The durability of concrete - Essay Example

? REPORT ON THE DURABILITY OF CONCRETE Table of Contents Concrete, as a Construction Material 2. Definition of Durability 3. Processes of Deterioration 3.1 Processes under Fluid State 3.2 Processes under Solid/Hardened State 4. Case studies of deterioration and their remedial measures 5. References Concrete, as a Construction Material Concrete is a basic construction material, it is made of mainly four constituents, cement, fine aggregate(sand), coarse aggregate and water. With the addition of water to the cement and other components in dry form, this addition of water makes the concrete fluid and this flow able mixture enables us to pore in the desired shape and size which is dictated by the moulds and formwork. During this the water chemically bonds with the cement and hydration takes place. The ultimate strength of concrete is a factor of water cement ratio and this is the reason a precise mix ratio of concrete is calculated and mix design is prepared. Finally the dry components are combined to form hardened matrix. This final product, the concrete is good in compressive strength but weak in tensile strength. To finish this weakness we provide steel reinforcement which has high tensile strength. This concrete is called the reinforced concrete. The concrete’s quality is determined by the quality and care taken while the production phase of the concrete. As we are well aware of the fact that elasticity of concrete is all most the same at low stress levels but tends to decrease as the higher stress levels are attained as matrix cracking develops. At the very same time the concrete is very weak in thermal expansion and has a low value of coefficient of thermal expansion. The content of cementitious material, along with the water content determine the final strength, durability, abrasion resistance, shrinkage values and many other properties. But the most important property that is under discussion is the Durability of concrete. Definition of Durability: Concrete durability has been defined by the American Concrete Institute as “its resistance to weathering action, chemical attack, abrasion and other degradation processes” Durability can also be defined as “the quality of materials or structures of continuing to be useful after an extended period of time and usage”. Whereas according to PCA(Portland Cement Association),” Durability is the ability to last a long time without significant deterioration”. Summarizing the above definition the durability is the ability of concrete to maintain it size, shape and all other properties against adverse environment with respect to time. Now the next issue is that what factors affect the durability of concrete and how can the material be improved to get the highest efficiency and life. There are several aspects that govern the strength of concrete and its life but the most effective way to calibrate this is to understand the process of deterioration in the first place. The most common proof of the deterioration process is the formation of the cracks and the detail study of the types of cracks will enable us to decide and deduce the reason, mechanism and solution to the problem. These cracks can be formed by during the plastic state or the hardened state. The reason of which will be discussed in details in the next sections. Processes of Deterioration There are several ways in which the deterioration of the concrete can be studied and processes explained but I believe that the most comprehensive pattern to understand this process is to first understand the cracks and types of cracks and then the reasons for their formation. Concrete has two forms during its formation and service. Firstly it is in fluid state and then the hardened state. Now we will discuss the processes under the two states. Fluid State deterioration and Crack propagation The reasons for these cracks are mainly due to the poor workmanship and handling. The main reasons among this are plastic shrinkage, settlement and lastly movement of formwork and subgrade during the construction work. The reasons for the movement of the formwork are faulty design, substandard material, lack of proper supervision and careless workmanship. The subgrade movement can be caused by the segregation of concrete and it might be due to the excessive vibrations or falling of concrete from some height. When such process happens, the aggregates are displaced and the shape of the final set is uneven, leading to cracks. After this the major cracks seen in the fresh state are due to the plastic shrinkage, this type of cracks are generally seen in slabs and in near vicinity of the reinforcement. There are few solutions to eliminate the plastic shrinkage cracks after the setting of concrete and it is better to prevent these cracks by power trolleling and floating as soon as the cracks appear they are filled. This is generally avoided by better workmanship. Then another reason for these types of cracks is the rapid drying of the concrete where there is less thickness of concrete such soffits and slabs. It can be concluded that the cracks caused by the plastic shrinkage can be avoided by the proper workmanship. Then another major process of cracks which lead to deterioration of concrete is the plastic shrinkage cracks. This is a specific crack that is formed due to excessive settlement and bleeding of concrete during its plastic state. The main points where these cracks appear are the top of the columns and at joints. These cracks are because of air entrainment between the particles of the concrete and later these voids form a failure cracks. The solution to these cracks is that reduction in bleeding which can be attained by the introduction of fibrous materials or by using the air entraining admixtures. The plastic settlement can also be prevented by revibration of concrete. By effective revibration the compressive strength can be increased upto 25 % more(Technical report No 22, 1995). Processes of Deterioration in Hardened State There are several ways to sub categorize these processes of deterioration, as a general convention these are sub divided into Mechanical Processes and Chemical Processes. But in my point of view a better and more comprehensive subdivision of processes of deterioration is given in the Technical Report No 22, produced by concrete Society Working Party. The report mentions that the sources of the deterioration and crack propagation are subdivided into physical, chemical, thermal and structural aspects. The first which would be under our discussion would be Physical processes of deterioration and the shrinkage of aggregates, drying shrinkage and crazing is the main process which are the reason for the deterioration. When the concrete is in fresh state the volume of the aggregate is high but with time the water particles evaporates and after time long interval there is further decrease in volume due drying there is shrinkage. The factors affecting the long term drying shrinkage and consequently the durability of the concrete include temperature variations, curing methods, humidity and exposed surface. On the other hand the constituents of the concrete affect the durability. Firstly water as a constituent affects because more the water available more will be the evaporation and more void swill be formed. Then the type of cement also has an impact where it is seen that the increase in fineness will increase the tendency to shrink. The quality of aggregate also defines its shrinkage value, where as some different aggregates found in Scotland areas have very high shrinkage value during there life duration. Usage of admixtures directly influence the water requirement and the voids ratio, in addition it affects the water and air entrainment in the life span of the concrete. Then the humidity in the air will be also determine the amount of evaporation and drying of the concrete. The remedial measures for long term drying shrinkage are attained by using highest possible aggregate size, accurate mix design, using appropriate admixture according to environment of the concrete and proper workmanship and curing processes. Then another physical deterioration process is crazing which is due to the differential movement of water and moisture and high or uneven concentration of water content. Another reason can be the discontinuity of composition of material near the surfaces of concrete. Crazing is how ever not a dangerous aspect as far as durability is concerned but when the there is extra or accidental loading the spaling of concrete is seen. The next most important process is the Chemical Processes. The first among which is the corrosion of concrete. This process is sub divided into two processes, firstly leaching/acid attack and secondly corrosion of the steel reinforcement. Leaching of concrete is basically the dissolution. Normally the water used for concrete production is undersatured in CSH and Ca(OH)2, consequently its dissolves(solution produced) and this is called leaching. This process leads to higher porosity and lower strength gain. Then acid attack is another process which causes corrosion of concrete. As we are aware that concrete contains Ca(OH)2 which is basic and when the concrete comes in interaction with an acidic environment then concrete will corrode. The corrosive solution include HCl, HNO3, H2SO4, CaCl2, MgCl2, and NaCl. Lastly the corrosion of the steel reinforcement is also a process in which the steel chemically reacts with air trapped in the concrete and when the cracks are propagated over the reinforcement and air comes on interaction with steel and it oxidize and corrode. Carbonation of concrete is also another process in which Ca(OH)2 becomes CaO3 and this results into lowering of pH and then this results into the corrosion of steel reinforcements. Then sulfate attack is also another mechanism in which the sulfate ions attack the components of the cement. There are several sources of sulfates, which include the ground water, sewerage and waste disposal water, high sulfate content soil, sea water, agricultural waste water and lastly the aggregates that contain high sulfates content. Other chemical processes that deteriorates the concrete includes alkali-silica reaction, alkali-cabonate and some others. On the other side under the thermal processes of deterioration, freezing and thawing is a process which water expands and contracts due to change in temperature in the environment. This produces inwards and outwards pressure inside the concrete matrix. There are two types of the freezing and thawing cycles one is on the horizontal areas and the other is due to freezing of the water within the concrete. The first one is normally seen in parking plazas and wide roofs where the water stays and stagnates at the top of the horizontal surface and the freezing and thawing of the water results into cracks on the surface and the water then percolates into cracks and then apply pressures. At the same time it also corrodes the steel reinforcements. The effects of water going in the cracks and then freezing are very severe and immediate. Other mechanism of the deterioration include the creep effect which due to the deformation of the structure under constant loading. This produces cracks and then the environments effects on the concrete and its components are the same as mentioned in the discussion made earlier. Then another reason which results into enormous cracks and some times result into complete failure from service of the structure is the accidental load. These loads include heavy wind load, earthquake loads, demolition in near vicinity and etc. When such loads are applied they result into cracks which create many other issues and decrease the structures serviceability. Case Studies Of Deterioration Case Studies# 1 Building Beam As shown in the photograph below, a beam just above the door has deteriorated and the naked corroded reinforcement is visible. It is evident that the component has plastic settlement cracks of type A over its reinforcement. Then the loss of concrete also indicates that the concrete cover was insufficient to accommodate the slight movement/deflection of the member and as a result all the reinforcement was bared off. Another reason for the deterioration at this point could be the crazing which might have been caused by the improper formwork as it is a possibility because the edge is just above the opening in the wall and as a result proper curing and finishing could not have been provided. Remedial Measures: The remedial measures would be to remove some spoiled corroded concrete and make proper grooved surface and then grout the whole space with rich concrete and then paint the surface with a proper weather shield paint so that it can look good and have some life as well. Case Studies# 1 Building columns The second example is a deteriorated column of a Building. It is clear from the picture that it is a plastic settlement crack which is of type B and under due to the arching effect in the column. The reason of which are clear that this happened because the concrete has bleed excessively and early drying was experienced by the concrete in its fresh/fluid state. Then during the deterioration in the hardened state structure has been subjected to early thermal contraction which is the type G and is present in the external restraints. The reason of which are very much depend on the climate of Leeds in specific and Uk in general where the sudden cooling of concrete in the winter time construction is very common and the problem is very common in structures which are open to environment like the one selected. Then another deterioration that is seen is the corrosion in the steel reinforcement, which is because of the fact that poor concrete is used and less cover is given to the reinforcement where they tend to show high value of deflections such as the one in the top of the column. Remedial Measures: The first line of defense against the plastic settlement cracks and deterioration is the proper workmanship, revibration and usage of air entrainment admixtures or any other possible means to stop or reduce bleeding of the concrete. Then as the cracks are very enormous and a large amount of concrete has deteriorated hence we have to clear all the damaged concrete and replace it with good rich fine concrete. After the finishing work is done the surface is treated with thermoplastics, as mentioned before the possible cause of deterioration is the thermal contraction and temperature variation of the environment hence proper insulation is required to avoid the reemerging of the problem. Hence its is suggested that thermoplastic material such as rubber modified asphalts or advanced materials such as pitch PVC could be used as well. Case Studies# 3 Parking Plaza As a case study, there are three examples being taken from the near vicinity of the university and in the Leeds areas. The first one is the parking plaza site and the photo taken from there is as under. It can be deduced after having a detailed look at the photograph that the cracks and deterioration is in the beam of the parking plaza. It is proposed that the cracks tension bending cracks and the concrete has spalled and broken away from the corner resulting into naked corroded reinforcement. This proof that this is a type of plastic shrinkage crack which is seen under the reinforcement. There can be several reasons for the plastic shrinkage cracks when the concrete was in plastic state which could be the early drying of concrete, may be less water was used than the requirement. Another reason could be the less bleeding of the concrete. Then it is evident that the corner has broken down which is a indicates that proper reinforcement cover was not given as the parking might have been under heavy sudden loading the less cover had facilitated the breakage. The corroded reinforcement also substantiate the fact that less concrete cover was provided or it might have been because of low quality of concrete being used. Remedial Measures: The first target would be to decide whether the crack is dangerous or not and does it have any structural significance and then the remedy is suggested accordingly. In this case the as we see the problem is in a beam of a parking ramp where the ramp is rising and it is evident that the beam is a load bearing component in this case. At the same instance it is seen that the reinforcement has also rusted excessively. Suggesting that repair work must be made immediately. The first line of action in this case would be to remove the deteriorated concrete in the effected areas and then a repair cavity is made. The crack seen in the picture comes under the category of factures as they are more than 6mm long. So the cracks are opened out and mortar with fine cement is applied. It should be noted that the content of water in the fine concrete should be as less as possible. After applying the fine concrete it is suggested that the finished surface is first treated with some cost effective elastomer, such as polysulphites, silicon based resin or acrylics material. Lastly the final surface is painted with weather shield paint to improve the life of the repair and better aesthetics. References 1. Concrete cracking – who is to blame? (2005). Concrete cracking – who is to blame?. technical director, unibeton ready mix. 1-32. 2. PCA. (2010). Durability, Stands up to natures forces. Available: http://www.concretethinker.com/solutions/Durability.aspx. Last accessed 26-02-2012. 3. Concrete Society Working Party (1992). Concrete Society, Technical Report No 22 . 3rd ed. UK: The Concrete Society . 1-48 4. Dr. Jan Bisschop (2012). Concrete Durability, Concrete corrosion, sulfate attack, salt scaling, and strength reduction. Zurich: Swiss federal Institute of Technology, Zurich. 1-20. 5. Cement Concrete & Aggregates Australia. (2004). Concrete Basics. In: . A Guide to Concrete Practice . Australia: .. 45-49. Read More