Cure all your cracked concrete!
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Almost all of Americas’ bridges today are over 50 years old; the average age of bridges is 42 years. They were made of concrete. And the Department of Transportation claims that the majority are obsolete or structurally deficient. But ordinary concretes’ deficiencies don’t just apply to bridges. Indeed, the American Society of Civil Engineers estimates that $2.2 trillion is needed to promote US infrastructure from its current status of “D grade” to “B grade” infrastructure. But would it be possible to save some of this money if concrete didn’t break as easily? Concrete is an incredibly strong material; its main ingredients are cement, gravel, sand, and water. But the main problem is that it is brittle and tends to break easily. Furthermore, concrete needs steel inside for tensile strength. This steel expands and contracts depending on the weather and causes cracks in the concrete. But heavy trucks and machinery can also easily cause cracks. These cracks let in water and salt which causes steel to rust. Ultimately the whole pad crumbles.
Traditional concrete cracks easily and calls for expensive repairs. Self-healing concrete repairs itself by releasing bonding agents upon formation of cracks. The main question being asked is whether self-healing concrete can be made; and if it can, should the bonding agents be added to the mixture initially when the concrete is first made or when the cracks start to appear? This will be determined by comparing a traditionally repaired concrete to concrete that has been self-healed by a variety of bonding-agents in stress and durability tests. Hypothesis: If different samples of “self-healing concrete” are made from calcite, sodium silicate, a mixture of soil and milk, concrete mortar (control), and simple glue, then the calcite sample will endure the most stress, most cost effective, and will be most durable. The control is going to receive the same treatment as all other samples except no additional material (stated above) will be added to it. The reason that these agents were used is that various universities in the past have done research on these materials. Delft University in Netherlands researched using bacillus pasteurii and a food such as calcium lactate. Since these materials were not available, the bacteria was replaced by soil—which has a high concentration of this bacteria—and milk—which has a high concentration of calcium lactate. The bacteria and food are supposed to produce calcite when in contact with water. Calcite was also tested separately in case the soil and lactate did not produce it. Sodium silicate is otherwise known as “water-glass” or “liquid water.”
The hypothesis was confirmed. If different samples of “self-healing concrete” are made from calcite, sodium silicate, a mixture of soil and milk, concrete mortar (control), and simple glue, then the calcite sample will endure the most stress, will be most durable, and cost effective. This hypothesis was confirmed. As per the soil and milk, the mixture did not create calcite that could make the concrete repair. In fact when soil and milk were added after the crack formed, the samples could only hold up 0.1 kg. and when they were added before the crack formed, the samples could hold up a little more. As per the Calcium Carbonate samples, when the agent was mixed internally it performed worse than if the agent was added after cracks were made. This indicates that using calcite to fill cracks would actually strengthen the overall concrete mixture. This makes it effective for self-healing concrete. The glue samples produced similar results however the calcium carbonate were more effective. The calcite is perhaps the most effective agent tested. Even though the sodium silicate was stronger than it, the calcite is more cost efficient than the sodium silicate. But the sodium silicate preformed the best out of all samples. In the sodium silicate samples, when the agent was mixed internally, the concrete fared much better than the control. Even when concrete was repaired with the agent, the samples performed better than the controls. However, even though the sodium silicate is effective, it is not economically efficient. At $4.00 /100g the sodium silicate is by far much more expensive than any other agents. Eventually, this makes the sodium silicate unpractical. If the price of this agent does fall then the agent would be highly effective but today it is not effective.
This project has huge ramifications. It proves that even though sodium silicate may be the best agent to repair cracks, it is highly ineffective because of its high price. Therefore, calcite is the second best option. It proves that one should use calcite to fix any cracks. Furthermore, if calcite is used, it will not reduce the strength of the concrete (this is proven in step 3). in comparison, other agents may be effective in fixing cracks but the agents may actually reduce the overall strength of the concrete.
The “self-healing concrete” aspect of this experiment was not able to be expressed. In this experiment, the soil should be replaced with bacillus pasteurii and the milk should be replaced with calcium lactate. If these two things meet then the bacteria is known to produce calcite. If this is done then perhaps ‘soil and milk’ agent will be the most effective. This will truly make the concrete “self-healing.”
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