ABSTRACT: Traditional concrete cracks easily and calls for expensive repairs (currently ~ $2.2 Trillion in US). Self-healing concrete repairs itself by releasing bonding agents when run-off water (including salt) seeps through 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 multiple durability tests. There were five bonding agents: 1—calcium carbonate, 2—sodium silicate, 3—glue (modeling a polymer), 4—a mixture of soil and lactate (modeling bacillus pasteurii), and 5—concrete mortar (control). Two major experiments were performed (N=3 trials each). In the first, concrete was cracked and then the bonding agents were used to repair it (as traditionally done). In the second part, the bonding agents were added before the concrete solidifies; this ensured that the bonding agents do not negatively affect the overall strength of the sample. It was seen that the sodium silicate sample sustained more weight (17.69kg—Part1 and 6.80kg—Part2) than any other agent including control (4.88kg in Part 1&2); however, sodium silicate is unpractical because of high costs ($4.00/100g). As a result, calcite was thebest overall bonding agent; it repaired the cracked concrete in part 1, did not significantly reduce the overall strength of the sample in part 2 (held 3.33kg), and was cost-effective ($0.11/100g). Ultimately, self-healing concrete made of calcite has the economical and engineering potential to save trillions.  

Background:

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.

Question:

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.”

Materials:

• Box divided into 5”x3” blocks with construction sticks
• Store bought concrete mix and water
• Sodium Silicate
• Calcium Carbonate (calcite—limestone)
• Elmer’s glue bottle
• Soil and 2% concentrated milk

Controls:

• All blocks will be same size: 5×3 inches
• All blocks will receive same amount of cement in each
• All samples will be produced in same place and time—in same conditions
• All samples will receive the same amount of water
• All samples will be tested with similar weights and procedure.

Procedure:

Step 1: Control

1. Add water to concrete
2. Make 5×3 inch subdivisions of control concrete
3. Let concrete dry and cure
4.  To test the samples, suspend each sample between two objects. There should be a gap of about 1.5” between the two objects. Refer to the diagram below for further images.

5. Put a pencil on top of the sample so that when the weight is added, all the weight is concentrated on one section of the sample
6. Keep on adding weights until the concrete sample cracks. Record the weight at which the concrete sample cracks.
7. Repeat this procedure 12 times.

Step 2: Testing strength of concrete when agent added after cracks form:

1. Take 12 blocks of concrete and then create cracks in each of them. This can be done by dropping a similar hammer from a height of 3 feet. This will ensure that crack sizes are the same.
2. Add calcite, sodium silicate, glue, or soil and milk to the cracks of each of three samples. Refer to the diagram below for clarification.

1. Add water on top of each of the samples. 
2. Repeat steps 1, 2, and 3.

Step 3: Test strength of samples when concrete is made with bonding agents (this will ensure that bonding agent does not actually reduce strength of concrete)

1. Make new concrete mixture and add each of 4 agents (Calcite, sodium silicate, glue, or glue and soil) to it. These agents should be added to the concrete initially when it is first being produced and hasn’t solidified. Add a ratio of 1:3.
2. Repeat steps 1, 2, and 3. 

Step 4:

1. In addition, the cost-effectiveness of each samples of concrete will be determined. Based on the results mentioned in step #3 and prices that the substances were bought at, this can be determined. For example, was the sample in which sodium silicate was added before the concrete solidified more cost-efficient than the sample in which regular mortar was added before the concrete solidified? Similar questions such as this one can be asked for each sample.

Data:

 

Graphs:

Conclusion: 

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.

Future Uses: 

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.

Next Steps: 

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.”

Photos:

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