Concrete facts and fairy tales
Concrete facts and fairy tales.
Once Upon a Time….. Or so every good fairy tale tells us and it seems there are just as many fairy tales around some building materials, for their ability to preform miracles in service, take concrete for example, a product which was invented over 2000 years ago and is the go to product of choice for home owners, builders and architects today used primarily because of it’s almost unlimited durability and versatility, from Driveways, walls, floor slabs, roof’s and ‘new’ bench tops and polished floors? Although how many of us remember the ‘Old’ concrete washtubs of our grandparents? Everything old is new again.
Some myths about concrete need to be put to bed if it’s to do its job properly and give a useful service life for many generations. The first is one I learned whilst managing a large steel reinforcing prefabrication team in Brisbane, where we were designing, building and supplying elements used for everything from Stormwater Pit covers, large structural arch supports and Bridge Columns for the Clem7 Tunnel, Northern Bus Way and Hornibrook Bridge and that is ‘Reinforced Concrete Shouldn’t Crack!’ The fact is Structural reinforcement does not prevent cracking of concrete due to volume changes. Volume changes caused by moisture and temperature cycles are a natural part of concrete’s service life. Concrete restrained from movement may crack because it is weak in tension the steel reinforcement or ‘Reo’ does not prevent cracking of concrete but acts to ‘hold’ the crack faces together which gives the structure additional torsional strength that concrete alone could not achieve. Concrete has complementary compressive strength plus the lime in the concrete acts are a ‘rust’ inhibitor to the steel, further helping with the life of reinforced concrete.
Freshly poured concrete that is finished ‘Flat and Level’ will remain that way after drying!
The fact is concrete can and does change shape whilst curing, factors such as temperature, humidity and variations in the concrete itself can cause changes in the finish of concrete, ‘Curling’ of the concrete can occur at the edges of concrete as well as at joints and cracks, curling can be caused by differences in temperature and moisture content of the concrete between the top and bottom of the slab.
Concrete is very dense and impervious to water! Again this isn’t the case, concrete absorbs and expels moisture over its lifespan, it will draw moisture from the ground its been placed on (an area of high relative humidity) and transfer it through to the surface an area of low relative humidity, bringing salts and other minerals along with it, this is sometimes seen as ‘Efflorescent Salt Stains’ or damp patches on floors, walls and soffits. Ways to reduce the permeability or porosity of concrete is to specify concrete with a lower cement to water ratio, uniform aggregate sizing and to have chemical plasticiser additives included in the mix as well as a vapour barrier placed on the ground prior to pouring the slab to prevent moisture transfer. All these methods should be considered at the planning stage of any works.
The higher the concrete strength the more durable it will be! Concrete has a relatively high compressive strength when compared with other materials; however, in service use and design measures determine the durability of concrete. Uncontrolled cracking can lead to moisture penetration and corrosion of the reinforcing steel, resulting in deterioration also known as ‘Spalling or Concrete Cancer’. Another form of failure is Alkali-silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition. ASR is caused by a reaction between the hydroxyl ions in the alkaline cement pore solution in the concrete and reactive forms of silica in the aggregate. A gel is produced, which increases in volume by taking up water and so exerts an expansive pressure, resulting in failure of the concrete. In unrestrained concrete (that is, without any reinforcement), ASR causes characteristic ‘map cracking’ or ‘Isle of Man cracking’. The next is sulphate attack caused by salts drawn up into the concrete. All these problems are caused by uncontrolled moisture being drawn into the concrete.
So next time someone starts to tell you a story about building materials and the way they keep failing or shouldn’t be used in a particular way ask yourself if there are other factors not considered in the application, design and limitations of the product to ensure you get all the facts before discounting them or using something which may not be fit for purpose.
Senior Building Inspector