Science Buddies: "Ask an Expert"

Question: How does the ratio of sand to cement affect the strength of concrete?

Hypothesis: If the ratio of sand to cement is increased or decreased then the strength of the concrete will decrease because the proper ratio of sand to cement is needed to allow the cement to chemically bond to sand.

Our experiment didn’t provide us with results that we can test.

We filled vinyl tubes with four different “mixtures” of concrete (differing amounts of aggregate {sand}). None of the 16 samples solidified (including the one with the proper ratio of sand to cement). Therefore we cannot test the strength. I am not sure why this happened but it did. My presumption is possibly there was not enough air to cure the mixture inside the “semi” sealed vinyl tubes? Again, I am not sure.

So what now?

Do we document what happened and therefore provide no conclusion?
Do we try to come up with a different “mold” or “vessel” to try again?
If such is the case, we have a 12/1 data submission date so any ideas?


Please help!

Thanks

Hi!
This is a common occurrence during science experiments- things going not quite as expected. However, in any case, there should ALWAYS be a conclusion. Although you may not have gathered the results you would have liked to, you can still analyze what possibly went wrong, thus including any errors with the experimental design and procedure that might have caused the deviation from the expected results. If you have time, you should definitely try a less-sealed vinyl tubing and see what happens! In any case, you can still have a conclusion detailing everything you did and possibilities for future work and improvement upon the experiment. Best of luck!

Most likely the “semi sealed" tubes are the culprit. How about trying cardboard tubes, such as paper towels come on? A quick online search for "concrete tube forms" gave me this information about a product for homeowners:

Easy-to-use cardboard tube, scaled-down version of rigid fiber forms that heavy-construction contractors use

. You probably can't quickly get access to rigid fiber forms, but there are lots of sources for cardboard tubes.

Keith

You need to understand that "mortar" (mixture of portland cement, sand, and water) has three components and requires physical mixing to insure the chemical reaction is uniform throughout the batch. If you add too much water, the mix will take longer to set up and the chemical reaction will be slowed down by thermal losses. The typical mixing methods are to start with an insufficient amount of water and slowly add water until you get everything wetted and have a stiff mix. In order to prevent the mix from drying too fast by the form drying out the mix, you typically wet down the form.

You need to understand that "concrete" (mixture of portland cement, sand, gravel, and water) has four components and again requires physical mixing to insure the chemical reaction is uniform throughout the batch. The amount of portland cement needed for any given strength mix will be affected by the sand to gravel volume ratios. Typically when there is more sand than gravel, you need more portland cement.

Hope this helps you analyze things by thinking about ALL the components in the mix.