Transport properties of concrete pavements with excellent long-term in-service performance

Excellent long-term performance in concrete pavements is associated with both concrete strength and durability properties like permeability and chloride ion penetration resistance. Water and air permeability are investigated, as is the rapid chloride permeability test (RCPT). Strong relations between compressive strength and permeability and chloride ion penetration resistance of typical ordinary Portland cement (OPC) concrete pavements are observed. Effects of environmental exposure on permeability and RCPT results are explained. Through-depth variation of permeability and RCPT results within a pavement slab are shown and discussed.     

Influence of metakaolin on the properties of mortar and concrete: A review

Supplementary cementing materials (SCM) have become an integral part of high strength and high performance concrete mix design. These may be naturally occurring materials, industrial wastes, or byproducts or the ones requiring less energy to manufacture. Some of the commonly used supplementary cementing materials are fly ash, silica fume (SF), granulated blast furnace slag (GGBS), rice husk ash (RHA) and metakaolin (MK), etc. Metakaolin is obtained by the calcination of kaolinite. It is being used very commonly as pozzolanic material in mortar and concrete, and has exhibited considerable influence in enhancing the mechanical and durability properties of mortar and concrete. This paper presents an overview of the work carried out on the use of MK as partial replacement of cement in mortar and concrete. Properties reported in this paper are the fresh mortar/concrete properties, mechanical and durability properties.     

Identification of residual gas-transport properties of concrete subjected to high temperatures

The pore space of porous materials strongly affects both mechanical and transport properties. As regards building materials such as concrete which may be exposed to severe environmental conditions, these properties determine the service life of concrete and concrete structures. Recently, the gas permeability of concrete was identified as the key parameter controlling explosive spalling and, thus, the integrity of concrete structures under fire loading. In this paper, an experimental setup for determination of the residual permeability of concrete subjected to high temperatures is presented. Two different experimental procedures, characterized by either a constant or a decreasing pressure history at the pressurized side of the specimen, are considered. Transport properties are identified on specimens made of in-situ as well as laboratory-cast concrete with or without additional polypropylene (PP) fibers after cooling from high temperature, covering a range of permeability over four orders of magnitude. The obtained results highlight the effect of additives and the production process on transport properties and, thus, on the performance of concrete under fire attack.     

An experimental study on the water-purification properties of porous concrete

The results of an experiment on the compressive strength and water purification properties of porous concrete are reported in this paper. Two sizes of coarse aggregate were used, namely 5 to 10 mm, and 10 to 20 mm. Three absolute volume ratios of paste-aggregate were used, namely 30%, 40% and 50% for a given size of aggregate. The compressive strength is found to be higher when the size of the aggregate is smaller, and when the paste-aggregate ratio (P/G, vol.%) is smaller. In the water purification experiment, the amount of organisms attached on the porous concrete surface is indirectly examined by the consumption of the dissolved oxygen (DO, mg/l). Water purification of the porous concrete is evaluated by the removal amount of the total phosphorus (T-P, mg/l) and total nitrogen (T-N, mg/l). A porous concrete with a smaller size of aggregate and a higher void content was found to have superior ability of the removal of the T-N and T-P in the test water. This is due to the large specific surface area of the porous concrete. Results from this study show that porous concrete using industrial by-products is able to purify water efficiently.     

Performance of recycled aggregate concrete monitored by durability indexes

The report of an investigation into the performance of concrete manufactured with recycled aggregate (RA) using durability indexes as indicators is presented in this paper. Durability indexes, such as chloride conductivity, oxygen permeability and water sorptivity, of three different concrete mixes containing 0%, 50% and 100% RA were monitored at ages 3, 7, 28 and 56 days. The results show that durability quality reduced with increase in the quantities of RA included in a mix; however, as expected, the quality improved with the age of curing. At the age of 56 days, increases in index value of a concrete mix made with 100% RA over that made with 100% natural aggregate were 86.5% and 28.8%, respectively, for chloride conductivity and water sorptivity. The corresponding value of oxygen permeability index (OPI) for the same concrete mixes was a reduction of 10.0%. For 50% RA concrete, the reductions in chloride conductivity and water sorptivity indexes at the curing age of 56 days compared to 3 days were 62.7% and 42.7%, respectively. The corresponding figure for OPI was an increase of 37.6%. The poor performance of the RA concrete is associated with the cracks and fissures, which were formed in RA during processing, thereby rendering the aggregate susceptible to permeation, diffusion and absorption of fluids.     

Effect of high temperature or fire on heavy weight concrete properties

Temperature plays an important role in the use of concrete for shielding nuclear reactors. In the present work, the effect of different durations (1, 2 and 3 h) of high temperatures (250, 500, 750 and 950 °C) on the physical, mechanical and radiation properties of heavy concrete was studied. The effect of fire fitting systems on concrete properties was investigated. Results showed that ilmenite concrete had the highest density, modulus of elasticity and lowest absorption percent, and it had also higher values of compressive, tensile, bending and bonding strengths than gravel or baryte concrete. Ilmenite concrete showed the highest attenuation of transmitted gamma rays. Firing (heating) exposure time was inversely proportional to mechanical properties of all types of concrete. Ilmenite concrete was more resistant to elevated temperature. Foam or air proved to be better than water as a cooling system in concrete structure exposed to high temperature because water leads to a big damage in concrete properties.     

Research on improving the heat insulation and preservation properties of small-size concrete hollow blocks

To solve the problems of poor thermal insulation and heat preservation properties that exist in the walls made of small-size concrete hollow blocks, a new type of compound small-size concrete hollow blocks has been developed. As shown in tests and calculations, the compound blocks not only improve significantly the thermal insulation effect, heat preservation properties, and impermeability of the walls, but also show good decorative results.     

Permeation properties of self-compacting concrete

Permeation properties, which include permeability, absorption, diffusivity etc., have been widely used to quantify durability characteristics of concrete. This paper presents an experimental study on permeation properties of a range of different self-compacting concrete (SCC) mixes in comparison with those of selected traditional vibrated reference (REF) concretes of the same strength grade. The SCC mixes with characteristic cube strength of 40 and 60 MPa were designed containing either additional powder as filler or containing no filler but using a viscosity agent. The results indicated that the SCC mixes had significantly lower oxygen permeability and sorptivity than the vibrated normal reference concretes of the same strength grades. The chloride diffusivity, however, appeared to be much dependent on the type of filler used; the SCC mixes containing no additional powder but using a viscosity agent were found to have considerably higher diffusivity than the reference mixes and the other SCCs.     

Permeability characteristics of carbonated concrete considering capillary pore structure

During carbonation process, the calcium phases present in cement are attacked by CO₂ and converted into CaCO₃ and the permeability of concrete is changing due to the change in porosity. The rate of carbonation depends upon porosity and moisture content of the concrete. Especially in underground reinforced concrete structures, the interior portion of concrete surface may be exposed to carbonation and the exterior portion of concrete surface exposed to wet soil or underground water. As carbonation proceeds from outer surface into internal portion of concrete, microstructure is also changed continuously from outer surface into internal portion of concrete. Even the deteriorations in the structures due to the carbonation have been reported more, research on permeability characteristics of concrete considering carbonation and micro-structural information is very scarce.In this study, the permeability coefficient in carbonated concrete is derived by applying a capillary pore structure formation model in carbonated cement mortar and assuming that aggregates do not affect carbonation process in early-aged concrete as a function of porosity. The permeability obtained from the micro-level modeling for carbonated concrete is verified with the results of accelerated carbonation test and water penetration test in cement mortar.     

Mechanical properties of recycled aggregate concrete under uniaxial loading

In this paper, the compressive strength and the stress-strain curve (SSC) of recycled aggregate concrete (RAC) with different replacement percentages of recycled coarse aggregate (RCA) are investigated experimentally. Concrete specimens were fabricated and tested with different RCA replacement percentages of 0%, 30%, 50%, 70% and 100%, respectively. Uniaxial compression loading is applied in the experiments. Special attention of the analysis is devoted to the failure behaviour and the influences of the RCA contents on the compressive strength, the elastic modulus, the peak and the ultimate strains of RAC. Analytical expressions for the peak strain and the stress-strain relationship of RAC are given, which can be directly used in theoretical and numerical analysis as well as practical engineering design of RAC structures.     

Poroplastic properties of calcium-leached cement-based materials

Asymptotic calcium leaching of cement-based materials produces a new material composed of C-S-H with low C/S ratio on the order of 1 and a high porosity generated by the dissolution of Portlandite (CH) crystals, which creates a new pore-size family in the micrometer range. This paper investigates the role of these two phenomena in the multiaxial inelastic and hardening deformation behavior, in compression, of calcium-leached cement pastes and mortars. From triaxial tests and SEM microscopy, it is shown that the low C/S C-S-H matrix is highly plastically deformable, which is consistent with the high degree of polymerization and the effect of the C/S ratio on the intrinsic cohesion of C-S-H. The validity of the effective stress concept is experimentally proven for calcium-leached cement paste and mortar and provides evidence that the low C/S C-S-H solid phase of the cement paste is a pure cohesive incompressible material. In turn, the large pores created by the CH dissolution provides expansion space for the incompressible solid during compressive loading. Once this porosity is filled, the volume deformability is exhausted, and the material dilates to failure. In a similar way, the early tendency of mortars to dilate is found to be a consequence of a competition between plastic material behavior of the matrix (plastic hardening) and porosity-controlled structural deformation (geometrical hardening) triggered by frictional dilation mechanisms in the Interfacial Transition Zone (ITZ).     

A method for assessment of the freeze-thaw resistance of preformed foam cellular concrete

The growing use of cellular concrete for building materials and geotechnical fills brings forth the question of suitable durability and performance standards. Of particular importance is the performance of cellular concrete in freezing and thawing environments. Since the macrostructure of cellular concrete or cellular control low-strength material is not like that of normal-weight concrete, a modified procedure is needed to specify the required characteristics of cellular concrete that lead to freeze-thaw durability. This research investigated the freeze-thaw durability of cellular concrete and developed a modified freeze-thaw test procedure, based on ASTM C666. Physical properties related to freeze-thaw durability were measured for each mixture and compared to the initial properties. As a result of these comparisons, recommendations are made regarding the production of freeze-thaw-resistant preformed foam cellular concrete exposed to freeze-thaw environments. The results of the study show that depth of absorption was a key predictor in developing freeze-thaw-resistant concrete. Compressive strength, depth of initial penetration, absorption and absorption rate are the important variables in producing cellular concrete that is resistant to cycles of freezing and thawing. Density and permeability were shown not to be significant variables.     

Compared imbibitions of ordinary and high performance concrete with null or positive water pressure head

A method to simultaneously measure the moisture diffusion coefficient, D_θ, of unsaturated concrete, and the saturated concrete hydraulic conductivity, K_l, was developed for cylindrical specimens placed on a container filled with water that could be maintained at a given hydraulic pressure. Ordinary Portland cement Concrete (OPC) with a moderate and High Performance Concrete (HPC) with a low water to cement ratio were tested. The time dependent distribution of water content in the specimens was measured using a non-intrusive method based on gamma-ray attenuation. The measurements were conducted with varying hydraulic head (positive or null). Boltzmann's transformation was used to analyze the experimental results obtained at different hydraulic pressures and the difference between the null (or atmospheric) and positive pressure results is used to accurately determine K_l and also D_θ . This paper will present the results obtained using this original method, possible interpretations and future research.     

Characterization of the water flow through concrete based on parameter estimation from infiltration tests

When considering the durability and performance of concrete structures, the characterisation of the moisture distribution becomes a question of major concern. In this paper, a new approach for the characterization of the flow parameters, either under saturated or partially saturated conditions, is presented. The method is based on numerical back-analysis of simple and inexpensive infiltration tests. The satisfactory results that were obtained support the validity of the methodology developed and point towards a promising new line of research. Further examples covering a wider casuistic of concrete types must be performed before the applicability of the method can become fully consolidated.     

Influence of thickness on mechanical properties and crack-bridging ability of coatings for concrete

Different variables can influence the crack-bridging ability (CBA) of a coating. These parameters can be related to: (i) the substrate, i.e. the way the crack opens and develops in the concrete; (ii) the substrate/coating interface, i.e. the adhesion of the organic coating to the concrete; (iii) the coating, i.e. its thickness and mechanical properties; (iv) the external conditions, i.e. the temperature in field and the mechanical solicitation of the structure. In the present paper, some experiments were performed to analyse the third point, i.e. to find possible relationships between the variation of the CBA (in situ failure of the coating applied to the concrete substrate) and the mechanical properties (free-film failure) of coatings with thickness. Experimental results suggest that the value of strain at break is a key factor in the variation of CBA of a coating with thickness.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.     

An experimental study on the retrofitting effects of reinforced concrete columns damaged by rebar corrosion strengthened with carbon fiber sheets

This paper presents the results of an experiment study on the structural behavior of reinforced concrete (RC) columns damaged by rebar corrosion and the retrofitting effects of damaged RC columns strengthened with carbon fiber sheets (CFS). In the experiment, a cyclic horizontal loading test was carried out using RC columns damaged by different degrees of rebar corrosion and strengthened with CFS. As a result, it was revealed that the deterioration of their structural behavior was mainly caused by the decline in the confining effect due to the falling off of concrete cover and the reduction of mechanical properties of corrosion rebar. In addition, the test proved that shear strengthening using CFS is an very effective retrofit technique that prevents bond splitting cracks and shear cracks from growing and improves the ductility of RC columns with corroded rebars due to the confining effect of CFS.     

Physical and mechanical properties of concrete with added dry sludge from a sewage treatment plant

One of the main objectives of this study was to evaluate an alternative final destination for the growing production of sludge from sewage treatment plants. At present, a significant quantity of dry sludge is obtained in Catalonia. That is why this paper examines the possibility of adding this waste product to plain concrete with Portland cement.

Taking into account the authors' previous work, this study focused on making specimens of concrete containing different percentages of sludge from a sewage treatment plant, studying both their physical properties (density, porosity, and absorption capacity) and their mechanical properties (compressive strength, flexural strength, and elastic modulus) over time.

The results confirmed that up to 10% of treatment plant sludge can be added to concrete for use in certain very specific applications.     

Effect of lime putty addition on structural and durability properties of concrete

The effect of lime putty addition on main structural and durability properties of concrete was studied. Different types of cement were used for concrete preparation: a Portland cement, a pozzolanic cement and a Portland cement with the addition of 20% fly ash. The measured concrete properties were compressive strength, setting times, length change, porosity, carbonation depth and degree of steel bar corrosion. It was found that the lime putty addition has a positive effect on the properties of concrete that contain pozzolans and a slightly negative effect on the properties of pure Portland cement. This behavior was correlated with the availability of active silica of cementitious materials. The active silica of pozzolans reacts with the added calcium hydroxide giving constituents, which improve the concrete stability and durability.