The water/cement ratio: a fragile relationship

The water/cement ratio: a fragile relationship

It goes without saying: relationships are no walk in the park. Whether it’s with your partner, your parents or your children (or all of the above), they require tender loving care to bear fruit. But did you know that, as far as relationships go, concrete is also affected? Although you may find it hard to believe, the water/cement ratio, a fragile connection between two essential ingredients, plays a fundamental role in the makeup of concrete. Join us as we uncover what makes them click, or not.

W/C ratio 101: a backgrounder

The water/cement ratio refers to the ratio between the weight of water and the weight of cement in a concrete mix.  The concept was developed by famed American researcher Duff A. Abrams in 1918 – you may know him from such classic inventions as the slump test for measuring the consistency of fresh concrete – and lays the ground rules in the relationship between the water/cement ratio and hardened concrete’s compressive strength.

A delicate balance or why you shouldn’t add water uncontrollably

“Add some water, it’s coming out hard” – those right there are some all too common words at jobsites, whereby an operator, a concrete pump driver or any other professional involved in the process, adds water to the concrete by aiming a hose inside the mixer (unless of course they have a Putzmeister machine, in which case the process is carried out with the touch of a button). And you know what; it’s not good news, as the damage to the concrete is real. By adding the wet stuff to the mix, what they’re doing is altering the fragile water/cement ratio. And this can have serious repercussions as far as final quality is concerned:

  • Loss of compressive strength

 

One m3 of concrete is, as a rule, made up of 400-450 kg of cement. For simplicity’s sake, let’s say we have 425 kg/m3 and a water/cement ratio of 0.45. That means that in one m3 of concrete, we have:

  • 425 kg cement
  • Water: 425 kg cement x 0.45 kg water/kg cement = 191.25 kg water

Great! So now imagine that the operator (or other) in question grabs the hose because he thinks the concrete is coming out hard. In a matter of minutes (if we bear in mind that a hose can pump out anywhere from 30 to 90 litres/minute), he’s succeeded in adding between 60 and 180 litres of water to the mixer, which has a capacity of 6 m3. This means that our water/cement ratio has been significantly altered: from 0.45 to 0.473-0.52.

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And that, my friends, directly affects the strength of the concrete, which has gone from having 30 N/mm2 to having 24.8 N/mm2 – or a 17% decrease in its compressive strength at 28 days (check out the graph at the bottom of this blog post for further reference).

  • Loss of durability

 

Similarly to compressive strength, durability is also compromised when the water/cement ratio is altered. For example, when the ratio is high, the cement particles are dispersed.

This means that when the concrete hardens, the cement’s hydration products don’t cover the entire space, resulting in a porous finish.  The higher the water/cement ratio, the more porous the concrete and, consequently, the less durable it is.

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Conversely, with a lower water/cement ratio, the less porous the concrete, and the more durable it is.

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Conclusion: better to hold off

Like with any relationship, the water/cement ratio is complex – and now you know why. So instead of adding water uncontrollably, you’re better off not giving in to the temptation. And if your concrete doesn’t come with the relevant specifications, make sure to send the mixer back to the plant.

 

 

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