Woven fabric reinforcement of cement matrix

The performance of woven fabric reinforced cement was studied to resolve the influence of the structure of the fabric on its reinforcing efficiency. Special fabrics were produced for this study in which the longitudinal (warp) yarn density was kept constant and the perpendicular (fill) yarn density was varied, in the range of 0 to 22 yarns per centimeter. Specimens for flexural and pull out testing were produced from the fabrics and crimped yarns untied from the fabric. Scanning electron microscope tests were carried out to resolve the microstructure of the composite, in particular at the yarn-matrix interface. The results point to three main conclusions: (1) Woven fabric structure improved the bonding capacity as compared to polyethylene monofilament fibers and cement matrix. (2) The crimped structure of the yarns in the fabric plays an important role in this improvement of the bond, providing mechanical anchoring between the woven fabric and the cement matrix. (3) There is an optimal density of fill yarn in the fabric, which causes the higher flexural strength within the tested densities and matrix formulations. This optimum is achieved in the fabric with five fill yarns per centimeter, and it may be accounted for by the fact that at higher density the matrix does not penetrate efficiently into the spaces in the fabric.     

Fiber reinforcement of a biomimetic bone cement

In this study we investigated the influence of electrospun polymer fibers on the properties of a α-tricalcium phosphate/gelatin biomimetic cement. To this aim, we added different amounts of poly(L-lactic acid) and poly(lactide-co-glycolide) fibers to the cement composition. Fibers enrichment provoked a significant reduction of both initial and final setting times. Moreover electrospun polymer fibers slowed down the conversion of α-tricalcium phosphate into calcium deficient hydroxyapatite. As a result, the final cements were more compact than the control cement, because of the smaller crystal dimensions and reduced crystallinity of the apatitic phase. The compressive strength, σ(b), and Young's modulus, E, of the control cement decreased significantly after 40 days soaking in physiological solution, whereas the more compact microstructure enabled fiber reinforced cements to maintain their mechanical properties in the long term.     

Chloride corrosion of steel fibre reinforcement in cement mortar

This paper presents an experimental study on the corrosion resistance of steel fibres and steel bar reinforcement in cement mortar. The mortar matrix incorporated various amounts of calcium chloride from 2 to 10%, and the rate of corrosion was monitored by the electrode potential method. The structure of the mortar and the steel surfaces were examined by scanning electron microscopy. The results showed that the addition of calcium chloride modified the microstructure of the mortar matrix, both its water absorption capacity and its porosity increased with increasing amounts of calcium chloride. The electric potential measurements showed that while the bar reinforcement displayed corrosion at 2% calcium chloride, the fibres did not indicate any harmful corrosion until the chloride content was 6%. Chloride admixtures added to concrete may thus be less harmful to steel in steel fibre concrete than in reinforced concrete.     

纤维水泥制品的抗裂增强材料

本文介绍了水泥制品存在的问题,纤维的种类及性能,及纤维对水泥制品的改善作用。     

Status of fiber-reinforced plastic reinforcement development and cement based research needs

This article examines the state of development of fiber-based reinforcement for concrete members in the United States. The term fiber-reinforced plastic (FRP) is used to identify this type of primary reinforcement used for prestressed and nonprestressed concrete members. The article focuses on the current state of development and the research needed to characterize and assure the performance of FRP reinforcement in concrete structures. The current development includes a basic understanding of glass FRP reinforcement and a lesser understanding of alternative fiber types and prestressing applications. Research needs include strength, ductility, compatibility with cement, bond, and thermal characteristics of the FRP reinforcements. Of these research areas, the compatibility of FRP reinforcement with cement based materials is one of the most critical areas for further development.     

On the use of R-PET strips for the reinforcement of cement mortars

We study the alkali resistance and the flexural response of a cement-based mortar reinforced through polyethylene terephthalate (PET) strips obtained through hand cutting of ordinary post-consumer bottles. On considering 1% fiber volume ratio and different strip geometries, we show that the analyzed reinforcing strips owe remarkable alkali resistance and are able to markedly improve the toughness of the base material. Comparisons are established with the outcomes of a recent study on a similar reinforcement technique of a cement–lime mortar.     

Improved mechanical properties of acrylic bone cement with short titanium fiber reinforcement

Acrylic bone cements are widely used in total joint arthroplasties to grout the prosthesis to bone. The changes in the tensile properties and fracture toughness of polymethylmethacrylate (PMMA) bone cements obtained by the addition of control and heat treated short titanium fibers are studied. Heat treatment of titanium fibers is conducted to precipitate titania particles on the fiber surface to improve the biocompatibility of the metal. Control and heat treated short titanium fibers (250 μ long and 20 μ diameter) were used as reinforcements at 3 volume %. X-ray diffraction indicated the presence of a rutile form of titania due to the heat treatments. The tensile and fracture properties were improved by the addition of fibers. Bone cements reinforced with titanium fibers heated at 550^∘C for 1 h followed by 800^∘C for 30 minutes show the largest increase in fracture toughness along with the smallest changes in elastic modulus and needs to be further investigated.     

Improved mechanical properties of acrylic bone cement with short titanium fiber reinforcement

Acrylic bone cements are widely used in total joint arthroplasties to grout the prosthesis to bone. The changes in the tensile properties and fracture toughness of polymethylmethacrylate (PMMA) bone cements obtained by the addition of control and heat treated short titanium fibers are studied. Heat treatment of titanium fibers is conducted to precipitate titania particles on the fiber surface, which may improve the biocompatibility of the metal. Control (non-heat treated) and heat treated short titanium fibers (250 μm long and 20μm diameter) were used as reinforcements at 3 volume %. X-ray diffraction indicated the presence of a rutile form of titania due to the heat treatments. Results indicate that the tensile and fracture properties of unfilled bone cement were improved by the addition of control and heat-treated fibers. The fracture properties of bone cements reinforced with control titanium fibers were at least 10% higher than those reinforced with heat treated titanium fibers. Therefore, we recommend further studies on the use of non-heat treated titanium fibers to reinforce acrylic bone cement.     

Effectiveness of combined nanoadditives for cement systems

We have developed a process for the sol–gel synthesis of a SiO₂-based combined additive in the presence of superplasticizers and investigated the hydration kinetics, phase composition, microstructure, and strength characteristics of a modified cement stone. The results demonstrate that combined nanoadditives accelerate hydration processes, without significantly changing the structure of the modified cement stone. The observed strength gain in the samples is due to the rapid accumulation of cementitious substance during the hardening system formation process. The results of a detailed study of modified cement systems have been used to optimize the composition of the combined additive.     

水泥厂窑尾除尘设计

根据某水泥厂生产规模、粉尘的性质和现场实际排放情况,在现有设计规范的基础上,设计出包括集气罩、风管、除尘器和风机在内的一整套窑尾通风除尘系统.针对水泥窑尾的粉尘排放情况,提出了详细的集气罩和送风管网的设计方案,仔细分析比较了窑尾电、袋式除尘器的优缺点及处理效果,选择了合适的除尘器和风机.讨论了实际应用中可能遇到的问题,并给出了解决方法,可作为水泥窑尾除尘系统的设计和分析的借鉴.     

Some effects of cement and curing upon carbonation and reinforcement corrosion in concrete

Experimental data are presented to illustrate the effects of cement type and curing upon the depth of carbonation and reinforcement corrosion in cover concrete after exposure for 18 months at 20°C and 60% relative humidity. Three curing periods (1, 3 and 28-days) and 17 cements, with various proportions of granulated blastfurnace slag or limestone, were used to make concretes, at 0.59 water/cement ratio, with 28 day strengths in the range 26 to 46 MPa. The depth of carbonation after 18 months was 64% greater than after 6 months and was affected more by cement type than by curing. The depth of carbonation increased when Portland cement clinker was replaced by 19% or more of limestone or granulated blastfurnace slag. The depth of carbonation after 18 months correlated better with the air permeability of cover concrete, initial weight loss (an indicator of moisture diffusion rate in cover concrete) or the cube strength 8 days after the end of curing than it did with 28-day cube strength. The rate of reinforcement corrosion increased steeply when the carbonation front approached the reinforcing steel, and it was still increasing after the carbonation front had completely passed the reinforcement. For a given unneutralised remainder (i.e. cover depth minus the depth of carbonation), curing had little effect upon the rate of corrosion but higher rates were observed when the cement contained granulated blastfurnace slag. The results were broadly consistent with a simple engineering strategy in which the rate of carbonation was related to the air permeability of cover concrete, and the rate of any subsequent reinforcement corrosion was largely dependent upon moisture conditions, without any obvious influence of the cover depth or the permeability of the cover concrete. The results also suggested that estimation of the rate of reinforcement corrosion could be improved by taking account of the cement type and treating the unneutralised remainder as a variable.     

Detection of nanostructural anomalies in hydrated cement systems

Monitoring the flow of helium gas into the structure of hydrated cement systems has proven to be a useful method for following nanostructural changes in the C–S–H phase of hydrated cement systems. The method is sensitive to changes that occur on removal of structural water from the layered silicates. The helium-inflow method was applied, in this study, to normally hydrated low-water–cement ratio (w/c) Portland cement pastes (w/c < 0.38) and to low w/c autoclaved cement systems containing fly ash and elemental sulfur. Unusually, high amounts and rates of inflow were observed for these pastes. It was postulated that inflow occurred into both interlayer and other spaces in the latter. The inflow into the other or ‘trapped’ space was unexpected and considered anomalous in absence of a widely accepted explanation. The structural differences which were observed at the nanoscale for the low w/c preparations were consistent with behavioral aspects for published structural models of layered C–S–H. These include the models of Richardson and Jennings and concepts involving the existence of two types of C–S–H. Arguments for the existence of ‘trapped’ space between aggregates of C–S–H layers are advanced. Evidence for the preservation of C–S–H structures (similar to those formed during normal hydration) for the autoclaved systems containing fly ash and sulfur is presented. The evidence is compatible with the existence of ‘trapped’ space within layered agglomerates and the collapse of C–S–H structure on removal of water from interlayer space, typical of normally hydrated pastes.     

Effects of fiber length and volume fraction on the reinforcement of calcium phosphate cement

A self-setting calcium phosphate cement (CPC) transforms into solid hydroxyapatite during setting at body temperature, and has been used in a number of medical and dental procedures. However, the inferior mechanical properties of CPC prohibits its use in unsupported defects, stress-bearing locations or reconstruction of thin bones. The aim of the present study was to strengthen CPC with fiber reinforcement, to examine the effect of fiber length and volume fraction, and to investigate the reinforcement mechanisms. Previous studies employed either short fibers for random distributions, or continuous fibers that were as long as the specimen size with preferred orientations such as unidirectional alignment. In the present study, a novel methodology was developed in which fibers several times longer than the specimen mold size were randomly mixed with the CPC paste to approximate the isotropy associated with short fibers, and at the same time achieve the high reinforcement efficacy associated with continuous fibers. Carbon fibers of 8 m diameter were used with fiber lengths ranging from 3 mm to 200 mm, and fiber volume fraction from 1.9% to 9.5%. A three-point flexural test was used to fracture the specimens. Scanning electron microscopy was used to examine crack-fiber interactions and specimen fracture surfaces. The composite containing fibers of 75 mm in length at a volume fraction of 5.7% achieved a flexural strength about 4 times, and work-of-fracture 100 times, greater than the unreinforced CPC. It is concluded that randomly mixing the CPC paste with carbon fibers that were several times longer than the specimen mold size resulted in substantial improvements in strength and fracture resistance; the reinforcement mechanisms were crack bridging and fiber pullout; and fiber length and volume fraction were key microstructural parameters that determined the cement properties.     

Properties and performance of silane: blended cement systems

The paper presents the results of a study dealing with the performance of water repellents on hardened blended cement pastes. Since on the European market Portland cement does not play the dominant role anymore and due to the new national and European policies concerning Greenhouse Gases and sustainability, cement manufacturers produce more and more blended cements (CEM II–CEM V). Nevertheless, the majority of experience concerning the efficacy of water repellents is gained from Portland cement; therefore knowledge in regard to the interactions of blended cement with water repellent agent is minimal. Two silane-based products were applied on ‘fresh’ and carbonated cement substrates containing limestone, fly ash, slag and trass, and were investigated in terms of their functionality. The evaluation of the treatments’ performance and effectiveness were assessed using various laboratory measurements. Hydrophobicity, water absorption, colour changes and the penetration depth of silanes into the substrate were evaluated before and after artificial aging experiments. Moreover, the outdoor weathering test was performed to shed light on treated surface appearance in a ‘real’ outdoor environment. The results showed that surface wettability was independent on water ingress or colour variations, especially for cement specimens artificially aged by accelerated carbonation. Cement pastes containing slag and trass seemed to more distinctly affect the water repellents’ surface performance.     

Tensile behaviour of mortar-based composites for externally bonded reinforcement systems

Textile reinforcements applied with inorganic matrices are currently receiving great attention for strengthening reinforced concrete and masonry structures, especially when preservation criteria need to be fulfilled for safeguarding cultural heritage. As the development of mortar-based reinforcements is still at an early stage, their mechanical properties need to be investigated and standardized testing methodologies have to be defined. The paper presents an experimental study on the tensile behaviour of strengthening systems comprising two different textiles and five mortar matrices. Various clamping methods and testing setups have been experimented and their effect on the results is discussed. Monotonic and cyclic tests have been carried out to derive strength and stiffness, crack pattern, failure mode, and response stages under tension, which have been related to the mechanical properties and the layout of the matrix and the textile.     

A new ductility model of reinforced concrete beams strengthened using Fiber Reinforced Polymer reinforcement

Despite the superior performance of Fiber Reinforced Polymer (FRP) as compared with conventional steel bars in terms of high strength-to-weight ratio, corrosion resistance, and high fatigue performance, FRP strengthened beams exhibit lower ductility due to the linear elastic response of the FRP reinforcement. Several ductility and deformability models were developed in order to account for the elastic behavior, i.e. high elastic energy, of FRP when used for strengthening existing steel reinforced concrete or for new construction. In this paper, a new ductility expression that relates the deformability of a reinforced concrete (RC) structure strengthened using FRP to the energy dissipated, was developed. The developed expression also considers the type of loading, static or fatigue. The new expression was validated against experimental test results of RC beams strengthened using prestressed Near Surface Mounted (NSM) carbon FRP un-fatigued and fatigued beams. Furthermore, the ductility index at which the deformability of the structure equals the ratio of total energy dissipated to elastic energy, defined as the optimum ductility index, was investigated for both the un-fatigued and fatigued beams. The prestress strain corresponding to the optimum ductility index was found to be 2830 με (the strain value can be determined using an accepted arbitrary test such as monotonic test) while no optimum ductility was achieved for the case of the un-fatigued beams. It is noteworthy that the optimum ductility index is subject to the variability of design, beam geometry, and prestressing level. Therefore, the ductility evaluation of the NSM CFRP strengthened beams was meant to give only an insight into the problem and not to propose certain limits.