Influence of interface properties on fracture behaviour of concrete

Hardened concrete is a three-phase composite consisting of cement paste, aggregate and interface between cement paste and aggregate. The interface in concrete plays a key role on the overall performance of concrete. The interface properties such as deformation, strength, fracture energy, stress intensity and its influence on stiffness and ductility of concrete have been investigated. The effect of composition of cement, surface characteristics of aggregate and type of loading have been studied. The load-deflection response is linear showing that the linear elastic fracture mechanics (LEFM) is applicable to characterize interface. The crack deformation increases with large rough aggregate surfaces. The strength of interface increases with the richness of concrete mix. The interface fracture energy increases as the roughness of the aggregate surface increases. The interface energy under mode II loading increases with the orientation of aggregate surface with the direction of loading. The chemical reaction between smooth aggregate surface and the cement paste seems to improve the interface energy. The ductility of concrete decreases as the surface area of the strong interface increases. The fracture toughness (stress intensity factor) of the interface seems to be very low, compared with hardened cement paste, mortar and concrete.     

Size effects in concrete fracture – Part II: Analysis of test results

This paper presents an analysis of the extensive experimental program aimed at assessing the influence of maximum aggregate size and specimen size on the fracture properties of concrete. Concrete specimens used were prepared with varying aggregate sizes of 4.75, 9.5, 19, 38, and 76mm. Approximately 250 specimens varying in dimension and maximum aggregate size were tested to accomplish the objectives of the study. Every specimen was subjected to the quasi-static cyclic loading at a rate of 0.125mm/min (0.005in./min) leading to a controlled crack growth. The test results were presented in the form of load-crack mouth opening displacement curves, compliance data, surface measured crack length and crack trajectories as well as calculated crack length, critical energy release rate, and fracture toughness (G ₁). There is a well pronounced general trend observed: G ₁ increases with crack length (R-curve behavior). For geometrically similar specimens, where the shape and all dimensionless parameters are the same, the R-curve for the larger specimens is noticeably higher than that for the smaller ones. For a fixed specimen size, G ₁ increases with an increase in the aggregate size (fracture surface roughness). For the same maximum aggregate size specimens, the apparent toughness increases with specimen size. It was clear that the rate of increase in G ₁, with respect to an increase of the dimensionless crack length (the crack length normalized by the specimen width), increases with both specimen size and maximum aggregate size increase. The crack trajectory deviates from the rectilinear path more in the specimens with larger aggregate sizes. Fracture surfaces in concrete with larger aggregate size exhibit higher roughness than that for smaller aggregate sizes. For completely similar specimens, the crack tortuosity is greater for the larger size specimens. The crack path is random, i.e., there are no two identical specimens that exhibit the same fracture path, however, there are distinct and well reproducible statistical features of crack trajectories in similar specimens. Bridging and other forms of crack face interactions that are the most probable causes of high toughness, were more pronounced in the specimens with larger maximum size aggregates.     

Processing and characterization of a lightweight concrete using cenospheres

A study has been conducted in which a lightweight concrete was processed using ceramic microspheres, known as cenospheres, as a primary aggregate. The mechanical properties, including compressive strength, tensile strength, flexural strength and fracture toughness, were tested and cataloged. It was determined that the addition of high volumes of cenospheres significantly lowered the density of concrete but was also responsible for some strength loss. This strength loss was recovered by improving the interfacial strength between the cenospheres and the cement. The interfacial properties were quantified using interfacial fracture mechanics techniques. These techniques were also employed to find a suitable surface modifier with which to improve this interface. The admixture silica fume and the coupling agent Silane were found to be suitable candidates and both performed well in small-scale compression testing. Silica fume was eventually isolated as a prime candidate. The concrete produced with this admixture was tested and compared to a concrete with an equal volume fraction of cenospheres. The addition of silica fume improved the compressive strength of cenosphere concrete by 80%, tensile strength by 35%, flexural strength by 60% and fracture toughness by 41%.     

Rheological behaviour of fresh cement paste as measured by squeeze flow

A method is proposed for measuring the rheology of cement paste under conditions that suppress shear flow, i.e. squeezing. This method is based on squeezing samples in a servohydraulic compression-tension testing machine, and is different from the commonly used shear flow experiments. Possible artefacts such as the buoyancy of the piston that penetrates the paste, sedimentation of cement paste, geometry of the container, and friction at the interface between the top plate (or piston) and sample are investigated. Plots of stress versus apparent strain were obtained and compared with results from standard shear flow experiments. Because cement paste has both viscoelastic and viscoplastic characteristics, results are analysed in terms of both solid-like deformation and liquid-like flow behaviour. A first-approximation theoretical analysis is developed, based on the assumption that cement paste behaves as a non-Newtonian liquid, and results are compared with the experimental results.Nomenclature Shear strain rate in power law fluid model - _zr Shear strain converted from _zr - Shear strain rate - Normal strain rate - _zr Component of shear strain - _zr Component of shear strain rate - _zz Component of normal strain - Viscosity - Density of cement paste (3.2 g cm^–3) - _Cav Calculated average normal stress of cement paste - _Nav Calculated average normal stress of power law fluid - _m Measured normal stress of cement paste - _zz Normal stress in z direction - _eq Equivalent shear stress converted from normal stress - _rz Shear stress in momentum equation - a _i Coefficients in polynomial function of geometric factor for cement paste - B Buoyancy force - CGF Geometric factor for cement paste - d _o Amplitude of squeeze motion - F _N Load in normal direction - g Gravitational constant - h Sample height - h _o Initial sample height - Velocity of platen - k Order of polynomial function of geometric factor for cement paste - m Consistency in power-law fluid model - n Power index in power-law fluid model - P Pressure - P _a Atmospheric pressure - PGF Geometric factor for power-law fluid model - r Radial direction in cylindrical coordinates - R Radius of sample - s 1/n - V Volume of the top platen submerged into cement paste - v _r Velocity inr direction - v _z Velocity in z direction - z Vertical direction in cylindrical coordinates     

The fracture properties of glass polyalkenoate cements as a function of cement age

The fracture properties of two glass polyalkenoate cements based on a short chain-length and on a long chain-length poly (acrylic acid) have been studied as a function of the cement age. The stress intensity factor, K _I, increases with cement age for both cements. The un-notched fracture strength _f increases with cement age, largely as a result of an increase in the Young's modulus accompanying crosslinking of the polyacrylate chains by metallic ions. The toughness G _I remains approximately constant for the short chain-length cement, but reduces with cement age for the long chain-length cement. Analysis of the toughness data using a chain pull-out model leads to the conclusion that chains distant from the fracture plane are involved in fracture, and that the number of chains that take part in chain pull-out decreases as the crack opening displacement reduces with cement age.     

Evaluation of Fracture Parameters by Double-G,Double-K Models and Crack Extension Resistance for High Strength and Ultra High Strength Concrete Beams

This paper presents the advanced analytical methodologies such as Double- G and Double - K models for fracture analysis of concrete specimens made up of high strength concrete (HSC, HSC1) and ultra high strength concrete. Brief details about characterization and experimentation of HSC, HSC1 and UHSC have been provided. Double-G model is based on energy concept and couples the Griffith's brittle fracture theory with the bridging softening property of concrete. The double-K fracture model is based on stress intensity factor approach. Various fracture parameters such as cohesive fracture toughness (K_Ic^c), unstable fracture toughness (K_Ic^un) and initiation fracture toughness (K_Icⁱⁿⁱ) have been evaluated based on linear elastic fracture mechanics and nonlinear fracture mechanics principles. Double-G and double-K method uses the secant compliance at the peak point of measured P-CMOD curves for determining the effective crack length. Bi-linear tension softening model has been employed to account for cohesive stresses ahead of the crack tip. From the studies, it is observed that the fracture parameters obtained by using double - G and double - K models are in good agreement with each other. Crack extension resistance has been estimated by using the fracture parameters obtained through double - K model. It is observed that the values of the crack extension resistance at the critical unstable point are almost equal to the values of the unstable fracture toughness K_Ic^un of the materials. The computed fracture parameters will be useful for crack growth study, remaining life and residual strength evaluation of concrete structural components.     

Scratch test model for the determination of fracture toughness

We revisit the scratch test within the framework of linear elastic fracture mechanics. In the analysis, we employ an Airy stress function approach to determine stresses and displacement in the vicinity of the scratch-blade–material interface, which serve as input for the evaluation of the energy release rate by means of the J-Integral. In contrast to previous models, the energy release rate thus found scales with the sum of the applied forces squared. This entails a linear relation between the applied forces and , where w is the scratch width and d the scratch depth. This analytical scaling is validated using experimental scratch data on cement paste and sandstone, which shows that the proposed approach provides a convenient way to determine the fracture toughness from scratch tests carried out with different scratch widths and depths.     

Effect of coarse aggregate quantity on fracture toughness of concretes

The results of fracture toughness tests performed according to mode II of loading for basalt and limestone concretes of different aggregate contents are presented. The stress intensity factor K _IIC was determined. The fractographical and microscopic studies were performed in an attempt to relate the parameters of concrete structure to its fracture toughness.     

The interfacial zone and bond strength between aggregates and cement pastes incorporating high volumes of fly ash

The weak transition zone between aggregate and cement paste controls many important properties of concrete. A number of studies dealing with interfacial zone are available in the literature for normal concrete and concrete containing silica fume. High-volume fly ash concrete for structural applications was developed at CANMET in the 1980s, but to date there has been no information available for interfacial zone in high-volume fly ash concrete.In this paper, the orientation index and mean size of Ca(OH)₂ crystals in the aggregate-paste interfacial zone were determined by the X-ray diffractometer. The bond strength between the aggregate and paste was also investigated. It was found that, at the age of 28 days, there was no obvious transition zone between the aggregate and cement paste incorporating high volumes of fly ash. The higher the paste strength, the higher is the bond strength.     

Fracture toughness of concretes at high temperature

The fracture toughness of ordinary and refractory concretes in the range of 20–1300C was investigated, and the stress intensity factor, K _Ic, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K _Ic at 20C for both concretes are comparable, being equal to 0.64 MNm^–3/2 for ordinary concrete, and 0.72 MNm^–3/2 for refractory concrete, respectively. At 1100C, K _Ic has a value of 0.043 MNm^–3/2 for ordinary concrete, and for the refractory concrete at 1300C, K _Ic=0.34 MNm^–3/2. The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.     

Mechanical properties and size effects in lightweight mortars containing expanded perlite aggregate

The study presented here investigates the effect of density in cementitious mortar on its mechanical properties under quasi-static loading. The reduction in density was achieved through the addition of expanded perlite as a lightweight aggregate into cement paste by volume replacement of cement in the ratio from 0 to 8. This yielded a range of densities between 1000 and 2000 kg/m³. The compressive and flexural response of these mixes were determined for geometrically scaled specimens to study the size effect. Some mixes were reinforced with polymer microfibres and the Mode I fracture toughness parameters were evaluated through flexural testing of notched beams. When compared with a reference Portland cement paste, the compressive strength and elastic modulus scaled as the cube of the density, while the fracture toughness varied linearly with it. The study shows that the specimen size effect on compressive and flexural strength decreases with a drop in the density of the mix and also with fibre reinforcement. On the other hand, the specimen size effect on the critical crack mouth opening displacement was more pronounced at lower densities.     

Influence of water-cement ratio on micro-cracking of ordinary concrete

The influence of the water/cement ratio on fracture toughness of ordinary concrete has been investigated. The stress intensity factorK _IIc and fracture energyJ _IIc has been tested (Mode II, shearing). The concrete structure was examined by SEM and the influence of water/cement ratio on concrete cracking has been established.     

Strength and fracture properties of asbestos-cement mortar composites

The strength and fracture properties of random asbestos fibre-reinforced cement mortar composites are reported in this paper. The fibre content varies between 5% and 20% by weight. Both the ultimate tensile strength ( _t) and the modulus of rupture ( _b) increase with increasing fibre-volume fraction. These results are shown to agree satisfactorily with the law of mixtures modified for randomly oriented short fibre-reinforced composites. The critical stress intensity factor (K _c) and the specific work of fracture (R) have been determined using three-point bend edge-notched beams and grooved double-cantilever-beam (DCB) specimens. There is generally good agreement between these two physical quantities estimated from the two testpiece geometries. It is shown that the fibre pull-out mechanism is dominant in the fracture of asbestos cements and that the specific work of fracture can be reasonably well predicted by considering the energies absorbed in both the pull-out and the fibre/matrix interfacial debonding processes.     

On the factors affecting strength of portland cement

This paper reports mechanical property measurements for Portland Cement paste free from fabrication artifacts (e.g. bubble-type voids), and compares them to published results both for normal and new high strength cement. Removal of large voids (above 100m) by vacuum de-airing leads to an increase of 15% in mean flexural strength and a small decrease in fracture toughness. This increase in flexural strength is predictable from the tied-crack model previously proposed to explain the notch-sensitivity behaviour of hardened cement paste, and for which direct experimental evidence was obtained. It is suggested that factors such as moisture content are at least as important as large voids in controlling mechanical properties. It is concluded that the much increased strength of the new polymer-containing cements must result from improvements to the microstructure other than the simple elimination of voids.     

The examination of the fracture toughness of concretes with diverse structure

Examination of fracture toughness of concretes was made using Mode I (tension at bending) and Mode II (shearing) fracture. Subjected to examination were gravel and dolomite concretes in their natural states and the same concretes as made from paraffinated aggregates. Gravel and dolomite conretes with diverse water–cement (W–C) ratios were also examined. The values of the stress intensity factors, KIc and KIIc, and those of fracture energy, JIc and JIIc, were determined. In the case of concretes with variable W/C ratios, regression equations were also determined that described the dependence of the stress intensity factors and fracture energies on the W–C ratio. The paraffination of aggregates resulted in a considerable drop in the stress intensity factors studied as compared with those of concretes made from non-paraffinated aggregates. This drop was 34% for gravel as examined according to Mode I fracture, and 27% as examined according to Mode II fracture. For dolomite concrete drops were 19 and 28%, respectively. An increased W–C ratio caused a dramatic drop of both stress intensity factors. By addition of a super-plasticizer to the concrete mixture an evident improvement in the strength properties of both types of concrete occurred. Microstructural examinations performed have clearly confirmed the relationship between the type of aggregate used for concrete making and the microstructure of the concrete, particularly within the area of the contact layer between the aggregate and the cement paste. © 1998 Chapman & Hall     

Strengthening of alloy 8090 and its fracture mechanics parameters

An investigation has been made of the tensile properties, impact-, initial fracture toughness and fracture mode of an aluminium-lithium 8090 alloy at room temperature and 77 K, depending upon the heat treatment and orientation. The peak-aged material exhibited an excellent combination of strength and toughness, equal to or exceeding that shown by the high-strength aluminium alloys of the 2000 and 7000 series. The superior strength and toughness of peak-aged plates, including that of 3% stretched material, compared to underaged material seems to be associated with the lower content of coarse insoluble precipitates, a higher density of S-precipitates in a matrix ligament (grain) which promote ductile fracture. The impact toughness of the peak-aged specimens increased at 77 K only in the L-T plate orientation, while in the T-L orientation it was somewhat lower or remained the same. The toughness increase at 77 K is discussed in terms of the role of the matrix and (sub)grain-boundary precipitates, freezing of low-melting point impurities of sodium and potassium alkaline metals at (sub)grain boundaries and the occurrence of the fine crack divider delamination toughening. The yield strength, R _o.2, increase on ageing was accompanied by a corresponding increase in initial crack divider fracture toughness, K _lc, opposite to the trends obtained for some traditional high-strength aluminium alloys. Changes of K _lc versus R _o.2 depending on orientation are discussed using models for ductile fracture toughness behaviour of aluminium alloys, based on the criterion that a critical width of the heavily strained zone at the crack tip approximates the average ligament width, d _p, i.e. the thickness of the elongated grain in the L-T and T-L plate orientations. It was also found that, for constant chemical composition and fabrication practice of the alloy, a critical plate thickness exists B 0.1 6 t _i, where _i is the initial thickness of the rolling ingot, for which the tensile strength properties in the L-T orientation are the same as that in the T-L orientation, while the plasticity (measured by elongation to failure) of the plate is a maximum. Two types of laminated cracks were observed on fracture surfaces of the specimens: large, >1 mm deep (the number of these cracks remains the same as the number of hot-rolling passes), and fine <0.4 mm (shallow laminated cracks, the number of which significantly increases with decreasing temperature, 77 K).     

Impact of particle packing mix design method on fracture properties of natural and recycled aggregate concrete

The fracture properties of four types of concrete prepared using natural coarse aggregate and recycled coarse aggregate and conventional and particle packing method (PPM) of mix design approaches are studied. The three‐point bending (TPB) test is performed using three different sizes of single edge notched beam. The fracture energy is calculated from the load‐CMOD curve obtained in the TPB test, and in this process the load‐CMOD curve is curtailed at 2% of the depth of the beam. Based on CTOD_c and w₁ relationship, appropriate softening function is used to estimate the double‐K fracture parameters. The fracture energy and fracture toughness parameters of recycled aggregate concrete (RAC) is inferior to the natural aggregate concrete (NAC). The PPM mix design improves the fracture properties of concrete in comparison to the conventional mix design approach. The fracture properties of PPM mix designed RAC are comparable to that of NAC prepared using conventional method.     

Fracture and fracture toughness of stoichiometric MgAl2O4 crystals at room temperature

The fracture toughness and path of stoichiometric spinel (MgAl₂O₄) crystals were determined at 22 °C for key low-index planes by double cantilever beam, as well as fractography of flexure specimens failing from either machining or indentation flaws. These results are compared with other single and polycrystalline MgAl₂O₄ fracture toughness values measured by various techniques, as well as single crystal versus polycrystal results for other materials. Evaluation of experimental and theoretical results shows (1) the fracture toughness of the spinel {110} plane is only a limited amount (e.g. 6%) higher than for the {100} plane (1.2 MPa m^1/2), (2) fractography of machining flaw fracture origins was the most effective source of K _IC results, and (3) caution must be used in applying fracture toughness techniques to single crystals. Cautions include accounting for possible effects of elastic anisotropy (especially for double cantilever beam and probably double torsion tests), the nature of failure-initiating flaws (especially for notch-beam tests), and the frequent lack of symmetric plastic deformation and fracture (especially for indentation techniques).Retired.     

Determination of saturated surface-dry condition of clay–sand mixed soils for soil–cement concrete construction

Saturated surface-dry condition of soil needs to be known when soil–cement concrete mix is rationally designed. Based on the effective water concept, determination of the saturated surface-dry moisture content (hereafter referred to as w _ssd) of clay–sand mixed soil, necessary for determining unit water of soil–cement concrete construction, is dealt with in this study. The saturated surface-dry condition of soil, where soil water exhibits the same chemical potential as that of cement paste, can be determined both with drying rate method and pF method. Mixed soils were prepared in combination with six cohesive soils, standard sand and a recycled fine aggregate. Drying rate method and pF method were proven to be effective in determining w _ssd of any combinations of the soils. Relationship between volume fractions of cohesive soil and sand and w _ssd was found to be a linear equation.     

Application of the Griffith criterion to fracture of boron fibres

Tensile tests were performed on boron fibres and the resulting fracture stresses _f recorded. The fracture surfaces were investigated by SEM, with emphasis on the type of flaw nucleating the fracture. It was found that pre-existing, so-called proximate voids, in the boron mantle near the core-mantle interface, have transverse irregularities, large enough to create stress concentrations. The radial extensions, 2c, of fracture initiating proximate voids in mirror zones were measured from the micrographs. The obtained 2c _f relation was found to be in good agreement with the Griffith criterion for brittle fracture. In addition, the experimental log 2c-log _f plot gives some information on the value of the surface energy,, of boron.