Microstructural and chemical effects of wet/dry cycling on pulp fiber-cement composites

The microstructural and chemical mechanisms responsible for pulp fiber-cement composite degradation during wet/dry cycling are being investigated through environmental scanning electron microscopy (ESEM), energy dispersive spectroscopy (EDS), and mechanical testing. Based on these results, a three-part progressive degradation mechanism for cast-in-place kraft pulp fiber-cement composites is proposed, which involves: (1) initial fiber-cement or fiber interlayer debonding, (2) reprecipitation of needle-like or sheath-like ettringite within the void space at the former fiber-cement interface or between the S1 and S2 fiber layers, and (3) fiber mineralization due to reprecipitation of calcium hydroxide filling the spaces within the fiber cell wall structure. This investigation also revealed that kraft pulp fibers exhibit poor resistance to degradation due to their inferior dimensional stability, as compared to thermomechanical pulp (TMP) fibers. TMP fibers contain significant amounts of lignin, which is alkali sensitive. Despite this, TMP fiber-cement composite exhibit improved resistance to degradation during wet/dry cycling. It is proposed that this improvement in durability may be attributed to the presence of lignin in the cell wall restricting fiber dimensional changes during wetting and drying, and hence, minimizing fiber-cement debonding. Additionally, it is proposed that lignin acts as physical barrier to calcium hydroxide formation within the fiber cell wall, minimizing fiber mineralization of TMP fibers.     

Wood fiber surface treatment level effects on selected mechanical properties of wood fiber-cement composites

The objective of this study was to determine the effects of sodium (N) silicate, potassium (K) silicate, and silane (Si) treatment levels on newspaper and unbleached kraft fibers for enhancing selected mechanical properties of wood fiber-cement composites compared to untreated wood fiber-cement composites. Both wood fiber types were treated with selected aqueous solution strengths, air dried, and mixed with water and cement. The bending and compression properties of the specimens were determined after 28 days of hydration. Results of this study indicated that the aqueous chemical treatments of the wood fibers enhanced some of the mechanical properties of wood fiber-cement composites compared to the untreated wood fiber-cement composites. The enhancement depended on chemical treatment and wood fiber type. All three chemical treatments of newspaper fiber enhanced the normalized toughness values compared to the untreated newspaper fiber-cement composites. In addition, higher treatment levels using N silicate with newspaper fiber improved the compressive strength and bending modulus of the composites compared to the untreated newspaper fiber-cement composites. Kraft fiber treated with all three chemicals enhanced the compressive strength, bending modulus and bending strength compared to the untreated kraft fiber-cement composites. However, only silane-treated kraft fiber improved the normalized toughness values compared to the untreated kraft fiber-cement composites. The results of the study indicated that certain chemical treatments react better with different wood fiber types resulting in selected mechanical property enhancements.     

Supplementary cementitious materials for mitigating degradation of kraft pulp fiber-cement composites

Kraft pulp fiber reinforced cement-based materials are being increasingly used where performance after exposure to environmental conditions must be ensured. However, significant losses in mechanical performance due to wet/dry cycling have been observed in these composites, when portland cement is the only cementitious material used in the matrix. In this research program, the effects of partial portland cement replacement with various supplementary cementitious materials were investigated. Binary, ternary, and quaternary blends of silica fume, slag, Class C fly ash, Class F fly ash, metakaolin, and diatomaceous earth/volcanic ash blends were examined for their effect on the degradation of kraft pulp fiber-cement composite mechanical properties (i.e., strength and toughness) during wet/dry cycling. After 25 wet/dry cycles, it was shown that binary composites containing 90% slag, 30% metakaolin, or greater than 30% silica fume did not exhibit any signs of degradation, as measured through mechanical testing and microscopy. Ternary blends containing 70% slag/10% metakaolin or 70% slag/10% silica fume were also effective in preventing degradation. A reduction in calcium hydroxide content and the stability of the alkali content due to supplementary cementitious material addition were shown to be primary mechanisms for improved durability.     

Autogenous healing of engineered cementitious composites at early age

Autogenous healing of early ages (3 days) ECC damaged by tensile preloading was investigated after exposure to different conditioning regimes: water/air cycles, water/high temperature air cycles, 90%RH/air cycles, and submersion in water. Resonant frequency measurements and uniaxial tensile tests were used to assess the rate and extent of self-healing. The test results show that ECC, tailored for high tensile ductility up to several percent and with self-controlled crack width below 60 μm, experiences autogenous healing under environmental exposures in the presence of water. However, the recovery for these early age specimens is not as efficient as the recovery for more mature specimen, for the same amount of pre-damage and exposure to the same environment. Even so, the self-healing for these early age specimens demonstrates high robustness when the preloading strain is limited to 0.3%. This conclusion is supported by the evidence of resonant frequency and stiffness recovery of the healed ECC materials.     

Durability of a multiscale fibre reinforced cement composite in aggressive environment under service load

The LCPC has developed and patented a new ultra high performance fibre reinforced cement composite (UHPFRCC); the introduction of three steel fibre sizes leads to a multiscale fibre reinforced cement composite (MSFRCC) with multi-cracking and hardening behaviour in uniaxial tension (f_t more than 20 MPa). An innovative test of durability is presented. Pre-cracked thin slabs are damaged by fatigue under loading corresponding to service load (30 MPa in bending test) then maintained under bending at the same level. A part of these slabs undergoes 30 weekly wetting-drying cycles in a chloride solution (NaCl 5%, 20 °C). A reloading to failure is led. One notes an absence of corrosion in the micro-cracked area even for cover lower than 300 μm and a strength increase for pre-damaged slabs by fatigue. Under service load and in the presence of chloride water, a quasi-total recovery of initial stiffness is possible; it is accompanied by an increase of the pseudo-elastic behaviour. The development of a better matrix/micro-fibres synergy and a diffuse micro-cracking explains this result.     

Effects of flocculants and sizing agents on bending strength of fiber cement composites

Wood fiber is used to replace asbestos in the manufacture of fiber cement due to its high availability, low cost and good reinforcement properties. The different chemical composition of the cellulose fibers makes its compatibility with the cement much more complex than that of asbestos fibers. In the Hatschek process a suitable flocculant is needed when using cellulose fibers. The right selection of the flocculant is crucial due to its effect on mineral fines retention, dewatering and formation and, as a consequence, on the overall efficiency of the machine. This paper shows how anionic poly-acryl-amides (A-PAM), the most common flocculants used in Hatschek machines, have a negative effect on the bending strength properties of fiber cement sheets. In order to overcome this problem fiber surface treatment, with sizing agents, is proposed in this paper. Sizing with styrene-acrylate copolymers and alkyl ketene dimer produces an increase in bending strength properties.     

Combined time domain reflectometry and AC-impedance spectroscopy of fiber-reinforced fresh-cement composites

Time domain reflectometry and conventional AC-impedance spectroscopy were combined to investigate the impedance response of fiber-reinforced fresh-cement composites over a broad frequency range (0.1 Hz to 1 GHz). With proper attention to inductive effects at high frequency, conventional AC-IS can be employed to evaluate important fiber dispersion issues (fiber orientation, segregation, and clumping) in fresh-cement matrix composites.     

混凝土结构用纤维增强塑料筋的耐久性评述

钢筋锈蚀在房屋建筑、公路、桥梁、大坝等混凝土结构中大量存在,是混凝土结构耐久性破坏的主要形式之一。纤维增强塑料筋(FRP筋)作为一种钢筋替代材料在混凝土工程中的应用是解决钢筋锈蚀问题的新途径之一。本文对FRP筋及型材在多种环境条件下的耐久性问题的国内外研究进展现状进行了分析,并对今后FRP筋及型材的耐久性研究趋势作了展望。     

Properties of ceramic fiber reinforced cement composites

Mechanical properties and preliminary durability of ceramic fiber reinforced Portland cement composites tested with wet-hot accelerating method were investigated. The results showed that the flexural strength of mortar could be increased obviously by adding ceramic fiber into it, but the effect of the flexural reinforcement was influenced by various factors, including fiber length, fiber content and kinds of matrices; the preliminary durability of ceramic fiber in ordinary Portland cement tested with wet-hot accelerating method was much better than that of alkali-resistant (AR) glass fiber. The mechanism of the durability of ceramic fiber in ordinary Portland cement is also discussed.     

Modeling of the link between microstructure and effective diffusivity of cement pastes using a simplified composite model

The knowledge of the relationship between porosity and transport properties of concrete is a point of major importance to run properly the models coupling chemistry, transport and mechanics in order to simulate the engineered barrier degradations in the context of the nuclear waste deep repository. The present work proposes a simplified composite model aiming at linking microstructure and effective diffusivity of cement pastes. The proposed analytical method allows the estimation of the evolution of effective diffusivity of such materials submitted to porosity opening or plugging, at the scale of the Representative Elementary Volume (REV). The method is then applied to Ordinary Portland Cement (OPC) pastes. The porosity-diffusion evolutions determined from the composite model for various OPC pastes are implemented into simplified chemo-transport simulations aiming at describing the leaching of cementitious materials. Using these evolutions, OPC paste leaching simulations are in good agreement with the available experimental data, indicating a good reliability of the simplified composite model.     

Durability of concrete with addition of dry sludge from waste water treatment plants

This paper evaluates the use of dry sludge as an additive for concrete, for which it must be guaranteed that the resulting concrete has the appropriate mechanical strength and durability.In earlier work on the subject, it was shown that the addition of sludge reduces the mechanical strength of concrete. With the addition of 10% sludge in proportion to the amount of concrete, the mechanical strength decreases significantly, making it unsuitable for medium- to high-strength reinforced concrete.One possible area of application would be in the preparation of low-strength mass concrete that could be used for bases and subbases of roads with light traffic, as filler, etc.We subjected the concrete specimens to different types of accelerated attack in order to evaluate long-term performance and compare them with the reference concrete (not containing sludge).The following tests were made:

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Combined wet-dry cycles using fresh water, seawater and water containing 5% sulphates

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Accelerated ageing in an autoclave

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Accelerated carbonation

The performance of the concrete containing sludge was acceptable and comparable to the results obtained for the reference concrete not containing sludge.     

Interaction between loading, freeze-thaw cycles, and chloride salt attack of concrete with and without steel fiber reinforcement

In this paper, the deterioration of concrete subjected to the combined action of four-point bending—loading, freeze-thaw cycles, and chloride salt attack—is discussed. Test results show that concrete tested in chloride salt solution scaled much more severely than in fresh water, and its weight loss in chloride salt solution was twice that in water. However, dynamic modulus of elasticity (DME) of concrete in chloride salt solution dropped more slowly than that in water due to supercooling resulting from chloride salt. It is also shown that the degradation process of concrete simultaneously exposed to loading, freeze-thaw cycles, and chloride salt attack was significantly accelerated. The higher the stress ratio exerted, the lesser the freeze-thaw cycles that concrete could resist and, consequently, the shorter the service life. When a relatively high steel fiber content is introduced (1.5 vol.%), the deterioration process of concrete subjected to the three damaging processes is considerably reduced.     

Dynamic behaviour and visco-elastic damage model of ultra-high performance cementitious composite

The dynamic behaviour of ultra-high performance cementitious composite (UHPCC) with compressive strength of 200 MPa with different steel fiber volume fractions was studied under impact using the split Hopkinson pressure bar. Three aspects of the testing: a gimbal device, wave shaping and direct strain measurement, were used to increase experimental accuracy. Results indicate that UHPCC has obvious strain rate effects. The peak stress, peak strain, elastic modulus and the area under the stress–strain curve increase with increasing strain rate. When the strain rate exceeds a threshold value, specimens with and without fibers begin to fracture. At high strain rate the unreinforced specimens fracture into small parts while fiber reinforced ones only have fine cracks on the edges. A visco-elastic damage model of UHPCC is proposed based on a nonlinear visco-elastic model (the ZWT model) and the material damage measured by the ultrasonic wave velocity method.     

Crack effects on gas and water permeability of concretes

The relationship between load-induced cracking and concrete permeability is studied. Ordinary concrete (OC) and high-performance concrete (HPC), including steel fiber-reinforced concrete (HPFRC), are used. Two discs, 50 mm-thick slices, cut from 110-220 mm cylindrical specimens are diametrically loaded, as for a normal splitting test. The lateral displacement, also called the crack opening displacement (COD) is monitored for each loading cycle. After unloading, gas and finally water permeability tests are both performed, using constant head permeameter, to compare the influence of the percolating fluid and the COD. Due to the wide range of measured gas flow, Klinkenberg's and Dupuit-Forcheimer's laws are applied to compute the intrinsic gas permeability. Results suggest it increases proportionally to the cube of the COD and it matches water permeability, if only the first water percolating time is considered. The roughness parameter of the cracks induced in each concrete, is compared and discussed.     

Durability prediction of p-urethane clearcoats

The durability of topcoats is dependent on a large number of factors such as polymer composition, stabilization package and the conditions during the weathering process. For obvious reasons, prediction of the long-term (5–10 years) durability of coatings is very important. Some background information concerning the weathering process as well as methods to trace the degradation during exposure is given. The rate-determining factor for the degradation of PUR coatings is photo-oxidation. The photo-oxidation rate (POR) is controlled by the polymer structure but also stabilizers such as hindered amine light stabilizers (HALS) have a large influence. The prediction of the durability of clearcoats is based on tracing of the POR and of the HALS longevity during exposure. The POR is measured using FTIR-PAS. The HALS longevity is determined by ESR. The results show that degradation can be detected much earlier compared to methods as gloss loss and time-to-cracking. Moreover, detection of differences between systems after short exposure times as well as prediction of the long-term durability are possible. Service life prediction (SLP) of clearcoats based on these chemical–analytical methods become more reliable as with the classical approach.     

Bending and compressive behaviours of a new cement composite

The Laboratoire Central des Ponts et Chaussées (LCPC) has recently developed and patented a new cement composite, the CEMTEC_multiscale, which is stress hardening in tension and has a very high uniaxial tensile strength, more than 20 MPa. This paper is about the determination of the compressive and bending behaviors of the CEMTEC_multiscale used in the frame of ribbed slabs.The principal results obtained are the following:

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the characteristic modulus of rupture is equal to 42 MPa for the “slab” function;

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the characteristic modulus of rupture is equal to 48 MPa for the “rib” function;

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the ultimate tensile strain is around 5 10⁻³;

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the characteristic strength and ultimate strain in compression are equal to 205 MPa and 4 10⁻³, respectively; and

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the Young modulus is equal to 55 GPa and the Poisson coefficient is equal to .21.

     

Engineered cementitious composite with characteristic of low drying shrinkage

This paper reports a new class of engineered cementitious composite (ECC) with characteristics of low drying shrinkage, tight crack opening and high tensile strain capacity. Research emphasis is placed on the influence of different cementitious matrix on drying shrinkage, tensile property and early age cracking behavior of the composites. Experimental results show that drying shrinkage of the composite is greatly reduced as using the low shrinkage cementitious material in matrix, while the composite remains strain-hardening and multiple cracking characteristics. The measured drying shrinkage strain at 28 days is only 109 × 10^− 6 to 242 × 10^− 6 for low shrinkage ECCs. For traditional ECC, the shrinkage strain at 28 days is nearly 1200 × 10^− 6. The average tensile strain capacity after 28 days curing is 2.5% of the low shrinkage ECC with tensile strength of 4-5 MPa. Further, in the strain-hardening and multiple cracking stage, cracks with much smaller width compared to the traditional ECC are formed in the low shrinkage ECC.     

Origin of the thermoelectric behavior of steel fiber cement paste

Carrier scattering dominates the origin of the Seebeck effect in steel fiber cement. The scattering sites include the fiber-matrix interface, which is like a pn junction, since the fiber and cement paste have opposite signs of the absolute thermoelectric power. The scattering results in positive and negative values of the absolute thermoelectric power, depending on the fiber content.     

Effect of carbon addition on the ferroelectric hysteresis properties of lead zirconate-titanate ceramic-cement composites

Piezoelectric composites made from lead zirconate-titanate ceramic and cement have recently been developed as sensors in smart concrete structures due to their compatibility with the host concrete structure. However there is difficulty in poling the composite due to the insulating nature of the cement phase. In this work, carbon powder was used as a conducting phase in the composites at up to 2.0% by volume. The ferroelectric properties of the composites were investigated. For the same electric field, composites with added carbon were found to have higher polarization values. Therefore, this suggests that carbon could be added as a conducting phase to lead zirconate-titanate ceramic-cement composites in order to improve their polarization.     

Restraining effects of fibers during non-uniform drying of cement composites

Fiber reinforcement is often added to concrete materials to reduce shrinkage strains due to drying of the cement-based matrix and the associated potential for crack growth. The role of fibers in restraining shrinkage is complicated due to their semi-random distribution within the material, non-uniform drying of the cement-based matrix phase, and the three-dimensional geometry and boundary conditions of the structural component. This work describes an irregular lattice approach for explicitly modeling individual fibers, and their collective actions, within fiber reinforced cement composites (FRCC) subjected to drying environments. The spatial and orientation distributions of the fibers conform to the structural boundaries and could account for other aspects of the production process. The stresses in the fibers differ greatly depending on whether uniform or non-uniform drying is considered, the latter case being more realistic. Whereas the emphasis is on modeling the pre-cracking actions of the fibers, this paper briefly discusses the potential for using this discrete type of approach to model fracture of FRCC.