Do you remember the white sugar cubes our old fashion parents/relatives used to drop into the oceans of their tea or coffee? Now picture those in a much bigger size but made from sand, cement, stones, and water. As a result, those mixtures combined give us a material commonly known as concrete. You scoop the concrete into a mould and allow them to cure/dry for a day. The result is a concrete cube. I must admit, once they cure for a day, they become stronger to break. After all the word concrete means strong.
You might be wondering, why go through the troubles of doing cubes of concrete? To name a few: houses, buildings, roads, and bridges have a significant portion of concrete in them mainly because of the strengths it could offer. For example, if you are building a bridge, it is necessary to know if it would withstand the weight of the cars and not just collapse. Hence the material of choice is vital. Concrete has a funny way of reacting to weight by means of compression and tension. Therefore, the purpose of doing a concrete cube test is to validate the strengths of the concrete to ensure its in line with what the Designer/s (A Civil Engineer but specialised in design & software calculations) designs for that specific job.
Imagine your doctor recommends three doses of paracetamols a day for 7days. Then two doses of paracetamols from day 7 to 14. Followed by one dose of paracetamol from day 14 to day 21. Lastly, half a dose of paracetamol from day 21 to day 28. He also recommends during this period, you drink 2 liters of water a day, eat three specific dietary meals a day and abstain from drinking coffee. If you abide by this health guidance, you’d see a significant improvement in your health and be at your strongest on day 28. However, failure to adhere to your doctor’s guidance could lead to a dent in your well-being, health ,and cost implications.
Just like humans, when we were younger, our bones were weaker and became stronger the older we get. Concrete has the same behaviour. Therefore, as Civil Engineers we’ve studied this behaviour and came up with a time vs strength chart/diagram that could be used as the basis on which all projects involving the use of concrete could be derived from to provide Engineering safety assurance for all projects.
The concrete cubes would be at its strongest on day 28. Failure to achieve the required strengths could be due to poor curing methods, insufficient or incorrect materials (sand, cement, stones & water), room temperature etc.
Concrete has a pattern of increasing with strengths over a period of 7, 14, 21, 28 days. Therefore we conduct these concrete cube samples, extract them from the mould, cure them in water, test their strengths in the lab using a compressive testing machine and record the results. Concrete become stronger with water over time hence, we put these cubes in a water-curing tank at room temperature. In addition, during the making of these samples, we also consider what is known as a slump test. An indicator to determine whether the concrete has been mixed properly.
One of my roles as a start-up Civil Engineering Graduate is to conduct these samples on site and send them to the lab – subject to the project you are working on. Before the real-world experience, I studied the theory and practical aspects of concrete cubes samples at university – but that’s a story for another day. At university, we had to mix the materials ourselves to produce concrete (See slides below),whereas in the real world the material is already mixed by the concrete supplier. Throughout my Graduate scheme, I have worked on various projects and conducted concrete samples. To ensure we are compliant with the clients and project requirements. Again, I must admit I have made over 50 concrete cubes and a few ended up failing in the lab – so don’t be hard on yourself when this happens.
If they fail, this could lead to cost, time and safety implications. In other words, the entire concrete foundation of your bridge or house needs to be dug out and started from scratch. Therefore, it is important and necessary to conduct and obtain concrete sample the correct way by doing so contributing to the success of projects – It keeps Everyone Happy!!!
NOTE 1 : Safety first. Ensure you have the correct personal protective equipment and supervision (if necessary). Concrete causes skin irritation which could led to Dermatitis – ‘Don’t forget to protect those precious hands with gloves’
NOTE 2: Did you know that Concrete becomes, even more, stronger when combined with steel? i.e reinforced concrete. The two are like best friends.
NOTE 3: In the real world, the concrete cubes are often collected a day later by a sub-contractor (often a company specialising in concrete cube testing) and taken to their lab for curing and testing.They will then send the results, back to us via email.
Tip 1: An enemy of concrete is air voids. So do not forget to whack your concrete cube samples 27 times (before curing) with a tamping rod to get rid of the air in the concrete. Otherwise, your cubes would end up with holes in them.
Tip 2 : Always have spare cube/s, just in case your results from the lab for the other cubes are unsatisfactory. The spare cube/s might prove otherwise.
Tip 3: Always use a marker pen to write the dates on your cubes and distinguish which ones are day 7, 14, 21 or 28. My suggestion is you make 3 cube samples for each day – the more the merrier.
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