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Structural Concrete, Vol. 9, no. 3, September 2008

Effect of grout proportions on strength of two-stage concrete

Hakim S. Abdelgader, Department of Civil Engineering, Al-Fateh University, Tripoli, Libya 
Abdurrahman A. Elgalhud, El-Mergib University, Al-Khoms, Libya


The objective of this paper was to study the manufacture of suitable grouts for two-stage (pre-placed aggregate) concrete using different admixtures to improve flowability of grout and hence the strength of two-stage concrete. The hypothesis of two-stage concrete is different from conventional concrete, because the particle-to-particle contact of coarse aggregate shrinkage is lower than that of normal concrete. The most important aspect of two-stage concrete strength still remains unclear. The strength of two-stage (pre-placed aggregate) concrete has been described in different ways, by experiment or theory. As no reliable data are available showing the strength of two-stage (pre-placed aggregate) concrete, it was necessary to investigate its strength at different mixes. This paper deals with the effect of different types of admixture on the concrete strength of two-stage (pre-placed aggregate) concrete. The paper also presents experimental results of pre-placed crushed aggregate concrete using 36 different grout mixture proportions. A total of 360 standard concrete cylinders were tested in unconfined compression and tensile strength at 28 days. On the basis of these results, a relationship between the tensile strength and compressive strength of two-stage (pre-placed aggregate) concrete has been statistically derived. 

Structural Concrete, Vol. 9, no. 2, June 2008

Mechanical properties of high-volume fly ash self-compacting concrete mixtures

P. Dinakar, Indian Institute of Technology, Chennai, India
K. G. Babu, CBRI, Uttranchal, India
M. Santhanam, Indian Institute of Technology, Chennai, India

Self-compacting concrete (SCC), a recent addition to the concrete scenario, is gaining popularity worldwide owing to the ease of its placement without any need for compaction. This paper describes the results of an investigation aimed at producing and evaluating SCC mixtures made with high volumes of class F fly ash. Eight fly ash SCC mixtures of various strength grades (20 - 100 MPa) were designed at the desired fly ash percentages of 0, 10, 30, 50, 70 and 85%, and were compared with five different mixtures of normal vibrated concrete mixtures (20 - 100 MPa). Tests were carried out on all mixtures to obtain the properties of fresh concretes in terms of viscosity and stability. The mechanical properties of hardened concretes such as compressive strength, splitting tensile strength and elastic modulus were also determined. The different amounts of paste caused differences in the properties of the two types of concrete. Test results indicated that the use of high volumes of class F fly ash in SCC mixtures decreases its 28-day compressive strength. However, the strength results showed continuous and significant improvement at the ages of 90 and 180 days, which was most probably owing to the pozzolanic reaction of fly ash. Self-compacted fly ash concrete mixtures exhibited higher splitting tensile strengths and lower elastic modulus compared with normal vibrated concretes. 

Structural Concrete, Vol. 9, no. 3, September 2008

Bond behaviour of NSM FRP strips in service

Kurt Borchert, Bantrel Co., Calgary, Alberta, Canada
Konrad Zilch, Technical University, Munich, Germany 


Most fibre-reinforced polymer (FRP) systems for strengthening of existing structures use an epoxy resin-based adhesive. Chemically, epoxy resins are defined as plastics. Compared with concrete, epoxy resins show material properties more sensitive to service conditions, for example creep during sustained loading and increased temperature, respectively. These effects can become critical for the durability and the load-carrying capacity of the strengthening. In consideration of applications under elevated temperatures or strengthening systems based on permanent bond stresses the benefit may be limited by the material properties of the epoxy resin. In order to predict reliably the long-term performance of adhesively bonded FRP reinforcement it is important to develop experimentally validated design models. This paper reports experimental long-term tests investigating the properties of epoxy resins and their impact on the bond behaviour of near-surface-mounted (NSM) FRP strips. Based on these experimental results material models, bond models and a simplified calculation method are developed and presented in this paper. 

Structural Concrete, Vol. 9, no. 2, June 2008

An experimental investigation on beam supported reinforced concrete skew slabs

K. U. Muthu, M S Ramaiah Institute of Technology, Bangalore, India
M. U. Aswath, Bangalore Institute of Technology, Bangalore, India
A. Prabhakara, University Viswesvaraya College of Engineering, Bangalore, India


An experimental programme has been designed to cast and test 18 beam supported reinforced concrete skew slabs subjected to distributed loading. The variables include edge rigidity, skew angle and coefficient of orthotropy. The load deflection behaviour, ultimate strength and the effect of cracking were studied and the results are presented. The current work highlights the effect of membrane action on skew slabs with lateral restraints and its signficance. 

Structural Concrete, Vol. 9, no. 2, June 2008

A finite element formulation for concrete structures in plane stress

G. Bertagnoli, Department of Structural and Geotechnical Engineering, Politecnico di Torino, Italy
V. I. Carbone, Department of Structural and Geotechnical Engineering, Politecnico di Torino, Italy

A comprehensive, very compact non-linear finite element is proposed, which is able to describe the behaviour of two-dimensional concrete structures from serviceability conditions up to collapse. The formulation of this finite element takes into account all the material non-linearities typical of concrete structures such as cracking, non-linear behaviour in compression, tension and compression softening and shear transmission along cracks. The robustness of the finite element derives from its compactness and from the reduction of the number of input parameters that control the structural response, but whose values very often cannot be properly introduced. The proposed finite element is calibrated by reproducing a wide range of well-known experimental tests, carried out both on simple panels and complex two-dimensional structures; it is then tested with reference to three additional cases, where it shows a satisfactory capability to predict reinforced concrete two-dimensional structural behaviour. 

Structural Concrete, Vol. 9, no. 2, June 2008

Strength and durability of high-volume fly ash concrete

G. Baert, Magnel Laboratory for Concrete Research, Ghent University, Belgium
A.-M. Poppe, Magnel Laboratory for Concrete Research, Ghent University, Belgium
N. De Belie, Magnel Laboratory for Concrete Research, Ghent University, Belgium

The effects of replacing 10, 40 or 60% of the cement content by low-calcium fly ash on the compressive strength and durability of the concrete were investigated. An appropriate amount of (super)plasticiser was added to the mix to obtain good workability. At an early age the compressive strength decreases with increasing level of cement replacement. After 28 days the compressive strength increased relatively more for high-volume fly ash concrete than for the control concrete. Concrete with fly ash performed better in lactic/acetic and sulphuric acid during accelerated experiments. The chloride diffusion coefficients resulting from accelerated chloride migration tests were significantly lower for concrete with fly ash than for the control concrete, except for the mixture with 60% replacement of the cement content. The resistance to frost/thaw cycles was similar for all concrete mixtures. The carbonation depth after 9 weeks in a 10% carbon dioxide (CO2) environment increased with increasing fly ash content. High volumes of fly ash also decreased significantly the resistance against the combined action of frost and de-icing salts (3% sodium chloride (NaCl) solution). From these results it can be concluded that high-volume fly ash concrete has a potential for commercial use in particular applications. 

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