Key Engineering Materials Vol. 677

Abstract: In the article the possibility of utilization of two waste materials: Recycled Concrete Aggregate (RCA) fraction 4-16 mm and Class F fly ash (from coal burning power plant) in high perfor-mance concrete (HPC) was presented. Concrete with RCA were made with varying amount of cement and Suplementary Cementing Materials (SCM). The specimens of concrete were tested to compare mechanical properties as well as some properties related to the durability of concrete. Compression strength values up to 71.40 MPa were achieved and good values of properties determinig durability of reinforced concrete structures.

Abstract: This article deals with concrete containing waste material originating in a coir fibres extraction from outer protective peel of a coconut. Coir pith is used as cement replacement in amount of 5% and 10%. The influence of moisture content on the thermal properties of studied concrete is determined by means of experimental measurements of basic physical properties, hygric transport parameters and thermal characteristics. From achieved results it can be concluded, that the higher amount of coir pith is used, the better values of thermal characteristics material show. But at the same time thermal characteristics are more influenced by moisture content.

Abstract: The article analysed the results of comparative sampling of selected properties of concretes with addition of aggregates from sanitary ceramic wastes. For the need of the analyses there were created concretes of various w/c coefficients (0,35, 0,50, 0,75) and with diverse percentage participation of ceramic wastes aggregates (exchange of 50% and 100% coarse aggregates into ceramic). The range of performed samples and analyses included sampling of mechanical and physical properties of concretes and the aggregates used. What was under analysis: for aggregates – sampling of bulk density, absorbency, resistance to crushing; for concretes from ceramic wastes – sampling of density, absorbency, compressive strength, tensile strength and compressive strength after roasting. Roasting was performed according to defined temperature cycles: cycle I - from 0 to 150^OC (vaporisation of free water), cycle II - from 150 to 550^OC (separation of chemically combined water), cycle III - beyond 550^OC (change of hydraulic combination into ceramic, sintering).

Abstract: Self-consolidating concrete (SCC) has recently drawn attention to the construction industry in hot weather countries, due to its high fresh and mechanical properties. The slump flow is routinely used for quality control of SCC. Experiments were conducted by the current authors to investigate the effects of hot weather conditions on the slump flow of SCC. Self-consolidating concrete mixtures were prepared with different dosages of fly ash and superplasticizer and under different ambient temperatures. The results showed that the slump flow of SCC is sensitive to changes in ambient temperature, fly ash dosage, and superplasticizer dosage. In this paper, several artificial neural networks (ANNs) were employed to predict the slump flow of self-consolidating concrete under hot weather. Some of the data used to construct the ANNs models in this paper were collected from the experimental study conducted by the current authors, and other data were gathered from literature. Various parameters including ambient temperature and mixing time were used as inputs during the construction of ANN models. The developed ANN models employed two neural networks: the Feed-Forward Back Propagation (FFBP) and the Cascade Forward Back Propagation (CFBP). Both FFBP and CFBP showed good predictability to the slump flow of SCC mixtures. However, the FFBP network showed a slight better performance than CFBP, where it better predicted the slump flow of SCC than the CFBP network under hot weather. The results in this paper indicate that the ANNs can be employed to help the concrete industry in hot weather to predict the quality of fresh self-consolidating concrete mixes without the need to go through long trial and error testing program.Keywords: Self-consolidating concrete; Neural networks; Hot weather, Feed-forward back-propagation, Cascade-forward back propagation.

Abstract: The use of coarse recycled aggregate in concrete is already accepted in the Czech Republic but with specific restrictions. The problem is the use of fine recycled aggregate (FRA) in concrete. This paper is focused on behavior of concrete containing FRA. FRA, which originated from recycling plant in the Czech Republic, was used in an experimental part. FRA was obtained from demolished and crushed concrete structures. Four concrete mixtures were designed and prepared. The first concrete mixture was reference which did not include FRA. Natural sand was replaced by FRA in other concrete mixtures in varying ratio. There were tested physical, mechanical and deformation properties of concrete. According to test results it is possible to say that the use FRA as partial replacement of natural sand in concrete mixtures.

Abstract: The problem of recycling of construction wastes is important and at the same time difficult to deal with. One of the possible ways of using the construction wastes coming from the demolition of concrete constructions is to re-use them in the production of construction concretes as recycled concrete aggregates RCA. Determining the concrete composition with the use of RCA demands conditioning its different from the natural aggregates NA physical and mechanical properties. In the procedure of projecting the concrete composition with three equations theory the assumption of consistency class of concrete mixture is demanded. Having accepted it, the water demand of aggregates and cement is determined. In case of natural aggregates NA the formulas of Sterne's and Bolomey's are used in which aggregates water demand is conditioned from its kind and granulation and also from concrete mixture consistency. In case of RCA, there is lack of such data and each time it is necessary to determine the water demand empirically after performing a trial batch. There also exists a necessity to determine the relation between RCA water demand and its other properties which are easy to be determined in laboratory conditions and in short time. Such a property can be measured with the crushing rate w_rm resistance of aggregates to crushing. Crushing rate w_rm was used to qualify the recycled aggregates from recycling with the point of their potential of being re-used in constructive concrete production. It was determined a relation between crushing rate w_rm and the coefficient A_RCA taking place in the modified strength equation of Bolomey and thus it became possible to use the method of three equations to project the concrete composition on recycled aggregates.

Abstract: For the last 15-20 years in Europe and in the USA the new technologies allowing to reduce temperature of hashing and consolidation of the asphalt concrete mix prepared on viscous bitumen by 20-40 °C were developed. Thus characteristics of a covering in comparison with use of traditional hot asphalt concrete don't worsen.One of ways of achievement of effect is application of special chemical additives.Results of researches of influence of the additives developed by authors together with the enterprise of "Selena" for production of warm asphalt concrete of the series «RAA WAC» on properties of bitumen and asphalt concrete in comparison with a known additive of Evotherm J1 are given in the real work.It is established that the studied additives at increase of their concentration in knitting a little reduce conditional viscosity of bitumen, raise tensile properties at a temperature of 25 °C, practically don't influence softening temperature.

Abstract: High traffic load and temperature are the two critical issues that affect the quality of asphalt pavement. Due to these issues asphalt skeleton is used, spilled with cement paste. Rising asphalt and cement concrete, which due to the high strength and high elasticity can withstand high static loads and temperature changes. However, an important factor for sporadic use and incidence are composite's financial demands. The most expensive item is utilization of cement. Therefore, financially demanding cement can be replaced by secondary raw material. Thanks to composition and prerequisite of additional hydration recycled concrete was chosen in the form of fine ground powder.

Abstract: This paper deals with behavior of timber-concrete composite structures with mechanical connection systems. The paper is focused to two different connection systems: using dowel-type fasteners and using special surface connector. Behavior of dowel-type connection system is based on modification of Johansen ́s equations valid for timber to timber connections. Behavior of connection system with special surface connector is evaluated by experiments and numerical simulations.

Abstract: The presented study was focused on the influence of the micronized recycled a concrete sleeper on the mechanical properties of fine-grained concrete, specifically on dynamic modulus of elasticity. The tested material consisted of cement CEM I 42.5R (Radotín), crushed bricks fraction 2-5 mm, 0-4 mm sand and micronized recycled concrete fraction 0-0.125 mm. Individual material samples differed in the quantity of cement and finely ground recycled concrete (milled at Ltd. Lavaris, Libčice – Czech Republic). Cement was replaced by the recycled concrete in an amount of 30, 50 and 70 weight percent. Testing was performed on beams of dimensions 40 × 40 × 160 mm by using non-destructive testing – pulse method. The article compares the values obtained between 2-nd and 28-th day.