Showing papers in "Magazine of Concrete Research in 2006"

Microstructure analysis of hardened recycled aggregate concrete

Abstract: Fifty percent of the total weight of construction and demolition waste is concrete. Consequently, the utilisation of crushed waste concrete must be considered as imperative in concrete production. Recycled aggregates are formed basically by adhered mortar and original aggregates. Adhered mortar is a very porous material, with a high absorption capacity and high permeability. Four different percentages (0, 25, 50 and 100%) of wet recycled coarse aggregates were used in the production of the concrete tested, the same compressive strength being obtained in all four mixes. The microstructure of recycled aggregate concrete was analysed using a fluorescent light optical microscope, which made it possible to observe the high porosity of recycled aggregates with and without adhered mortar, the increment of air void percentage in new cement paste when high amounts of recycled aggregates are used and the effectiveness of new interfacial transition zone in recycled aggregate concrete.

Mechanical properties of concrete containing recycled asphalt pavements

Abstract: Recycled asphalt pavement (RAP) is the removed and/or reprocessed pavement material containing asphalt and aggregate. The use of RAP in asphalt pavement has become common practice in the constructi...

Recycled concrete exposed to high temperatures

Abstract: Concrete exposure to high temperatures produces changes in its internal structure, for instance loss of its strength and deformation capacity, in extreme cases risking the service life of the structure. The damage level will depend on the temperature reached, exposure time, cooling velocity and type, size of the structural element and the material's own characteristics. In this paper results of conventional concretes (CC) made with a natural coarse aggregate and different water/cement (w/c) ratios exposed to high temperature are compared with those obtained with recycled concretes (RCs) of similar characteristics produced by employing 75% by volume of recycled coarse aggregate (crushed waste concretes). The concretes were evaluated by ultrasound and resonance frequency, and further determination of the static modulus of elasticity and compressive strength was possible. In all cases the parameters evaluated in concretes exposed to high temperatures are lower than in others with the same characteristics and...

Comparing performance of modified two-stage mixing approach for producing recycled aggregate concrete

Abstract: Exhaustion of landfill areas coupled with the extensive redevelopment programme in Hong Kong has prompted the use of recycled aggregate. However, the inferior quality of recycled aggregate (RA) has restricted its use to low-grade applications such as roadwork sub-base and pavements, while its adoption for higher-grade concrete is rare because of the lower compressive strength and higher variability in mechanical performance of RA. A new concrete mixing method, that is the two-stage mixing approach, was advocated to improve the quality of RA concrete (RAC) by splitting the mixing process into two. The current paper describes two modified mixing methods with some alteration to the two-stage mixing approach by proportioning ingredients of cement and water with the percentage of RA added in the first mix, referred to as the two-stage mixing approach(proportional-1) (TSMAp1), and proportioning the cement content (without water) with the percentage of RA used in the first mix, referred to as the two-stage mixin...

Effect of geopolymer cement on microstructure, compressive strength and sulphuric acid resistance of concrete

Abstract: The effect of geopolymer cement on the compressive strength and resistance to sulphuric acid of concrete made with type 10 and type 50E cements was studied. Hydration products and microstructure of various concrete mixtures incorporating geopolymer cement were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmitted light microscopy in order to identify the mechanisms underlying the effects of the geopolymer cement on concrete properties. Experimental results indicate that using geopolymer cement as partial replacement for ordinary Portland cement (OPC) effectively improved both the compressive strength and resistance to sulphuric acid. Replacement of 50% OPC by geopolymer cement increased the 28-day compressive strength by about 50% and reduced the mass loss of concrete specimens subjected to eight weeks of immersion in sulphuric acid solutions with pH of 0·3 and 0·6 by 42% and 30%, respectively. The effect of geopolymer cement on the compressive strength and sulphur...

Bond behaviour and flexural performance of structural synthetic fibre-reinforced concrete

Abstract: This study established the optimum fibre geometry for structural synthetic fibre-reinforced concrete. This was achieved through bond testing of structural synthetic fibres that fully utilised matrix anchoring without fracturing, while maintaining the maximum pullout resistance. The experimental procedure was divided into three parts. First, eight deformed structural synthetic fibres with widely different geometries were investigated and bond tests were conducted. Crimped structural synthetic fibres performed significantly better than the other fibres in terms of interface toughness and pullout load. Second, additional bond tests were performed on various sizes of crimped structural synthetic fibres to determine their optimum dimensions, which were at a height of 1·8 mm and an amplitude of 6·0 mm. Third, flexural tests were performed on concrete reinforced using the optimum structural synthetic fibres. It was demonstrated that structural synthetic fibres improved the flexural toughness of concrete.

Autogenous deformations and viscoelasticity of UHPFRC in structures. Part I: experimental results

Abstract: Autogenous shrinkage is the main driving force for internal deformations in ultra-high performance fibre-reinforced concretes (UHPFRC). Autogenous deformations and viscoelasticity are determined wi...

Fracture energy of high-performance concrete at high temperatures up to 450°C : the effects of heating temperatures and testing conditions (hot and cold)

Abstract: The fracture energy of high-performance concrete (HPC) with a compressive strength of 67·1 MPa was studied by conducting three-point bending tests on eighty notched beams of 500×100×100 mm at high temperatures up to 450°C (hot) and in cooled-down states (cold). The temperatures in the furnace and inside the concrete and the weight loss of concrete were continuously monitored. If the exposure time was long enough, 16 h in this study, both thermal and hygric equilibriums could be reached. The fracture energy sustained a decrease–increase tendency with the increasing heating temperature for the hot concrete, and an increase–decrease tendency for the cold concrete. The modulus of rupture generally decreased with the heating temperature for the hot concrete but sustained an increase–decrease tendency for the cold concrete. There was a sudden drop at 105°C for the hot concrete owing to high vapour pressure inside the concrete. The residual compressive and tensile strengths both decreased with the increasing hea...

Strain of heated concrete during two thermal cycles. Part 1: strain over two cycles, during first heating and at subsequent constant temperature

Abstract: This paper presents the strain behaviour of three nuclear reactor type plain unsealed (50–60 N/mm2) concretes exposed under 0% and 20% compressive loads to a 14-day two-thermal-cycle test (test temperatures ranging from 100°C to 600°C). A generic approach is taken for wider application and for the in-depth understanding of the behaviour of heated concrete in general. Concrete during the two thermal cycles experiences both expansive and contractive strains. The former includes ‘thermal expansion’, cracking expansion and chemically induced expansion. The latter includes drying shrinkage, load-induced thermal strain (LITS) during virgin heating, ‘delayed’ LITS at constant temperature and time-dependent creep. The overall strains during heating, at constant temperature and during cooling reflect both contractive and expansive influences. LITS is absent during cooling and a second heat cycle. A summary of the influences of material and environmental factors upon the thermal strain and LITS of concrete is given...

Residual bond strength of corroded plain round bars

Abstract: This paper reports results of an experimental programme to measure changes in bond characteristics of plain round reinforcement as a result of corrosion. Principal parameters included in the investigation are the amount of corrosion, the presence of confining reinforcement in the form of links, cover thickness and the initial condition of the reinforcement. In the absence of confining links, bond strength of bars cast near the bottom of a pour is found to reduce with increasing corrosion. However, although top-cast bars were initially weaker, small amounts of corrosion were sufficient to raise bond strength to that of bottom-cast bars. With increasing corrosion, strength reduced in a similar manner to bottom-cast bars. Confining reinforcement had little influence on bond strength prior to the onset of corrosion, but plays an important role in maintaining strength once corrosion becomes established. Results from these tests suggest that surface crack width may provide a better parameter than section loss or corrosion penetration through which to assess residual bond strength. The reasons for this are explained, and empirical relationships for residual strength are derived.

Passing ability and segregation stability of self-consolidating concrete with different aggregate proportions

Abstract: An extensive experimental programme has been carried out to study the effects of fine:10 mm:20 mm aggregate proportions on the passing ability and segregation stability of self-consolidating concrete. A total of 18 trial concrete mixes with water/cementitious materials ratios ranging from 0·30 to 0·36 and different combinations of fine and coarse aggregates were produced. In each mix, the paste volume was set at 35%, and pulverised fuel ash, condensed silica fume and a third-generation superplasticiser were added at dosages of 25%, 5% and 3% respectively. All the mixes were tested to have slump flow well above 700 mm. However, not all have satisfactory passing ability and segregation stability, as revealed by J-ring, U-box and sieve segregation tests. The cohesiveness and coarse aggregate content appeared to be the major factors as the passing ability and segregation stability were generally better for mixes with higher cohesiveness and lower coarse aggregate content. Nevertheless, some of the mixes may b...

Richter's laws at the laboratory scale interpreted by acoustic emission

Abstract: In the present work the acoustic emission (AE) technique is applied to examine some aspects influencing concrete failure in compression. By monitoring a structure by means of the AE technique, it proves possible to detect the occurrence of stress-induced cracks. Cracking, in fact, is accompanied by the emission of elastic waves that propagate within the bulk of the material. These waves can be received and recorded by transducers applied to the surface of the structural elements. This technique can be used for diagnosing structural damage phenomena. The current paper presents a mixed experimental and theoretical approach to evaluate the energy density dissipated during compression at each value of strain. Through this method, the two semi-empirical Gutenberg–Richter (GR) laws, well-known in seismology, are verified, the fractal interpretation results of which are very close to the essence of the AE phenomenon. Furthermore, the mentioned approach allows the parameter of ‘magnitude’ to be identified, appear...

Test method for determination of the transmission and anchorage lengths in prestressed reinforcement

Abstract: This paper describes a new test method to characterise the bond of prestressed reinforcements to concrete. In order to analyse the bond behaviour in terms of transmission and anchorage, this test m...

Influence of steam curing on hardened properties of recycled aggregate concrete

Abstract: The effects of steam curing on the hardened properties of recycled aggregate concrete were investigated. In this study, two series of concrete mixtures with water/cement ratios of 0·55 and 0·45 were prepared. Recycled aggregates were used as 0, 20, 50 and 100% by volume replacements of natural coarse aggregate in the concrete mixtures. The concrete specimens underwent standard water-curing and steam-curing regimes. The test results showed that the strength of concrete decreased as the recycled aggregate content increased. An initial steam-curing regime increased the one-day strength of the concrete, but the corresponding 28- and 90-day strengths decreased. Furthermore, steam curing reduced the static modulus of elasticity of concrete compared with that of the water cured concrete. However, the negative effect of steam curing diminished as the recycled aggregate content increased. Moreover, steam curing decreased the drying shrinkage and increased the resistance against chloride ion penetration of the recy...

Transient thermal creep of concrete in accidental conditions at temperatures up to 400°C

Abstract: In this work, the transient thermal strains of high-performance concrete (HPC) under constant load and increasing temperatures up to 400°C in accidental conditions are studied experimentally. Heating is applied at a rate of 1·5°C/min until successive constant temperature levels are reached. These temperature stages are maintained for 24 h to ensure the stabilisation of internal temperature and the physico-chemical thermo-dependent process. The analysis of experimental measures is concentrated on the so-called transient thermal creep. Test results confirm the existence of transient thermal strains during heating even for small loads. The transient thermal creep (TTC) values were estimated for four temperature levels: 150, 200, 300 and 400°C.

Plastic shrinkage cracking in concrete slabs. Part I: a numerical model

Abstract: In the current paper, an analytical model that can predict the occurrence of plastic shrinkage cracking in concrete slabs is introduced based on the balance between bleed water and bleed water evaporation. To estimate the rate and amount of bleeding in concrete slabs, the consolidation model proposed is used after comparing the analytical results with the experimental results. In the presented model, the effect of temperature variation caused by the hydration heat of the concrete matrix is taken into consideration when estimating the evaporation rate of the bleed water. Then, the time at which the concrete surface begins to dry, which is closely related to the time at which plastic shrinkage cracking begins to occur, can be estimated using the two analytical models mentioned above. Through correlation studies between experimental data and analytical results, it is verified that the proposed model can predict the occurrence of plastic shrinkage cracking with relative precision.

Plastic shrinkage cracking in concrete slabs. Part II: numerical experiment and prediction of occurrence

Abstract: In the present paper, the influence on plastic shrinkage cracking of many factors related to concrete mixture properties and weather conditions are analysed through parametric studies using the num...

Mechanisms of long-term decay of tension stiffening

Abstract: Details of an extensive collaborative project to study tension stiffening have been recently published in a number of papers. Among other factors, the project investigated the loss of tension stiffening with time. It has been concluded that the reduction of tension stiffening to a constant long-term value is fairly rapid, being complete in 10–30 days. This paper explores the mechanisms that may operate to cause the reduction of tension stiffening with time. It is concluded that the loss is mainly attributed to cumulative damage resulting from either the formation of additional surface cracks or the formation or extension of internal cracks. It would appear that the final tension stiffening is only minimally dependent on concrete strength.

Seismic performance of repaired RC columns

Abstract: Steel or fibre-reinforced polymer (FRP) composite jackets have been utilised in retrofitting reinforced concrete (RC) bridge columns and have shown to be effective in enhancing seismic performance of the structures. However, to date, only a few researches have been conducted on the behavioural characteristics of the repaired RC columns using steel or FRP jackets. In the present paper, the comparative performance of repaired RC columns using steel and CFRP jackets is presented. Also, the effect of transverse reinforcement ratio on the behaviour of the steel and the CFRP repairing is investigated. Monotonic and cyclic load tests are conducted on nine RC column specimens with different repairing strategies and transverse reinforcement ratios to compare the ultimate and the hysteretic behaviours. From the tests, it is observed that both steel and CFRP jacket repairing can significantly increase the ductility and the ultimate capacity of damaged columns. Notably, the steel jacket repaired columns show better e...

Shear deformation in reinforced concrete beams with thin web

Abstract: In this paper the main results of an experimental campaign, carried out to analyse the shear deformation and interaction between moment and shear on the global and local deformations of reinforced ...

Strain of heated concrete during two thermal cycles. Part 2: strain during first cooling and subsequent thermal cycle

Abstract: The strains of three nuclear reactor type concretes during first cooling from constant test temperatures of 110°C to 600°C and during a second heat cycle have been analysed in detail. Drying shrinkage and load-induced thermal strain (LITS) are largely absent from the strains measured during the subsequent three thermal transients because the ‘delayed’ components were largely (but at lower temperatures not totally) dissipated in the five-day period at constant temperature. An important conclusion derived from the strain measurements during the three last thermal transients is that they provide a sensitive indication of the thermal stability of the concretes as deduced from thermal cycling tests with and without compressive loading. It is from these results that the concept of the ‘concrete-specific critical temperature’ emerges, and it is being used for the first time in the current paper. Up to this ‘critical’ temperature, a second thermal cycle would not cause significant additional damage. At higher tem...

Strain of heated concrete during two thermal cycles. Part 3: isolation of strain components and strain model development

Abstract: This paper follows on from the two previous papers in the series dedicated to the strain behaviour of three nuclear reactor type concretes during a 14-day two-thermal cycle While the trends were described in detail in the previous papers, the present paper is dedicated to the separation and quantification of strain components during the various stages of the heat cycles in order to develop a predictive strain model for use in finite element modelling The total strain is shown to be the superposition of individual strain components, each related to the specific strain inducing mechanism within the concrete Having established the principles of strain components, the strains measured during the first thermal cycle under load were isolated, and quantified, according to the strain type and heating stage: (a) virgin heating under load: load-induced thermal strain (LITS) and shrinkage components; (b) constant temperature: creep, shrinkage and crack-induced strain; and (c) cooling: contractive thermal strain,

A comparative study on the interpretation of concrete core strength results

Abstract: Cores measuring 94 mm and 144 mm in diameter and having length-to-diameter ratios of 2·0, 1·0 and 0·75 were removed from beam specimens produced from different concrete mixtures. Standard cubes and cylinders were also produced from the same mixtures. Compressive strength tests were conducted on 216 standard and 530 core specimens. The test results obtained from core specimens were compared with those of standard specimens. The effects of maximum aggregate size in concrete mixture, core diameter and type and size of the standard specimens on the interpretation of the core test results were examined. The test results showed that the maximum size of the aggregate affects the strength of the cores even when the diameter of the core is larger than three times the maximum size of the aggregates. The age of the concrete was found to be an important factor in the interpretation of the core strength results. Although the cores with a length-to-diameter ratio of 0·75 gave higher strength values, they may be used fo...

Cyclic behaviour of interior post-tensioned flat plate connections

Abstract: In high seismic regions, post-tensioned (PT) slab–column frames are commonly used to support gravity loads in conjunction with a lateral-force resisting system (LFRS) such as a core wall. The LFRS is designed to resist 100% of the design lateral forces as well as to limit lateral displacements to an acceptable level, whereas the slab–column frame must sustain the gravity loads under the expected (design) displacements. Given the relatively sparse data on the seismic performance of PT flat plate slab–column frames, cyclic tests of four interior PT slab–column connections were conducted. Primary test variables were the level of gravity shear at the slab–column connection and the slab tendon arrangement. Test results indicate that both the test variables strongly influence the cyclic behaviour of the PT connections, and that the use of slab bottom reinforcement at the slab–column connection was effective in resisting positive moment developed under lateral loading as well as improving the hysteretic energy d...

An analytical method for prediction of the elastic modulus of concrete

Abstract: Experimental and theoretical research has shown that in predicting its elastic modulus, concrete should be modelled as a three-phase material at a mesoscopic level and the proportions, mechanical properties and interaction of these three-phase constituents should all be considered in the prediction. The current paper attempts to develop an analytical method for prediction of the elastic modulus of concrete. A three-phase composite circle model for the concrete matrix is proposed to represent the heterogeneous nature of concrete. Based on the classic theory of elasticity, a closed-form solution to the plane strain bulk modulus of concrete is derived. After verifying the developed method with both experimental and numerical results, the effects of the aggregate area fraction, the elastic modulus of aggregate, the elastic modulus of interfacial transition zone (ITZ), the maximum aggregate diameter, the maximum cement diameter and the aggregate gradation on the elastic modulus of concrete are examined in a qu...

Behaviour of concrete columns with drilled holes

Abstract: Holes drilled out to install additional services or equipment, such as for ducts through columns, beams or walls, can lead to loss of strength and possible structural failure. Until now little work has been done on holes in columns and, hence, the present study aims to examine the amount of strength lost owing to the presence of holes in columns. The reported experimental work deals with different parameters such as the number and dimensions of the holes and their relative position. It is shown that, for large diameter holes, a section capacity loss of up to 50% is possible.

Constitutive behaviour of high-performance ferrocement under axial compression

Abstract: The increased utility of ferrocement for stressed skin surfaces such as domes, arches, funicular shells etc, necessitated further research and development for better performance. The aim of the cur...

Fire resistance of RC and FRP-confined RC columns

Abstract: The retrofit of columns by fibre-reinforced polymer (FRP) confinement has become a popular technique over the past decade but when this technique is adopted to strengthen reinforced concrete (RC) c...

Durability properties of low-resistance self-compacting concrete

Abstract: The building industry is turning increasingly to the use of self-compacting concrete (SCC) in order to improve many aspects of building construction: SCC offers several advantages in technical, economic, environmental and human terms. However, there are still some problems with regard to its durability, in terms of physical and chemical properties. The purpose of this research project is to study various durability characteristics of SCC compared with reference samples of vibrated concrete (VC) with similar low compressive strength. For this purpose, SCC and VC mixes were prepared using the same ingredients in identical proportions, the only difference being that limestone filler was used for the SCC mixes. Tests carried out on these samples revealed that there was no significant difference in the physico-chemical properties (oxygen permeability, chloride diffusion, water absorption, carbonation and leaching by ammonium nitrate) of the two types of concrete.

Modelling of bond between three-wire strands and concrete

Abstract: The bond between strands and concrete is of importance for prestressed concrete. The research presented in the current paper aims at a better understanding of the bond mechanism, and of how different detailings of the strand interface affect the behaviour. A bond model for three-wire strands was established and calibrated by use of pull-through tests. The results from finite element (FE) analyses with the bond model and the tests were used in parallel. It was found that adhesion, friction and the ability to develop normal stresses determine the bond response of the strand; consequently, they were used as input parameters in the bond model. How different detailing of the strand surface affects these parameters, and the influence on the bond mechanism, are shown. For example, adhesion has the strongest influence on the initial bond response in the cases of smooth and indented strands. Regarding indented strands, the maximum bond capacity is determined by the strand indentation. The knowledge gained can be u...