Showing papers in "Cement & Concrete Composites in 1994"

Measurement and prediction of the strength of rubberized concrete

Abstract: Illegally discarded piles of automobile tires are sources of potential hazards. Current disposal methods are wasteful and costly as they require either consumption of landfill space or continuous costly maintenance. A solution to the problem of scrap-tire disposal is the potential use of tire chips and crumb rubber as mineral aggregate substitutes in Portland cement concrete mixes. In this study, some of the engineering properties of rubberized concrete were examined and a neural network was developed to predict its compressive and tensile strengths. Rubberized concrete was found to possess good esthetics, acceptable workability, and a smaller unit weight than normal concrete. However, rubberized concrete did not perform as well as normal concrete under repeated freeze-thaw cycles. It exhibited lower compressive and tensile strength than that of normal concrete. Unlike normal concrete, rubberized concrete had the ability to absorb a large amount of plastic energy under compressive and tensile loads. It did not demonstrate the typical brittle failure, but rather a ductile, plastic failure mode. Test results were analyzed so that a model can be developed to predict the strength of rubberized concrete. Two neural network models were developed to predict the reduction in the compressive and tensile strength as a result of replacing mineral aggregate with rubber aggregate. A maximum difference of 9·2% between test results and model prediction was detected during the testing of the neural networks.

Properties of hardened concrete containing treated expanded polystyrene beads

Abstract: This paper reports the results of an experimental investigation into the properties of hardened concrete containing chemically treated expanded polystyrene beads. The results showed that the strength, stiffness and chemical resistance of polystyrene aggregate concrete of a constant density were affected by the water to cement ratio. Drying shrinkage after 84 days of drying for polystyrene concretes, having 10 mm coarse aggregate and a nominal density of 1300 kg/m3, were 730 and 655 microstrains. Empirical equations were developed to relate the strength and pulse velocity and to predict the modulus of elasticity from its strength.

Strength and reliability of carbon-fiber-reinforced cement composites

Abstract: The effect of carbon fibers on the mechanical properties of cement paste composites is studied. The addition of polyacrylonitrite-based carbon fiber to a cementitious paste matrix results in a significant improvement in the tensile and flexural properties of the composites. The uniaxial tensile strength results are obtained using the novel cementitious composites axial tensile technique. The addition of 1,2 and 3 %vol. of carbon fiber to a cement matrix results in an increase in the uniaxial tensile strength of 32, 48 and 56%, respectively. The enhancement of the composite flexural strength was more significant, as compared to the uniaxial tensile strength. The flexural strength of cement matrix increased by 72, 95 and 138% with the addition of 1,2 and 3 %vol. of carbon fiber, respectively. Weibull statistics indicate that reliability in flexure was not enhanced by fiber addition and there was no correlation between the percent fiber loading and the reliability of the composites. However, the reliability of the carbon-fiber-reinforced composite in tension was greatly improved and there is a positive correlation between fiber loading and the Weibull modulus, m.

Air-cured banana-fibre-reinforced cement composites

Abstract: In this paper techniques for the fabrication of banana-fibre-cement composites are reported. The physical and mechanical properties of air-cured fibre-reinforced cement composites, containing banana reinforcement prepared by several different methods are discussed. It was found that kraft pulped banana fibre, at a loading of between 8 and 16% by mass, resulted in composites with flexural strengths in excess of 20 MPa. At a fibre loading of 14% by mass, the flexural strength is 24·92 MPa and the fracture toughness value is 1·74 kJm−2, properties adequate for the production of commercially viable building materials. Thus, in countries like India, China, and within South East Asia, where there is a lack of softwood fibre (the natural fibre most preffered as an alternative to asbestos), but in which banana fibre occurs in abundance, a viable alternative fibre resource is available for commercial consideration.

Effect of increased tensile strength and toughness on reinforcing-bar bond behavior

Abstract: The research reported here investigated the pull-out behavior of deformed reinforcing bars embedded in fiber-reinforced-concrete (FRC) and high-performance- fiber-reinforced-concrete (HPFRC) matrices exhibiting increased tensile strength and toughness. Increased strength and toughness of the embedding matrix resulted in a significant increase in pull-out strength, strain capacity, and over-all ductility, as well as more stable crack development. Additionally, when sufficient lateral constraint (i.e. cover thickness) was provided, the use of an HPFRC matrix exhibiting strain-hardening behavior resulted in a slip-hardening pull-out response.

Municipal wastewater sludge as cementitious and blended cement materials

Abstract: The potential for using wastewater sludge in combination with limestone to produce cementitious building materials is investigated. A ground mixture of the raw materials was incinerated to produce the cement clinker. The effects of raw materials mix compositions, incineration temperatures and durations on various properties of the cement were examined. The experimental results show that under controlled incineration, it is possible to produce the so-called ‘Bio-cement’ from wastewater sludge that would satisfy the strength requirements for masonry cement. When being used as a partial replacement material for portland cement, the Bio-cement can replace up to 30% by weight of ordinary portland cement without deteriorating the mortar strength. Blended cement with up to 10% replacement level not only shows higher strength than control portland cement, it also exhibits a higher rate of strength development at early ages. This paper presents properties of the cements as well as their strength characteristics.

Ferrocement laminates for strengthening RC T-beams

Abstract: The flexural behaviour of reinforced concrete T-beams strengthened with thin ferrocement laminates attached to the tension face was investigated. Twelve simply-supported beams were tested under concentrated loads at mid-span. The ferrocement laminates were attached using ‘L’-shaped mild steel round bars as shear connectors. The parameters that were varied include the spacing of the shear connectors, method of surface preparation and the volume fraction of reinforcement in the ferrocement laminates. Performance was gauged in terms of cracking characteristics, mid-span deflection, stiffness, and ultimate load capacity. The results showed that use of closely spaced shear connectors and proper surface preparation resulted in improved serviceability and flexural capacities.

Microstructure, chloride diffusion and reinforcement corrosion in blended cement paste and concrete

Abstract: This paper presents some results from a wide-ranging investigation of porosity and pore-structure, chloride-diffusion, and reinforcement-corrosion studies in blended cement paste and concrete. The cement-replacement materials used were pfa, slag, and microsilica. Mercury-intrusion-porosimetry experiments were carried out on pastes with w/c of 0·45. Chloride-diffusion studies and corrosion investigation on rebars were carried out by using concrete mixes of proportions by weight 1:2·5:1·2 and w/c of 0·58. In all these experiments, initial curing for at least 14 days was under relatively low-humidity environments instead of idealised wet curing. The results show that microsilica cement pastes have a larger pore volume than other pastes used in the investigation. The chloride penetration, however, is the minimum in silica-fume concrete. The chloride-binding capacity of pfa concrete is less than that of normal concrete in the surface zone, but, at a greater depth (> 20 mm), the binding capacity of pfa concretes exceeds that of plain concrete. The results generally indicate a higher corrosion activity of reinforcement with increasing chloride concentration in the pore fluid. Microsilica was by far the most effective cement-replacement material in increasing the corrosion resistance of reinforcement in concrete.

The anticorrosive effect of fly ash, slag and a Greek pozzolan in reinforced concrete

Abstract: The influence of the addition of 15% and 30% fly ash, 15% and 30% of a Greek natural pozzolan and 50% granulated blastfurnace slag to ordinary Portland cement on the corrosion resistance of the reinforcing bars was studied in a program of long-term exposure to seawater. The use of blended cements resulted in a decrease in the corrosion rate, especially after long exposure times. The most effective protection was rendered by the 30% fly ash mix. This performance was related to the chloride content and the chloride binding capacity of the blended cements.

Dispersant composition for cement having excellent property in inhibition of slump-loss

Abstract: A cement admixture which when added to a cement composition, such as cement mortar or concrete inhibits slump-loss of the cement composition and improve its workability and applicability. The admixture is composed of copolymers from an alkenyl ether, a polyalkenyl ether and maleic anhydride.

Pore structure and chloride ion permeability of mortars containing silica fume

Abstract: The addition of silica fume in concrete causes a remarkable increase in strength and a drastic reduction in chloride ion permeability. These effects may be due primarily to microstructural changes both in the cement paste phase and in the interfacial zone around aggregates. The standard method of test for rapid determination of the chloride permeability of concrete, AASHTO T 277–831, has increasingly been used to evaluate the permeability of concrete. However, for the concrete containing silica fume, the results of the AASHTO T 277–831 test, which is expressed in terms of electrical charge passed, do not necessarily reflect the real diffusion index of chloride ion through the concrete. There seems to be factors other than the pore structure which govern the results of the AASHTO T 277–831 test in the concrete containing silica fume. In this study, the effects of silica fume to reduce the chloride ion permeability of the mortar were investigated based on the results of pore size distribution measurements, X-ray diffraction analysis, SEM observations and pore solution extraction. The application of the AASHTO T 277–831 test to the evaluation of the chloride ion permeability of the concrete containing silica fume was discussed.

Trade-off between strength and ductility of random discontinuous fiber reinforced cementitious composites

Abstract: It is well known that fiber reinforcement is an effective way of improving toughness of brittle materials. However, it is less clear how fiber reinforcement (particularly for random discontinuous fibers) can contribute to strength and ductility enhancement of the composites. In this paper, we address this issue from a micromechanics point of view. The conditions for multiple cracking, which is the mechanism of pseudo ductility, are briefly reviewed in terms of micromechanical parameters such as fiber aspect ratio, matrix toughness, and interfacial bond strength. Special attention is given to examining competing effect of these microparameters on composite strength and ductility. Experimental results of direct tensile tests of several random discontinuous fiber-reinforced high strength concretes and OPC paste are presented and discussed in light of recent advancements in pseudo strain-hardening theory.

Testing Concrete for AAR in NaOH and NaCl solutions at 38°C and 80°C

Abstract: Low-alkali and high-alkali concrete prisms were made with a water/cement ratio of 0·55 and two very alkali-silica reactive aggregates, and tested at 38 and 80°C in 1 M NaOH and NaCl solutions, in water and in air at about 100% RH. The results obtained indicate, at least for the concretes tested, that: (1) ion migration by diffusion is relatively rapid between the immersion solution (alkaline solution or water) and the concrete pore solution; (2) in the presence of reactive aggregates, two expansive mechanisms seem to superimpose when testing in 1 M NaCl, e.g. alkali-aggregate reaction and formation of chloroaluminates; (3) testing in water gives very low expansion as a result of alkali dilution and consequent pH reduction in the pore solution; (4) when testing in 1 M NaOH, the rate of expansion is affected by the initial concrete alkali content; (5) testing concrete prisms in 1 M NaOH at 80°C is the most rapid testing procedure but is unreliable for determining the potential alkali-reactivity of aggregates; and (6) testing in 1 M NaOH at 38°C appears the most promising procedure among those investigated, for a number of reasons. For instance, compared with the concrete prism method CSA A23-2-14A (e.g. testing in air at ≈ 100% RH), the test duration could be reduced from 1 year to 6 months, and the concrete specimens under test are not subjected to alkali leaching and humidity variations. Some results also suggest that low-alkali concretes exposed to salt solutions could be protected for some long period of time against AAR.

Chloride permeability of concrete subjected to freeze-thaw damage

Abstract: The effects of freezing and thawing cycling on the chloride permeability of normal weight and lightweight concretes were investigated by using the AASHTO T277 chloride permeability test method and the freeze-thaw test method similar to that recommended by ASTM C666. The results showed that the chloride permeability of the normal weight concretes having an air content of at least 5·3% changed little with the repeated cycles of freezing and thawing up to 618 cycles, irrespective of the presence and the type of mineral admixtures. It was also found that lightweight concretes made with fully-saturated expanded shale aggregates exhibited an extremely high chloride permeability at any air content, when they were subjected to a single freeze-thaw cycle. Furthermore, the type of microcracks developing in normal weight concretes exposed to the repeated freeze-thaw cycles was discussed.

Effects of added alkali hydroxides in mix water on long-term SO42− concentrations in pore solution

Abstract: In ASR (alkali silica reaction) studies it has become common to pre-dissolve NaOH or KOH in the mix water to simulate the effects of high alkali content cements. It has been found that such treatment does not result in the expected augmentation of the OH− ion concentrations, and much or all of the dissolved alkali hydroxide is immediately converted to alkali sulfate by reaction with gypsum. Much of the dissolved sulfate so produced remains in the pore solution under long-term exposure at 38°C, the common temperature employed in ASR studies. The concentration of SO42− ions produced at a given alkali hydroxide dosage varies with the specific hydroxide added, increasing in the order LiOH to NaOH to KOH. If ASR reaction is occuring, the SO42− ion concentration diminishes progressively with time. This effect was not due to progressive incorporation of sulfate in the ASR reaction product, as had been suspected. It may be due to secondary ettringite precipitation in ASR-induced cracks.

Durability and moisture sensitivity of recycled wastepaper-fiber-cement composites

Abstract: The effects of moisture and weathering on the performance of recycled wastepaper-fiber-cement composites were investigated through accelerated laboratory tests simulating the effects of wet-dry and freeze-thaw cycles as well as carbonation and chemical reactions under natural weathering. Measures were developed and successfully evaluated for the control of aging effects in composites through changes in matrix composition.

Corrosion condition evaluation and corrosion protection of unbonded post-tension cables in concrete structures

Abstract: There is disclosed a method and apparatus for the corrosion condition evaluation of unbonded prestressing elements in post-tension concrete structures. The method involves locating a prestressing element in the structure and providing at least two openings in the structure at positions along the length of the element. One of the openings is an inlet port and the other is an outlet port, each of the ports permitting communication with the gaseous environment within a conduit surrounding the prostressing element. The gaseous environment is accessed through the outlet port by extracting a sample of gas therethrough. The sample is then measured to determine its humidity and thereby evaluate the corrosion condition of the prestressing element between the inlet port and the outlet port. A method and apparatus is also provided for the on-site corrosion protection of unbonded prestressing elements whereby the gaseous environments within the conduits are cyclically pressurized with a dry gas. The protection method may also include programmable control of the dry gas pressurization and feedback of the humidity levels in the conduits for adjusting the parameters of cyclical pressurization so as to maintain a predetermined maximum humidity value in the gaseous environments associated with the prestressing elements of the structure.

Application of the NaOH bath test method for assessing the effectiveness of mineral admixtures against reaction of alkali with artificial siliceous aggregate

Abstract: The use of the mortar bar expansion test in a 1 M NaOH solution at 80°C as a means of assessing the effectiveness of fly ash and silica fume against alkali-silica reaction was investigated using fused quartz as reactive aggregate. Long-term ASTM C227 test results were utilized for comparative purposes. The NaOH bath test provides reasonable results about the minimum contents of fly ash and silica fume needed to prevent deleterious expansion in mortars containing fused quartz. Particularly for silica fume, agreement between the minimum admixture contents predicted by the two test methods is found only when the expansion limits of 0·25% at 12 days and 0·10% at 1 year or, alternatively, 0·15% at 14 days and 0·05% at 1 year are used for the NaOH bath and ASTM C227 test methods, respectively. A consistent relationship exists between the expansion of mortar bars and the Na + ion concentration in the pore solution after 14 days immersion of the specimens in the NaOH solution. With both test methods there is no direct correlation between the performance of fly ashes and their total and available alkali contents.

Strength variations in lightweight concrete beams

Abstract: The distribution of strength within a series of five 2·2 m × 0·5 m × 0·3 m laboratory reinforced concrete beams has been examined, with the aid of non-destructive and partially destructive testing techniques, as well as cores. Four different types of lightweight concrete have been tested and compared with a normal weight gravel concrete. Results have also been compared with tests on standard cube specimens. Results demonstrate the influence of concrete type upon in-place strength distribution and variability, as well as the relationship to standard cube strength. Some discrepancies have been found between the results from different test methods, and these are considered in detail. It is concluded that whilst materials factors of safety commonly used in design are likely to allow adequately for strength differences between cubes and in-place concrete, planning and interpretation of in-situ testing programmes may in some cases require modification from the established procedures for normal weight concretes.

Corrosion inhibiting repair and rehabilitation treatment process for reinforced concrete structures

Abstract: A repair and rehabilitation treatment process for reinforced concrete structures involves the removal of concrete from above rebar or other metal reinforcement material in the concrete structure. After removal of concrete, the metal reinforcement materials are saturated with corrosion inhibiting agents. Saturation is best achieved by multiple spray applications of the corrosion inhibitor. The cavity in the concrete structure with the treated rebar or other metal reinforcement materials is then backfilled and/or overlaid with repair concrete. Preferably, the repair concrete includes corrosion inhibitors which will diffuse to the rebar over time or is a low permeability concrete that reduces the rate of diffusion of chloride corrosion causing agents to the rebar. The repair and rehabilitation process significantly increases the concrete structure's service life.

Acrylic fibres as reinforcement for cement pastes

Abstract: The possibility of employment of acrylic fibres as reinforcing material in cement paste, mortars and concretes has been studied by the authors of the present paper according to two features characterizing this kind of composite: 1. (a) Reinforcing capacity of poliacylonitrile fibres 2. (b) Resistance to chemical attack of the aforementioned fibres The present paper is the first part of a study projected on the long run; accordingly, in the future more data could be furnished in order to study more closely some specific aspects that can complement these first results. Thus for instance, in pastes of Portland cement, it has been proved that the presence of this type of fibre improves their impact resistance upto around 30–40 times whereas flexural resistances for 1 1·5% wt of addition of fibres have an increase, with respect to those obtained for pure paste, of around 50% in some cases. Simultaneously, it has been proved that the chemical resistance or attackability of acrylic fibres in high alkaline media is quite good.

Autoclave method for identification of alkali-reactive carbonate rocks

Abstract: Based on the rapid test method suggested by the authors for determining the alkali reactivity of siliceous rocks, an autoclave test method has been developed for determining the reactivity of carbonate rocks. The main modified test conditions are changing the ratio of cement to aggregate from 10:1 to 1:1, the size of aggregate from 0·15–0·75 mm to 5–10 mm, and 10 × 10 × 40 mm3 to 5–10 mm, and 10 × 10 × 40 mm3 mortar bar to 20 × 20 × 60 mm3 microconcrete bar. By this rapid test method, the reactivity of carbonate rocks can be determined in 2–3 days. Experimental results also proved that the autoclave method may be used to assess rock cylinders for alkali reactivity.

Interfacial and mechanical behavior of fiber-reinforced calcium phosphate cement composites☆

Abstract: One basic drawback of hydrothermally synthesized calcium phosphate cements (CPC) is that they are brittle and fragile. To solve this problem, we investigated the interfacial chemistry, microstructure development, and toughness-related physical properties of CPC composites reinforced with strong fibrous materials (carbon fiber, glass fiber, and Kevlar). The most ideal interfacial structures between fiber and CPC were observed in carbon-fiber-reinforced composites, i.e., intermediate layers that were created by a moderate interfacial interaction of carbon with CPC promoted cross-linking, which tightly interconnects the fibers and thereby significantly improves ductility and toughness. In contrast, an undesirable interfacial mode was identified in Kevlar- and glass-fiber-reinforced CPC composites that were autoclaved at 250°C: the alkali-catalyzed hydrolysis of Kevlar in the composite prevented adherence to the CPC, and an extensive interaction between CPC and glass fiber resulted in the formation of porous intermediate layers. These negative factors were correlated directly to a lack of toughness and to catastrophic failure.

Finite element analysis and parametric study of steel-concrete composite beams

Abstract: A finite element model is presented to predict the stresses and deformations in steel-concrete composite beams. The model takes into account the effect of cracking and tension-stiffening in the tensed concrete, and of longitudinal slip between the steel beam and the concrete slab due to the ‘partial interaction’ of theconnectors. Some comparisons with experimental data available in literature are reported to validate the efficiency of the proposed model. Finally, a parametric study was done to investigate the effects of the geometric and mechanical variables as boundary conditions and the slip modulus of the connectors.

Plate impact testing method for GRC materials

Abstract: The paper reports on the impact performance evaluation of glass fibre reinforced cement (GRC) materials. The paper includes a review of the principles of impact testing methodology and a proposal for a new test method consisting of a drop weight instrumented impact on plate specimens. The significance of impact test data is discussed with regard to the parasitic effects on impact load versus time records. An application of digital filtering treatment provided the load-displacement characteristics reflecting the specimen response alone. The following impact features of the GRC elements were determined: the maximum impact load, the energy absorbed up to the maximum load and the energy absorbed up to total failure. The energy absorption capability of GRC plates was studied as a function of plate thickness and impact velocity. A comparison of the impact data and the reference quasi-static tests yielded a ratio of impact-to-static energy absorption of 1·7–1·8 for the GRC elements considered.

Shrinkage of fibre and reinforced fibre concrete beams in hot-dry climate

Abstract: The influence of steel fibre inclusion on the shrinkage of 16 full-size plain and reinforced concrete beams was assessed. Shrinkage measurements, at three levels over the depth of the beams, were carried out for 200 days. Half of the beams were cured in a controlled laboratory environment and the other half cured under hot, dry and windy climatic conditions. Test results show that under laboratory curing conditions adding 1% by volume of steel fibres reduced the ultimate shrinkage at the top, mid-height, and bottom of the plain concrete by 16, 23, and 28%, respectively. However, in the reinforced concrete beam the presence of longitudinal reinforcement rendered it less significant. Under the uncontrolled severe curing environment, the addition of 1% by volume of fibres produced a reduction of 30% in shrinkage at the bottom level of both the plain and the reinforced concrete beams. At the top level, however, the geometry constraints and the compaction techniques influenced the fibre contribution to shrinkage.

The resistance of steel fibre concrete to VTOL engine jet blast

Abstract: A two stage investigation was carried out to assess the suitability of steel fibre concrete as a pavement material for VTOL aircraft. Conventional pavement concretes can spall when subjected to jet blast during take-off, and to a lesser extent landing. Initial laboratory tests produced a fibre mix which complied with PSA pavement quality concrete specifications and which could be suitably finished. In two tests series, twenty-nine slabs were subjected to jet blast simulating Harrier take-off/landing conditions. Thermal imaging and video camera recordings were used to establish the surface temperatures and residence times to first spall for the fibre mix and two control plain mixes under jet exit temperatures in the range 500 to 800°C. The presence of steel fibres marginally increased the residence time to cause spalling for jet temperatures above 650°C. Steel fibres did not appear to influence the rate of increase of surface temperature and resulted in slightly higher spalled areas.

Suitability of two rapid test methods for determining the alkali reactivity of sands

Abstract: Five sands have been used in a study aimed at determining their reactivity potential using a number of test methods. The sands were used either in mortar bars or in concrete prisms containing non-reactive coarse aggregate, at various alkali levels, and subjected to accelerated test procedures. Expansion of mortar bars stored in 1 M NaOH solution at 80°C indicated the sands to be potentially reactive, whereas those made at 2·5 or 3·5% cement alkali levels and subjected to autoclaving for 4 or 5 h at 127°C did not produce deleterious expansions, except in the case of one sand. Microstructural studies have indicated that the mechanisms operating in the two methods could be different. Although some of the sands had petrographic features that could indicate potential reactivity, unfortunately, no controlled long-term experimental or field data are available related to the service performance of the sands in concrete structures. Further investigation is required to establish or verify the suitability of the rapid test methods for determining the reactivity of sands with more confidence.

Constitutive relation for steel fibre concrete under biaxial compression

Abstract: This paper presents an analytical model for predicting the response of steel fibre concrete under biaxial compression up to failure. The model treats the steel fibre concrete as an orthotropic material for which the constitutive relation based on a total stress-strain formulation is derived. In the derivation, the biaxial compressive strength of steel fibre concrete is first evaluated by treating it analogous to a plain concrete under triaxial compression, with a confining pressure due to the fibres in the third direction. By defining equivalent uniaxial stress-strain relations and establishing the secant moduli in the principal directions, the stress-strain relation for steel fibre concrete under biaxial loading is obtained by a stepwise procedure. The stress-strain relations so derived are found to compare well with available experimental data.