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Opinion: Concrete creep, stress and strain

05 February 2020

The action of creep in concrete structures has to do with the movement of water within the concrete under stress. Creep, in effect, ‘relaxes’ the concrete and lets the structure and the concrete get on with doing the job it was designed for.

There is a different kind of creep that gives any concrete technologist or engineer that knows what he is doing, more stress and strain than listening to someone who calls concrete, cement. Let’s just get that one out of the way, if you were wondering, there is a difference between the two. Cement is a grey powder and only an ingredient in concrete. You don’t mix or throw cement, you mix and cast concrete. Throwing cement in your driveway will result in a lot of hazardous dust, until the first rains.

Back to creep. The creep in our standards results in 30-year-old engineers looking like they have been working in the sun for 50 years, judging from the strain on their faces. This type of creep happens when, for example, 25 MPa concrete is specified and the test result is just a little bit low. It is true that the acceptance criteria have to be agreed well before the project starts, but in the absence of an agreement, the engineer is presented with a result of 24,5 MPa. As this result is very close to 25 and when you round up, the result might be accepted. Then on the next pour a result of 24 MPa is produced and because it is close to the previous result of 24,5 MPa, even this result is accepted.

Very soon the argument is made that if you accepted 24, you should be accepting 23. At this point we are well beyond the specification and accepting a substandard product and/or workmanship. The creep in acceptance criteria and the standard of products accepted can have far reaching results, beyond the costly creams to diminish the wrinkles that stress and strain leave on engineers’ faces (shout out to my wife!)

Just to clarify, the excuse for such behavior cannot be “but it shouldn’t be”. This is akin to someone driving 80 km/h in a 60 zone and, when pulled over by the cops, expecting not to be fined, because the speed limit is wrong. The fact remains that you are outside the specifications. Either get on to the committees which write the specifications and change the specifications, or adhere to the specifications. Get in the game with the rest of us, don’t stand on the sideline and criticise the referee and the players.

Know your specifications, make sure they are measurable and don’t be the engineer who sends a full load of 90-mmslump readymix back, because he measured a 92 mm slump, having not spun the truck for 5 minutes, not testing the truck within 30 minutes from arrival on site (that’s the truck, not the engineer) and not sampling according to SANS.

This is Construction Thor, reminding you to know the rules, agree before the pour and stay within the specifications. Put your hammer down, bring the lightning and make a difference!

Concrete Trends: 2019 August Edition


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