B. Bhattacharjee

Porosity, pore size distribution and in situ strength of concrete

In this study, in situ strength of concrete was determined through compression test of cores drilled out from laboratory cast beams. The apparent porosity and pore size distribution of the same concrete were determined through mercury intrusion porosimetry, performed on small-drilled cores. The normal-strength concrete mixes used in the experimental investigation were designed to exhibit a wide variation in their strengths. To ensure further variation in porosity, pore size distribution and strength, two modes of compaction, two varieties of coarse aggregates, different levels of age, curing period and exposure condition of concrete were also introduced in experimental scheme. With the data so generated, an appraisal of the most frequently referred relationships involving strength, porosity and pore size of cement-based materials was carried out. Finally, a new empirical model relating the in situ strength of concrete with porosity, pore size characteristics, cement content, aggregate type, exposure conditions, etc., is presented.

Study on some factors affecting the results in the use of MIP method in concrete research

Effects of rate of pressure application and forms and type of sample on porosity and pore size distribution of concrete estimated through mercury intrusion porosimetry (MIP) are presented in this experimental work. Two different forms of concrete sample, namely, crushed chunks of concrete and small core drilled out from the concrete beam specimens, were used for this study. The results exhibit that the rate of pressure application in mercury porosimetry has little effect on porosity and pore size distribution of concrete. It is also demonstrated that small cores drilled out from large concrete specimens are preferable as samples for performing porosimetry test on concrete.

Some aspects of evaluation of concrete through mercury intrusion porosimetry

Through an experimental investigation, statistical distribution of three identified parameters of mercury intrusion porosimetry which are closely related to the strength and performance of concrete, have been worked out. It has been demonstrated that distribution of these three parameters are normal for different mix proportions of concrete. The minimum number of samples to be tested while evaluating performance of concrete through these parameters of mercury intrusion porosimetry have been estimated using the experimental data for various permissible errors at 95% confidence level. Further, the number of samples to be tested for permissible errors to be within 15% of the true value are also suggested. In the second part of the paper an investigation on the form of sample to be adopted for MIP study of in-situ concrete has been presented, whereby it has been demonstrated that it is preferable to use mortar adhered with aggregate, rather than mortar devoid of aggregate from the same parent in-situ concrete.

Modeling of Chloride Diffusion in Concrete and Determination of Diffusion Coefficients

In this paper, a new form of mathematical model for chloride ingress in concrete is presented to review the process of chloride ingress in saturated and unsaturated concrete. Material parameters reviewed are the ion diffusion coefficient and solution diffusion coefficient, both of which have been shown to be concentration dependent, rendering the chloride diffusion process to be nonlinear. A simple experimental methodology and a calculation procedure are proposed enabling the estimation of chloride diffusion coefficients from experimentally determined chloride and solution profiles in a short time.

Influence of Size Fraction of Ponded Ash on its Pozzolanic Activity

The paper examines the role of different size fractions of ponded ash, characterised in terms of their physical nature and chemical composition, on the lime-reactivity strength of ash-lime-sand mortars. The paper also presents SEM and MIP characterisation of different size fractions. The results of the investigation show the strong influence of the fine particles on their physical properties and lime-reactivity strength of mortars. Ponded ash contains both reactive, small particles and non-reactive or poorly reactive large particles, due to which it loses its overall pozzolanicity. Its use as a pozzolan in cement concrete will only be possible if the non-reactive large sized particles are separated from it. The paper concludes that the practice of wet disposal of fly ash adopted mostly in India is detrimental to the pozzolanic activity of the ash.

EXPERIMENTAL SERVICE LIFE PREDICTION OF REBAR-CORRODED REINFORCED CONCRETE STRUCTURE

In this investigation, an experimental methodology for service life prediction of rebar-corroded reinforced concrete (RC) structure has been suggested. This methodology is based on the cumulative damage theory. The final failure is assumed to be the result of the effects of the two modes of failures. The first mode considered is the natural corrosion of rebar from the time of depassivation followed by the second mode, which is the accelerated corrosion of rebar caused by anodic electrolysis under the impressed anodic current for a short period. The actual cracking of the specimens after applying an optimal anodic current for a given period has been carried out by splitting under physical load.

A SIMPLE ARRANGEMENT AND PROCEDURE FOR IN- SITU MEASUREMENT OF CORROSION RATE OF REBAR EMBEDDED IN CONCRETE

Abstract An arrangement for the in-situ measurement of the corrosion current density, i.e. corrosion rate, of rebar embedded in concrete through a linear polarisation technique is described. It incorporates a simple experimental method being suggested for determination of the ohmic resistance of concrete, which enables the ohmic drop to be eliminated mathematically from the polarisation data. The error due to non-uniform distribution of the applied electric signal through a small size counter electrode to the working electrode (rebar) is removed by using the transmission line model. A calculation procedure for estimating Tafel slopes has also been suggested for precise estimation of the Stern—Geary constant. The utility of the arrangement and the experimental procedure suggested for in-situ corrosion rate measurement is demonstrated through a carefully planned experimental scheme.

Role of Steel and Cement Type on Chloride-Induced Corrosion in Concrete

This paper reports the findings of a comprehensive experimental investigation where the corrosion performance of different types of steel and cement were studied in different concrete mixtures contaminated with admixed chloride. Three different types of steel, namely cold twisted deformed (CTD) bars and two varieties of thermo-mechanically treated (TMT) bars have been used as steel reinforcement. Three types of cement used in this investigation are ordinary portland cement (OPC), portland pozzolana cement (PPC), and portland slag cement (PSC). Corrosion current density, half-cell potential values, and the relative concrete resistivity have been measured together with free and total chloride concentrations and pH values of the concrete mixtures. From the study, it was concluded that PSC performed best in increasing the corrosion initiation period while PPC performed best in extending the propagation period among the cement types. Similarly, Tempcore TMT steel performed best both in the initiation and propagation period than the other two types of steel.

Neural Networks as Decision Support System for Energy Efficient Building Design

This paper outlines the possibility of the application of neural networks to energy efficient building design. The case of thermal design of building elements has been chosen as an illustration. The neural network employed for this purpose is feed forward, non-recurrent, and multilayered. The training of neural network is based on the back propagation algorithm. The network was trained for an input set of desirable thermal parameters namely thermal transmittance, time lag and decrement factor. The output parameters constituted the material and section properties of the building element. Once trained, the net was then tested for the patterns for which it had not been trained, and the error percentage was found to be within a permissible limit.

Application of SAP and PEG as curing agents for ordinary cement-based systems: impact on the early age properties of paste and mortar with water-to-cement ratio of 0.4 and above

The feasibility of utilising “Superabsorbent Polymer (SAP)” and “Polyethylene glycol (PEG)” as curing agents for ordinary cement-based systems with water-to-cement ratio of .4 and above has been investigated. The investigation considers water-to-cement ratios of .4, .5 and .6 and curing ages of 3, 7 and 28 days. A commercially available SAP of 300 μm average particle size and a laboratory grade PEG with an average molecular mass of 6000 have been used as curing agents. The impact of SAP and PEG dosages on consistency, setting time and degree of hydration of paste samples has been evaluated. The evolution of strength, water absorption and water retention characteristics has been studied using mortar samples and compared with respect to water- and air-cured control. SAP doses in the range of .3–1.0% of cement mass have been found to be effective over the full range of water-to-cement ratio tested. PEG admixed at the rate of .5–2.0% has been found to be beneficial at .50% dose for water-to-cement ratio of .4, thus indicating its suitability for low water-to-cement ratio systems. Observations of this preliminary study indicate the viability of implementing SAP and PEG as self-curing agents albeit with certain limitations requiring further investigation.