Living Game: Concrete Technical Liner Notes

A small step in the quest to find a place to be in Hoshisato's "Living Game".

In Volume 7 Chapter 10 of Hoshisato's Living Game, Fuwa asks the architect Sugita about concrete. The Hawks translated the lines as

Fuwa: If you reduce the grain size in a concrete mix, you need to increase the quantity of the cement to reach the same hardness, right?
Sugita: No good. The problem isn't the hardness or durability of the concrete. As you reduce the grains, the melding contraction also increases.

The terminology in Japan might be different or the manga writer might have used the incorrect term or the terms got lost in translation, but what they are saying is correct. I would have used terms that are more familiar to American engineers.

Fuwa: If you reduce the stone size in a concrete mix, you need to increase the quantity of the cement to reach the same hardness, right?
Sugita: No good. The problem isn't the hardness or durability of the concrete. As you reduce the stone size, the shrinkage also increases.

The key ingredients for concrete are stone, sand, cement, and water. Cement is NOT concrete, it is a powder used to make concrete. The stone and sand make up the bulk of the concrete. The water and cement react to glue the stone and sand together. When the water and cement react, they form a compound; the water stays locked with the cement particle.

That's why engineers say that cement does not dry. Drying means that the water must leave for the hardening reaction to take place. This is not true; in fact, it is best to keep the concrete damp for at least a day after the pouring. For sidewalks and other small pours, this means damp burlap and plastic sheets. I'm not sure what builders use on large dams or foundations.

The cement-water paste is usually the weak component in the concrete, not the stone. Up to a certain point, increasing the amount of cement increases the strength of the concrete. Too much cement means that there isn't enough water to react with the cement.

Over the lifetime of a structure, water does leave cement-water compound, unless the concrete is in a highly humid environment or immersed. The water doesn't need to leave for the hardening reaction to take place, but the concrete starts out wetter than surrounding air and dries naturally. When the water leaves, the concrete decreases in volume. This is shrinkage. The process is slow and a tell tale sign are hairline cracks on the concrete.

Back to Fuwa's statement. By decreasing the size of the individual stones, the surface area increases. This increased surface area needs more of the cement-water paste to coat and glue the stones together.

An easy way to understand the increase in surface area is to imagine a square cake, whichever type you want. Now imagine putting frosting on the top and the four sides. Once you have that image in your mind, cut the cake down the middle. Once you push the two halves apart, you have two unfrosted sides, one on each half. These unfrosted sides are the increase in cake surface area. More frosting would be needed to cover these exposed faces.

Once you substitute stone for cake and cement-water paste for frosting, you should understand why Fuwa suggests increasing the amount of cement.

Sugita tells him that the solution is no good. The stone and sand won't change volume from water loss; the cement-water is the culprit. Increasing the amount of cement-water also increases the potential for volume change.

I was tickled by the exchange, because it's a fairly advanced topic. One prof told me that most engineers know 3 things about concrete:
1. It's gray
2. It's hard
3. The 28 day strength, that's found by breaking a sample in a lab.

From what I understand about Japan construction, most Japanese practicing architects are engineers as well. In America, the training for the disciplines have grown apart. Another growing separation is between the design professionals (architect and engineer) and actual act of construction.

The exchange between the owner and building contractor come frighteningly close to the truth in some instances. Owners, builders, architects, and everyone down the line insist that they are looking for value without compromising safety or performance. Sugita brings up an excellent point that cost is king to the renter. In theory, the market should be able to sort out the poor quality. However, if you read Dilbert you understand how efficiency and effectiveness can sometimes fall by the wayside in favor of bean counting, territoriality, and general incompetence. And then you reach situations that surreally resemble the Living Game.