ABSTRACT
To understand the weakness of concrete and the impact of adding reinforcement to concrete on its strength under stress. We first build two identical concrete cylinders, one for the compression test and another one for the tensile test. The result shows that concrete’s strength is much stronger in compression stress than in tension stress. Then to test out the impact of adding reinforcement to concrete on its strength under stress, we build two similar concrete beams except one has reinforcement( rebar) cast inside the beam and one does not. Then we put them in the press, the result comes out that the beam that has rebar in it not only can take more stress than the regular beam with no rebar but also the rebar beam’s cracking mode changed from brittle to ductile mode. But this passive reinforcement will only take the tensile stress after the concrete cracking, which some people might want to avoid. Hence, we make two more beams to demonstrate active reinforcement, for one of the beams, we put tension on the rebar before the concrete is cured, and for another beam, we cast plastic sleeves into the concrete.The steel rods can slide smoothly in these sleeves.Once the beam is cured, we tightened nuts onto the rods to tension them. under these two ways, the rebar can take up tensile stress instantly without waiting for the concrete to crack, making it an excellent structure for construction.
INTRODUCTION
Speaking of concrete, I think everyone is familiar with it and knows it as a great construction material since we can see it occurs in every construction space. And when we are asked why we are using concrete, most people will just come up with the answer.” Because it is strong.” Sure, concrete indeed is a super strong material. But only in a certain way. Concrete itself is very strong in taking compression stress, but it can barely take little tensile stress and it will crack instantly. So why people nowadays are still using concrete as the fundament of building construction?
In this experiment, we tested the impact of adding reinforcement to concrete on its strength under press. Because the concrete is providing strength against compressive stress while the reinforcement provides strength against tensile stress. So I expect concrete with reinforcement can take heavier loads.
Materials & Methods
We are making out four beams of the same size, the same structure with the same ratio of concrete. But for the second beam, we cast a couple of rebars, a common type of reinforcement made out of deformed steel, cast into the lower portion of the concrete. In the third one, we put tension on the steel reinforcement tendons as the concrete are cast. In the fourth one, we
cast plastic sleeves into the concrete.The steel rods can slide smoothly in these sleeves. Once the beam cured, we tightened the nuts onto the rods to tension them. Then, we put these four beams once at a time under the press machine and start to apply stress to the beam, keep increasing the stress until the beam breaks, and record the weight that is applied to the beams.
Results
The video does not really give out a specific number of the weight that makes each beam breaks. But it does say that the second beam is much stronger than the first beam without any reinforcement in holding stress. Also, instead of the instant cracking from the bottom in pressing the first beam, the cracks in the second beam come pretty slowly and ductile. While the third beam and the fourth beam are not any stronger than the second beam, it just takes more pressure for these two beams to have cracks.
Discussion
The result supports my hypothesis that concrete with reinforcement can take heavier loads. Because concrete itself is actually not a great structural material under press since it is strong under compression stress but weak at tensile stress. And normally when the first concrete beam is being pressed, the top will experience compression stress while the bottom is experiencing tensile stress, hence due to concrete’s property, the beam cracks from the middle bottom at a brittle movement. However, for the second beam, the addition of the reinforcement is really helping the beam to take up the tensile stress because it goes through the whole beam and is placed at the bottom, the area where most tensile stresses are applied. Hence, when the stress is applied, the beam will crack, providing space for the reinforcement to take up the tensile stress. So the second beam not only can take up heavier loads than the first beam but also, the second beam cracks in a very ductile way, it gives people time to react and save it in real life. In addition to that, in order to avoid the cracks, we build the third and the fourth beam with active reinforcement, in which we apply tension stress to the reinforcement before it is cast into the beam, so the reinforcement can start taking tensile pressure at the beginning without waiting for the beam to crack to provide it space. This operation actually doesn’t really make it stronger than the second beam, but it does make the cracks come at a later time.
Conclusion
Concrete itself is not a great structural material, although it is strong in taking compression stress it is weak at taking tensile stress, and it is brittle. This means once the building that makes out of pure concrete cracks, it will leave people with no time to react. Hence, thanks to engineers, nowadays we have different types of reinforcement cast into the concrete to make it a strong composite structural material since now it not only can take more stress than the regular beam with no rebar but also the rebar beam’s cracking mode changed from brittle to ductile mode, giving people time to react on it.
