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A comparative spatial analysis of surface hardness of structural concrete is introduced. Main objective of the paper is to make a repeatability comparison of three types of the still most popular non-destructive testing devices for concrete: L-type original Schmidt rebound hammer, N-type original Schmidt rebound hammer and N-type Silver Schmidt rebound hammer. Results indicate that the surface hardness measurement uncertainty is related to the weight of the hammer mass and is apparently not related to the impact energy of the rebound hammer devices. It is observed that the measure of surface hardness for the Silver Schmidt rebound hammer (Q-value) does not have positive correlation to the original rebound index (R). Results indicate the best performance of the N-type original Schmidt rebound hammer in terms of stability and normality of data. Geostatistical analysis of the measured data (in terms of empirical semivariograms) highlights different statistical behaviour for the mechanical recording rebound hammers and for the electro-optical recording rebound hammer.

Surface hardness testing of materials can be considered as the oldest method to get information about strength related material properties. In recent decades the rebound hammer has been the most popular surface hardness testing device for concrete uniting the advantages of its predecessors. In the technical literature numerous proposals are available for simple, two-parameter regression analyses of rebound surface hardness vs. compressive strength relationship of concrete. The remarkable diversity of the proposed curves implies the need of the more than two-parameter regression techniques to reveal the most pronounced parameters governing hardness behaviour. The objectives of present experimental studies were to carry out dynamic and static hardness tests, Young’s modulus and compressive strength tests on concrete specimens. From the development of the tested properties with time it can be concluded that the rebound hammers provide a hardness value for high strength concretes connected to the Young’s modulus rather than the compressive strength. Present paper includes a parametric simulation and a parameter fitting of the verified phenomenological constitutive model of the authors which recognizes the w/c ratio as the main driver of the interrelated material properties and gives a realistic formulation for the time dependent behaviour of the rebound surface hardness of concrete.

Supplementary cementing materials (SCMs) contribute to the hydration of Portland cement by physical phenomena (e.g. nucleation effect) or by chemical reactions (e.g. pozzolanic activity). The partial substitution of Portland cement with SCMs can significantly reduce the CO 2 emission during the production of concrete and, therefore, can make concrete a more sustainable and environmental-friendly material. Durability of concrete is considerably improved by the incorporation of SCMs. Due to the pozzolanic activity and the filling effect, use of SCMs can result high performance concrete having both enhanced mechanical characteristics and reduced permeability that leads to improved durability. Combined use of SCMs of ternary composition of SiO 2 -Al 2 O 3 -CaO (e.g. blastfurnace slag and fly ash) can lead to extraordinarily favourable durability performance of concretes at even large cement substitution ratios, however, very few experimental data exist on the combined use of silica dominated and alumino-silicate dominated SCMs (e.g. silica fume combined with metakaolin) at increased cement substitution ratios. Most important aim of the present research was to reveal if there is any advantage of the combined use of silica fume and metakaolin. [ABSTRACT FROM AUTHOR]

Present paper studies metakaolin as a supplementary cementing material (SCM ) for concrete in terms of compressive strength development and k-value concept of CEN /TR 16639:2014. Sixteen different concrete compositions are studied in the range of water-to-cement ratio of w/c = 0.50-0.65. Results indicate that the effectiveness of metakaolin depends on the applied w/c-ratio and the applied cement substitution ratio; the smaller the w/c-ratio, the more effective the metakaolin; the higher the cement substitution ratio, the less effective the metakaolin. The k-value decreases by the increase of w/c-ratio, independently of the age of concrete. The higher the cement substitution ratio, the smaller the k-value. The k-value tends to reach a final value in time. [ABSTRACT FROM AUTHOR]

Reinforced concrete is a widely used construction material. However, corrosion of steel reinforcement might be a danger to its durability. In this paper the corrosion mechanism of steel reinforcement in concrete is introduced. Literature and case studies are reviewed on concrete crack width and its effect on reinforcement corrosion. The results found in different sources proved to be inconsistent. Crack width variation within concrete cover is presented as a reason. The influencing factors of reinforcement corrosion in concrete are collected with special regard to crack geometry. [ABSTRACT FROM AUTHOR]

A comparative spatial analysis of surface hardness of structural concrete is introduced. Main objective of the paper is to make a repeatability comparison of three types of the still most popular non-destructive testing devices for concrete:L-type originalSchmidt rebound hammer,N-type originalSchmidt rebound hammer andN-type SilverSchmidt rebound hammer. Results indicate that the surface hardness measurement uncertainty is related to the weight of the hammer mass and is apparently not related to the impact energy of the rebound hammer devices. It is observed that the measure of surface hardness for the Silver Schmidt rebound hammer (Q-value) does not have positive correlation to the original rebound index (R). Results indicate the best performance of the N-type original Schmidt rebound hammer in terms of stability and normality of data. Geostatistical analysis of the measured data (in terms of empirical semivariograms) highlights different statistical behaviour for the mechanical recording rebound hammers and for the electro-optical recording rebound hammer. [ABSTRACT FROM AUTHOR]

Surface hardness testing of materials can be considered as the oldest method to get information about strength related material properties. In recent decades the rebound hammer has been the most popular surface hardness testing device for concrete uniting the advantages of its predecessors. In the technical literature numerous proposals are available for simple, two-parameter regression analyses of rebound surface hardness vs. compressive strength relationship of concrete. The remarkable diversity of the proposed curves implies the need of the more than two-parameter regression techniques to reveal the most pronounced parameters governing hardness behaviour. The objectives of present experimental studies were to carry out dynamic and static hardness tests, Young's modulus and compressive strength tests on concrete specimens. From the development of the tested properties with time it can be concluded that the rebound hammers provide a hardness value for high strength concretes connected to the Young's modulus rather than the compressive strength. Present paper includes a parametric simulation and a parameter fitting of the verified phenomenological constitutive model of the authors which recognizes the w/c ratio as the main driver of the interrelated material properties and gives a realistic formulation for the time dependent behaviour of the rebound surface hardness of concrete. [ABSTRACT FROM PUBLISHER]

Preliminary results of a two years long research and development project is introduced in this paper. One novel type of 2D deformed steel fibre (with the product name of PetiX) and four different types of 3D deformed steel fibres (PetiX V45; PetiX V90; PetiX S45; PetiX S60) were designed, manufactured and tested under laboratory conditions. The laboratory tests showed promising results. Superior performance of certain types of 3D deformed steel fibres was observed. Considerable improvement of the bond performance of steel fibres was realized in concrete and increased flexural toughness of SFRC prismatic specimens tested in flexure was demonstrated. [ABSTRACT FROM AUTHOR]

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Statistical analysis has been performed on rebound index data collected in-situ at the bottom surface of a concrete floor over 130 m2 of testing area. Different measures of inherent variability were calculated. Correlations between inherent variability measures were studied. It has been shown that skewed distributions can be fit with the best goodness of fit for the vast majority of the local statistical parameters studied. Application of the Peirce's criterion filtering was demonstrated. Inferior local normality of the distribution of rebound index was found for the insitu rebound hardness test results. [ABSTRACT FROM AUTHOR]

Highlights: [•] Repeatability of rebound hammer test depends on type of cement, w/c-ratio and age. [•] Repeatability parameter distributions have positive skewness for rebound hammer test. [•] Rebound index coefficient of variation cannot estimate that of compressive strength. [•] Repeatability and precision claims (ACI 228.1R-03; ASTM C 805) need reconsideration. [•] Basic curve and assessment method of EN 13791 need reconsideration for rebound test. [ABSTRACT FROM AUTHOR]

Variability of the in-situ rebound hammer tests is composed of the inherent variability of the measuring method and the spatial variability of the concrete performance properties. In-situ, Schmidt rebound hammer tests were performed at the bottom surface of a concrete slab, with dimensions of 25.0 m × 7.5 m. The studied region on was 22.0 m × 6.0 m. The analysis of the inherent variability of the rebound hammer tests was limited to the comparison of the statistical location parameters; mean, median and mode values. The spatial variability was analysed by contour plots and semivariograms of the statistical location parameters. It was demonstrated that the geostatistical methods may be applicable for the much smaller scale concrete structures than the scale of geological formations. Semivariograms and semivariogram models seem to be useful tools for the characterisation of spatial variability of concrete structures. [ABSTRACT FROM AUTHOR]

The rebound hammer test of concrete can provide an alternative to drilled core tests for estimating the in-situ compressive strength of concrete. The variability of concrete strength is recognised during the strength assessment and the design strength of concrete is specified as its characteristic compressive strength by taking the variability of the strength of concrete into account. In the present paper, particular topics of the inherent variability of the rebound hammer tests are discussed. Different types of Schmidt hammers are compared in this sense. The precision of the original N-type Schmidt hammer is demonstrated to be superior to original L-type or Silver Schmidt N-type hammers. Observations are confirmed by normality tests. Results imply the need for further research. Measurement errors made by the operators of the rebound hammers are analysed. Differences among the repeatability and reproducibility are demonstrated. As a closing remark the need of revealing the connection between the variability of compressive strength and rebound index, as well as the need of the spatial variability analysis of rebound index are highlighted [ABSTRACT FROM AUTHOR]

Development and application of Fibre Reinforced Polymer (FRP) reinforcements in concrete structures have been accelerated in the last three decades. Durability of FRP reinforcements is expected to exceed that of conventional steel reinforcements as FRP reinforcements do not tend to corrode. Present paper intends to review experiences on characteristics of fiber reinforced polymers (FRP) for civil engineering applications and to discuss design considerations by applying FRP bars or tendons embedded in concrete as prestressed or non-prestressed reinforcement. Test results on FRP indicate high tensile strength, high fatigue strength as well as low relaxation and creep. Design recommendations are based on work carried out more than fifteen years ago on first generation reinforcing products. Design guidelines are necessary to enable the use of new materials, they should be made flexible enough to accommodate future developments and facilitate the quick adoption of innovative products. Present paper, therefore, introduces the most recent European research action (COST TU1207) that aims to develop a new generation of design guidelines for FRP materials based on European Standards. [ABSTRACT FROM AUTHOR]

Abstract: Surface hardness testing of concrete is a long established NDT method for in situ strength estimation. Nowadays, the rebound hammer is the surface hardness testing device for concrete of the most widespread use. Based on a comprehensive literature review it was realized that no general theory was developed in the last more than 50years that could describe the relationship between surface hardness and compressive strength of concrete. The diversity of the numerous empirical proposals that can be found in the technical literature needs to be explained. It can be even found in some publications that the method is suitable only for assessing the uniformity of concrete. There is long a time need for a model that can clarify the rebound surface hardness of concrete. Present paper introduces a phenomenological constitutive model (SBZ-model) that can be formulated for the surface hardness of concrete as a time dependent material property. The generating functions of the model are based on the time dependent development of the capillary pore system of the hardened cement paste in concretes that is characterised by the water–cement ratio as a practical simplification. The modelling assumptions and the use of the model would add to the fundamental understanding of the rebound surface hardness of concrete. An extensive experimental verification study clearly demonstrated the reasonable application possibilities of the SBZ-model. [Copyright &y& Elsevier]

The article presents information on the 3rd International Symposium on Bond in Concrete held on November 20-22, 2002 in Budapest, Hungary. The Symposium was organised by the Hungaran Group of fib at the Budapest University of Technology and Economics, officially sponsored by fib and supported also by American Concrete Institute and Japan Concrete Institute. The principal aim of the 3rd Symposium on Bond in Concrete was to bring together researchers and practitioners from across the whole area of concrete applications. The conference has reviewed the advances in understanding and application of all aspects of bond in concrete achieved over the last decade.

To assess concrete strength in a structure, nondestructive technique (NDT) like rebound hammer is combined with destructive technique (coring tests) in order to implement a relationship ‘‘conversion model” between the compressive strength and, NDT measurements. The conversion model is used to estimate the local strength value at each test location using the corresponding NDT value. Then the estimated mean strength and/or estimated strength standard deviation (concrete strength variability) values are calculated. However, the reliability of these estimated values is always a questionable issue because of the uncertainties associated with the strength predictions based upon NDT measurements. To improve the reliability, the uncertainties must be reduced by specifying and controlling their influencing factors. The objective of this paper is to study the reliability of assessment by analyzing the effects of several influencing factors: number of test locations used to identify a conversion model between strength and rebound measurement NC (number of cores), true value of concrete strength variability, within-test variability of rebound measurements, accepted uncertainty level, quantity to be assessed (mean strength, strength variability), model identification approach (like regression) and the way of selection core locations (random or conditional i.e. selection based on NDT measurements from preliminary investigation). To this end, a large campaign of laboratory studies datasets (1700 test results) was considered for the analysis in the present study. Results show that NC , within-test variability of rebound measurements and true concrete strength variability have significant effects on the assessment reliability. Conditional selection of cores has also an important effect on improving the reliability so it is strongly recommended. [ABSTRACT FROM AUTHOR]