I have seen so much crumbling concrete here that I decided that it would be a good idea to hold a couple of workshops on making concrete. But first I needed a quick and easy test for the strength of concrete and an inexpensive device to do the testing. We did not need to test to an absolute standard but simply to compare the relative strengths of different batches of concrete. A challenge and an opportunity presented itself when a group of ninth graders came to me asking for help with a science project. Since we had only two weeks in which to complete the project, it had to be done with materials that we had on hand, and it had to be something that we could do in a short time. My concrete idea seemed to fill the bill, and we could present it as a demonstration of the scientific method to test a hypothesis.
This is the device that I threw together from materials that I had lying around the shop.
A measured force could be applied to the lever by slowly pouring a measured amount of water into the bucket.
The force appled to the sample could be calculated from the law of levers
For this experiment, we made a single batch of concrete using 5 parts of sand to one part of Portland cement (by volume). We added enough water to mix to a plastic consistency.
For molds we used one-quart plastic yogurt containers and added the same measured amount of mix to each container to give us a disk 3.5 cm deep and 9.5 cm in diameter. After adding the mix, the containers were tapped on the table several times to firmly settle the mix.
In this experiment there were four treatments and three replications of each treatment.
The treatments were:
1. Rapid drying
A common error locally in making concrete is to leave it uncovered to let it air dry, sometimes even in the direct sun. They even use the term "drying" instead of "curing" or "setting" to describe the hardening of concrete. Therefore, for this treatment we let the samples harden uncovered in the containers overnight, and the next day we removed them and let them air dry for the one-week curing period.
2. Covered
People here rarely cover their fresh concrete to cure it, partly because of the additional cost and effort and partly because they don't understand its importance. In this treatment, we covered the plastic containers with tight-fitting lids for the duration of the curing period.
3. Under water
The general belief is that concrete won't harden under water. I have been continually frustrated when my workers put their shovels and trowels into a bucket of water overnight rather than cleaning them, because they are going to use them the next day and they think that the water will keep the adhering concrete soft. In this treatment, after leaving the concrete to set for an hour, we carefully poured a layer of water 3 cm deep over the top of the now firm concrete and left it covered with water for the duration of the curing period.
4. Extra water added
One of the common practices here is to make a soupy mix with far too much water. Since most of the concrete is mixed by hand with shovels rather than in a mixer, it is much easier to turn a soupy mix than a stiff one. In this treatment, we took the same mix that we used in all of the other treatments and added more water until the mix was quite liquid. After the concrete had set (but still had a little layer of water on top), we carefully added water on top as we did with the "under water" treatment.
The next day...
All the samples were left to cure for 7 days, when the students returned to test them. Each sample was removed from the mold and carefully placed in the device so that the bolt pressed precisely in the center of each disk.
A measured amount of water was then slowly poured into the bucket until the disk fractured.
The students recorded the data and took them back to school, where they were to do all the calculations and write a report.
I had requested a copy of their report, but they never brought me one, so I don't have the data to present here. But based on our observations as we did the measurements, we can draw the following conclusions:
1. Concrete hardens perfectly well under water.
2. The samples cured under water and the samples covered with a waterproof seal yielded the strongest concrete.
3. The samples that were dried quickly yielded the weakest concrete.
4. Adding enough water to make a "soupy" mix significantly weakened the concrete.
I am now thinking of repeating this exercise with other youth groups. (I have come to the conclusion that it is a wasted effort to work with adults, who already have their beliefs and ways of doing things and are not open to change.)
There are several other errors that I commonly see that could be addressed in different experiments:
a. A dirty mix. The sand and gravel used in making concrete here is dug from river beds, and depending on when and where it was dug can carry varying loads of silt and debris. I have never seen it washed or even tested for silt content. When transport of clean water is difficult, the water used in the mix often is simply dipped from a muddy ditch or stream. Cement mixers are rare here, and most often concrete is mixed directly on the ground. A patch of ground is scraped clear of plants and debris, the sand and gravel are piled on the bare patch, a bag of Portland cement is dumped on the pile, and the whole thing is mixed by hand with shovels. In the process, significant amounts of soil are added to the mix.
I propose an experiment with five treatments:
1. Clean water and washed sand mixed on a clean surface
2. Clean water and sand dug directly from the river mixed on a clean surface
3. Muddy water and washed sand mixed on a clean surface
4. Clean water and washed sand mixed directly on the ground
5. Muddy water and sand dug from the riverbed mixed directly on the ground
b. A cement poor mix. A common practice is to measure the ingredients in "paladas" (shovelfuls). The proportions of cement, sand, and gravel therefore can vary depending on the moisture content and the texture of the sand and gravel and the care of the person doing the mixing. Because of the cost of Portland cement, there is a tendency to try to make the cement go further by adding a little extra sand. Extra sand is also added to stiffen the mix if too much water is accidently added.
I propose an experiment with 4 treatments:
1. A mix of 4 parts of sand to 1 part of Portland cement
2. A mix of 5 parts of sand to 1 part of Portland cement
3. A mix of 6 parts of sand to 1 part of Portland cement
4. A mix of 7 parts of sand to 1 part of Portland cement
c. Remixing partially hardened concrete. The usual practice is to mix a whole bag of cement at once, regardless of the time it will take to use it all up. If the mix becomes a little too stiff as it is being used, water is added and the concrete is remixed, sometimes several times throughout the day. It is not uncommon for workers to leave a pile of mix hardening while they break for lunch. Upon their return, they simply add water, remix it, and get on with their work. If a little bit of hardening concrete is left from one batch and they need another, they often just mix the old in with the new batch.
I propose an experiment with four treatments:
1. Put a portion of the mix directly into the molds, cover them, and leave them to cure
2. After a half hour, add sufficient water to make the stiffening mix plastic again, put another portion into the molds, cover them, and leave them to cure
3. After another half hour, add water to the mix a second time, mix it, put another portion into the molds, cover them, and leave them to cure
4. After another half hour, add water to the mix a third time, mix it, put another portion into the molds, cover them, and leave them to cure
d. Failure to reinforce. In a resource-poor country, steel is expensive, and despite the saying here, "lo barato sale caro" (cheap ends up expensive), a whole lot of concrete is poured with no reinforcement.
I propose an experiment with two treatments:
1. Samples with no reinforcement
2. Samples reinforced with a disk of chicken wire
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