What are the Main Causes of Damaged Concrete?May 29, 2015
Concrete is made by mixing together cement, gravel, sand and water. The particles in the mixture are bound tightly together chemically, so that the concrete sets hard, simulating the properties of rock. Concrete is the most frequently used building material on earth, but various factors cause it to deform or degrade over time, reducing its longevity.
The makeup of a concrete mix – that is, the proportions of cement, aggregates and water – depends on the appearance, performance and strength that the finished product requires. Modern concrete mixes can be complex, and failure to work consistently to the ratio of ingredients can weaken the concrete and compromise its durability.
Volumetric concrete mixing, in which the three basic ingredients are stored in separate compartments and mixed together on-site, allows the mix to be adjusted to different situations or conditions without wasting raw materials. By contrast, traditional barrel mixed, or drum mixed, concrete is ready mixed at the concrete plant and delivered ready to use, so offers no such option.
Even if concrete is mixed, placed and finished correctly, plastic shrinkage cracks, also known as crazing, may occur if the concrete isn’t cured properly. Curing refers to the process by which the concrete is maintained at an adequate temperature and protected from moisture loss in the early stages. Without it, a pattern of narrow, shallow cracks will form on the surface of the concrete. More importantly, the ingredients will not react to produce the expected strength and durability.
Chemical & Mechanical Damage
Unfortunately, concrete is also susceptible to attack from a variety of chemical, mechanical and physical sources. Properly graded aggregates, thorough mixing, good handling, placement and finishing practices and thorough curing increase the resistance of concrete to chemical attack. However, acids and soluble sulphate salts, particularly those of calcium, magnesium and sodium, are among the chemical agents that are potentially harmful to concrete.
Mechanical damage can be caused, before hardening, by settlement within the concrete mass before hardening or, after hardening, settlement of the surface on which the concrete is laid. More obvious causes of mechanical damage include abrasion, explosion, impact, overloading and vibration.
One of the main sources of physical attack on concrete is frost and successive freeze-thaw cycles can cause cracking, crumbling, expansion and scaling of a concrete surface. Ironically, the use of chemicals, such as sodium chloride, to remove ice and snow from concrete surfaces exacerbates the problems caused by the freeze-thaw cycle.
Typically, concrete aggregates are chemically unreactive, or inert. However, some aggregates do react with alkali hydroxides in the concrete mixture, causing expansion and cracking over extended periods. The effects of this reaction, known as the Alkali Silica Reaction (ASR), can be reduced by adding byproducts from other industrial processes to the concrete mixture. Silica fume, a byproduct of producing silicon metal, fly ash, produced during the combustion of coal and ground granulated blast furnace slag, a byproduct of iron and steelmaking, have all been successfully employed in this respect.
Written by Roadmaster Concrete Mixers, the leading supplier of volumetric concrete mixers across the UK and Europe. Get in touch for more information.