility of nuclear concrete containments subjected to the material degradation due to alkali silica reaction (ASR). Current guidelines for the seismic probability risk assessment of nuclear power plants suggest that the time-dependent change and degradation of material properties should be taken into account for the seismic fragility analysis. However, further studies need to develop the methodology to estimate the effects of material degradation on the seismic capacity of concrete containments and their uncertainties. Recently, the ASR damage was observed in nuclear power plants with a long operation period and thus is gaining more attention. In this study, the material deterioration due to ASR in concrete is simulated using the open-source software Opensees, and the seismic fragility analysis is conducted considering nonlinear time-history behavior. The numerical model implements the distribution of multi-directional ASR expansions using a weight function assigning a higher strain rate to unconfined directions. The distribution of the ASR expansions in each case is validated with the results reported in the literature from accelerated ASR experiments. The maximum drift at the cycle shown the maximum shear force is selected as the damage index for the limit state from the seismic analysis results. From the seismic fragility analysis, the median acceleration capacity of the case-study concrete containment turns out to decrease by 5.4% due to the ASR damage.
Understanding Mineralogy of Incineration Ashes via Raman Imaging
Presented By: Hamza SAMOUH
Affiliation: University of Illinois Urbana Champaign
Description: The concrete industry is responsible for ~8% of the global CO2 emissions. Different actions can be taken to reduce these emissions ranging from the substitution of clinker to the development of new types of cements. Supplementary Cementitious Materials (SCM) have been highly successful in the partial replacement of cement clinker. However, several traditional SCMs (specifically coal-based fly ashes) are becoming increasingly scarce due to declining coal production in many parts of the world. One potential solution to this ongoing scarcity of fly ashes is to consider Municipal Solid Waste Incineration (MSWI) ashes which are produced post-combustion of solid waste in a waste-to-energy facility. However, these MSWI ashes have complex mineralogy and contain new chloride and sulfate-based phases whose influence on cement hydration is far from understood. In this work, we aim to use Raman imaging as a potential tool for understanding the complex mineralogy of these ashes so they can be used as SCMs in the future. This research could pave the way to divert these MSWI ashes from landfills to beneficial applications.