Concrete resists weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and the properties desired.
Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of the concrete. The design service life of most buildings is often 30 years, although buildings often last 50 to years or longer. Because of their durability, most concrete and masonry buildings are demolished due to functional obsolescence rather than deterioration.
However, a concrete shell or structure can be repurposed if a building use or function changes or when a building interior is renovated. Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired.
For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor. High Humidity and Rain: With little to no organic content, concrete is resistant to deterioration due to rot or rusting by in hot,humid climates.
Moisture can only enter a building through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Walls need to breathe or concrete will dry out if not covered by impermeable membranes. Portland cement plaster stucco should not be confused with exterior insulation and finish systems EIFS or synthetic stucco systems that may have performance problems, including moisture damage and low impact-resistance.
Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. While repair may be justified to preserve the architectural legacy of iconic 20th-century buildings, such as those designed by reinforced concrete users like Frank Lloyd Wright , it is questionable whether this will be affordable or desirable for the vast majority of structures.
The writer Robert Courland, in his book Concrete Planet , estimates that repair and rebuilding costs of concrete infrastructure, just in the United States, will be in the trillions of dollars — to be paid by future generations. Steel reinforcement was a dramatic innovation of the 19th century.
The steel bars add strength, allowing the creation of long, cantilevered structures and thinner, less-supported slabs. It speeds up construction times, because less concrete is required to pour such slabs. These qualities, pushed by assertive and sometimes duplicitous promotion by the concrete industry in the early 20th century, led to its massive popularity.
Reinforced concrete competes against more durable building technologies, like steel frame or traditional bricks and mortar. Around the world, it has replaced environmentally sensitive, low-carbon options like mud brick and rammed earth — historical practices that may also be more durable. Early 20th-century engineers thought reinforced concrete structures would last a very long time — perhaps 1, years.
In reality, their life span is more like years, and sometimes less. Building codes and policies generally require buildings to survive for several decades, but deterioration can begin in as little as 10 years. But there is still a lack of knowledge about their composite qualities — for example, in regard to sun-exposure-related changes in temperature. The many alternative materials for concrete reinforcement — such as stainless steel, aluminium bronze and fibre-polymer composites — are not yet widely used.
Conceptually, a durability design is based around safety, where the structure must resist failure by various hazards it is exposed too. Safety has typically been applied to structural mechanics; however, we should not be so restricted in our design when dealing with the performance of materials. The use of this technique is increasingly advocated for dealing with durability and service life problems Siemes, Vrouwenvelder and van den Beukel, By incorporating time into the design, we can now value the degradation of the materials as part of the overall problem.
This time-based design has to set performance related requirements to ensure that the structure fulfils its long term service life goals for safety. This then has the effect of forcing the designer to ensure that the material selection will achieve the long-term durability requirement for the service life goals.
Table 1. When considering a durability design, a key understanding of the environment and the exposure of the materials is essential in achieving a good design. Table 2. Exposure classes and class descriptions BS EN Although both these tables differ overall, it can be seen that they both consider the degradation of the materials.
ACI categories are broken down by a damage mechanism and then class, with a variation of severity. As with BS EN , the class is broken down based on wet and dry cycles, with various corrosion risks associated with a letter in the exposure class. As with most situations, a series of environments and contaminants can coexist on the same structure, so the design engineer must pay attention to this in the durability design.
This can be extremely complex on some structures but, if overlooked, can result in a degradation failure in an unacceptable period, which can cost many millions of dollars. As we repair structures today, durability and the environment are not considered enough in their role in leading to premature and costly maintenance repairs.
Figure 2 provides an overall holistic approach to the considerations required to provide durable concrete structures. Figure 2. Holistic approach to durability. By reducing QA on a construction project due to cost and budget constraints, the long term performance of a building or a structure can be drastically affected. Building codes and policies generally require buildings to survive for several decades, but deterioration can begin in as little as 10 years.
But there is still a lack of knowledge about their composite qualities — for example, in regard to sun-exposure-related changes in temperature. The many alternative materials for concrete reinforcement — such as stainless steel, aluminium bronze and fibre-polymer composites — are not yet widely used. The affordability of plain steel reinforcement is attractive to developers.
But many planners and developers fail to consider the extended costs of maintenance, repair or replacement. Cheap and effective, in the short term at least. There are technologies that can address the problem of steel corrosion, such as cathodic protection , in which the entire structure is connected to a rust-inhibiting electric current.
There are also interesting new methods to monitor corrosion, by electrical or acoustic means. Another option is to treat the concrete with a rust-inhibiting compound, although these can be toxic and inappropriate for buildings.
Fundamentally, however, none of these developments can resolve the inherent problem that putting steel inside concrete ruins its potentially great durability. This has serious repercussions for the planet.
Concrete is the third-largest contributor to carbon dioxide emissions, after automobiles and coal-fuelled power plants. Concrete also makes up the largest proportion of construction and demolition waste, and represents about a third of all landfill waste. Recycling concrete is difficult and expensive , reduces its strength and may catalyse chemical reactions that speed up decay.
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