Differential Expansion of the Concrete Causing Stress - Paper Example

Phase changes of the concrete components.

Heating of the reinforcements, with loss of mechanical resistance, dilatations. The concrete structures of the buildings must provide a maintenance time of resistant capacity in case of fire R for a certain time.

There are two approaches to providing a reinforced concrete element with the required strength: prescriptive and performance. In the case of the prescriptive method, adopted either by the IBC (International Building Code of the United States) or the European Concrete Model Code, conservative limits are established for the coatings and thicknesses of the sections. These values have led in practice to the fact that the problems with high temperatures in concrete structures are minimal and that this material is universally recognized as an excellent material against fire and high-temperature demands.

Regarding the information collected from the spread sheets indicating the condition of surface appearance of concrete (plaster/finish, colour, crazing), structural conditions and further correlated with the results of NDTS and laboratory tests, the structural members have been designated with various damage classifications. Combined with the personal experience of the expert, the visual inspection and the various tests present a fairly accurate condition of the damaged structural element. Since the reinforced concrete is a highly variable matrix, sometime the results of different tests appear to give somewhat contradictory results but with experience, these can be reconciled.

CHAPTER ONE: INTRODUCTION

1.1 Introduction

Concrete is affected by several environmental degradation factors and these factors limit its service life. For quality assurance and condition evaluation, tests are necessary (preferably non-destructive). Non destructive tests (NDTs) can be described as tests which maintain the original properties of concrete. NDT methods for fire damaged steel include Hardness test and metallurgical microscope. Hardness test is simple and easy, but the hardness test results should not be used to guarantee the material to an appropriate specification, for which coupon tests are required. Microscopic test requires specialist personnel and equipment. It is used only when it is essential, e.g. to provide information on the micro-structure of metal so as to establish an accurate picture of the heating environment.

There are several properties which make concrete different from other construction materials. One of them is that it is possible to manufacture concrete from different combinations of materials. However, the overall features of concrete will depend on the treatment that the concrete goes through before it gets to the site. NDT tests for steel has progressed at a much faster rate compared to NDT tests of concrete. This could be attributed to the fact it is easier to test steel compared to concrete. There is no standard definition for nondestructive tests when it comes to concrete. Mostly, they are tests that do not alter the concrete. For others, they are simply tests that do not alter or change the function of a structure, in which case the drilling of cores is considered to be a NDT test. They can also be described as tests that do less manage to the structure than does drilling of cores.

There are several ways through which fire damage to concrete can be measured. Some of these ways include visual classification, DT tests, NDT tests (such as thermoluminescence, ultrasonic pulse velocity) and pink indication method. It is important to choose the appropriate testing method before detailed assessment starts. The Schmidt hammer test is a simple and cheap approach, but it only gives an indication of the concrete properties near the surface of the structural element and its results may contain large scatters; the ultrasonic pulse velocity method is also easy to use and may be used to detect internal cracks of concrete, but it requires access from both sides of the structure; the thermoluminescence method involves taking a small amount of concrete through a considerable depth; through analysis of the thermoluminescence lost due to heating, the temperature profile through the concrete depth may be established, from which the mechanical properties of concrete may be obtained.

Builders use more concrete in construction compared to other man-made materials present. The history of concrete is traced back to the ancient Egyptians. Nowadays concrete mixing is a science about materials that play an important role in civil engineering. Engineers design concrete mixes with steel reinforcement to meet the structural and temperature requirements of any project under construction. The hot climates generate many problems to work with concrete. This includes complications from heat, high reinforced concrete temperatures, low humidity and solar radiation. Any of these variables can cause the premature evaporation of water from the concrete, resulting in shrinkage, cracking and a decrease in the strength of the concrete. Engineers sometimes use ice in the mix design to keep the temperature low.

Reinforced concrete contains steel fibers and rods (bars) in the structure to generate force. This allows the steel to take the tensile stress in the concrete structure; however, the steel does not interfere with the thermal expansion of the concrete. Many types of aggregates can be used in the design of the concrete mix to add strength to the structure while it cures. A bit of crushed rock is always used in most concrete mixtures since it has a smaller number of crystalline quartz structures. If a crystalline aggregate is added to the mixture, the thermal expansion increases with temperature to a maximum temperature of 700 degrees Celsius. As the quartz crystals expand and break with the heat of hydration, the concrete disintegrates and begins to detach in small parts. In nature, this occurs mainly with granite. The detachment of the layers is called exfoliation.

Several of the characteristics dependent on the temperature of the concrete is due to the type of aggregate used and the composition of the concrete mixture. The thermal conductivity is directly related to the ambient temperature. If the heat conditions rise, the thermal conductivity of the reinforced concrete drops. During the initial curing stage, the temperature of the concrete rises rapidly with the hydration process and can dry quickly, resulting in a thermal break. In special structures, for example, those that contain a high fuel charge or when you want to optimize the design to fire, it is possible to adopt the performance approach. In this, an analysis of the tensional state of the structure is made, including the effect of the temperature on both the concrete and the reinforcement bars and all the possible failure modes of the structure are analyzed as well as the redistribution mechanisms of the structure efforts in the structural elements.

The study of the microstructure of materials in engineering has led to remarkable advances in the knowledge of their properties and durability. Concrete is among the most widely used construction materials globally. Unlike other materials used by civil engineering, it has an extremely complex structure that is far from being a homogeneous material, both microscopic levels as in the meso level. In this compound a porous matrix surrounds a set of inclusions of different rigidity and size; pores and inclusions are randomly distributed along with a series of defects mainly constituted by micro and macrocracks. Although in the concrete many properties are related to the microstructural characteristics, the response of the material depends closely on the characteristic aspects of the mesostructure as the presence of defects, pores of different sizes, and mainly of the larger aggregates and th...