Meysam Najimi

Sulfate Resistance of Nanosilica and Microsilica Contained Mortars

Presented is a side-by-side comparison study intended to identify the effects of nanosilica (nS) on chemical sulfate attack resistance of portland cement (PC) mortars and its effectiveness in comparison to similar replacement levels of the more widely implemented microsilica (mS). Several mortar mixtures were prepared with a 4.1 and 7.2% tricalcium aluminate (C₃A) PC by progressive cement replacement with nS or mS. The mortars tested were measured for expansion, compressive strength, and mass loss. Results indicated that nS replacement benefited the studied mortars. However, in the dry powder form and method of mixing used in this study, poor dispersion and agglomeration of the nS was suspected to hinder mortar permeability in comparison to mS and low-C₃A cement mortars. Replacement with nS in aqueous dispersion, however, proved to be significantly more effective than equivalent replacement of dry powder nS and mS.

Numerical study on the feasibility of dynamic evolving neural-fuzzy inference system for approximation of compressive strength of dry-cast concrete

For the first time, dynamic evolving neural-fuzzy system (DENFIS) was used to predict the compressive strength of dry-cast concretes.For comparison purposes, 6 nonlinear regression, 6 neural network, 5 ANFIS, 3 online first-order TSK DENFIS, 3 offline first-order TSK DENFIS, and 3 offline high-order TSK DENFIS models were developed.DENFIS model with high-order TSK inference system was found to be more robust than first-order TSK online and offline models.High-order DENFIS model could be trained to produce more reliable prediction results in comparison with neural network, ANFIS and nonlinear regression models. This paper assesses effectiveness of dynamic evolving neural-fuzzy inference system (DENFIS) models in predicting the compressive strength of dry-cast concretes, and compares their prediction performances with those of regression, neural network (NN) and ANFIS models. The results of this study emphasized capabilities of online first-order and offline high-order Takagi-Sugeno (TSK) type DENFIS models for prediction purposes, whereas offline first-order TSK-type DENFIS models did not produce reliable results. Comparison between the produced results of an elite high-order DENFIS model with those predicted by the selected NN, regression and ANFIS models showed effectiveness of DENFIS model than the regression model, while its performance was similar to or slightly better than the other artificial prediction tools.

Impact-Compacted Noncement and Vibratory-Placed Noncement/Partial-Cement Concretes Containing Fluidized Bed and Pulverized Coal Combustion Residues

The two-part research study presented herein evaluates fresh and hardened properties of vibratory-placed noncement/partial-cement concretes, as well as impact-compacted noncement concretes, containing various dosages of pulverized coal combustion fly ash and fluidized bed combustion spent bed. The results of Part I of this study on vibratory-placed noncement/partial-cement concretes revealed lower early strengths than those of reference concrete, whereas their late strengths (90 and 180 days) were similar to or higher than those of reference concrete. The selected noncement/partial-cement concretes produced higher expansion and lower drying shrinkage than those of the reference concrete. Abrasion resistance of noncement and partial-cement mixtures under dry conditions were better than that of the reference concrete, whereas opposite results were obtained under wet conditions. Laboratory results of Part II on impact-compacted noncement concretes revealed higher compressive strength and resistance to internal sulfate attack with increasing coarse aggregate content. The strength, stiffness, and resistance to abrasion and freezing/thawing of impact-compacted concretes improved with decreases in fluidized bed combustion (FBC) spent bed to pulverized coal combustion (PCC) fly ash ratios, whereas opposite results were obtained for sulfate-induced expansions.

Effects of Blaine and Tricalcium Aluminate on the Sulfate Resistance of Nanosilica-Containing Mortars

AbstractThis study set out to determine the effect of dry powder nanosilica (nS) on the sulfate resistance of mortars when paired with portland cements (PCs) of contrastingly different fineness and...

Influence of Dispersion Methods on Sulfate Resistance of Nanosilica-Contained Mortars

AbstractThis study evaluates the influence of various dispersion methods on the sulfate attack resistance of nanosilica (nS)-contained mortars. Multiple mechanical or ultrasonic dispersion methods,...