Proposal 

Introduction

Structure-property relationships are at the heart of modern material science.  Concrete has a highly heterogeneous and complex structure.  Progress in the field of materials has resulted primarily from recognition of the principle that the properties of a material originate from its internal structure; in other words, the properties can be modified by making suitable changes in the structure of a material.  Although concrete is the most widely used structural material, its structure is heterogeneous and highly complex.  At microscopic level, the complexities of the concrete structure begin to show up and it consists of mainly three phases i.e. HCP (hydrated cement paste), aggregate phase and the transition phase, which lies between the aggregate phase, and HCP.  It becomes obvious that the phases of the structure are neither homogeneously distributed with respect to each other, nor are they themselves homogeneous.  For instance, in some areas the HCP mass appears to be as dense as the aggregate while in other it is highly porous.  Also, if several specimens of concrete containing the same amount of cement but different amounts of water are examined at various time intervals it will be seen that, in general, the volume of capillary voids in the HCP would decrease with decreasing w/c ratio or with increasing age of hydration or by increasing the curing time. Hence, the porosity of the concrete would be reduced, which would lead to the better service life of the concrete with optimum amount of w/c ratio and the curing time.    
    Nowadays admixtures are used to modify the properties of concrete or mortar to make them more suitable for the work at hand, or for economy, or for such other purposes as saving energy and also for increasing the strength and durability of concrete. Cement requires high energy during its manufacturing process whereas the fly ash, blast furnace slag and silica fume are easily available industrial by-product, requiring little processing and have inherent cementitious properties.  In recent years significant attention has been given to the use of the silicafume as a concrete property enhancing material, as a partial replacement for Portland cement, or both.  The interest in the silicafume started with the strict enforcement of air pollution controls in many countries. Silicafume is a by-product resulting from the reduction of high purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon or Ferro silicon alloys. To find solutions to this problem, studies were initiated and after some investigations on the performance of silicafume in concrete have been conducted in various countries. Silicafume are very fine smooth spherical particles with a surface area of approximately 20,000 m2/kg. The particle size distribution of a typical silica fume shows most particles to be smaller than 1 micrometer with an average diameter of 0.1micrometer. This is approximately 1/100 of the size of an average cement particle, due to which it surrounds the cement particles. This small silicafume spheres act as fillers and they occupy some of the space between the relatively coarser cement grains, which can be otherwise occupied by water. This improvement in the distribution of the hydration products results in dramatic improvement of fine pore structure of the hardened concrete that then leads to denser, stronger and less permeable concrete. The beneficial action of silicafume is it reduces the porosity of the transition zone between the cement paste and the aggregates that increases the strength and impermeability of the concrete. The reaction between the high-density cement particles and low-density water results in hydration product consisting of solid of intermediate density and a residual pore system. Pores are therefore inherited to hydration reaction. The characteristics of the pore system, which affect the properties of cement-based materials, are the porosity and pore structure. The porosity, which is a measure of degree of porous ness of materials, is defined as, the ratio of volume of pores to that of the total volume of materials. The pore structure of concrete, perhaps more than any other characteristic of material affects the behaviour of concrete. The pore structure of material can be represented through a relationship between cumulative pore volume and the pore radius known as pore size distribution. There are basically two types of pores i.e. gel pores and capillary pores.       
The unique feature of microstructure of a cement-based matrix is the presence of a connected and finely divided pore system. Several methods have been reported in the literature for the study and evaluation of pore structure of cement based materials. Mercury Intrusion Porosimetry (MIP) is most commonly adopted method for the study of porosity and pore structure characteristics of cement based materials. The engineering properties such as strength, permeability, resistance to aggressive environments, etc., of concrete are mainly a function of capillary porosity and its distribution that can be easily assessed by MIP method.
The data available from the MIP can be used to study:   

  • Pore Size Distribution Curves
  • Estimation of Surface area and Surface Area Distribution
  • Estimation of Pore Populations
  • Estimation of equivalent Pore Size/Threshold Diameter/Critical Diameter
  • Distribution of Total Porosity, Free Porosity and Trapped Porosity.


Objectives
1.    To study the pore structure and porosity of concrete.
2.    To study the effect of curing and w/c ratio on pore structure of concrete.
3.    To study the effect of addition of silica fumes on pore structure of concrete.
4.    To predict the strength and permeability from MIP results using established models.

Need For Study
1.    To understand the microstructure of concrete.
2.    To see whether MIP test can be used as an additional NDT to predict the inside strength of concrete.


Scope Of Work
1.    The study is restricted to M20 grade of concrete
2.    Two levels of water cement ratio 0.35 and 0.5 were adopted.
3.    Two levels of curing periods 7 days and 28 days were adopted.
4.    10% replacement of cement with silicafume.


Methodology
1.    Literature study from library.
2.    Laboratory study along with experiments.
3.    Pore structure study through MIP.
4.    Analysis, conclusion and recommendations.


Hypothesis
1.    Silicafume decreases the porosity of the concrete and simultaneously increases the strength and impermeability.
2.    Water/cement ratio and curing period affects the microstructure of concrete.
3.    MIP can be used as an additional NDT for predicting the in-situ strength of concrete.
 

 

 

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