Skills and Expertise
Research Items (32)
The influence of time on the mechanical behavior of concrete after exposure to elevated temperatures has been studied. Twenty-one self-compacting high-performance concrete mixtures with different incorporation amounts of coarse recycled concrete aggregate (RCA) and three unprocessed waste powder materials have been tested at age of 270 days for residual compressive and flexural strength after exposure to fire. The results have been compared to the results for the same concrete, which have been studied at age of 90 days. A new parameter has been introduced for comparing the responses of concrete to elevated temperatures at different ages; this parameter was the “heat resistance” which expresses the total area under the curve of the relative residual strength (compressive or flexural) after exposing to six temperature degrees (20, 150, 300, 500, 600, and 800 °C). The results showed that the age of concrete has an influence on the response of concrete to elevated temperatures. The heat resistance of compressive strength enhanced with age but the concrete behaved with a tendency different to that at the age of 90 days. The heat resistance of flexural strength has not been affected or slightly decreased but not with more than 10% to that at the age of 90 days. The used waste powder materials were unprocessed waste fly ash, waste cellular concrete powder and waste perlite powder; they proved that using any of them up to 15% as a replacement for cement with 0% or 25% of RCA enhanced the concrete resistance for the fire with time. The main two reasons for changing of residual strength with the time were the changing of water content and the proceeding of hydration of the binder. In general, long ages testing properties of concrete simulate the real behavior of concrete structures accurately.
It is a challenging task to take into account the type and form of aggregate in the numerical modelling of concrete. Discrete Element Method (DEM) offers the possibility to consider these properties via its model parameters. However, to get a usable model for further purposes, these model parameters first have to be calibrated precisely, based on laboratory measurement data. The model calibration is a time consuming process that requires high computational resources. In this study, an estimation model is proposed to define the model parameters of a DE model in function of the properties (density, compressive strength, particle size distribution, etc.) of the real material. The proposed model contained a number of factors and constants which were optimized based on our measurements. The measurements were carried out on samples produced from seven different concrete mixes. The mixes differed from each other in the type of the aggregate (quartz gravel, crushed stone, expanded clay) or in their particle size distribution (A or C grading curves, no-fines). The estimation model, with the optimized parameters, was applied on a test data set. The results indicated that the estimation model was able to estimate the DE model parameter properly. By estimating the DE model parameters more precisely at the start of the calibration process, it is possible to save significant amount of time and make the modelling more productive. The accuracy of the estimation model can be further increased by considering the aggregate-to-cement (a/c) ratio of concrete which is difficult to be determined in case of a hardened concrete sample whose concrete mix composition is unknown. An image processing technique is applied in this study to determine the a/c ratio. The results showed that the thresholding method is an appropriate alternative to determine the a/c ratio of various concrete mixes.
- Jan 2019
The sustainability of engineering products has become a basic requirement instead of a mere choice because the harmony between economic activity and the earth’s ecosystem must be seriously considered. The influence of using three unprocessed waste powder materials as cement replacing materials (CRMs) and/or coarse recycled concrete aggregate (RCA) as a partial replacement of coarse natural aggregate (NA) on fresh and mechanical properties of self-compacting high-strength concrete (SCHSC) is investigated in this study. The activation index of the CRMs on the cement paste is tested as an initial step. The CRMs, namely, waste fly ash (WFA), waste perlite powder (WPP) and waste cellular concrete (WCC), are tested through 21 mixtures allocated by seven different series with three mixes of each. The mechanical properties of the 21 concrete mixes are determined after one, three and nine months of curing. Results of compressive strength, splitting tensile strength, flexural strength and modulus of elasticity are presented. This work shows that the mechanical and environmental performance of SCHSC can be improved by the replacement of NA by RCA of up to 50% and the replacement of cement by WPP or WFA of up to 15%. Using WCC is not recommended to be reached 15% and using WFA is preferable to be with incorporating RCA rather than NA alone. Findings indicate that incorporating waste materials can be valuable in SCHSC, thereby potentially leading to an increasingly green environment and paving the way for advancements in sustainable construction.
Advances in recycling can exceed traditional practice in that a non-renewable resource can at least become a partially renewable resource. This research attempts to support the cradle-to-cradle concept by testing the effect of reused recycled concrete aggregates (RRCA) on the fresh, physical, mechanical, and microstructure properties of self-compacting high-strength concrete (SCHSC). Two generations of recycled aggregate concrete (RAC) are produced; the first and second are created by recycled concrete aggregate (RCA) and by RRCA, respectively. For each generation, two replacement amounts are used (25% and 50% by mass). Results show that using RRCA does not affect the fresh properties of SCHSC, and up to 50% RRCA replacement amount by mass can be adopted for enhancing the mechanical and microstructure properties of reused recycled aggregate concrete (RRAC). Computed tomography indicates that the porosity of RRCA is less than that of RCA.
Discrete Element modelling of concrete requires the precise calibration of model parameters that is a long lasting and computationally expensive task in case of complex models. Present study introduces a method for estimating a model parameter (normal strength of parallel bonds) for concretes with different particle size distribution and aggregate type. The parameter estimation leads to an optimization problem based on physical measurement data. In the present paper, a model and its parameters are proposed to estimate normal strength of parallel bonds based on the density and compressive strength of concrete.
The aim of our research project was to investigate the effect of different construction waste materials on the strength and durability of normal concrete. Construction waste materials were applied as supplementary material. 10% by mass (relative to the cement amount) of cellular concrete powder or powdered clay brick were used in the mixes, which were compared with a reference mix (without any supplementary material) and with a mix that contained air-entraining agent. The application of air-entraining agent in concrete is frequently recommended to increase the durability of concrete. Standard cubes with 100 mm of edge length were casted, and after 28 days a compressive strength test was done on three cubes from every mix. The durability of the concrete was investigated with two different methods. First the cubes were subjected to 0, 50, 100 and 150 cycles of freezing then the weight and strength loss of the samples were measured. Besides that, the freeze-thaw scaling resistance of the specimen was investigated by applying 7, 14, 21 up to 56 cycles of freeze-thaw scaling. All the applied methods were carried out in compliance with the recommendation of the CEN/TS 12390- 9:2007 standard.
Present research program is dealing with the effects of cellular concrete powder (CCP) on the compressive strength and durability of concrete. CCP is an industrial by-product, which was applied as supplementary cementitious material (SCM) in the concrete mix. In the previous research of the authors of this article, it was found that the cellular concrete powder, applied as an additive to the concrete mix, can increase the compressive strength and the resistance against freezing of concrete (Gyurkó, Szijártó, Nemes, 2017). In that paper, the results were compared to a reference mix and a mix that contains air-entraining agent, which is a typical solution to increase concrete durability. Increasing of durability can be done by the application of supplementary materials, like silica-fume or metakaolin. Present paper is dealing with the comparison of concrete with metakaolin and cellular concrete powder, on the basis of compressive strength and freeze resistance, using them as a cement substituting material. Different amount of metakaolin and CCP were added to the concrete mix, by substituting a given percentage (3, 10 or 17%) of cement with them. The results show the positive effect of both supplementary materials and based on present research the maximum amount of useful CCP can be approximated for the given concrete mix.
Az utóbbi évtizedekben számos épületkatasztrófa igazolta, hogy a tűzteherre való méretezés kérdései továbbra is aktuálisak. A különböző hőmérsékleti tartományokban a betonban lejátszódó legfontosabb fizikai és kémiai folyamatok jelentősen befolyásolják a beton magas hőmérséklet hatására kialakuló anyagtulajdonságait. A beton tűzterhelés hatására bekövetkező tönkremenetele alapvetően két okra vezethető vissza a beton alkotóelemeinek kémiai átalakulására, illetve a betonfelület réteges leválására. A beton szilárdsági tulajdonságainak változása magas hőmérsékleten a következő paraméterektől függ: a cement típusától, az adalékanyag típusától, a víz-cement tényezőtől, az adalékanyag-cement tényezőtől, a hőterhelés módjától és a lehűtés módjától. Magas hőmérséklet hatására a beton szerkezete megváltozik, jelen cikkben ezen változásokat ismertetjük
- Feb 2015
Durability is one of most important requirements of concrete design. From this aspect is arising the required minimal compressive strength class of conventional concrete in order to ensure durability. The porosity of cement mortar matrix and consequently the liquid and gas permeability of concrete may be reduced. But the strength of lightweight aggregate concrete depends primarily on the crushing resistance of lightweight aggregate (LWA). With low strength LWA may be achieved higher concrete strength by high strength cement mortar matrix. Therefore we can not consider durability simply on the basis of the compressive strength of concrete. The most important durability factors are closely related to the porosity and strength of the cement mortar matrix but for example the abrasion resistance depends more significantly on the aggregate type. Lightweight aggregates usually do not have high abrasion resistance, but they can be advantageous in case of restoration of bridge pavement. The most important factors of durability regarding pavements are frost scaling and abrasion resistance. These parameters (especially abrasion resistance) are not available in the literature when using LWA.
- Sep 2014
Perfect bond between concrete and reinforcement, which is described by the bond-slip curve, is the basic assumption for reinforced concrete structures. When using special or new materials, detailed, experimental verification is necessary to thoroughly characterize the bond-slip behaviour. The fib Model Code 1990 (MC1990) and 2010 (MC2010) as well as the Eurocode 2 (EC2) include in detail the bond properties of reinforcing steel bars of smooth and ribbed surface, however, no information is given for prestressing strands. Strands consisting of seven prestressing wires of smooth surface, which are arranged in twisted position around the longitudinal axis of the strand, are widely used in various fields of building industry. The goal of this research was to experimentally describe the bond properties of the above prestressing strands embedded in two types of lightweight aggregate concrete with expanded clay pebbles as lightweight aggregate (= 63.4 MPa, 1800 kg/m and = 86.5 MPa,1950 kg/m) as well as in usual high-strength etalon concrete (= 85.2 MPa) made of normal-weight aggregate. Strength of the applied concretes were tested by standardized tests; the bond properties of strands were tested by pull-out tests. The test results were compared with the relevant expressions and recommendations of the MCs and the EC2. The primary goal of this paper was to experimentally describe the bond-slip behaviour of prestressing strands of seven wires with pull-out tests.
For normal weight concrete we have conversion factors, when testing not at the age of 28 days or the curing of specimens is not according to the standard. Lightweight aggregate concrete may be very different from normal weight concrete from this point of view. The properties mainly depend on the properties of the lightweight aggregate like particle density, water absorption capacity. The aim is to study the impact of the time under water storage on compressive strength and fracture tests results.
Expansion and bonded anchors respond different ways in case of the special compound or the increase of temperature. Bond strength of an anchor is influenced not only by the strength of concrete, but also by its composition. The behaviour of expansion and bonded anchors is different in normal weight concrete (NWC) and lightweight aggregate concrete (LWAC). Five different concrete mixtures were studied. The compressive strength of NWC and one of LWAC were the same. And the composition of LWAC was changed. In our experimental study torque controlled expansion anchors as well as bonded anchors (vinyl ester or vinyl ester with cement adhesive) were tested. In case of temperature loading the anchors were installed at room temperature in concrete blocks, than were previous heated up to 150°C or 300°C. Reference tests were also carried out on specimens stored continuously at room temperature. Our experimentally study was carried out with two different concrete strengths.
- Oct 2006
Until recently, it was not possible to make structural concrete using expanded glass aggregate. This was because of the relatively low strength of the expanded glass aggregate, and also because the lightweight aggregates had a high water absorption that can make concrete placing difficult. Expanded glass aggregate with relatively high crushing resistance and low water absorption is now available from a manufacturing plant in Hungary. The new aggregate is similar to conventional lightweight aggregates, but the ratio of density of concrete to compressive strength is much more favorable to making structural concrete. This paper summarizes the results of experiments on the mechanical properties of concrete made of this new expanded glass aggregate.