Porosity as an Intrinsic Property Influencing Durability - Porosity of construction materials

Porous Concrete later get changed to propagated cracks affecting durability of the system

POROSITY IN MATERIALS                                                                                               

Certain factors that are highlighted as issues in the sustainability of building construction are the durability of construction materials, the air quality within indoor and the health conditions of buildings. These issues are found important in both social and in the economic view. As per the International Energy Agency (IEA)2007, it has been stated that the materials that are used for construction internally and externally have influence in the occupant's health conditions in addition to the energy consumption that is carried out during the service of the building. This brings a point, to state the importance of selection of construction materials at the primary stage in bringing buildings that possess high environmental quality.

NF X 50-501, 1982 gives set of normative rules about the durability of construction and reconstruction activities. It defines durability as the ability of an entity to carry out its function based on the provided conditions of maintenance till the limit that is stated is reached. Or it is the ability to perform the way it is required to, throughout its service life.

The sustainability of building has a long connection with its durability. The diffusion of certain harmful species into the structures ( constructed building components) is a factor in which the durability shows its concern. In more clear words, the diffusion of chlorides into the wood or into the reinforced concrete structural components is a great concern, as it affects the overall performance of the structure. This is clearly the permeability property that the building material possess that bring the life of the structure to question. Anyway, both the parameters, the durability and the permeability are interrelated. The direct impact of these parameters on the structure is an unclear theory.

Physical Properties Of Materials influencing the Durability

This unlike other notes on physical properties of material, the microscopic, as well as macroscopic detailing of materials and influence in building performance, is studied. Among all the physical properties that define a material, porosity is an important property. The morphology of each material is shown by its porosity. The porosity has a great influence on the durability and hence the sustainability of the structure. This article is more into the porosity of building materials in a broader view.
The porosity whether 2D or 3D that is observed in the material can be defined as the ratio of the area(volume) of pores in the material to the total area(volume) of the whole material.

When the porosity of the material is concerned, there are two types of it, that are observed. They are:

  • · Absolute Porosity or Total Porosity
  • · Accessible Porosity or The effective Porosity

The absolute porosity is the designation given for the porosity that defines an ideal porous medium. An ideal porous medium is a medium in the material where all the pores are interconnected. The pores that are closed are also included in this list. The closed pores are those that are not filled up by the fluids. Hence it is also called as the total porosity.

The effective porosity is the overall ratio of the pores that are connected to each other. This connection is the pathway for the flow that would take place in the medium. Only if the porous medium's structure is known, the term is used.

Based on the study carried out by Ollivier and Benboujdjema(2012) a figure representing the different types of pores that are present in the material are shown in fig.1. It is to be kept in mind that, the interconnection between the voids that are open will bring a drastic effect on the materials diffusivity properties.

Fig.1.Different Configurations of Porosities 

Porosity in Cement Paste

We will discuss the porosity in a detailed manner, by taking into consideration the cement paste. The cement paste is a heterogeneous material. The porosity that is seen in this material has a contribution from the air bubbles, the capillary pores, and the porosity through hydration. At different scales, the cement paste possesses different porosities. There are mainly two types of pores 

  • Capillary pores: These pores have a size ranging from 0.01 to 50μm. Based on Bouny (2007), the cement paste that has different water-cement ratios show structural differences due to the presence of these capillary pores. 
  • Gel C-S-H Porosity: These pores have a size ranging from a few angstroms to nanometers ranging from 50 to 100nm. These have a smaller dimension compared to the capillary pores. These pores are formed as a result of hydration reaction and are considered as their intrinsic property. These cracks are later by time decomposed to interlamellar pores. This with the action of loads or disturbances converts to intro- lamellaires cracks. 

Mortar and Concrete Porosity

The existence of pores in the mortar and the concrete are all the results of the evolution of micro-cracks during the early stages. The porosity of the aggregates does have influence in the contribution of pores. 
The figure below shows the variation of porosity for a cement paste, without silica fume and water cement ratio equal to 0.4. The contact interface, due to the presence of aggregates tends to increase the porosity through a destabilizing effect. The graph below is expressed for different aggregate content. When the composition incorporates silica fume, this porosity increase with the addition of aggregates is less visible.

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