Concrete is a complex composite material composed of cement, fine aggregates (sand) and coarse aggregates (brick or stone chips) mixed with water that hardens with time. Generally, OPC (ordinary Portland cement) concrete cannot ensure more than 45MPa compressive strength. As a result, various types of other ingredients are added with the OPC to get high strength concrete along with additional functional properties, if necessary. Concrete is a vital material for construction of almost all types of structures, namely residential buildings, industrial structures, dams, roads, tunnels, skyscrapers, bridges, sidewalks, superhighways, etc. After casting, concrete starts to gain its strength with time, which gradually reaches to the maximum level (towards 100% of the targeted strength). Achieving 100% strength is very difficult and that the required curing period for this achievement is really not very certain. However, it is well established that the rate of gain of concrete compressive strength is higher during curing of its first 28 days after the casting and then it slows down. Depending on the compressive strengths, concrete may be categorized as follows:
Any durable concrete lasts for a long time without significant deterioration in service properties and helps the nature in conserving its resources. It also reduces waste formation related to frequent repair or replacement of old structures. Finally, durable concrete helps ensure reduced pollution, better safety and promising economic growth. The consequences of non-durable concrete structures are presented in Fig.1.
Now, what are the controlling parameters for the durability of the concrete? Concrete durability is not just strength dependent property. It is the ability of the concrete to challenge various environmental actions, e.g., chemical attack, carbonation, abrasion, freezing/thawing cycles, etc while maintaining its mechanical properties in desired level. Frankly speaking, it is not a luxury feature only for expensive structures or infrastructure projects. For all types of structures, durable concrete have plenty of benefits to the environment, people and the national economy. For example, additional ten years in the service life of various structures in any country may result billions of dollars saving to its national economy. This might also help look forward for other priority projects of the nation. Many factors may affect the durability of concrete structures such as structural design, concrete quality, workmanship, structure usage, nature of environmental exposure, etc. But, durability is greatly influenced by concrete permeability, i.e. the porosity level of the concrete structures. Many mixtures, designed with widely different materials and mix proportions, can produce concretes of equal strength, but with different permeability levels. Concrete that meets only the strength requirement may fail to fulfil the expected durability if the permeability or porosity level is high. So, in a very simple term, it might be quoted that the durability of a concrete is affected by its porosity level, which also affects its compressive strength. So, having the similar level of porosity, high strength concrete is arguably more durable. The importance of strength for durability of a concrete structure has also been mentioned in ACI 318-14 code. As per this code, a minimum compressive strength of 5000 psi (35MPa) is required to make the concrete durable to some extent. For a very long life, concrete must have high strength and minimum porosity.
High strength concrete is obviously more expensive than any conventional concrete in a direct volume comparison. However, as the strength level increases the concrete structures having similar level of load bearing capacity demand less volume of materials (concrete as well as steel reinforcement). This ultimately reduces the overall weight of the structures and their construction costs as well. To visualize this, the materials used and cost involvement for the construction of the Two Union Square Skyscraper in Seattle, USA (Fig.2) can be considered. This 58 storied 226m high rise building was built in 1988 with concrete of 131MPa compressive strength and 780MPa yield strength steel reinforcement (ASTM A1035). Compared to a similar size of building built with normal concrete (e.g., strength level 35MPa) and reinforcing steel bar (e.g., ASTM A615 grade, yield strength 400MPA), this skyscraper is around 50% lighter. Not only material saving, the overall construction cost was also 15% lower because materials saving means overall cost saving. So, it is really not true that construction with high strength building materials is an expensive affair and thus not suitable for poor or developing countries.
Besides the longer service life of RCC structures made with high strength concrete there are some other advantages also. Because of quick hardening behaviours, the overall construction time decrease drastically. Significantly high compressive strength concrete means more capability to support load. So, for a similar size of building, relatively lower column section will be required leading to expanded and valuable floor expanse. The possibility of longer spans with high strength concrete could decrease the amount of expected piers and pier support remarkably in the case of bridge applications. It also lowers down the overall steel bar consumption making the RCC structures lighter, which is the one of the seven required conditions to be earthquake safe. With increase in the concrete strength the possible decrease in column section and steel bar consumption is shown in Fig.3. The predicted less frequent maintenance could also add further cost advantages. So, it’s time for our policy makers as well as design engineers to think for mandatory use of high strength concrete, especially for making residential apartments, commercial buildings, flyovers, bridges, etc.
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