Andreas Lampropoulos

Concrete Shrinkage Effect on Columns Strengthened with Concrete Jackets

Placing reinforced concrete (RC) jackets around RC columns is a common strengthening technique, particularly in seismic regions. The influence of concrete shrinkage on columns strengthened with a RC jacket has not been investigated yet. This paper presents an analytical procedure to calculate the stresses induced by shrinkage of the new concrete. A variable modulus of elasticity with time and relaxation due to creep are taken into consideration in the procedure. Finite element analysis method is used to perform parametric numerical simulations. From the results, it is found that jacket concrete shrinkage reduces the strength of composite columns. This strength reduction increases as the shrinkage strain values increase. For example, a value as low as 0,6 of monolithic behaviour was found for a normalised axial load of 0,4 and a concrete free shrinkage strain of 1600 microstrains. It is concluded that the effect of concrete shrinkage must be considered when strengthening RC columns, as it induces slip at the interface between the old and the new concrete and tensile stresses in the jacket concrete.

Ultra-High-Performance Fiber-Reinforced Concrete under Cyclic Loading

Ultra-high-performance fiber-reinforced concrete (UHPFRC) is a novel cementitious material with enhanced strength in tension and compression, and significantly high energy absorption in the postcracking region. The application of UHPFRC for the earthquake strengthening of existing structures could considerably improve the performance of existing structures due to its superior properties. There are published studies where the direct tensile and the flexural behaviors of UHPFRC have been investigated and the superior tensile strength and post-crack energy absorption have been highlighted. However, there are not any published studies on the performance of UHPFRC under cyclic loading. In this paper, the results of an extensive experimental program on UHPFRC under direct tensile cyclic loading are presented and a constitutive model for the response of UHPFRC under cyclic loading is proposed. The accuracy of the proposed model is validated using experimental results from various loading histories and for different percentages of fibers, and the reliability of the proposed model is highlighted.

Monolithic coefficient values for design when seismically strengthening RC columns with jackets

Monolithic coefficients correlate the behavior of strengthened reinforced concrete columns to that of respective monolithic columns and can be used to estimate strengthened column response. Finite element analyses are performed with different values of normalized axial load, jacket thickness, and jacket concrete strength. Four- and three-sided jackets are examined. Monolithic coefficients for strength, stiffness, and displacement at yield and failure are obtained by comparing results of strengthened and monolithic specimens. Calculated values are compared to existing code values and useful conclusions and practical guidance concerning the effect of the examined parameters on monolithic coefficient values are presented.

Biaxial stress due to shrinkage in concrete jackets of strengthened columns

The effect of restrained concrete shrinkage on the strength of concrete used to strengthen existing structures is investigated in this study. A series of compressive tests are performed on square prism concrete specimens where steel plates are placed in order to restrain free shrinkage. Results indicate that there is a significant reduction of concrete strength (20 to 30%) due to a biaxial stress state induced by the restrained concrete shrinkage. The experimental results are used to validate a numerical procedure proposed to simulate concrete shrinkage. A numerical investigation is then conducted on models that represent the concrete of a reinforced concrete jacket applied to an original column for strengthening purposes where the degree of restraint is different to that of the examined square prism specimens. It was found that that there is a further reduction of concrete strength (30 to 40%).

Numerical Study Of The Effects Of Preloading, Axial Loading And Concrete Shrinkage On Reinforced Concrete Elements Strengthened By Concrete Layers And Jackets

In this study, the technique of seismic strengthening existing reinforced concrete columns and beams using additional concrete layers and jackets is examined. The finite element method and the finite element program ATENA is used in this investigation. When a reinforced jacket or layer is being constructed around a column it is already preloaded due to existing service loads. This effect has been examined for different values of the axial load normalized to the strengthened column. The techniques of strengthening with a concrete jacket or a reinforced concrete layer on the compressive side of the column are examined. Another phenomenon that is examined in this study is the shrinkage of the new concrete of an additional layer used to strengthen an existing member. For this investigation, a simply supported beam with an additional reinforced concrete layer on the tensile side is examined. The results demonstrate that the effect of preloading is important when a reinforced concrete layer is being used with shear connectors between the old and the new reinforcement. It was also found that the shrinkage of the new concrete reduces the strength of the strengthened beam and induces an initial sliding between the old and the new concrete.

Effect of Undensified Silica Fume on the Dispersion of Carbon Nanotubes within a Cementitious Composite

The synergistic effect of multi-walled carbon nanotubes (MWCNTs) and Undensified Silica Fume (USF) on the microstructure of cementitious composites has been studied. In the current work, USF was used to enhance the dispersion of nanotubes throughout the composite and prevent the re-agglomeration of nanotubes by providing a physical barrier of particles of small size. Ultrasonication was employed to disperse MWCNTs in water in the presence of polycarboxylate-based superplasticizer (PCE) as a dispersion agent. The results indicate that incorporation of USF considerably improves the dispersion of nanotubes in the composites, with subsequent enhancement of composite packing density. This enhancement can be attributed to the synergistic effect of MWCNTs and USF in reducing the volume of pores through the cementitious composites.

Effect of connection procedures on the behaviour of RC columns strengthened with RC layers and jackets

This study examines the behaviour of reinforced concrete (RC) columns strengthened with either a RC jacket or an additional RC layer. The main object of this work was to evaluate the behaviour of the strengthened specimens for different connection procedures used to bond the old and new concrete. For the column strengthened with a concrete jacket, the effect of the roughness of the interface on the behaviour of the strengthened specimen was examined. For the column strengthened using an additional RC layer, the presence of shear connectors between the old and new reinforcement was also examined to provide adequate connection between the two concrete members. The strength degradation of the interface because of the cyclic loading was taken into account. Using the results of the analyses, monolithic coefficients (these are special coefficients that correlate the behaviour of the strengthened specimens to the respective monolithic) were calculated for the different connection procedures examined.

Experimental Investigation on Thermal Conductivity and Mechanical Properties of a Novel Aerogel Concrete

Concrete is one of the most commonly used construction material worldwide (Kosmatka et al. 2008) with relatively low cost and good durability. Concrete has high compressive strength, high fire resistance, and also it is easy to be cast in any desire shape. On the other hand, the main drawbacks of concrete are related to its low tensile strength, high carbon dioxide emissions as a result of the use of Portland cement, and high thermal conductivity properties.

Numerical Investigation of Strengthened Unreinforced Masonry (URM) Structures with Ultra High Performance Fibre Reinforced (UHPFRC) Layers

A novel strengthening technique has been examined in this study using additional Ultra High Performance Fibre Reinforcement Concrete (UHPFRC) for the improvement of the structural performance of existing masonry structures. UHPFRC is a material with enhanced cementitious matrix and high percentage of steel fibres which is characterised by considerably improved compressive strength, tensile strength, and ductility.

Shrinkage Effect on Beams Strengthened with Additional Concrete Layers

A common technique used to increase the flexural capacity of reinforced concrete beams is theaddition of concrete layers in the compressive or in the tensile side. Until now, there are limitedstudies for the effect of the concrete shrinkage on the performance of the strengthened beams. Inmost of the published studies, concrete shrinkage is ignored and perfect connection is assumedbetween the old and the new concrete which is not conservative. In this study full scale beamsstrengthened with additional concrete layers in the tensile side have been investigated. The resultsof an experimental investigation together with numerical and analytical results are presented. Thecrucial effect of the concrete shrinkage on the slip of the interface is highlighted and the interactionbetween the slip induced by concrete shrinkage and the slip due to bending loading is presented.