Analysis and cost optimisation of prestressed concrete joists in beam-block floors
Vanessa Cristina de Castilho, Universidade Federal de Uberlandia, Brazil Maria Cristina Vidigal de Lima, Universidade Federal de Uberlandia, Brazil
The overall production costs of precast concrete elements are important to quantify. Analyses of the constructive stages of these elements, however, may more realistically lead to a function representing production, transport, assembly and casting costs. This article investigates the cost minimisation problems of joists in beam-block floors using a heuristic method known as genetic algorithm (GA) and a conventional method known as generalised reduced gradient (GRG2). The continuous variables considered in the problem were prestressed reinforcements in the three levels, distance between reinforcements, concrete layer strength, distance between joists, concrete layer thickness and degree of redistribution of the continuous slabs' negative moments. Based on the results, the heuristic method was found to be effective in identifying the solution for precast element cost minimisation problems. The performance of the cost function vis-à-vis market alterations was evaluated considering the influence of concrete, reinforcement and labour prices.
Load carrying capacity of cracked concrete railway sleepers
Håkan Thun, Lulå University of Technology, Sweden Sofia Utsi, Luleå University of Technology, Sweden Lennart Elfgren, Luleå University of Technology, Sweden
The load carrying capacity of damaged prestressed concrete railway sleepers has been investigated in order to obtain information about how the cracking influences the remaining load carrying capacity compared with an uncracked sleeper. The following tests have been performed: bending capacity of the midsection, the rail section, horizontal load capacity of the fastener, control of the concrete properties and fatigue capacity in bending of the rail section. A visual inspection and classfication of the damages are also presented. The test results have been compared with calculations according to the Swedish railway code for sleepers and show that railway sleepers are quite robust. Small cracks do not seem to influence the load carrying capacity and it is first when the cracking is very severe that the load carrying capacity is reduced significantly.
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.
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.
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.
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.
