Adverse effects of high temperatures and freeze-thaw cycles on properties of HFRHSCs containing silica fume and metakaolin

Adverse effects of high temperatures (HTs) and freeze-thaw (F-T) cycles on the properties of hybrid fibers reinforced high strength concretes (HFRHSCs) containing silica fume (SF) and silica fume + metakaolin (SF + MK) were investigated in this study. Concrete mixtures without and with hybrid fibers (HFs) up to 20% replacement by the weight of Portland cement (PC) with SF and SF + MK were made with the water-binder ratio of 0.25. Experimental studies were conducted on the specimens prepared from these mixtures to determine the ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (fct,fl) and splitting tensile strength (fct,sp) values of concretes without and with HFs exposed to ambient temperature (AT) and HTs. Moreover, the experimental studies were performed to determine the Upv and fc values of concretes exposed to F-T cycles. Microstructure analyses were also conducted on the specimens to examine the changes in the matrix, interface zone and aggregate of concretes without HFs exposed to AT, HTs and F-T cycles. Experimental results have revealed that replacement ratios of SF and SF + MK have important effects on the properties of concretes without and with HFs. The experimental results have also shown that the fc, fct,fl and fct,sp values of the concretes with HFs are higher than the concretes without HFs. The Upv, fc, fct,fl and fct,sp values of the concretes with HFs have been affected more than that of the concretes without HFs due to the adverse effects of HTs. © 2018 Elsevier Lt

Modeling of elastic modulus of concrete containing fly Ash by gene expression programming

4th International Conference on Sustainable Construction Materials and Technologies, SCMT 2016 -- 7 August 2016 through 11 August 2016 -- -- 137590The gene expression programming that is one of the artificial intelligence techniques, have been commonly used to model some of civil engineering applications. In this study, two models in the gene expression programming for predicting the elastic modulus of concrete containing fly Ash have been developed. First model is proposed for the elastic modulus prediction from compressive strength of concrete containing fly Ash, and second model is proposed for the elastic modulus prediction from amount of fly Ash and compressive strength of concrete containing fly ash. For the aim of building these models, the experimental results for 259 specimens presented with 132 different concrete mixtures were collected from the literature. The training and testing sets of these models are divided without prior planning from the experimental results. These models are also validated with 122 data of experimental results other than the data used in training and testing sets. Moreover, the results obtained from these models are compared with the experimental results and formulas results given by some national building codes. These comparisons revealed that the equations of these models appeared to well concur with the experimental results and found to be very reliable. © 2016 International Committee of the SCMT conferences. All rights reserved

Mechanical and microstructural properties of HFRHSCs containing metakaolin subjected to elevated temperatures and freezing-thawing cycles

Mechanical and microstructural properties of hybrid fibers reinforced high strength concretes (HFRHSCs) containing metakaolin (MK) and metakaolin + silica fume (MK + SF) subjected to elevated temperatures and freezing-thawing (F-T) cycles were investigated in this paper. In the concrete mixtures without and with hybrid fibers, MK and MK + SF were replaced with eight percentage of cement by weight. A total of sixteen concrete mixtures were produced at a water-to-binder ratio of 0.25. Ultrasound pulse velocity (U pv ), compressive strength (f c ), splitting tensile strength (f sts ) and flexural strength (f fs ) tests were performed to evaluate the properties of HFRHSCs subjected to ambient and elevated temperatures (300, 400 and 500 °C). The U pv and f c tests of HFRHSCs subjected to F-T cycles were also conducted. Moreover, the alterations in the matrix, interface zone and aggregate of concretes without hybrid fibers subjected to ambient temperature, elevated temperatures and F-T cycles were analyzed by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) spot and polarized light microscope (PLM). The results of tests have shown that the replacement levels of MK and MK + SF have important influences on the mechanical and microstructural properties of high strength concretes. Moreover, the mechanical results have shown that the concretes with hybrid fibers have higher f c , f sts and f fs than other concretes. The results have also shown that the U pv , f c , f sts and f fs values of concretes without and with hybrid fibers decrease, as the temperature increases. © 201

Mechanical and microstructural properties of alkali-activated slag and slag?+?fly ash mortars exposed to high temperature

In this study, changes in the mechanical and microstructural properties of alkali-activated blast furnace slag (BFS) and BFS + fly ash (FA) blended mortars exposed to 25, 400, 600 and 800 °C temperatures are investigated. The alkali-activated mortars using the replacement ratios of 0, 25 and 50% FA by weight of BFS together with the activators are produced in addition to the control mortars produced by only cement. Three different optimum Ms (SiO2/Na2O ratio of activators) values of 0.50, 0.75 and 1.00 are determined for the alkali-activated mortars. The FA substitution by BFS increases the high temperature resistance of alkali-activated BFS mortars. © 2019 Elsevier LtdÖmer Halisdemir ÜniversitesiThe SEM, EDS and XRD analyses of this research were performed by using the devices in Central Laboratory of Niğde Ömer Halisdemir University. This research study was supported by Niğde Ömer Halisdemir University project “Investigation of the properties of alkali-activated lightweight concretes”, project code FEB2008/10-BAGEP

Weldability of Zinc Coated Automotive Steel Sheets by Resistance Spot Welding

APAY, Serkan/0000-0003-4624-9082WOS: 000208889900011In a given study, resistance spot welding of automotive steel sheets, which are widely used in the house-goods and car manufacturing industry, is studied. The shear strength, hardness and microstructure of the welded parts are measured in order to find the effects of welding period, welding current, and thickness of the galvanizing under constant applied electrode force pressure. Welding periods are chosen as 22 and 27 cycles, and welding current increased from 4.5 kA to 5.5 kA. The electrode-applied forces are kept constant at 3.5 Bar. The samples are exposed to tensile shear tests, hardness tests and metallographic examinations, and suitable welding parameters are advised to users. The results show that, increasing welding period and thickness of coating, the tensile shear strength and hardness of the welding nugget are increased