Development of Design Procedures for Flexural Applications of Textile Composite Systems Based on Tension Stiffening Models

The Aveston Copper and Kelly (ACK) Method has been routinely used in estimating the efficiency of the bond between the textile and cementitious matrix. This method however has a limited applicability due to the simplifying assumptions such as perfect bond. A numerical model for simulation of tensile behavior of reinforced cement-based composites is presented to capture the inefficiency of the bond mechanisms. In this approach the role of interface properties which are instrumental in the simulation of the tensile response is investigated. The model simulates the tension stiffening effect of cracked matrix, and evolution of crack spacing in tensile members. Independent experimental results obtained from literature are used to verify the model and develop composite tensile stress strain response using alkali resistant (AR) glass textile reinforced concrete. The composite stress strain response is then used with a bilinear representation of the composite obtained from the tensile stiffening model. The closed form and simplified equations for representation of flexural response are obtained and used for both back-calculation and also design. A method based on the average moment-curvature relationship in the structural design and analysis of one way and two way flexural elements using yield line analysis approaches is proposed. This comprehensive approach directly shows the interrelation of fundamental materials characterization techniques with simplified design equations for further utilization of textile reinforced concrete materials

Analytical and Quantitative Aspects of Surface Moisture Transport and Plastic Shrinkage Cracking

The drying rate of porous materials such as hydrating cement paste during early hydration stages is studied using analytical and experimental procedures. Effects of micro and macro fibers as they change the nature of restrained shrinkage cracking are also documented. A methodology based on vacuum drying experiments is developed to measure the rate of evaporation from the surface of fresh paste and mortar mixtures that leads to restrained shrinkage cracking. Stages of microcrack coalescence due to plastic shrinkage cracking are quantitatively analyzed using digital image correlation. A model for internal moisture transfer simulates initial constant drying rate followed by a vapor diffusion transport phenomena. A fluid mechanics approach for water evaporation from the boundary-layer in terms of mass transfer, diffusion, and convection is used. Effect of temperature, wind speed, and relative humidity are studied. Results of these two experiments are then integrated with an analytical approach for the restrained ring specimen which correlates the moisture diffusion in the specimen with the rate of evaporation and shrinkage strain. The model incorporates key influential parameters of diffusion, evaporation, shrinkage, creep, aging, and microcracking, in the stress analysis of a restrained concrete section. The formulation addresses the effect of geometry of the specimen, the humidity and shrinkage conditions, and the restraint offered by stiffness of the steel ring. Finally the modelling is extended to simulate a slab on ground and predict multiple transverse cracking as well as slab curling using a finite difference model

Ultra high Performance Concrete - Materials Formulations and Serviceability based Design

[EN] ABSTRACT Materials and mechanical design procedures for ultra-high performance cement composites (UHPC) members based on analytical models are addressed. A procedure for the design of blended components of UHPC is proposed using quaternary cementitious materials. The blending procedures are used using a packing and rheology optimization approach to blend high performance mixtures using non-proprietary formulations. Closed-form solutions of moment-curvature responses of UHPC are derived based on elastic-plastic compressive model and trilinear strain hardening tension stress strain responses. Tension stiffening behavior of UHPC due to fiber toughening and distributed cracking is then incorporated in the cross-sectional analysis. Load-deflection responses for beam members are obtained using moment-area, and direct integration approach. The proposed models provide insights in the design of SHCC to utilize the hardening properties after cracking. Using proper parameters, generalized materials model developed are applicable to both SHCC and strain softening cement composites such as steel fiber reinforced concrete (SFRC), textile reinforced concrete (TRC) and ultra-high performance concrete (UHPC)Yao, Y.; Arora, A.; Neithalath, N.; Mobasher, B. (2018). Ultra high Performance Concrete - Materials Formulations and Serviceability based Design. En HAC 2018. V Congreso Iberoamericano de hormigón autocompactable y hormigones especiales. Editorial Universitat Politècnica de València. 1-13. https://doi.org/10.4995/HAC2018.2018.8263OCS11

Advances in Natural Fiber Cement Composites: A Material for the Sustainable Construction Industry

The need for economical, sustainable, safe, and secure shelter is an inherent global problem and numerous challenges remain in order to produce environmentally friendly construction products which are structurally safe and durable. The use of sisal, a natural fiber with enhanced mechanical performance, as reinforcement in a cement based matrix has shown to be a promising opportunity. This work addresses the development and advances of strain hardening cement composites using sisal fiber as reinforcement. Sisal fibers were used as a fabric to reinforce a multi-layer cementitious composite with a low content of Portland cement. Monotonic direct tensile tests were performed in the composites. The crack spacing during tension was measured by image analysis and correlated to strain. Local and global deformation was addressed. To demonstrate the high performance of the developed composite in long term applications, its resistance to tensile fatigue cycles was investigated. The composites were subjected to tensile fatigue load with maximum stresses ranging from 4 to 9.6 MPa at a frequency of 2 Hz. The composites did not fatigue below a maximum fatigue level of 6 MPa up to 106 cycles. Monotonic tensile testing was performed for composites that survived 106 cycles to determine its residual strength

First Principles-Based Design of Economical Ultra-High Performance Concrete

This paper presents a novel strategy to design the binder phase of ultra-high performance concrete (UHPC) from commonly available cement replacement (fly ash, slag, microsilica, metakaolin) and fine filler (limestone) materials. A packing algorithm is used to extract the number density, mean centroidal distance, and coordination number of the microstructure. Similarly, rheological studies on the pastes provide yield stress, plastic viscosity, and mini-slump spread. The selection criteria involves using the three microstructural and three rheological parameters individually or in combination to define packing and flow coefficients. The selection criteria is flexible enough to allow users modify the constraints depending on the application. The binder with the desired packing and rheological features is combined with aggregate sizes and amounts chosen from a compressible packing model based on maximum packing density. A fiber volume fraction of 1% is also used, along with accommodations for wall and loosening effects. The model is programmed in a userfriendly environment to enable engineers select aggregates from locally available materials. Compressive strengths greater than 150 MPa are obtained for the selected UHPC mixtures after 28 days of moist curing. The strength-normalized cost of such mixtures is only a fraction of that of proprietary UHPCs

Reinforcing Efficiency of Micro and Macro Continuous Polypropylene Fibers in Cementitious Composites

The effect of the microstructure of hydrophilic polypropylene (PP) fibers in the distribution of cracking associated with the strengthening and toughening mechanism of cement-based composites under tensile loading was studied. Using a filament winding system, continuous cement-based PP fiber composites were manufactured. The automated manufacturing system allows alignment of the fiber yarns in the longitudinal direction at various fiber contents. Composites with surface-modified hydrophilic macro-synthetic continuous polypropylene fibers and monofilament yarns with different diameters and surface structures were used. Samples were characterized using the tensile first cracking strength, post-crack stiffness, ultimate strength, and strain capacity. A range of volume fractions of 1–4% by volume of fibers was used, resulting in tensile first cracking strength in the range of 1–7 MPa, an ultimate strength of up to 22 MPa, and a strain capacity of 6%. The reinforcing efficiency based on crack spacing and width was documented as a function of the applied strain using digital image correlation (DIC). Quantitative analysis of crack width and spacing showed the sequential formation and gradual intermittent opening of several active and passive cracks as the key parameters in the toughening mechanism. Results are correlated with the tensile response and stiffness degradation. The mechanical properties, as well as crack spacing and composite stiffness, were significantly affected by the microstructure and dosage of continuous fibers

Development of Design Procedures for Flexural Applications of Textile Composite Systems Based on Tension Stiffening Models

The Aveston Copper and Kelly (ACK) Method has been routinely used in estimating the efficiency of the bond between the textile and cementitious matrix. This method however has a limited applicability due to the simplifying assumptions such as perfect bond. A numerical model for simulation of tensile behavior of reinforced cement-based composites is presented to capture the inefficiency of the bond mechanisms. In this approach the role of interface properties which are instrumental in the simulation of the tensile response is investigated. The model simulates the tension stiffening effect of cracked matrix, and evolution of crack spacing in tensile members. Independent experimental results obtained from literature are used to verify the model and develop composite tensile stress strain response using alkali resistant (AR) glass textile reinforced concrete. The composite stress strain response is then used with a bilinear representation of the composite obtained from the tensile stiffening model. The closed form and simplified equations for representation of flexural response are obtained and used for both back-calculation and also design. A method based on the average moment-curvature relationship in the structural design and analysis of one way and two way flexural elements using yield line analysis approaches is proposed. This comprehensive approach directly shows the interrelation of fundamental materials characterization techniques with simplified design equations for further utilization of textile reinforced concrete materials

Flexural Analysis and Design of Textile Reinforced Concrete

A model is presented to use normalized multi-linear tension and compression material characteristics of strain-hardening textile reinforced concrete and derive closed form expressions for predicting moment-curvature capacity. A set of design equations are derived and simplified for use in spreadsheet based applications. The model is applicable for both strain-softening and strainhardening materials. The predictability of the simplified model is checked by model calibration and development of design charts for moment capacity and stress developed throughout the cross section of a flexural member. Model is calibrated by predicting the results of Alkali Resistant Glass and Polyethylene fabrics. A case for the flexural design of Glass Fiber Reinforced Concrete (GFRC) specimen as a simply supported beam subjected to distributed load is used to demonstrate the design procedure

Testing of Concrete Under Closed-Loop Control

Closed-loop testing systems provide the ability to directly control the deformation of the loaded specimen. This considerably enhances the precision, stability, and scope of the experiments. Closed-loop machines can be used to determine the stable response of test specimen or structure by monitoring and controlling the physical quantities that are sensitive to its behavior. The importance of the various components of the closed-loop controlled system and the test configuration is reviewed in the paper. The most critical aspect of designing the test is the choice of the controlled variable. With appropriate controlled variables and good system performance, several interesting and intricate testing techniques can be developed, as seen in the examples presented here