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Formulating alkali-activated binders requires a clear understanding of the role of activator concentration and source material composition on product formation and strength gain. Alkali activation of a low-calcium fly ash (FA) binder with slag replacements at 30% and 50% by mass was evaluated. The reactivity and compressive strength gain of the activated binders were evaluated using activating solutions with different sodium hydroxide molarities (SHMs). An increase in SHM produced a higher compressive strength of the activated binder. The primary reaction product formed in the activated binder was calcium silicate hydrate with aluminium substitution (C-(A)-S-H). An increase in SHM led to an increase in C-(A)-S-H content and a reduction in porosity. The slag contributed to early reactivity of the binder and the initial strength development, while the FA contributed to the later age strength by silica enrichment of the C-(A)-S-H. While increasing the SHM produced an increase in early reactivity, the sodium did not directly contribute to the formation of the reaction product. The sodium from the alkaline activator was not chemically bound to the C-(A)-S-H and could be removed by leaching in water. An increase in basicity with SHM enhanced the contribution of the low-calcium FA, leading to higher silica enrichment of the C-(A)-S-H.

The rheological behaviour of alkali-activated low-calcium fly ash (AAF) pastes is evaluated. The sodium-based, alkali-silicate activating solutions are proportioned for specific reactive oxide ratios in the paste considering the reactive content of the fly ash. Alkali-silicate activating solutions with different silica contents, with an MS value (the mass ratio silica to sodium oxide (SiO2 : Na2O)) of less than 1.80 are used. The activating solutions exhibit a Newtonian behaviour with higher viscosities at larger silica contents or lower MS. The yield stress and the viscosity of the AAF pastes increase with the viscosity of the activating solution. The zeta potential of the fly ash particles in the alkaline solution with dissolved silica is lowered on decreasing its MS. The yield stress of the AAF paste increases sensitively on decreasing the MS of the activating solution due to the combined influences of reducing the zeta potential of fly ash particles and increasing the viscosity of the activating solution. There are counteracting effects of increasing the viscosity of the activating solution and raising the zeta potential of fly ash, produced by increasing the silica content in the activating solution. The yield stress of AAF paste is less sensitive to the silica content in the activating solution.

The fracture response and resistance to chloride penetration of concrete reinforced with discrete steel fibers (SF) and hybrid blend of steel and macro polypropylene fibers (HyFRC) are evaluated. A significant improvement in fracture response is obtained in HyFRC compared to SF. Cohesive stress-crack opening (σ-w) relationships are obtained for the concrete with and without discrete reinforcement from the measured fracture test responses. The hybrid fiber blend has a higher tensile strength and a significantly higher stress resistance to crack opening when compared to an identical volume fraction of steel fibers. HyFRC also exhibit a lower chloride penetration when compared to control and SF concrete mixtures. The hybrid blend of steel and polypropylene fibers provides improved structural performance due to enhanced resistance to crack opening and an improvement in the durability by reducing the chloride penetration in concrete.

The flexural behaviour of sandwich beam panels with integrated reinforcement and expanded polystyrene (EPS) core is evaluated at different shear span to depth (a/d) ratios. The sandwich beam is a lightweight, thin member consisting of two reinforced concrete structural wythes with diagonal truss-type connectors. In this experimental study, the strain profiles are measured accurately using digital image correlation (DIC) to study the level of composite action in flexure. The measured strain profiles indicate a significant level of independent bending in the two wythes that are accommodated within the shear deformation of the core. The cracking moment of the beam panels depends on the a/d ratio. The stresses obtained from sectional analysis and composite beam theory do not account for the shear deformation of the EPS core and significantly over-estimate the initial stiffness and cracking load. Flexural response predicted by sandwich beam theory considering an anti-plane core is derived and used to estimate the stress and strain states in the beam panel cross-section. The sandwich beam theory gives a correct representation of the experimentally measured strain profile in flexure and an accurate estimate of the cracking load. The dependence of the flexural cracking moment on the a/d ratio is also accurately predicted. The displacement profiles from DIC indicate that the flexural cracking in the sandwich panel produces a hinge-type opening of the crack. The progressive opening of the hinge produces a localized flexural failure, and the conventional sectional analysis derived from composite beam theory gives a conservative prediction of the ultimate flexural capacity.

The fracture response of macro polypropylene fiber reinforced concrete (PPFRC) and hybrid blend of macro and micro polypropylene fiber reinforced concrete (HyFRC) are evaluated at 1, 3, 7 and 28 days. There is an improvement in the early-age fracture response of HyFRC compared to PPFRC. The changing cohesive stress-crack separation relationship produced by ageing of the concrete matrix is determined from the fracture test responses. An improved early-age cohesive stress response is obtained from the hybrid blend containing micro and macro fibers. The hybrid fiber blend also has a higher tensile strength at early age when compared to an identical volume fraction of macro polypropylene fibers.

The fracture behavior of concrete with steel and macropolypropylene hybrid fiber blends (HyFRC) is evaluated and compared with steel fiber reinforced concrete (SFRC). Fiber blends at identical volume fraction as SFRC are shown to produce an improvement in the fracture response. At small crack separation, immediately after cracking, higher cohesive stress is produced in HyFRC compared to SFRC. Polypropylene fibers in concrete contribute cohesive stresses at larger crack separation. Polypropylene fibers in the concrete matrix improve the efficiency of the steel fibers by mobilizing a higher resistance at the initiation of the pullout. The initial stiffness, peak load, and the residual frictional resistance of the steel fiber pullout are increased with the addition of the polypropylene fibers. The higher reinforcing efficiency of steel fibers in the presence of polypropylene fibers is due to improved fiber–matrix bond, which results in a higher cohesive stress at smaller crack openings.

An X-ray diffraction (XRD)-based evaluation of the crystalline and amorphous phases in slag hydrating in an alkaline environment is presented. A method is developed for the quantification of the amorphous phases present in hydrating slag in a sodium hydroxide solution. In hydrating slag, the amorphous reaction product is identified as calcium aluminosilicate hydrate. A water-soluble sodium-based amorphous reaction product is also produced. The XRD-based quantification method relies on the direct decomposition of the XRD intensity pattern of the total amorphous phase present in partially hydrated slag into the intensity patterns of the amorphous unreacted slag, the hydrate and the sodium-based product. The unreacted slag content in partially hydrated slag is also determined from the decomposition of the intensity signature of the total amorphous phase. An independent verification of the amorphous unreacted slag content in hydrating slag is obtained from measurements of blends of unhydrated and partially hydrated slag. The XRD-based phase-quantification procedure developed here provides a basis for evaluating the extent of reaction in hydrating slag.

Early damage detection in concrete structures is of vital importance since its improve the safety and ensures durability of the structure. In this study, an actuator-receiver (AR) mode of the surface mounted Lead Zirconate Titanate (PZT) transducers is implemented to monitor the stress induced localized damage in a concrete structure. The crack is induced in a notched beam in a controlled manner. The extent and width of the crack are measured using full-field image analysis technique. The received stress waves from the AR pair is analyzed in coherent and diffuse field methods. The coherent field of the received stress wave was analyzed in time and frequency domain while the diffuse filed was analyzed using time-frequency analysis. Delay in the arrival time of maximum energy of the received stress wave is computed using time-frequency analysis of the time-normalized stress wave and is observed to be sensitive to increase in depth of the crack. It is also shown that the attenuation factor (frequency domain) which is derived from the coherent part of the received stress wave is very sensitive to change in crack depth while the change in time of flight (time domain) is least sensitive.

The link between the fracture behavior of fiber–reinforced concrete composite and the underlying mechanisms that influence the pullout behavior of fiber are studied. The influence of the fi...