Designing the Concrete for Durability - Term Paper Example

 Contents Page No. Introduction 3 Causes of danger to the durability of concrete 5 The strategic modern approach towards increasing the durability of concrete 7 Conclusion 10 References 12 Designing the concrete for durability: Introduction: Modern age is an age of technology and development. Sky-high concrete buildings with beautiful architectural designs are a concrete proof of the advancement of any nation. Today we see new designs of buildings, bridges, airports and towers. These structures are fundamentally made up of concrete. This is because of the unusually high compressive strength and durability offered by concrete. The extent of durability achievable in the concrete can be estimated from the fact that in the past, concrete structures have been designed for a service life of up to 100, 120 and in some cases, 200 years, which is beyond the service life limits of most construction codes and standards. (www.cowi.com, 20-). Since the concrete is fundamentally a man-made material, it is quite possible to control its characteristics and behavior by altering its mix design specifications and adding admixtures to it. Hence, by the careful selection of appropriate materials for preparing the concrete, and close monitoring of its curing once it has been cast, the qualities of concrete can be significantly enhanced to meet the requirements of a particular structure. There are certain design requirements associated with every location and type of a structure. If the structure is made of concrete, the concrete needs to be customized in order to combat the challenges brought to the structure by the weather conditions and temperature of a particular location. This needs an appropriate evaluation of the required properties of concrete by studying the literature for the history of challenges associated with a particular climate. Also, the achievement of required results requires monitoring the whole process right from the start that involves concrete making, casting and finally the curing. Of these, the last two processes are majorly field-specific and require careful supervision from the workers in the construction site. The first part i.e. preparation of the mix design is the most influential process in achieving the desired durability in the concrete. This paper discusses the common methodologies used for the achievement of durability in concrete structures. The durability of concrete structures: Concrete has long been in use as a construction material for a large variety of structures including bridges. A number of long-lived concrete bridges have been constructed in the past. They include the Shakh Isa bin Salam Bridge in Bahrain and the bridge over the Lerez River at Pontevedra in Spain shown in the figure-1 and figure-2 below: “Figure-1: The Shakh Isa bin Salam Bridge in Bahrain”. (Paeglitis, n.d.: 1) “Figure-2: The bridge over the Lerez River at Pontevedra in Spain”. (Paeglitis, n.d.: 1) In his report, (Paeglitis, n.d.: 1) has mentioned that more than 94 % of all bridges in the present age are made of concrete. Their number is so large because of the ability of concrete to retain its strength over long periods of time. Causes of danger to the durability of concrete: Before developing an approach towards designing the concrete for safety, it is imperative that the potential causes of danger to the concrete are identified. Some of them are as follows: 1. The exposure of reinforcement in the concrete to chlorides in the atmosphere cause it to corrode and loose its strength. The reinforcement inside the concrete may be left exposed to the environment as a result of poor vibration of concrete while casting. Thus, there remain some areas inside the formwork where the concrete does not approach. Once the concrete has set, such areas show exposed steel without a proper protection of concrete cover. When this happens, the steel is likely to start to rust in no time. 2. Sulphate attack on concrete because of the reaction of concrete with sulphur in sulphur rich environments. 3. Other causes of damage include alternate freezing and thawing cycles which cause the development of cracks in the concrete structure, hence providing an ingress for the atmospheric chlorides. According to (Bruhwiler and Mivelaz, 1999: 210), the process of deterioration of concrete is fundamentally divided into two phases which are given below: The initiation phase: In this phase, chlorides in the atmosphere find their way to the steel inside the concrete through some cracks, or honeycombed areas resulting into carbonation of the steel. This phase ranges from the initial phases of construction to the ingress of atmospheric chlorides. The initiation phase of deterioration does not cause a loss of strength in any part of the structure. The propagation phase: This phase basically covers the duration of reinforcement corrosion. This duration is inversely proportional to the rate of corrosion of the steel inside the concrete. It is thought to continue till enough strength loss has occurred as a result of loss of the corroded portions of the structure. “A durable concrete structure obviously has both a long initiation phase and a slow corrosion rate.” (Bruhwiler and Mivelaz, 1999: 210). Ideally, for a structure to have a durable design approach, its initiation phase should be even longer than the service life for which the structure was principally designed. The technical service life of a concrete structure. (Paeglitis, n.d., 3). The process of corrosion: The corrosion of steel essentially comprises two processes, namely the anodic process and the cathodic process of corrosion. (www.phdonline.org, 2010: 2). Anodic process results from a deterioration of the protective concrete layer on the face of the embedded steel which causes the iron atoms in the steel to be converted into ferrous ions. Oxygen is reduced upon its reaction with water which results in the formation of the hydroxyl ions. This is referred to as the cathodic process of corrosion. It is the alkaline nature of the concrete because of which it serves to protect the steel from corrosion by preventing the iron atoms from converting into the ferrous ions. Thus, the process of corrosion is prevented. That is why, providing the steel with adequate thickness of concrete cover is an essential requirement of achieving durability in the concrete. The strategic modern approach towards increasing the durability of concrete: Conventionally, achievement of durability in the concrete is ensured by considering a modification of such factors as the minimum concrete cover, maximum water / cement ratio and the maximum allowable cracking. But the traditional approach towards making the concrete durable has little consideration of the service life of the concrete at the time of designing the structure. Modern approach towards making the concrete durable and long-lived is based upon two strategies which are as follows: 1. To eliminate the possibility of damage to the structure caused by the harshness of environment the structure is built in. 2. To choose such materials and specifications for making the concrete used in the structure that would ensure continued resistance of the structure to the process of deterioration. Of the two mentioned above, the first one emphasizes upon providing the structure with a complete protection which can principally be achieved in three different ways as noted by (Paeglitis, n.d., 4): 1. Altering the micro environment which includes the coating and membranes. 2. Using non-reactive type of steel in place of ordinary steel as reinforcement in the structure. Examples of such types of steel are stainless steel. Also, ordinary steel can be coated with a sealant or paint before covering it with concrete in order to achieve the inert effect. 3. Suppressing the process of corrosion of the embedded steel in the concrete or reducing the possibility of frost attack on the concrete by providing the structure with a proper air-void system. The second strategy focuses upon more careful design and selection of the constituent materials in order to achieve increased durability in the structure. This strategy makes use of practical tests performed on similar structures in the past to estimate the true variables involved in designing the structure for prolonged service life. In this strategy, the following factors are considered in the design of the concrete structures: 1. Relating the forms of deterioration under discussion to the right environmental factors involved. 2. Working out the most suitable material and specifications for both the steel and concrete in a reinforced concrete structure. 3. Elongating the initiation phase of deterioration by providing the structure with a thicker concrete cover of increased density and lower permeability. This tends to minimize the initial cracking of concrete thus increasing the duration of the initiation phase of the deterioration of the structure. The ingress of atmospheric chlorides and carbon dioxide is essential for the steel to corrode. If the concrete cover is made thick and impermeable, the penetration of atmospheric chlorides and carbon dioxide can be substantially minimized. This leads to the estimation of service life of the structure by making use of a birth certificate. The birth certificate is created with the construction of the concrete structure and mentions the properties of the constituent materials and guides on the limitations of exposure. Not only this, the birth certificate expresses the most likely way in which the structure would perform when it encounters hazardous conditions in its lifetime. Efficient QA / QC system: In order to achieve the desired durability in the concrete, it is imperative that the processes of its making, casting and curing are closely monitored by a team of trained and experienced professionals. Hence the need of a dominating and powerful quality control system is inevitable. Essential requirements and features of an efficient QC system are as follows: 1. First of all, the desired qualities to be achieved by the contractor should be stipulated in the form of specifications in the constructions drawings and documents by the designers / client. 2. Achievement of durability in concrete is a matter of national concern as any failures in this regard have the potential to result in huge loss of precious human life. It is, therefore, a responsibility of the government to take all necessary measures to ensure that good quality concrete is used in the construction. One such measure can be unannounced concrete quality tests. A team of government officials specially designed to address such issues should test the concrete in use in various construction sites and ensure that it is as per the specifications mentioned in the construction drawings and documents. 3. It is the responsibility of the quality control department designated by the client to ensure that the contractor has maintained a record of results of cube strength tests of all concrete cast in a project. 4. The curing of concrete must be ensured once it has set and the curing period should in no case be less than that stipulated in the method statement. The above measures, if taken, can ensure the achievement of a concrete that is durable and as per the requirements of the design of a structure. Conclusion: Designing the concrete for durability is a step towards a significant reduction in the overall costs associated with the design, construction and maintenance of a concrete structure. “Concrete will be durable if the specifications under which it is procured require the appropriate limits and the concrete is produced to comply with the requirements.” (Mather, 2003). The objective of designing the concrete for durability is ideally to make it so strong, that even the initiation phase of its deterioration takes longer to complete than its own service life. This can be achieved by covering the embedded steel with a thicker concrete cover, making it increasingly dense and impermeable with the use of appropriate admixtures, and designing it for much lesser cracks. One way to estimate the service life of the concrete structures, the extent of corrosion they would undergo in their life, the duration of their initiation and propagation phases of deterioration and their overall efficiency in overcoming the environmental challenges is by making use of the numerical models. It is imperative for the QA / QC department to test the concrete and make sure that the concrete prepared for casting is sufficiently impermeable in nature so as to keep the atmospheric chlorides and the carbon dioxide from penetrating into the structure. Measures should be taken to limit the development of cracks in the concrete as it hardens by manually minimizing the difference of temperatures between the concrete and the atmosphere. References: Bruhwiler, E. and Mivelaz, P., 1999. “From Corrosion of Existing to Durability of New Concrete Structures”. [pdf]. Available at: http://ibeton.epfl.ch/Pubs/199x/Bruehwiler99.pdf. [Accessed: 13 July 2010]. Mather, B., 2002. “Concrete durability”. [pdf]. Published by Elsevier Science Ltd. 26 (1). doi:10.1016/S0958-9465(02)00122-1. [Accessed: 13 July 2010]. Paeglitis, A., n.d. “Durability design approach for concrete bridges”. [pdf] Available at: http://www.balticroads.org/html/pdf/01_Paeglitis_LV_eng.pdf. [Accessed: 13 July 2010]. www.cowi.com, 20-. “Concrete technology and durability design”. [pdf]. Available at: http://www.cowi.com/menu/service/BridgeTunnelandMarineStructures/Documents/021-1700-031e-07a_Concrete%20tech_low.pdf. [Accessed: 13 July 2010]. www.pdhonline.org, 2010. “Concrete deterioration”. [pdf]. Available at: http://www.pdhonline.org/courses/s155/s155content.pdf. [Accessed: 13 July 2010]. Read More