Evaluation of axial strength in circular STCC columns using UHPC and UHPFRC

Le Hoang, An,Fehling, Ekkehard,Thai, Duc-Kien,Van Nguyen, Chau Elsevier 2019 Journal of constructional steel research Vol.153 No.-

<P><B>Abstract</B></P> <P>This study investigates the axial strength of circular steel tube confined concrete (STCC) columns with the employment of plain ultra high performance concrete (UHPC) and ultra high performance fiber reinforced concrete (UHPFRC) having compressive strengths of cylinders varying from 178.9 MPa and 198 MPa. UHPFRC used in this study had 1% and 2% volume fraction of steel fibers in the mixture. Eighteen specimens including both short and intermediate columns were cast and tested under concentric loading on the concrete core to generate the maximal confinement effect. Experimental results showed that the effect of steel fibers on the axial load versus axial strain responses, and on the enhancement of strength and ductility was insignificant, while the parameters of the steel tube and the concrete core length were found to play a major role. The ultimate loads of the tests presented herein were compared with those obtained from the predictions by seven analytical models and five existing design codes. The design expressions from these analytical models and codes were assessed. For the prediction of the ultimate loads of both short and intermediate column, the model by De Oliveira et al. [] was the most suitable approach among seven selected models, whereas the ACI provision [] was the best among five existing design codes. Furthermore, the experimental ultimate loads obtained from this study were combined with those from previous studies to examine the influence of different concrete strength ranges on the strength enhancement. Finally, unified formulae to predict the ultimate strength with a wide range of concrete strength up to 200 MPa were also proposed for both short and intermediate columns.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Tests on 18 circular STCC columns using UHPC and UHPFRC with compressive strength up to 200 MPa were conducted to evaluate the axial strength. </LI> <LI> The accuracy of seven analytical models and five existing design codes in prediction of the ultimate strength was assessed. </LI> <LI> The effect of concrete strength ranges including NSC, HSC, UHPC and UHPFRC on the strength enhancement of circular STCC columns was identified. </LI> <LI> The unified formulae for predicting the confined peak stress (<I>f</I> <SUB> <I>cc</I> </SUB>) and the ultimate load (<I>N</I> <SUB> <I>u</I> </SUB>) of circular STCC columns were proposed. </LI> </UL> </P>

Numerical analysis of circular steel tube confined UHPC stub columns

An Le Hoang,Ekkehard Fehling 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.19 No.3

In this paper, a finite element model (FEM) in ATENA-3D software was constructed to investigate the behavior of circular ultra high performance concrete (UHPC) filled steel tube stub columns (UHPC-FSTCs) under concentric loading on concrete core. The “CC3DNonLinCementitious2User” material type for concrete in ATENA-3D software with some modifications of material laws, was adopted to model for UHPC core with consideration the confinement effect. The experimental results obtained from Schneider (2006) were then employed to verify the accuracy of FEM. Extensive parametric analysis was also conducted to examine the influence of concrete compressive strength, steel tube thickness and steel yield strength on the compressive behavior of short circular UHPC-FSTCs. It can be observed that the columns with thicker steel tube show better strength and ductility, the sudden drop of load after initial peak load can be prevented. Based on the regression analysis of the results from parametric study, simplified formulae for predicting ultimate loads and strains were proposed and verified by comparing with previous analytical models, design codes and experimental results.

Analysis of circular steel tube confined UHPC stub columns

An Le Hoang,Ekkehard Fehling 국제구조공학회 2017 Steel and Composite Structures, An International J Vol.23 No.6

The use of ultra high performance concrete (UHPC) in composite columns offers numerous structural benefits, and has received recent research attention. However, the information regarding the behavior of steel tube confined concrete (STCC) columns employing UHPC has been extremely limited. Thus, this paper presents an overview of previous experimental studies on circular STCC columns with taking into account various concrete strengths to point out their distinctive features. The effect of the confinement factor and the diameter to thickness ratio on both strength and ductility in circular STCC columns employing UHPC was investigated. The applicability of current design codes such as EC4, AISC, AIJ and some available analytical models for concrete confined by steel tube was also validated by the comparison of ultimate loads between the prediction and the test results of Schneider (2006) and Xiong (2012). To predict the stress-strain curves for confined UHPC in circular STCC stub columns, a simplified model was proposed and verified by the comparison with experimental stress-strain curves.

A review and analysis of circular UHPC filled steel tube columns under axial loading

An Le Hoang,Ekkehard Fehling 국제구조공학회 2017 Structural Engineering and Mechanics, An Int'l Jou Vol.62 No.4

Ultra high performance concrete (UHPC) has aroused interest around the world owing to superior mechanical and durability properties over conventional concrete. However, the application of UHPC in practice poses difficulties due to its inherent brittleness. UHPC filled in steel tube columns (UHPC-FSTCs) are capable of restricting the brittle failure of non-reinforced UHPC columns and forming a high performance member with enhancement of strength and ductility. Currently, research on UHPC-FSTCs remains very limited and there is relatively little information about the mechanical behavior of these columns. Therefore, this study presents a review of past experimental studies to have a deeper insight into the compressive behavior of UHPC-FSTCs under axial loading on entire section and on concrete core. Based on the test results obtained from Schneider (2006) and Xiong (2012), an analysis was conducted to investigate the influence of the confinement index (ξ) and diameter to steel tube thickness ratio (D/t) on the strength and the ductility in short circular UHPC-FSTCs. Furthermore, the appropriateness of current design codes including EC4, AISC, AIJ and previous analytical models for estimating the ultimate loads of composite columns was also examined by the comparison between the predictions and the test results. Finally, simplified formulae for predicting the ultimate loads in two types of loading pattern were proposed and verified.

Assessment of stress-strain model for UHPC confined by steel tube stub columns

Hoang, An Le,Fehling, Ekkehard Techno-Press 2017 Structural Engineering and Mechanics, An Int'l Jou Vol.63 No.3

Ultra high performance concrete (UHPC) has recently been applied as an alternative to conventional concrete in construction due to its extremely high compressive and tensile strength, and enhanced durability. However, up to date, there has been insufficient information regarding the confinement behavior of UHPC columns. Therefore, this study aims to perform an assessment of axial stress-strain model for UHPC confined by circular steel tube stub columns. The equations for calculating the confined peak stress and its corresponding strain of confined concrete in existing models suggested by Johansson (2002), Sakino et al. (2004), Han et al. (2005), Hatzigeorgiou (2008) were modified based on the regression analysis of test results in Schneider (2006) in order to increase the prediction accuracy for the case of confined UHPC. Furthermore, a new axial stress-strain model for confined UHPC was developed. To examine the suitability of the modified models and the proposed model for confined UHPC, axial stress-strain curves derived from the proposed models were compared with those obtained from previous test results. After validating the proposed model, an extensive parametric study was undertaken to investigate the effects of diameter-to-thickness ratio, steel yield strength and concrete compressive strength on the complete axial stress-strain curves, the strength and strain enhancement of UHPC confined by circular steel tube stub columns.

Assessment of stress-strain model for UHPC confined by steel tube stub columns

An Le Hoang,Ekkehard Fehling 국제구조공학회 2017 Structural Engineering and Mechanics, An Int'l Jou Vol.63 No.3

Ultra high performance concrete (UHPC) has recently been applied as an alternative to conventional concrete in construction due to its extremely high compressive and tensile strength, and enhanced durability. However, up to date, there has been insufficient information regarding the confinement behavior of UHPC columns. Therefore, this study aims to perform an assessment of axial stress-strain model for UHPC confined by circular steel tube stub columns. The equations for calculating the confined peak stress and its corresponding strain of confined concrete in existing models suggested by Johansson (2002), Sakino et al. (2004), Han et al. (2005), Hatzigeorgiou (2008) were modified based on the regression analysis of test results in Schneider (2006) in order to increase the prediction accuracy for the case of confined UHPC. Furthermore, a new axial stress-strain model for confined UHPC was developed. To examine the suitability of the modified models and the proposed model for confined UHPC, axial stress-strain curves derived from the proposed models were compared with those obtained from previous test results. After validating the proposed model, an extensive parametric study was undertaken to investigate the effects of diameter-to-thickness ratio, steel yield strength and concrete compressive strength on the complete axial stress-strain curves, the strength and strain enhancement of UHPC confined by circular steel tube stub columns.

Simplified stress-strain model for circular steel tube confined UHPC and UHPFRC columns

An H. Le,Fehling Ekkehard,Duc-Kien Thai,Chau V. Nguyen 국제구조공학회 2018 Steel and Composite Structures, An International J Vol.29 No.1

The research on the confinement behavior of ultra high performance concrete without and with the use of steel fibers (UHPC and UHPFRC) has been extremely limited. In previous studies, authors experimentally investigated the axially compressive behavior of circular steel tube confined concrete (STCC) short and intermediate columns with the employment of UHPC and UHPFRC. Under loading on only the concrete core, the confinement effect induced by the steel tube was shown to significantly enhance the utimate stress and its corresponding strain of the concrete core. Therefore, this paper develops a simplified stress – strain model for circular STCC columns using UHPC and UHPFRC with compressive strength ranging between 150 MPa and 200 MPa. Based on the regression analysis of previous test results, formulae for predicting peak confined stress and its corresponding strain are proposed. These proposed formulae are subsequently compared against some previous empirical formulae available in the literature to assess their accuracy. Finally, the simplified stress . strain model is verified by comparison with the test results.