Rheology of low carbon fibre content reinforced cement mortar

It is recognised that the addition of carbon fibres to a brittle cement matrix results in a less dense composite with enhanced ductility, improved impact resistance and increased toughness. In addition, the reinforcing effect of fibres in the cement often produces superior flexural strength and marked improvements in post-cracking behaviour. Further, carbon fibres influence the electrical properties of the composite which could, potentially, make it a smart material, with a range of applications. Despite attention directed towards the mechanical and electrical properties of carbon fibre reinforced cement (CFRC), there is a dearth of information of the influence of fibre additions on the rheological properties of the resulting composite. To this end, this paper describes an investigation using the Viskomat NT into the influence of carbon fibre additions (fibre length in the range 3–12 mm and volume in the range 0–0.5%) on the rheological properties of CFRC. Within the limitations of the instrument and testing procedure it is shown that CFRC’s conform to the Bingham model: increasing fibre volume and fibre length increase both the yield stress and plastic viscosity.     

Performance of natural fibre reinforced mortar roofing tiles

Interest in natural fibres as reinforcement in developing countries arose (when the use of steel and glass fibres in the industrialized countries was being researched and developed in the late 1960s) mostly because of their lower cost and better availability in the developing world. The use of natural fibres as reinforcement for mortar to be used mainly for roofing has been debated for a long time. Initially, it was rightly thought that natural fibres will adversely affect the setting of Portland cement and as such they should not be used. Later on it was found that natural fibres are embrittled by the hydration products of Portland cement and made ineffective within a short time, especially in hot climates. While such discussions were going on, the initially adopted large corrugated roofing sheet (corrugated iron sheet size) was being found impractical (mainly because of handling and curing problems) and its size was being reduced gradually. The present paper discusses difficulties encountered in the production and use of natural fibre reinforced mortar roofing tiles and means of reducing the adverse effects of Portland cement hydration products on the performance of natural fibres in mortar.     

The spalling strength of ultra-fiber reinforced cement mortar

This is a research report about the effects of polypropylene fiber and wood fiber on mechanical properties of cement mortar. First, using advanced Hopkinson pressure bar (HPB) tests, it investigates the wave propagation in cement mortar comprised polypropylene fiber and wood fiber. Second, according to the experiment, the spallation position is recorded by high-speed camera. Thirdly, it analyzes the test data of ultra-fiber reinforced and common cement mortar by numerical method. Finally, it deduces the spalling strength of all kinds of cement mortar by integrating all experimental data above. The results indicate that, compared with the strength of common cement mortar, the dynamic spalling strength of ultra-fiber especially that of the polypropylene fiber reinforced cement mortar increases evidently. However, adding too much fibers will deteriorate the dynamic spalling strength of cement mortar specimen. So the results will provide a test basis for further optimizing performance of cement mortar.     

Plant-based natural fibre reinforced cement composites: A review

The quest for sustainability in construction material usage has made the use of more renewable resources in the construction industry a necessity. Plant-based natural fibres are low cost renewable materials which can be found in abundant supply in many countries. This paper presents a summary of research progress on plant-based natural fibre reinforced cement-based composites. Fibre types, fibre characteristics and their effects on the properties of cement-based materials are reviewed. Factors affecting the fresh and hardened properties of cement-based composites reinforced with plant-based natural fibre are discussed. Measures to enhance the durability properties of cement-based composites containing plant-based natural fibres are appraised. Significant part of the paper is then focused on future trends such as the use of plant-based natural fibres as internal curing agents and durability enhancement materials in cement-based composites. Finally, applications and recommendations for future work are presented.     

Predicting the tensile strength of natural fibre reinforced thermoplastics

The tensile strength of short natural fibre reinforced thermoplastics (NFRT) was modeled using a modified rule of mixtures (ROM) strength equation. A clustering parameter, requiring the maximum composite fibre volume fraction, forms the basis of the modification. The clustering parameter highlights that as fibre loading increases, the available fibre stress transfer area is decreased. Consequently, at high volume fractions this decrease in stress transfer area increases the brittleness of the short fibre composite and decreases the tensile strength of the material. A key parameter, the interfacial shear strength, was determined by fitting the micromechanical strength model to tensile strength data at low fibre loading (10 wt%) where there is minimal fibre clustering.To test the modified ROM strength model, compression molded specimens of high-density polyethylene (HDPE) reinforced with hemp fibres, hardwood fibres, rice hulls, and E-glass fibres were created with fibre mass fractions of 10–60 wt%. The modified ROM strength model was found to adequately predict the tensile strength of the various composite specimens.     

Shear strength of reinforced aerated concrete slabs

The paper reports on the analysis of shear strength of reinforced slabs made of autoclaved aerated concrete without shear reinforcement. The test data are taken from eleven different investigations from six countries, in Europe and Japan, over a period of some twenty years and include 271 tests. The analysis of the test data results in regression expressions, suitably modified from a formula used for ordinary concrete members, and shows good agreement with test values. Appropriate expressions are suggested for design.     

高强钢丝绳网片-聚合物砂浆加固RC板抗爆性能试验研究

为了研究高强钢丝绳网片-聚合物砂浆对钢筋混凝土(RC)板的抗爆加固效果,对5块加固RC板和1块未加固RC板进行了野外现场爆炸试验,研究了砂浆强度、钢丝绳间距、钢丝绳预应力和界面增设销钉等因素对试件的破坏形态、裂缝分布及发展、跨中位移、钢筋应变等影响规律,并对爆炸试验后的试件进行了剩余承载力试验和爆炸损伤评估.研究表明:...     

Durability of slag mortar reinforced with coconut fibre

A comparative study of the microstructure of both new and in-use aged blast-furnace slag cement coir reinforced composite was performed. Aged samples came from internal and external walls of a 12-year-old house, built in São Paulo. The panels of the house were produced using 1:1.5:0.504 (binder: sand: water, by mass) mortar reinforced with 2% of coir fibre by volume. The binder was blast-furnace slag activated by 2% of lime and 10% of gypsum.Fibres were removed from the composite and subsequently cleaned with acid solution (chloridric acid, 10%) in an ultrasonic bath. Both aged and new fibres were studied under low pressure BSE SEM with EDS analysis. The lignin content of the fibre was measured by the acetyl bromide method and qualitatively evaluated by Weisner reaction using an optical microscope.Cement transformations were studied by X-ray diffraction and TG. Its pore water composition was also determined. Carbonation was measured by phenolphthalein. Composites were studied under low vacuum SEM. Interfaces and deposition of inorganic species in the fibre lumen were also investigated. Methods are described.After 12 years, the cement was fully carbonated. Fibres removed from the old samples seem to be undamaged when examined under SEM. Qualitative lignin content determination by Wiesner reaction suggests that old samples have lower content of guaiacyl lignin units. Nevertheless, the total lignin content of old fibres when measured by using the acetyl bromide method, is comparable to that reported in literature. No significant difference was found in the lignin content of fibres removed from external and those removed from internal walls.     

Flexural behaviour of small steel fibre reinforced concrete slabs

Influence of length and volumetric percentage of steel fibres on energy absorption of concrete slabs with various concrete strengths is investigated by testing 28 small steel fibre reinforced concrete (SFRC) slabs under flexure. Variables included; fibre length, volumetric percentage of fibres and concrete strength. Test results indicate that generally longer fibres and higher fibre content provide higher energy absorption. The results are compared with a theoretical prediction based on random distribution of fibres. The theoretical method resulted in higher energy absorption than that obtained in experiment. A design method according to allowable deflection is proposed for SFRC slabs within the range of fibre volumetric percentages used in the study. The method predicts resisting moment–deflection curve satisfactorily.     

Structural behaviour of basalt fibre reinforced glass concrete slabs

Small-scale slab tests at ambient and elevated temperatures, conducted on horizontally unrestrained simply supported slabs, are presented in this paper. The aim of this research is to investigate the structural behaviour of concrete produced from different percentages of glass sand (20, 40, and 60 % by weight) and reinforced with different volume fractions of basalt fibre (0, 0.1, 0.3, and 0.5 % by total mix volume), when subjected to large vertical displacement. The results were also compared against similar structural members with concrete that did not contain glass or fibres. The results showed that the fracture of the reinforcement was the mode of failure for all the slabs and the load carrying capacity was enhanced above the theoretical yield-line load. For the slabs tested at elevated temperatures, the enhancement due to membrane action was at least twice as high as that recorded in the ambient temperature tests. The slabs with higher glass sand and basalt fibre content also exhibited greater enhancement and failed at higher displacement. The results also showed that the enhancement in the concrete with glass aggregate and basalt fibre was greater than that in concrete that contained no glass or fibre by up to 26 and 31 % at ambient temperature and in fire respectively.     

Tests of continuous concrete slabs reinforced with carbon fibre reinforced polymer bars

Although several research studies have been conducted on simply supported concrete elements reinforced with fibre reinforced polymer (FRP) bars, there is little reported work on the behaviour of continuous elements. This paper reports the testing of four continuously supported concrete slabs reinforced with carbon fibre reinforced polymer (CFRP) bars. Different arrangements of CFRP reinforcement at mid-span and over the middle support were considered. Two simply supported concrete slabs reinforced with under and over CFRP reinforcement and a continuous concrete slab reinforced with steel bars were also tested for comparison purposes. All continuous CFRP reinforced concrete slabs exhibited a combined shear–flexure failure mode. It was also shown that increasing the bottom mid-span CFRP reinforcement of continuous slabs is more effective than the top over middle support CFRP reinforcement in improving the load capacity and reducing mid-span deflections. The ACI 440.1R–06 formulas overestimated the experimental moment at failure but better predicted the load capacity of continuous CFRP reinforced concrete slabs tested. The ACI 440.1R–06, ISIS–M03–07 and CSA S806-06 design code equations reasonably predicted the deflections of the CFRP continuously supported slabs having under reinforcement at the bottom layer but underestimated deflections of continuous slabs with over-reinforcement at the bottom layer.     

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.     

Behaviour of high volume fibre cement mortar in flexure

Tests are reported on the behaviour of high volume percentage steel fibre mortar specimens subjected to flexure. Flexural and cyclic load tests were conducted; in addition, comparison tests were made on conventional fibre mortar and ferrocement specimens. Both strength and deflection characteristics were studied. The results of the investigation indicate that with 8% high volume, steel fibre mortar specimens possess a flexural strength of about 40 MPa.     

Determination of bond strength in glass fibre reinforced cement using petrography and image analysis

The reinforcement in glass fibre reinforced cement (grc) is not present as discrete fibres, but as strands of about 200 filaments each. This configuration greatly complicates determination of the perimeter of the reinforcing elements, a crucial parameter in bond strength determination. Previous investigators have attempted to quantify strand perimeters but their methods have always involved at least one subjective step and are prone to operator bias. This paper describes an objective method of determining strand perimeters using digital analysis of images captured from petrological thin sections. The measured perimeters were found to be sensitive to the threshold value chosen for the analysis, and consideration of perimeter vs. threshold value curves eliminated subjectivity involved in the analysis. The perimeter value obtained, together with microscopic analysis of the crack patterns produced during tensile testing, were used for calculating bond parameters for different cement matrices. The development of bond with both ageing time and temperature were also studied. The method uses fundamental image analysis concepts and as such is readily adaptable to solve conceptually similar problems in a wide range of materials.     

Shrinkage of steel fibre reinforced cement composites

The paper presents results of an experimental investigation on the influence of steel fibres on the free shrinkage of cement-based matrices. Shrinkage tests were carried out on cement paste, mortar and two types of concrete mixes for a period of up to 520 days. Melt extract, crimped and hooked steel fibres were used for reinforcement at volume fractions ranging between 1 and 3%. The results indicate that fibres restrain the shrinkage of the various cement matrices to a significant extent, resulting in reductions of up to 40%. Crimped fibres are the most efficient in providing shrinkage restraint. The paper also presents a theoretical expression and an empirical expression which can be used to predict shrinkage strains of steel fibre reinforced cement matrices. The analysis requires a knowledge of the values of coefficient of friction, μ, at the fibre-matrix interface, which are also derived in this paper. The μ values for steel fibres in normal concrete, mortar and cement paste range between 0.07 and 0.12.     

Effects of ageing and weathering on the tensile strength of glass fibre reinforced high-alumina cement

The results of experimental studies on the variation of the tensile strength with time of high-alumina cement sheeting reinforced with glass fibres are presented. Over a period of one and a half years no significant deterioration has been found in specimens stored under either laboratory or outdoor conditions. Sheeting subjected to a severe artificial weathering cycle over a period of five months has shown some decrease in strength.     

Fatigue endurance of aged glass fibre reinforced cement

The resistance to flexural fatigue of glass fibre reinforced cement (GRC) stored in water for six years, has been studied. Peak stresses of between 6.0 and 18.2 MN m^–2 were used. At stresses of 10.0 and 18.2 MN m^–2 the median times before failure were 1.95×10⁵ and 2.0×10³ cycles, respectively. At a stress of 8.1 MN m^–2, six out of sixteen samples tested survived 4.65×10⁶ cycles. At a stress of 6 MN m^–2, all of the samples survived 1.75×10⁶ cycles. An unreinforced mortar specimen was also studied and its fatigue endurance showed greater scatter than the GRC samples.