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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.

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.

Bond of FRP reinforcement is expected to be more sensitive to temperature and rate of loading than conventional steel reinforcement. Present paper introduces experimental results on Carbon Fiber Reinforced Polymer (CFRP) wires of sand coated surface tested by pull-out tests in a multi-parameter laboratory test programme. In the experimental programme low, normal and high strength concretes, −25 °C, +20 °C and +65 °C testing temperatures, and static to impact loads were applied for the pull-out tests (with an order of 1000 in the increase of the rate of loading). Results indicate that the bond strength of the tested CFRP wires follows the change of temperature in a more pronounced manner than the concrete compressive strength does. A viscous-elastic, load rate dependent behavior can be realized in the bond response of the CFRP wires studied. Failure of bond is different for different concrete strengths, testing temperatures and loading rates. Results also indicate that the mathematical functions that are used for the modeling of bond of conventional steel reinforcements are not optimal for the CFRP wires studied.

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.

Durability of steel fibre reinforced concrete (SFRC) specimens is tested and evaluated. Concrete is mixed with moderately sulphate resistant CEM I 42.5 cement and does not contain airentraining agent. The aim of the research is to study how the dosage of high bond crimped steel fibre influences the durability damage kinetics of SFRC; freeze-thaw resistance, de-icing salt scaling resistance and resistance to water penetration are studied, completed with basic mechanical performance tests (compressive strength, modulus of elasticity, flexural tensile strength, splitting tensile strength, flexural toughness, fracture toughness, apparent porosity and vacuum water absorption). Results reveal the importance of the increased volume of the interface transition zone (ITZ) around the steel fibres in the first freeze-thaw cycles and the importance of the internal restrain activated by the steel fibres during later freeze-thaw cycles.

Strength assessment of concrete in existing structures is a key issue. Many non destructive techniques (NDT) are available which can provide information about the material condition and several approaches can be used to derive strength estimates from NDT test results. However, common practice suffers from many drawbacks: (a) it is difficult to ascertain the level of accuracy/confidence of concrete strength estimates, (b) one lacks established guidelines for estimating the concrete strength variability, (c) the best way to combine NDT methods and coring remains an open issue. The RILEM TC 249 “Non destructive in situ strength assessment of concrete” is addressing these problems. A benchmark was carried out in order to compare (a) how experts define and can carry out an NDT investigation program and (b) how experts derive strength values from the NDT measurements. The benchmark was based on synthetic simulations which reproduced a synthetic data set corresponding to a grid of twenty 3 m-high columns defining a single-storey building structure. The comparison of the various strategies and the analysis of results enabled identification of the most influential parameters that define an investigation approach and influence its efficiency and accuracy. A special emphasis was placed on the magnitude of the measurement error. A model of the investigation strategy is proposed, that will be detailed in future guidelines. The reader is invited to refer to a companion paper (Alwash et al. in Mater Struct, 1. doi: 10.1617/s11527-016-0962-x ), based on more extensive Monte-Carlo simulations of the same case-study, which analyzes and quantifies the efficiency of the investigation strategies with respect to both average strength and strength variability assessment, in order to draw conclusions based on rational analyses rather than on the element of chance.

Non-destructive testing methods for concrete structures require statistical validation of reliability model parameters. Present paper studies a comprehensive database of several thousand test locations for the better understanding of the statistical characteristics of rebound hardness of concrete and highlights that several gaps are found both in current technical literature and standardisation. Detailed evaluation is provided on repeatability and influencing parameters. Results of statistical analyses are presented for the within-test standard deviation, coefficient of variation, range and studentized range of rebound indices. Indications for the reconsideration of particular statements in the technical literature and standards are given.

The combined influence of service temperature and loading rate on the compressive strength of concrete has been studied over a wide range of compressive strength. New simplified prediction formulae are proposed to take the influence of temperature and loading rate into account. It is demonstrated that the new formulae are suitable to effectively predict the compressive strength of concrete under the combined influence of temperature and loading rate. It is also demonstrated that the Dynamic Increase Factor, DIF is not a practical tool in the representation of the combined influence of service temperature and loading rate on the compressive strength of concrete.

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.

Ecological and environmental benefits support the use of waste glass powder (WGP) as supplementary cementing material by the decrease of the amount of landfills, by the reduction of non-renewable natural resource consumption, by the reduction of energy demand for cement production (less cement is needed), and the reduction of greenhouse gas emission. Laboratory tests were carried out on cement paste specimens, in which waste glass powder (WGP) addition was used as a supplementary cementitious material. Cement was substituted with WPG at levels of 20% or 30% per mass. It was demonstrated that the WGP addition improves the workability of fresh pastes, and can be effectively used as cement replacement for compressive strength. It was also demonstrated that the particle size of the WGPs (specific surface area) has a stronger influence on the effectiveness of the cement replacement than the chemical composition. The effectiveness of the cement replacement increases as the specific surface area increases.

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.

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 50 years 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.

Vasbeton szerkezetek teherviselese akkor lehetseges, ha van tapadas a beton es a bebetonozott acelbetetek kozott. A tapadast kozel szaz eve vizsgaljak acelbeteteken, es a jelenseg kellő reszletesseggel ismert. A befolyasolo tenyezők hatasat egyenkent, illetve egyes tenyezők kombinalt hatasat egyuttesen a szakirodalom tobbnyire targyalja. Szamos mechanikai modell is letezik a beton es betonacel tapadasanak leirasara. Hianykent jelolhetők meg a hőmersekleti hatasokkal kombinalt, kulonfele terhelesi modok mellett elvegzett es kulonfele terhelesi sebessegeket alkalmazo vizsgalatok. Mivel az acelbetetek korrozioja betonban is bekovetkezhet, ezert evtizedek ota folynak kutatasok olyan iranyokban is, hogy mely anyagokkal lehetne a hagyomanyos acelbeteteket es feszitőbeteteket kivaltani, ha tartossagi szempontok ezt indokoljak. Az epitőmernoki gyakorlatban az 1990-es evek ota vannak jelen a szalerősitesű polimer (FRP) betetek erre a celra. A szalerősitesű polimer (FRP) betetek az elektrolitikus korrozionak teljes...

The present paper provides an overview of serviceability specifications given by the fib Model Code 2010 (MC2010 [1]). First the reasons of crack control and deflection control are discussed, then specific design rules are provided. Simple rules as well as detailed models are also presented. Numerical examples are also provided in order to help application of the design recommendations for crack control (reinforced and prestressed concrete elements) and for deflection control (reinforced and prestressed concrete elements), respectively. Simple rules mean indirect control of cracking or deflections without calculations. Indirect crack control may include limitation of stresses and selection of maximum bar diameter or maximum bar spacing. Indirect deflection control normally means limitation of the span to depth ratio. Detailed models are based on physical and mathematical approaches of cracking and deflections. The design crack width is expressed as the multiple of maximum bond transfer length multiplied by the mean strain between cracks. Deflection analysis can be provided by integration of curvatures or using a simplified or a refined method. Vibrations and numerical modelling of cracking are also briefly discussed. Crack width limits, tension stiffening, span-depth ratio

Crack formation presents a complex mechanical and geometrical question to be modelled. The available crack width formulations are often based on simplifications. A rigorous formulation of crack widths should be based on the integration of strain differences of reinforcement and concrete between cracks, due to the accumulated slips. In this paper an extensive literature review on crack widths and crack spacing is presented. The basic intention of the present paper is to summarise the development of flexural crack models and collect the most relevant formulae for crack spacing and crack width. It reports not only the possible improvement of design or research equations but also the appearance of new types of reinforcements with different characteristics from those of steel reinforcements. This state-of-the-art Report is a contribution to the work of fib TG 4.1 ‘Serviceability Models’.

In the present work, soda-lime glass cullet (flint, amber, green) and special glass cullet (soda-alkaline earth-silicate glass coming from low pressure mercury-discharge lamp cullet and incandescent light bulb borosilicate glass waste cullet) were ground into fine powders in a laboratory planetary ball mill for 30 minutes. CEM I 42.5N Portland cement was applied in mortar mixtures, substituted with waste glass powder at levels of 20% and 30%. Characterisation and testing of waste glass powders included fineness by laser diffraction particle size analysis, specific surface area by nitrogen adsorption technique, particle density by pycnometry and chemical analysis by X-ray fluorescence spectrophotometry. Compressive strength, early age shrinkage cracking and drying shrinkage tests, heat of hydration of mortars, temperature of hydration, X-ray diffraction analysis and volume stability tests were performed to observe the influence of waste glass powder substitution for Portland cement on physical and engineering properties of mortar mixtures.