Influence of Process Variables on Shrinkage in Low-Calcium Fly-Ash Geopolymers

The influence of process variables such as curing temperature and relative humidity (RH) on the shrinkage of alkali-activated fly ash (AAF) is examined in this work. The ambient conditions are varied after an initial accelerated moist curing at high-temperature. An analysis interlinking the effects of curing on AAF strength, shrinkage, reaction product content, and porosity is performed. Strength achieved and the pore structure formed for the different curing conditions depend upon the sodium alumino-silicate (N-A-S-H) gel content formed in the AAF. While water is not directly combined in the formation of N-A-S-H gel, its content is sensitive to the availability of moisture. The moisture loss due to drying during the geopolymerization reduces the N-A-S-H content formed in the AAF. Compared to the continuous moist curing at elevated temperature, there is a decrease in the N-A-S-H content on lowering the temperature or drying produced by the decrease in RH. Reducing temperature and RH following initial accelerated curing has the beneficial effect of reducing the shrinkage compared to drying at a higher temperature. Reduction in the N-A-S-H content due to decrease in temperature after the accelerated curing is more significant than the drying on lowering the RH to 50%. The autogenous shrinkage measured under sealed conditions contributes significantly to the total shrinkage in AAF. The shrinkage in the AAF is significantly lower than a comparable cement paste. While shrinkage is produced by drying, the moisture loss and shrinkage relationship is not unique. The shrinkage produced by moisture loss due to drying is primarily influenced by the pore structure formed in the AAF, which also depends on the N-A-S-H content. The influences of temperature and humidity on the strength, pore structure and shrinkage are determined by the N-A-S-H formed in the AAF

Influences of matrix strength and weak planes on fracture response of recycled aggregate concrete

The fracture behaviours of concrete made with natural aggregate and recycled coarse aggregate (RCA) derived from crushing old concrete are compared. The performances are evaluated using concrete proportioned for different compressive strengths. The RCA from crushed old concrete produces a composite aggregate with mortar and aggregate phases. An examination of the RCA shows pre-existing cracks in the aggregate phase. Crack propagation and crack opening profiles in the fracture response of concrete beams are evaluated using the Digital Image Correlation (DIC) technique. The displacement profiles across the beam obtained using DIC are evaluated to understand the crack growth in the concrete. The cohesive stress response and an energy measure determined from the fracture test are related to physical observations of the fracture surface. The crack path in the concrete and the contribution of the different interfaces depend on the strength of the matrix surrounding the aggregate. In concrete with lower cementitious and higher water contents, the new mortar interface with RCA and the pre-existing cracks in the RCA contribute to the fracture surface in the RCA. While a larger fracture surface area is created in concrete made with RCA, the energy measure and the cohesive stress determined from the fracture test are lower. In concrete proportioned for higher compressive strength, there is a densification of the RCA-mortar interface and the fracture plane is produced through the aggregates. The pre-existing cracks in the RCA create weak planes, which contribute to the fracture surface created. Improving the mortar-RCA interface does not result in an improvement in tensile strength or fracture characteristics since there is also a significant contribution of the weak planes in the aggregate phase of RCA to the failure surface. The measured fracture surface area does not correlate with the energy measure from fracture test response

Application of fracture mechanics to debonding of FRP from RC members

During the last two decades, externally bonded uni-directional fiber-reinforced polymer (FRP) composites have been widely used for strengthening, repairing, and rehabilitation of reinforced concrete (RC) structural members. The bond characteristics contribute to the effectiveness of the stress transfer achieved between the FRP composite and the concrete substrate. Debonding of the FRP composite reinforcement is the most critical concern in this type of application. Under monotonic and fatigue loading conditions, FRP-concrete shear debonding has been idealized as a Mode-II fracture problem along the bi-material interface. A cohesive material law is used to describe the interfacial stress transfer at the macroscopic level. The area under the entire curve represents the fracture energy and is related to the load-carrying capacity of the interface. In this paper, previous experimental results and literature are discussed to show how the fracture energy can be considered a true fracture parameter. In addition, a simplistic onedimensional numerical analysis of the direct shear test is presented with the intent of pointing out the effect of the fracture parameters related to the cohesive material law on the load carrying capacity. The results are instrumental to discuss the strain limits provided in the ACI 440.2R-08 document

Investigation of the Interface Fracture during Debonding between FRP and Masonry

The masonry-FRP interface fracture is a topic of considerable research interest due to the heterogeneity of the masonry subgrade. The size of the material inhomgeneity associated with the presence of mortar joints is significant compared with the length scale of fracture process. In this paper, the results of an experimental investigation into the shear debonding of FRP sheets from brick, mortar and masonry blocks are reported. The test procedures (Ali-Ahmad et al. 2006,2007) developed previously for obtaining the FRP-concrete cohesive fracture response are applied to study interface debonding from the three substrates. During each test, spatially continuous measurements of the surface strains on the FRP and masonry are obtained using an optical technique known as digital image correlation. The interface cohesive fracture response of FRP-brick and the FRP-mortar interfaces are obtained from the results of the strain analysis. The interface fracture energy associated with the FRPmortar interface is shown to be significantly smaller in magnitude than that of the FRP-brick interface. The contributions of the mortar and the brick to the overall load response of the masonry are analyzed using the cohesive material response of the FRP-brick and the FRP-mortar interfaces. It is shown that complete debonding is achieved at the FRP-mortar joint while the FRP is still attached to the bricks on either side of the joint. The cohesive crack front stretches across the fully debonded mortar joint before the cohesive crack completely crosses the joint. The local debonding at the mortar joint produces stresses higher than those associated with the main cohesive crack front in the brick-FRP interface close to the joint, thereby accelerating the crack advance

Porosity and pore structure control in cellular geopolymer using rheology and surface tension modifiers

Cellular geopolymer is produced by aerating Alkali-activated Fly ash (AAF) binder paste using Aluminum powder. The cellular structure created in the geopolymer depends on the AAF binder paste rheology and the surface tension of the activating solution. The constant strain rate rheological behavior of AAF binder paste varies between Maxwell flow and yield type depending on the activating solution content. Aeration with a stable bubble structure is achieved in an AAF binder paste with a yield-type constant strain-rate response. AAF binder pastes which exhibit a Maxwell-flow type constant strain rate response cannot retain bubbles in suspension. Addition of clay transforms the constant strain rate rheological behavior of the AAF binder paste from Maxwell-flow to yield type response producing a stable aerated paste. Increasing the clay content in the AAF binder paste increases its yield stress. The total porosity in the cellular geopolymer is controlled by the size and quantity of aluminum powder. Equivalent total porosity is achieved with a lower dosage of finer aluminum powder. Yield stress and surface tension have opposing influences on the pore size without influencing the total porosity. Increasing the yield stress of the AAF binder paste by adding clay produces an increase in the mean pore diameter. Adding surfactant to the AAF binder paste decreases the mean pore diameter in the cellular geopolymer. The porosity and the mean pore diameter in the cellular geopolymer can be controlled with the use of clay, and surfactant and by regulating the content and fineness of the aluminum powder. © 2022 Elsevier Lt

Extrusion-Based Three-Dimensional Printing Performance of Alkali-Activated Binders

Printable alkali-Activated fly ash-slag mixtures, which are homogeneous under pressure and achieve buildability in the extrusion-based three-dimensonal (3D) layer printing process, are developed. A baseline mixture of fly ash and slag with a sodium hydroxide activator is modified to achieve extrusion-based printing requirements, including printability, shape retention, and buildability. The role of additional dry constituents such as microsilica and clay in reducing phase separation under pressure for producing printable mixtures is evaluated. Phase separation in the mixture under pressure is sensitive to the particle size distribution. Printable mixtures, which do not segregate under pressure, have a narrower distribution of particle sizes, indicated by the Rosin-Rammler fit. The link between the rheological behavior of the mixture and its performance in printing is evaluated. The constant strain rate rheological response of the mixtures is distinguished between the yield-Type and Maxwell-flow behaviors. Mixtures that exhibit a Maxwell-flow type response produce a steadily continuing deformation and are not buildable. The distinction between Maxwell-flow and yield-Type behaviors is essential for identifying buildable mixtures. Alkali-Activated mixtures exhibit a viscoelastic response with both elastic and viscous components. The proportion of the storage to the loss modulus from rheological measurements provides an index of buildability. Achieving buildability with multiple layers depends on an internal structure capable of resisting elastic deformation, which is indicated by the development of the storage modulus with time. The role of additives on specific aspects of the rheological behavior of the mixtures is evaluated. The rheological behavior required for printing is achieved using carboxymethylcellulose (CMC), which produces a yield-Type behavior, and enhances the storage modulus and thixotropy of the alkali-Activated mixture. © 2021 American Concrete Institute. All rights reserved

Embedded smart PZT-based sensor for internal damage detection in concrete under applied compression

An embedded PZT-based sensor for monitoring internal damage in concrete is presented. An experimental program for producing controlled increments in levels of stress and damage in the concrete is developed. Formation and coalescence of microcracks are evaluated using digital image correlation. The electrical impedance (EI) measurements are recorded from the embedded sensor for different levels of stress and damage in the concrete. Changes in the EI signature associated with the resonant conditions of the PZT patch produced by stress and damage in the concrete are identified. The EI measurements provide a sensitive indication of the applied stress. Applied stress and damage in the concrete produce two counteracting effects on the EI resonant frequency. The internal damage in the concrete is detected from the EI measurement significantly earlier than the occurrence of surface cracking. Embedded sensors are more sensitive to the internal cracking in concrete compared to surface mounted PZT patches

Direct determination of cohesive stress transfer during debonding of FRP from concrete

Interface cohesive stress transfer between FRP and concrete during debonding is typically obtained using measured surface strains on the FRP, along the direction of the fibers. The cohesive material law is derived under a set of assumptions which include: (a) the bending stiffness of the FRP laminate is insignificant with respect to that of the concrete test block; (b) the strains in the bulk concrete produced by debonding are negligible, thus concrete substrate can be considered rigid; (c) there is stress transfer between FRP and concrete through the FRP\u2013concrete interface which is of zero thickness; and (d) the axial strain in the FRP composite is uniform across its thickness. In this paper, a test procedure for directly obtaining the through-thickness strains in the FRP and the concrete substrate during cohesive stress transfer associated with debonding is presented. The displacement and strain fields are measured on the side of a direct-shear specimen with the FRP strip attached on the edge. Based on the experimental results, the influence of the assumptions which have been introduced to determine the cohesive law is discussed. Within the stress transfer zone there is a sharp gradient in the shear strain. The location of the interface crack within the stress transfer zone and the cohesive stress transfer during the propagation of the interface crack are determined

Self-Leveling Geopolymer Concrete Using Alkali-Activated Fly Ash

Self-leveling concrete is developed with low-calcium alkali activated fly ash (AAF) binder paste. The rheological behavior of AAF pastes with different compositions is evaluated. AAF pastes are proportioned with alkali-silicate activating solutions to ensure specific reactive oxide ratios for comparable geopolymer strength. The yield stress and the viscosity of the AAF binder paste vary with the silica content and the silica modulus (SiO2/Na2O mass ratio) in the alkali-silicate activating solution. The slump and flow behaviors of concrete mixtures made with AAF paste are evaluated. The requirements of the AAF binder characteristics, paste content, and aggregate packing for achieving self-leveling flow characteristics under gravity-induced flow are assessed. The transition from a frictional to a flow-Type behavior in concrete mixtures depends on the AAF binder paste content. Self-leveling is achieved without the use of admixtures with an AAF binder paste of low yield stress and at a paste content of 45%. Improving the aggregate packing using the Fuller-Thompson curve and reducing the yield stress of the AAF binder paste increase the flow achieved in concrete mixtures. The specifications for cement-based self-consolidating concrete (SCC) are closely applicable for self-leveling AAF-based concrete

Experimental Investigation on Strengthening Of Soft Clay Brick Masonry Columns under Compression with Fiber-Reinforced Inorganic and Organic Matrixes

Masonry made with soft clay brick is commonly used in gravity load bearing of construction in India. The masonry piers and walls typically fail by vertical splitting. The purpose of this study is to improve the strength of masonry columns under compression using wrapping for additional confinement. The compressive load carrying performance and capacity of masonry columns wrapped with fiber reinforced composites in organic and inorganic matrixes are compared. For the purpose of overall improvements in cost and durability, glass and basalt fiber reinforcement is used. 30-40% improvement in the compressive performance of masonry prisms was achieved for both Organic and Inorganic matrixes. However, the specimens with inorganic matrixes were found to exhibit higher ductility compared to organic matrixes. Glass fibers were found to be more effective in wrapping masonry specimens compared to Basalt fiber specimens owing to its higher fiber count per unit length. Analytical models for predicting the compressive capacity of masonry columns with wrapping are verified against the experimental results. © 2022 Trans Tech Publications Ltd, Switzerland