Various types of point defects are: 1. Vacancy 2. Interstitialcy 3. Frenkel Defect 4. Impurity Atoms. A point defect is a very localised disruption in the regularity of a lattice. It is a defect of dimensions just like a point (zero dimensions). The size of the defect could be one atom, or two atomic diameters, which is just like a [...]
Vacancies have been seen to be thermodynamically stable in crystalline solids above 0°K, and that their number increase exponentially with the increase of temperature, as illustrated in Fig. 4.8 for aluminium. To understand this, the usual concept of Gibbs free energy for metal-system is used. In a perfect crystal, all lattice sites are occupied by atoms so that no vacancies [...]
Pores, voids and blow holes are defects in solid castings. Voids can be regarded as an agglomeration of vacancies. Pores are discontinuities or holes in solid parts. Such defects normally form during freezing due to one or combination of both the factors: (i) Evolution of dissolved gas at the solid-liquid interface. (ii) Decrease in volume that occurs during freezing in [...]
The cracking of concrete may be divided into the following categories: 1. Cracking of Plastic Concrete 2. Cracking of Hardened Concrete 3. Cracking Due to Chemical Reactions 4. Cracking Due to Weathering 5. Cracking due to Corrosion of Reinforcement 6. Cracking due to Faulty Construction Practices 8. Cracking Due to Error in Design and Detailing. Type # 1. Cracking of [...]
The treatment of concrete structures is carried out in the two stages: 1. Preparation of Surface for Repairs 2. Application of Repair Materials 1. Preparation of Surface for Repairs: Before the execution of any repair, one of the essential requirements common to all repair techniques is to remove the damaged or deteriorated concrete with the help of proper tools and [...]
In this article we will discuss about:- 1. Concrete Repair by Polymer Impregnation 2. Resin Based Repairs in Concrete 3. General Use of Epoxies in Concrete Construction 4. Repair of Cracks in Massive Structures 5. Common Types of Repairs. Concrete Repair by Polymer Impregnation: In this system of repair, the damaged concrete surface is cleaned thoroughly and dried. After cleaning [...]
In this article we will discuss about:- 1. Assessment of Cracks in Concrete Structures 2. Evaluation of Cracks 3. Identification of Weak Spots 4. Selection of Repair Procedure. Cracking is the most common indication of the defects in a concrete structure. It may affect appearance only or may indicate significant defect or lack of durability. Cracks may represent the extent [...]
In this article we will discuss about:- 1. Causes of Overheated and Burnt Steels 2. Detection of Overheated and Burnt Steels 3. Reclamation. Causes of Overheated and Burnt Steels: When steels are heated above the upper critical temperatures, austenite grains coarsen. If the coarsening is not excessive, normalising, or annealing can refine the grains to normal size. If the heating [...]
Some of the casting defects are described below: 1. Surface Roughness: Too coarse a moulding sand or when pouring temperature is high leads to rough or pebbly surfaces on castings. In steel castings, roughness is produced due to occurrence of surface reaction at mould-metal interface in which iron is oxidised and iron oxide reacts with silica to forms rough compound. [...]