Prestressed Concrete Thermal Behaviour

The structural fire safety capacity of concrete is very complicated because concrete materials have considerable variations. Constitutive relationships for prestressed normal-strength concrete (NSC) and high-strength concrete (HSC) subjected to fire are needed to provide efficient modelling and to meet specific fire-performance criteria of the behaviour for prestressed concrete structures exposed to fire. In this paper, formulations for estimating the parameters affecting the behaviour of unconfined prestressed concrete at high temperatures are proposed. These formulations include residual compression strength, initial modulus of elasticity, peak strain, thermal strain, transient creep strain and the compressive stressstrain relationship at elevated temperatures. The proposed constitutive relationships are verified with available experimental data and existing models. The proposed relationships are general and rational, and show good agreement with the experimental data. More tests are needed to further verify and improve the proposed constitutive relationships

Experimental and numerical study of time-dependent behaviour of reinforced self-compacting concrete slabs

University of Technology, Sydney. Faculty of Engineering and Information Technology.Developments in concrete technology provide engineers, designers, suppliers and contractors with new methods of approaching engineering problems. Many of these developments are engineered solutions to technical and commercial problems, by either improving the current practices or overcoming limitations in the existing technology. One of the developments is Self-Compacting Concrete (SCC). SCC refers to a ‘highly flowable, non-segregating concrete that can be spread into place, fill the formwork, and encapsulate the reinforcement without the aid of any mechanical consolidation’ as defined by the American Concrete Institute (ACI). SCC is regarded as one of the most promising developments in concrete technology due to significant advantages over Conventional Concrete (CC). Many different factors can influence a decision to adopt SCC over CC ranging from structural performance to associated costs. These decisions should be well informed and based on a sound understanding of such factors. In addition, Fibre Reinforced Self-Compacting Concrete (FRSCC) is a relatively new composite material which congregates the benefits of the SCC technology with the profits derived from the fibre addition to a brittle cementitious matrix. Fibres improve many of the properties of SCC elements including tensile strength, ductility, toughness, energy absorption capacity, fracture toughness and cracking. For a structure (made by CC, SCC and FRSCC) to remain serviceable, crack widths must be small enough to be acceptable from an aesthetic point of view, to avoid waterproofing and deterioration problems by preventing the ingress of water and harmful substances. Crack control is therefore an important aspect of the design of reinforced concrete structures at the serviceability limit state. Limited researches have been undertaken to understand cracking and crack control of SCC and FRSCC members. Since, the time-dependent mechanisms of SCC and FRSCC are still not completely understood; a reliable and universally accepted design procedure for cracking and crack control SCC and FRSCC members has not been developed yet. There exists a need for both theoretical and experimental research to study the critical factors which affect the time-dependant crack of SCC and FRSCC members. In this study cracking caused by external loads in reinforced SCC and FRSCC slabs is examined experimentally and analytically. The mechanisms associated with the flexural cracking due to the combined effects of constant sustained service loads and shrinkage are observed. One of the primary objectives of this study is to develop analytical models that accurately predict the hardened mechanical properties of SCC and FRSCC. Subsequently, these models have been successfully applied to simulate time-dependent cracking of SCC and FRSCC one-way slabs. Series of tests on eight prismatic, singly reinforced concrete one-way slabs subjected to monotonically increasing loads or to constant sustained service loads for up to 240 days, were conducted. An analytical model is presented to simulate instantaneous and time-dependant flexural cracking of SCC and FRSCC members. It should be emphasized that any analytical model developed for calculation of crack width and crack spacing of reinforced SCC and FRSCC slabs must be calibrated by experimental data and verified by utilizing Finite Element Method (FEM). The analytical predictions of crack width and crack spacing for the SCC and FRSCC one way slabs are in reasonably good agreement with the experimental observations

Effects of specimen size and shape on compressive and tensile strengths of selfcompacting concrete with or without fibres

Self-compacting concrete (SCC) can be placed and compacted under its own weight. Modifications in the mix design of SCC may significantly influence the material's mechanical properties. Therefore, it is vital to investigate whether all the assumed hypotheses about conventional concrete also hold true for SCC structures. This paper discusses an experimental programme that was carried out to study the effects of specimen size and shape on the compressive and tensile strength of SCC and fibre reinforced SCC. For this purpose, cube specimens with 100 and 150 mm dimensions and cylinder specimens with 100 3 200 and 150 3 300 mm dimensions were casted. The experimental programme examined four SCC mixtures: plain SCC, and steel-, polypropylene- and hybrid-fibre reinforced SCC. Compressive and tensile strengths were tested after 3, 7, 14, 28 and 56 days. The paper also investigates correlations between compressive and tensile strengths and the size and shape of the specimen

Mechanical characteristics of self-compacting concrete with and without fibres

Fibre-reinforced self-compacting concrete (FRSCC) is a high-performance building material that combines positive aspects of fresh properties of self-compacting concrete (SCC) with improved characteristics of hardened concrete as a result of fibre addition. Considering these properties, the application ranges of both FRSCC and SCC can be covered. A test program is carried out to develop information about the mechanical properties of FRSCC. For this purpose, four SCC mixes - plain SCC, steel, polypropylene and hybrid FRSCC - Are considered in the test program. The properties include compressive and splitting tensile strengths, modulus of elasticity, modulus of rupture, and compressive stress-strain curve. These properties are tested at 3, 7, 14, 28, 56 and 91 days. Relationships are established to predict the compressive and splitting tensile strengths, modulus of elasticity, modulus of rupture, and compressive stress-strain curve. The models provide predictions matching the measurements

Evaluation and Comparison of Analytical Models to Determine the Bond Characteristics of Steel Fibre Reinforced Self-Compacting Concrete

Steel fibre reinforced self-compacting concrete (SFRSCC) can be placed and compacted under its self weight with little or no mechanical vibration. It is at the same time cohesive enough to be casted without segregation or bleeding. Steel fibres improve many of the properties of self-compacting concrete (SCC) elements including tensile strength, ductility, toughness, energy absorption capacity, fracture toughness and cracking. Although the available research regarding the influence of steel fibres on the properties of SFRSCC is limited, this paper investigates the bond characteristics between steel fibre and SCC. This by comparison of the five analytical models including (i.e. Naaman et al. (1991a,b), Dubey (1999), Cunha (2007), Soranakom (2008) and Lee et al. (2010)) with the experimental results from the four recently conducted single fibre pull-out tests. The influence of the fibre end hook, embedded length, fibre orientation angle, on the bond characteristic between fibre and SCC are determined and discussed. The accuracy of each analytical model also has been examined

Creep and shrinkage of high-strength self-compacting concrete: Experimental and analytical analysis

In the present paper, a numerical and experimental study about creep and shrinkage behavior of a high strength selfcompacting concrete is performed. Two new creep and shrinkage prediction models based on the comprehensive analysis on the available models of both conventional concrete and self-compacting concrete are proposed for high strength self-compacting concrete structures. In order to evaluate the predictability of the proposed models, an experimental program was carried out. A concrete which develops 60 MPa within 24 h was used to obtain experimental results. Several specimens were loaded: (i) at different ages and (ii) with different stress-to-strength ratios. Deformation in non-loaded specimens was also measured to assess shrinkage. All specimens were kept under constant stress during at least 600 days in a climatic chamber with temperature and relative humidity of 208C and 50%, respectively. Results showed that the new models were able to predict deformations with good accuracy, although provided deformations overestimated slightly

A Comparison of the Bond Characteristics in Conventional and Self-Compacting Concrete, Part I: Experimental Results

Self-compacting concrete (SCC) is a very flowing material that can flow through the reinforcement and fill the formworks without any need of vibration during the concrete placement process. The material properties of SCC including bond characteristics must be well understood in order to use this type of high performance concrete in structural members broadly. This paper presents a comparison of the experimental results from the nine recent investigations on the bond strength of SCC and conventional concrete (CC). The comparison is based on the measured bond between reinforcing steel and concrete by utilizing the pullout test on the embedded bars at various heights in mock-up structural elements to assess the top-bar effect and on single bars in small prismatic specimens and conducting the beam tests. The investigated affecting parameters on bond strength are: the steel bar diameter, concrete compressive strength, types of bar (plain or deformed), embedded length of the bar, concrete type, concrete cover, curing age of concrete, casting direction of concrete and height of the embedded bar along the formwork

A Comparison of the Bond Characteristics in Conventional and Self-Compacting Concrete, Part II: Code Provisions and Empirical Equations

Self-compacting concrete (SCC) is a highly workable concrete that flows through complex structural elements under its own weight. It is cohesive enough to fill the spaces of almost any size and shape without segregation or bleeding. This makes SCC become more practical wherever concrete placing is difficult, such as in heavily-reinforced concrete members or in complicated formworks. Bond behaviour between concrete and reinforcement is a primary factor in design of reinforced concrete structures. This study presents a comparison between code provisions and empirical equations with the experimental results from the recent studies on the bond strength of SCC and conventional concrete (CC). The comparison is based on the measured bond between reinforcing steel and concrete by utilizing the pullout test on the embedded bars at various heights in mock-up structural elements to assess the top-bar effect and on single bars in small prismatic specimens; and conducting the beam tests. The investigated varying parameters on bond strength are: the steel bar diameter, concrete compressive strength, concrete type, curing age of concrete and height of the embedded bar along the formwork

Comparison of Creep Prediction Models for Self-Compacting and Conventional Concrete

Realistic prediction of concrete creep is of crucial importance for durability and long-term serviceability of concrete structures. To date, research about the behaviour of self-compacting concrete (SCC) members, especially concerning the long-term performance, is rather limited. Hence, the realistic SCC creep strain prediction is an important requirement of the design process of this type of concrete structures. SCC is quite different from conventional concrete (CC) in mixture proportions and applied materials, particularly in the presence of aggregate which is limited. This paper reviews the accuracy of the creep prediction models proposed by six international codes of practice, including: CEB-FIP 1990, ACI 209R (1992), Eurocode 2 (2001), AASHTO (2004), AASHTO (2007) and AS 3600 (2009). The predicted creep strains are compared with actual measured creep strains in 60 mixtures of SCC and 17 mixtures of CC. The affecting parameters on the creep of SCC including: the water to binder ratio, binder to aggregate ratio, sand ratio, and curing age are investigated and discussed

Bond characteristics of steel fiber and deformed reinforcing steel bar embedded in steel fiber reinforced self-compacting concrete (SFRSCC)

© Versita sp. z o.o. Steel fiber reinforced self-compacting concrete (SFRSCC) is a relatively new composite material which congregates the benefits of the self-compacting concrete (SCC) technology with the profits derived from the fiber addition to a brittle cementitious matrix. Steel fibers improve many of the properties of SCC elements including tensile strength, ductility, toughness, energy absorption capacity, fracture toughness and cracking. Although the available research regarding the influence of steel fibers on the properties of SFRSCC is limited, this paper investigates the bond characteristics between steel fiber and SCC firstly. Based on the available experimental results, the current analytical steel fiber pullout model (Dubey 1999) is modified by considering the different SCC properties and different fiber types (smooth, hooked) and inclination. In order to take into account the effect of fiber inclination in the pullout model, apparent shear strengths (τ(app)) and slip coefficient (β) are incorporated to express the variation of pullout peak load and the augmentation of peak slip as the inclined angle increases. These variables are expressed as functions of the inclined angle (φ). Furthurmore, steel-concrete composite floors, reinforced concrete floors supported by columns or walls and floors on an elastic foundations belong to the category of structural elements in which the conventional steel reinforcement can be partially replaced by the use of steel fibers. When discussing deformation capacity of structural elements or civil engineering structures manufactured using SFRSCC, one must be able to describe thoroughly both the behavior of the concrete matrix reinforced with steel fibers and the interaction between this composite matrix and discrete steel reinforcement of the conventional type. However, even though the knowledge on bond behavior is essential for evaluating the overall behavior of structural components containing reinforcement and steel fibers, information is hardly available in this area. In this study, bond characteristics of deformed reinforcing steel bars embedded in SFRSCC is investigated secondly