Cement-based Matrices Research Articles

Pull-out behavior of twin-twisted steel fibers from various strength cement-based matrices

This research explores a novel steel fiber made by twisting together two straight steel wires. The pull-out behavior of the twin-twisted steel fiber was investigated, proving its slip-hardening ability in cement-based matrices. The number of twists per unit length, fiber embedment length, matrix compressive strength, and fiber equivalent diameter were the experimental variables. Hooked-end fibers were also investigated for comparison. Pull-out test results of twin-twisted steel fibers quantified the mechanisms of frictional bonding and mechanical untwisting and their effects on the pull-out response and slip hardening of steel fibers. The maximum pull-out load increased with the increase of fiber twists per unit length, which increased the likelihood of damage to the matrix tunnel and had a negative effect on the energy absorption capacity. Defects at interfaces and scratches on fiber surfaces were examined using SEM imaging. Twin-twisted steel fibers reached an enhanced hardening response and energy absorption capacity compared to hooked fibers yet showed lower maximum bond strength. New bond strength and energy dissipation indexes were introduced and could effectively be used for fiber optimization.

Interfacial bonding characteristics of multi-walled carbon nanotube/ultralight foamed concrete

Abstract In the development of carbon nanotube (CNT)-reinforced cement-based matrices, one of the fundamental issues that investigators are confronting is CNT/cement-based matrix interfacial bonding, which determines the load transfer capability from the matrix to the CNT. In the present work, the stress transfer properties of multi-walled carbon nanotubes (MWCNTs) and ultralight foamed concrete matrices were studied using microscopic Raman spectrometry analysis. Two types of CNTs, such as MWCNT and MWCNT-COOH, were considered, wherein MWCNT-COOH was covered with fundamental COOH groups. The results show that the compressive and flexural strengths were 75 and 236% better for ultralight foamed concrete with a dry density of 200 kg/m3 with 0.4 wt% MWCNT-COOH addition, respectively. This indicates that the fundamental COOH groups of the MWCNT play an important role in determining the interfacial bonding characteristics between the MWCNT and the ultralight foamed concrete matrix. Therefore, the attachment of COOH groups with a reasonable concentration to the MWCNT surface may be an effective way to significantly improve the load transfer between the MWCNT and the ultralight foamed concrete matrix, leading to increased compressive and flexural strength values of composites.

Relevance of Specific Surface of Mixed Oxide Derived from Layered Double Hydroxides on the Rheological Properties and Porosity of Cement Pastes.

Cement-based products are the synthetic materials most used by humans, with consequent environmental impacts. One strategy that can assist in mitigating the adverse environmental effects of these materials involves the incorporation of multifunctional nanostructured additives. The objective of this work was to demonstrate the efficacy of incorporating mixed oxides (MO) derived from layered double hydroxides (LDH) to control the rheology and porosity of cement-based matrices. Thermal aging of LDH enabled the preparation of MO with different specific surface areas (SSA) for incorporation in different amounts in Portland cement. A low proportion of MO and low SSA increased workability by 22%. In contrast, a high proportion of MO and high SSA led to a 2.4-fold acceleration of cement consolidation and a 36.9% decrease of the porosity of the composite. These features could be attributed to additive-matrix interactions, with the LDH memory effect playing key roles in the cement crystal seed process and in competition for the absorption of free water within the cement paste. Therefore, the unprecedent results obtained suggest that the quantity and SSA of MO are key parameters to fine-tune the paste rheology and structure of hidrated cement. The MO materials showed easy adaptability and excellent potential for use as multifunctional additives in the production of eco-friendly, high-performance cement paste formulations with controllable properties according to the desired application.

Functionalized graphene-based materials for cementitious applications.

Graphene-based materials (GBM) are promising cementitious composite additives that can significantly improve the mechanical characteristics and durability of concrete due to their unique properties, such as high surface area and aspect ratio and excellent tensile strength, to name a few. To display their full potential, GBM have to be homogeneously dispersed into the aqueous environment of cement-based matrices. The present study addresses the issue of limited dispersibility in the aqueous media of GBM through the chemical functionalization of mono- and few-layer graphene structures with hydrophilic aryl sulfonate groups and shows that a series of mortar samples containing modified GBM exhibit increased flexural and compressive strength by up to 17% and 30%, respectively, compared to mortar references without additives.

Ballistic limit and damage assessment of hybrid fibre-reinforced cementitious thin composite plates under impact loading

Impact resistance of reinforced concrete (RC) structures can be significantly improved by strengthening RC members with thin composite layers featuring high damage tolerance. Indeed, to limit the well-known vulnerability of cement-based materials against impact loading, the synergistic effects of short fibres and continuous textile meshes as hybrid reinforcement has been proved to be highly beneficial. This paper addresses the characterisation of novel cement-based hybrid composites through accelerated drop-weight impact tests conducted on rectangular plates at different impact energies. Two distinct matrices are assessed, with particular interest in a newly developed limestone calcined clay cement (LC3)-based formulation. Important parameters quantifying energy dissipation capability, load bearing capacity and damage are cross-checked to compute the ballistic limit and estimate the safety-relevant characteristics of the different composites at hand. Although textiles alone can improve the damage tolerance of fine concrete to some extent, the crack-bridging attitude of short, well-dispersed fibres in hybrid composites imparts a certain ductility to the cement-based matrices, allowing a greater portion of the textile to be activated and significantly reducing the amount of matrix spalling under impact.

Effect of polymeric fiber coating on the mechanical performance, water absorption, and interfacial bond with cement-based matrices

The performance of sisal fibers in cement-based matrices is highly dependent on the interface efficiency between the fiber and the matrix. This behavior in turn is directly related to two main factors: the sisal fiber swelling tendency and fiber alkaline degradation. To enhance these properties and consequently the fiber–matrix adhesion, this study proposed the use of carboxylate styrene-butadiene rubber latex (XSBR) as a coating for sisal fibers. Different XSBR polymers and emulsions concentrations were adopted. The effects of this treatment on various properties of the fibers such as physical, chemical, and mechanical properties were studied. The physical properties of the modified fibers were investigated by means of scanning electron microscopy (SEM) and atomic force microscope (AFM) analyses. Also, changes in the moisture absorption capacity of the coated fibers were evaluated. To study the chemical influence of the polymer coating in the modified fibers, thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) techniques were adopted. Direct tensile and pullout tests were also assessed in natural and coated sisal fibers, to mechanically evaluate the modification in terms of fiber strength and adherence, respectively. The results evidenced the formation of a new polymeric layer around the fiber, responsible to reduce the water absorption tendency of the natural fiber. The increase in the fiber–matrix adhesion obtained in the pullout evaluation was attributed to the rougher superficial aspect of the modified fibers and increased chemical interaction between the polymer and the cementitious matrix. Moreover, it was proven that the polymeric layer reduced the fiber's alkaline degradation susceptibility resulting in a more stable sisal fiber.

Evaluation of the Alkali-Aggregate Reaction Mitigation Potential of Alkali-Activated Fly Ash-Based Matrices Containing Different Reactive Aggregates

The present study evaluated the use of alkali-activated fly ash-based matrices containing different reactive aggregates (quartz, basalt and limestone) to mitigate the alkali-aggregate reaction (AAR). The fly ash activation was performed with sodium hydroxide (NaOH) solution, with a molar ratio (Na2O/SiO2) of 0.4. Reference matrices were prepared using Portland cement type V (ARI). All composites were prepared with a mass ratio of 1:2.25:0.47 (binder: aggregate: water/agglomerant ratio). The petrographic analyses allowed the identification of potentially reactive phases in the three studied aggregates. From the results, it was verified that the Portland cement-based matrices with basalt and quartz aggregates confirmed the reactivity of these aggregates. On the other hand, the reactivity of the same aggregates was not manifested in the CVAA-based matrices, classifying the basalt and quartz aggregates as innocuous in the alkali-activated matrix. Therefore, CVAA-based matrices mitigated AAR when using reactive basalt and quartz aggregates. However, the limestone aggregate did not show reactivity in the matrices studied.

Influence of different bacterial strains on cracks self-healing in cement-based matrices with and without incorporated air

Influence of different bacterial strains on cracks self-healing in cement-based matrices with and without incorporated air

Effect of surface modification of sisal fibers with polyphenols on the mechanical properties, interfacial adhesion and durability in cement-based matrices

Effect of surface modification of sisal fibers with polyphenols on the mechanical properties, interfacial adhesion and durability in cement-based matrices

Biochar and recycled carbon fibres as additions for low-resistive cement-based composites exposed to accelerated degradation

Biochar (BCH) and recycled carbon fibres (RCF) were used as carbonaceous additions in low-resistive mortars/concretes. Their effects on mechanical, electrical, and durability properties were investigated. Tests were performed both during curing and accelerated degradation. The combined use of RCF and BCH decreased the electrical impedance of cement-based matrices, enabling the use of low-cost monitoring instrumentation, and improved their mechanical performance. Recycled carbon fibres and biochar additions increased carbonation and capillary water absorption but acting as a barrier they made water and chlorides penetrate less deeply.

Three sustainable polypropylene surface treatments for the compatibility optimization of PP fibers and cement matrix in fiber-reinforced concrete

Fiber-reinforced concrete (FRC) is a cementitious composite material that is gaining interest in the construction field to limit crack propagation and increase toughness. Polypropylene (PP) fibers are promising alternatives to steel fibers, primarily used in FRC, as they reduce weight and cost and are resistant to corrosion. However, the hydrophobic surface of PP inhibits a good adhesion with the surrounding cementitious matrix and so the exploitation of optimized mechanical performance. Though several chemical treatments have been studied, this work suggests more sustainable, cost-efficient, and fast surface treatments for the first time to increase the hydrophilicity of PP: UV-LED, picosecond UV-LASER, and corona discharge treatment. The treatments mainly provided a chemical functionalization without altering the surface morphology of PP, and their effectiveness in increasing PP hydrophilicity was confirmed by ATR-FTIR spectroscopy and optical contact angle measurements. The improved adhesion with the matrix was assessed by the pullout test of treated PP specimens from lime-based and cement-based matrices and by detecting the failure mode in the interphase zone through scanning electron microscopy.

Experimental assessment of the thermo-mechanical bond behavior of NSM CFRP with cement-based adhesives

Cement-based materials, which are incombustible and show low thermal diffusivity, show great potential as a valid alternative to polymeric matrices for bonding near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) systems for the strengthening of reinforced concrete (RC) structures. The performance of these materials in different loading conditions, including high temperature exposure, still needs to be better explored. This paper presents a study on the characterization of the bond behavior using cement-based adhesives and CFRP strips with innovative surface treatment. The CFRP strips used in this study were manufactured specifically with sand-treated surfaces to improve the bonding performance between CFRP and cement-based adhesive by taking advantage of the better grip of increased roughness surfaces. The results of the pull-out tests in ambient conditions, as well as exposed to elevated temperatures, are presented and discussed. The effect of using sand particles of different sizes, adhered with two different resins on the CFRP strips surface embedded in different cement-based matrices, was compared. The obtained results showed that by applying a sand treatment on the surface of CFRP strips, the maximum pull-out force was up to 12 times higher than when using smooth surfaced strips. Pull-out tests showed that, by using the NSM sand treated CFRP strips and CFRP bars with cement-based adhesive, an average bond strength up to about 9 MPa and 11 MPa, respectively, could be achieved. Thermo-mechanical pull-out tests showed the proposed strengthening system is able to withstand up to 30 min of heat exposure before bond failure.

Design of sustainable geopolymeric matrices for encapsulation of treated radioactive solid organic waste

Among radioactive by-products generated by nuclear technologies, solid organic waste is drawing attention because of difficult management and incompatibility with the disposal strategies traditionally adopted. Recently, geopolymers have been proposed as valid and green alternatives to cement-based matrices. In this work, novel geopolymeric formulations have been studied at laboratory scale to encapsulate ashes from incineration of surrogate solid organic waste and to further pursue sustainability and circular economy goals. Indeed, the most widely used precursor of literature geopolymers, calcined kaolin, has been totally replaced by natural raw materials and recycled industrial by-products. In addition, a highly zeolitized volcanic tuff has been chosen to further improve the intrinsic cation-exchange capacity of the geopolymer, hence enhancing waste-matrix interaction. The alkaline activation of the precursors, achieved without silicates of any metal, resulted in a promisingly durable geopolymeric matrix, whose chemical composition has been optimised to provide compressive strength above 10 MPa after 28 days of curing. A water-saturated sealed chamber provided the optimal curing condition to limit the efflorescence and improve the mechanical properties. At least 20 wt% loading of treated surrogate waste was achieved, without compromising workability, setting time, and compressive strength, the latter remaining within acceptable values. In order to demonstrate matrix durability, leaching behaviour and thermal stability were preliminarily assessed by immersion tests and thermogravimetric analyses, respectively. The leachability indices of constituent elements resulted far above 6, which is the generally agreed requirement for cement-based matrices. Moreover, the mechanical resistance was not worsened by the water immersion. The preliminarily obtained results confirm the promising properties of the new matrix for the immobilization of nuclear waste.

Emulsification of low viscosity oil in alkali-activated materials

This research aims to understand the mechanisms enhancing the fixation of low viscosity mineral oils, including tailings, in reactive inorganic matrices, by emulsification. To this purpose, significant amounts of a model low viscosity pure mineral oil (20%vol) are immobilized in alkali-activated materials (AAM), either based on metakaolin or blast furnace slag. In such case, Portland cement-based matrices are not adequate (emulsification delicate to manage and excessive setting retardation). Various surfactants are used to ease the oil emulsion.Visual observation and rheology evidence two distinct groups of surfactants. One contributes to structuring the oil/AAM fresh mix, with greater viscosity than without surfactant; the other includes non-structuring surfactants, without change in viscosity. Each group depends on the AAM considered.Whatever the AAM and the surfactant, the oil droplet size decreases significantly, without consistent correlation with the interfacial tension between oil/activating solution (AS). Interfacial tension alone does not explain the reduction in oil droplet size. Characterization of diluted ternary suspensions (solid particles – oil – AS) relates the structuring effect to interactions between oil and solid particles, through the surfactant polar head groups and non-polar hydrocarbon tails. A detailed mechanism explaining the oil stabilization and the mix structuring is discussed.

Clevis-Grip Tensile Tests on Basalt, Carbon and Steel FRCM Systems Realized with Customized Cement-Based Matrices

The tensile properties of fabric-reinforced cementitious matrix (FRCM) composites are experimentally investigated through clevis-grip tensile tests (according to AC434 provisions) on FRCM coupons realized with customized (ad hoc developed in this paper) cement-based matrices. The tested FRCM coupons are reinforced with basalt, carbon, or steel fabrics, and are prepared with three different matrices: one-component mortar incorporating dispersible copolymer powders of vinyl acetate and ethylene (matrices A and B) and two-component mortar with carboxylated styrene–butadiene copolymer liquid resin (matrix C). This has made it possible to investigate the mechanical compatibility between different mortar matrices and fabrics and the resulting tensile properties of FRCM composites in the uncracked, cracking, and fully cracked phases. Experimental results are critically analyzed in terms of stress–strain curves and failure mechanisms comparatively for the analyzed FRCM systems. It has been shown that the matrix B exhibits a good compatibility with the basalt pre-impregnated fabric, while the matrix C appears to be the most suitable candidate to optimize the interfacial stress transfer at the fiber–matrix interface for all fabrics, thus exalting the mechanical performances in terms of tensile strength and ultimate strain. The results of this experimental program can be useful for designing optimized mortar mixes aimed at realizing novel FRCM composites or at improving existing FRCM systems by suitably accounting for compatibility behavior and slippage at the fabric–matrix interface.

Degradation Kinetics and Durability Enhancement Strategies of Cellulosic Fiber-Reinforced Geopolymers and Cement Composites

The need to overcome the low energy-absorbing response and low tensile strength of geopolymers and cementitious materials has motivated researchers to develop fiber-reinforced composites. Interestingly, natural cellulosic fibers provide exciting prospects for advancements in bio-based materials compared to synthetic fibers. Incorporating fibers in such binders offer a crack limiting effect by creating multiple cracks than forming one major crack, usually leading to improved toughness and tensile strength of the composites. However, their sensitivity in alkaline media and environmental fluctuations deteriorates their durability and reinforcing capacity in composites. Therefore, exploring suitable mitigation strategies for such drawbacks of natural fibers based on previous research work and figuring out the existing gaps for further developments is extremely important. In this paper, cellulosic fiber degradation in geopolymer and cement-based matrices and the respective remedial actions have been reviewed based on a comprehensive analysis of the available literature. The influence of fiber modification and matrix treatment on the durability and strength characteristics of such composites is thoroughly discussed. Finally, recommendations and suggestions for further research in the field are presented.

Effect of alkali treatment on physical–chemical properties of sisal fibers and adhesion towards cement-based matrices

This work aims to evaluate the effect of alkali treatment on sisal fibers to improve the fiber properties and the effectiveness of their use as reinforcement in cement-based matrices. Alkaline solution concentration (1, 5 and 10 wt% NaOH) and post-treatment protocol (use of distilled water or acetic acid) influence were evaluated. The physical, chemical, thermal, wettability and surface characteristics were analyzed. Also, single-fiber direct tensile and pullout tests were performed. Results showed an increase in cellulose proportion with a reduction in hemicellulose and lignin content upon alkali treatment. Consequently, enhanced thermal stability and a decrease on the water uptake tendency were reached. Additionally, improved tensile properties, as well as an enhanced bond with the cementitious matrix, were found for alkali-treated fibers. Also, fibers became more stable when subjected to alkaline aggressive conditions. Thus, the alkali-treated sisal fiber seems to be an adequate and sustainable alternative as reinforcement for cement-based matrices.

Carbon Nanofibers Grown in CaO for Self-Sensing in Mortar.

Intelligent cementitious materials integrated with carbon nanofibers (CNFs) have the potential to be used as sensors in structural health monitoring (SHM). The difficulty in dispersing CNFs in cement-based matrices, however, limits the sensitivity to deformation (gauge factor) and strength. Here, we synthesise CNF by chemical vapour deposition on the surface of calcium oxide (CaO) and, for the first time, investigate this amphiphilic carbon nanomaterial for self-sensing in mortar. SEM, TEM, TGA, Raman and VSM were used to characterise the produced CNF@CaO. In addition, the electrical resistivity of the mortar, containing different concentrations of CNF with and without CaO, was measured using the four-point probe method. Furthermore, the piezoresistive response of the composite was quantified by means of compressive loading. The synthesised CNF was 5–10 μm long with an average diameter of ~160 nm, containing magnetic nanoparticles inside. Thermal decomposition of the CNF@CaO compound indicated that 26% of the material was composed of CNF; after CaO removal, 84% of the material was composed of CNF. The electrical resistivity of the material drops sharply at concentrations of 2% by weight of CNF and this drop is even more pronounced for samples with 1.2% by weight of washed CaO. This indicates a better dispersion of the material when the CaO is removed. The sensitivity to deformation of the sample with 1.2% by weight of CNF@CaO was quantified as a gauge factor (GF) of 1552, while all other samples showed a GF below 100. Its FCR amplitude can vary inversely up to 8% by means of cyclic compressive loading. The method proposed in this study provides versatility for the fabrication of carbon nanofibers on a tailored substrate to promote self-sensing in cementitious materials.

Tannic acid/ethanolamine modification of PE fiber surfaces for improved interactions with cementitious matrices

The paper at hand deals with the surface functionalization of polyethylene (PE) fibers by using tannic acid/ethanolamine (TA/EA) as an affordable and highly effective approach in enhancing the bonding characteristics in cement-based matrices. Surface morphology, charge formation, wetting behavior, and thermal properties of the fibers were investigated using environmental scanning electron microscopy (ESEM), electrokinetic measurements, water contact angle tests, and thermogravimetric analysis (TGA). Both one-sided and two-sided, single-fiber pullout tests were carried out to shed light on the effect of TA/EA deposition on the interactions between the cementitious matrix and the modified fibers as regards various EA concentrations and treatment durations. For the samples with the modified PE fibers notable enhancements were observed in interfacial shear strength, pullout energy, crack-bridging force, and crack-bridging energy when compared to the samples containing raw PE fiber. This is explained by the effective surface activation of PE fibers by TA/EA through introducing amino and hydroxyl groups onto the fiber surfaces, thus providing more reactive groups with the cement hydrates and yielding improved bonding in the fiber-matrix interfaces. Moreover, the TA/EA coatings not only showed no negative impact on the mechanical behavior of the fibers but even slightly enhanced their tensile strengths and moduli of elasticity.

Effect of Alkali Treatment on Physical-Chemical Properties of Sisal Fibers and Adhesion Towards Cement-Based Matrices

Effect of Alkali Treatment on Physical-Chemical Properties of Sisal Fibers and Adhesion Towards Cement-Based Matrices