The Effect of Bacteria on Early Age Strength of CEM I and CEM II Cementitious Composites

Despite being associated with lower carbon emissions, CEM II cementitious materials exhibit reduced early age strength compared to that of CEM I. Several studies have demonstrated early age strength improvements by incorporating bacterial cells in concrete. In this study, live vegetative bacteria and dead bacteria killed in two different ways were used to explore whether changes in strength are related to the bacteria’s viability or their surface morphology. Compressive and flexural strength tests were performed at mortars with and without bacteria for both CEM I and CEM II cement. Their microstructure, porosity and mineralogy were also examined. No net strength gain was recorded for either CEM I or CEM II bacterial mortars compared to non-bacterial controls, although changes in the porosity were reported. It is proposed that two phenomena, one causing strength-reduction and one causing strength-gain, took place in the bacterial specimens, simultaneously. It is suggested that each phenomenon is dependent on the alkalinity of the cement matrix, which differs between CEM I and CEM II mortars at early age. Nevertheless, in neither case could it be recommended that the addition of bacteria is an effective way of increasing the early age strength of mortars.

Continuous Reinforced Concrete Beams Strengthened with Fabric-Reinforced Cementitious Matrix: Experimental Investigation and Numerical Simulation

This paper aims to examine the nonlinear flexural behavior of continuous RC beam specimens strengthened with fabric-reinforced cementitious matrix (FRCM) composites through experimental testing and numerical modeling. A total of nine two-span RC beam specimens were constructed and tested. Test parameters included the type of FRCM (carbon (C-FRCM) and polyparaphenylene benzobisoxazole (PBO-FRCM), location of strengthening (sagging and hogging regions) and number of FRCM layers (two and four layers). Test results indicated that sagging strengthening resulted in a strength gain in the range of 17 to 29%, whereas hogging strengthening increased the load capacity by 9 to 17%. The use of C-FRCM resulted in a higher strength gain than that provided by PBO-FRCM composites. Specimens strengthened with PBO-FRCM exhibited, however, higher ductility and deformational capacity than those of their counterparts strengthened with C-FRCM. Doubling the number of FRCM layers resulted in no or insignificant increase in the load capacity but reduced the beam ductility. Specimens strengthened in the sagging regions exhibited moment redistribution ratios of 13 to 26% between the hogging and sagging regions. Insignificant moment redistribution was recorded for the specimens strengthened in the hogging region. Three-dimensional (3D) numerical simulation models, with and without an interfacial bond-slip law at the fabric–matrix interface, were developed. The inclusion of the bond-slip law in the modeling had an insignificant effect on predicted response. Although the models tended to underestimate the deflection, the predicted load capacities were within a 12% error band. Numerical findings were in agreement with those obtained from laboratory testing.

Effect of Accelerators on the Workability, Strength, and Microstructure of Ultra-High-Performance Concrete

The preparation of ultra-high-performance concrete (UHPC) with both high-early-strength and good workability contributes to further promotion of its development and application. This study investigated the effects of different accelerators (SM, alkaline powder accelerator; SF, alkaline powder accelerator containing fluorine; and AF, alkali-free liquid accelerator containing fluorine) on the workability and strength properties of UHPC. The microstructure of UHPC was also characterized by using XRD and SEM. Several dosage levels of accelerators (2%, 4%, 6%, and 8% by mass) were selected. The results indicate that the setting time and fluidity of UHPC are gradually decreased with an increase in accelerators dosage. Compared with fluorine-containing SF/AF, fluorine-free SM evidently facilitates UHPC early strength gain speed. However, the fluorine-containing accelerators have a higher 28 d strength ratio, especially AF. The maximum compressive and flexural strength ratios are obtained at a dosage of 6%, which are 95.5% and 98.3%, respectively. XRD and SEM tests further reveal the effect of different accelerators on the macroscopic properties of UHPC from the micro level.

Performance Evaluation of Additives to Make Abrasion Resistant Concrete: A Review

One of the most common forms of deterioration of concrete structures is by surface abrasion. Abrasion primarily depends upon the resistance level of materials, mix composition, strength gain and construction practices. Such deteriorations endanger the structural safety and increase the cost of repair and maintenance. In order to minimize the maintenance problems and to increase the serviceability of the structures, it is important that the structure should be resistant to abrasion. Understanding the physics of abrasion is critical for reducing its negative effects on concrete because abrasion is a typical degradation mechanism for concrete. An effort has been spent to identify the factors affecting abrasion of concrete structures and a review on different additives that can be added to achieve abrasion-resistant concrete is carried out. This paper reviews the research work carried out in the area of surface abrasion with works focusing within the last decade. Keywords: Abrasion Resistance, Additives, Concrete Abrasion, Degradation, Durability, Serviceability.

Pushover Tests on Unreinforced Masonry Wallettes Retrofitted with an Innovative Coating: Experimental Study and Finite Element Modelling

This paper investigates the mechanical contribution of an innovative coating applied on masonry wallettes compared to a traditional one. In both cases, the multifunctional coatings were insulating coatings intended for thermal refurbishment, but they could also be used to retrofit masonry. Uncoated specimens as well as coated ones were submitted to pushover tests to establish the strength gain. URM walls experienced brittle failures while the coated walls exhibited significant strength gains and strong ductility. The corresponding finite element models were developed. The behaviour of the URM walls was reproduced accurately in terms of strength and failure pattern. Models involving the coatings were used to partially retrieve the behaviour and to highlight the issues of a continuum approach.

Shotcrete using ternary binder made from coal combustion products: from lab tests to an application

The paper presents a ternary binder development and its utilization in shotcrete. The binder is made from fluidized bed combustion (FBC) fly ash, siliceous fly ash, and Ca(OH)2 addition, now available under the name Sorfix. XRD and TGA identified ettringite and C-S-H as two main hydration products. In addition, thermodynamic modeling verified robustness in terms of space-filling capabilities when varying input oxide composition. Since alkali-free accelerators produce mostly ettringite in Portland-based systems, a fraction of Portland cement was advantageously replaced with the ternary binder, forming early ettringite as well. Extensive testing led to 45% replacement of Portland cement, following J2 curve for early strength gain used commonly in shotcrete tunnel linings. The shotcrete was successfully tested in a mock-up experiment in a 2 m3 Prague served for the full-scale application, utilizing over 1000 tons of Sorfix and saving over 700 tons of CO2

INFLUENCE OF RECIPE AND TECHNOLOGICAL FACTORS ON THE PROPERTIES OF ALKALINE ACTIVATION BINDERS

В работе исследовано влияние микрокремнезема на кинетику набора прочности, изменение плотности и порового пространства цементного камня, рецептурно-технологических факторов и условий твердения. Обоснованы химические превращения микрокремнезема в условиях щелочной среды, приводящие к изменениям водородного показателя. Структура цементного камня характеризуется присутствием гидроалюмосиликатов натрия и кальция, кристаллических новообразований, близких к ларниту, и их гидратов, аморфной субстанции, сульфоалюминатов кальция, гидроалюминатов кальция. The paper investigates the effect of microsilica on the kinetics of strength gain, changes in the density and pore space of cement stone, formulation and technological factors and hardening conditions. The chemical transformations of silica fume under alkaline conditions, leading to changes in the pH value, have been substantiated. The structure of the cement stone is characterized by the presence of sodium and calcium hydroaluminosilicates, crystalline neoplasms similar to larnite and their hydrates, amorphous substance, calcium sulfoaluminates, calcium hydroaluminates.

APPLICATION FEATURES OF CONCRETE STRENGTH MONITORING SENSORS

The paper presents an algorithm of application of concrete strength monitoring sensors taking into account such features as a selection of sensor type, selection of concrete mixture calibration method according to regulated requirements, consideration of concrete maturity sensor location, degree of influence of hardening temperature on strength gain based on isotherms construction. This algorithm was reflected in practice, as the wireless sensor for concrete strength monitoring developed within the project was applied according to the selected scheme in real-time.

Tolerance to Intermittent vs. Continuous Blood Flow Restriction Training: A Meta-Analysis

The proven beneficial effects of low-load blood flow restriction training on strength gain has led to further exploration into its application during rehabilitation, where the traditional use of heavy loads may not be feasible. With current evidence showing that low-load blood flow restriction training may be less well tolerated than heavy-load resistance training, this review was conducted to decipher whether intermittently deflating the pressure cuff during rest intervals of a training session improves tolerance to exercise, without compromising strength. Four databases were searched for randomized controlled trials that compared the effect of intermittent versus continuous blood flow restriction training on outcomes of exercise tolerance or strength in adults. Nine studies were identified, with six included in the meta-analysis. No significant difference in rate of perceived exertion was found (SMD-0.06, 95% CI-0.41 to 0.29, p=0.73, I 2=80%). Subgroup analysis excluding studies that introduced bias showed a shift towards favoring the use of intermittent blood flow restriction training (SMD-0.42, 95% CI-0.87 to 0.03, p=0.07, I 2=0%). There was no significant difference in strength gain. Intermittent cuff deflations during training intervals does not improve tolerance to exercise during blood flow restriction training.