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Better Foundations, Part 2: Putting Concrete to Work

Written by Iver P. Cooper

 

In the world the up-timers left behind, the most widely consumed substance on Earth was water. What came second? Concrete. Indeed, concrete can be said to be, quite literally, the foundation of modern society. We depend upon it for shelter (concrete buildings), transportation (concrete highways), and energy (concrete dams providing hydroelectric power).

In Part I, we found out why the down-timers are going to rediscover the advantages of concrete as a structural material, and, by reinforcing it, take it—literally—to new heights.

In this part, we will first consider the nitty-gritty of proportioning, mixing, laying, curing and testing concrete, and then move on to the fun stuff—talking about what we can build with it.

Proportioning Concrete

 

Designing (proportioning) the concrete mix is an essay in the art of compromise. For plain concrete, we need to worry about the ratios of four ingredients: water, cement, coarse aggregate and fine aggregate. (For the moment, we will assume a Portland cement and no admixtures.)

An architect (or perhaps a building code) will have specified the minimum 28-day compressive strength of the concrete ("design strength"). The most important consideration is the ratio of water to cement (w/c). In 1919, Duff Abrams discovered that the strength (at a fixed age) increases as the w/c ratio decreases, provided the concrete can still be fully compacted (Camp; Cowan 128; Bauer 98). ACI figures that for plain concrete (non-air entrained), a 0.68 ratio yields a standard strength of 3000 psi, and 0.41, 6000 psi. (For air-entrained concrete, reduce the ratios by about 0.08-0.09.)

A lower w/c ratio also results in lower porosity of the cement paste. (Camp, Chap. 8). That correlates with lower permeability and hence higher durability (stops water from entering, bringing nasty chemicals in with it, or just freezing inside the concrete and stressing it).

Unfortunately, low w/c ratios ("stiff mixes") are also less "workable," because there is less water to act as lubricant between the particles. (It follows that high aggregate-to-water ratios also result in less workability.)

"Workability" refers to the ability to mix, place, compact and finish the concrete. Workability is usually measured by the "slump" test (see below). Slump is usually specified by the contract with the concrete supplier. The desired slump will depend on the type of construction, but it will usually be in the 1-4 inch range.

Typical w/c ratios are 0.4-0.7:1, and it is customary to prepare trial mixes and test whether, with the available cement and aggregates, they provide the desired strength and workability.

Another consideration is shrinkage. Shrinkage causes cracking. To reduce shrinkage, we need to reduce the amount of cement in the mix (sand and gravel doesn't shrink). But you need enough cement to mostly fill up the voids between particles. So that means that you have to find the right "grading," so the fine aggregate fills up the voids between the larger pieces.

Reducing the amount of cement is also economical; cement is typically the most expensive concrete ingredient.

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Several engineering handbooks contain extensive tables of trial mixes. Grantville's encyclopedias contain a number of recommendations on mix design and even some specific "starter" mixes. 1911EB "Concrete" says that the ratio of sand to cement can vary from 1:1 to 4:1, and that of gravel to cement is in the range of 3:1 (for very strong work) to 2:1 (for "unimportant" work). It says that the combined volume of the sand and cement should be sufficient to fill the void volume of the gravel. (The void volume can be crudely determined by filling a can of known volume with the gravel, and then seeing what volume of water the can will hold.)

A sample proportion mentioned in the article is one part cement, two parts sand, and five parts gravel. (Note that this gives a gravel/cement ratio of 5:1 which is higher than the 3:1 stated previously. I suspect that the intent of the writer was to refer to the ratio of gravel/sand, or gravel/sand+cement, and not to gravel/cement.) Insofar as water is concerned, it says that the amount required for the chemical reaction is 16% by weight, but that in practice more is needed to compensate for evaporation and other losses.

The Columbia Encyclopedia gives a "typical proportion" of "one part of cement, two parts of sand, and five parts of broken stone or gravel, with the proper amount of water for a pouring consistency." The Encyclopedia Americana "Concrete" article suggests, as a hand mix, one pound water, two pounds cement, four pounds sand, and five pounds coarse aggregate. And Time-Life's Masonry (26) suggests one part cement, two parts sand, and four parts gravel, with a half part water.

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Because fly ash improves workability, blending it with Portland cement allows use of a lower water/cement issue and hence the concrete can be of higher strength. It is no accident that Chicago, which is well supplied with fly ash, has a large number of concrete skyscrapers (Camp).

 

Estimating

 

A 94 pound (one cubic foot) bag of Portland cement might be combined with 188 pounds sand and 376 pounds gravel (ratio 1:2:4) . If the w/c ratio is 0.5:1, that calls for 47 pounds (5.64 gallons) of water. Air-free concrete with standard aggregate would weigh about 150 pounds per cubic foot. If the air content of the concrete is 4%, the density is 144 pounds/cubic foot, and you have about 715 pounds, or 4.95 cubic feet, of concrete per bag cement. If you know the volume of the slab, wall, column or beam you wish to pour, you can calculate how many bags of cement (and everything else) you need. One percent reinforcing steel would add about 4 pounds/cubic foot to the plain concrete.

 

Mixing Concrete

 

In 2000, there were several choices for mixing concrete. The simplest from the contractor's point of view was "ready-mix." The concrete batch is placed in a mobile mixer, and mixed while en route to the job site. Or the concrete is mixed in a stationary mixer at the ready-mix plant, and then transferred to a truck, which merely keeps it sufficiently agitated so it doesn't set.

While concrete was used in construction in the early nineteenth century (in a concrete bridge in Souillac, France in 1816), it was not until 1913 that ready-mixed concrete became available; it was hauled in a dump truck. That meant that the concrete had to be remixed upon arrival (Ali). Dedicated truck mixers were introduced in the 1920s. The earliest ones carried only one cubic yard of concrete; a modern truck mixer carries 10-12. The horizontal-axis revolving-drum mixer truck was introduced in 1930. (Kuhlman Corp.).

It remains to be seen how quickly ready-mix will be introduced in the new time line. The alternative in 2000 was to mix the ingredients at the job site. A contractor might have a portable mixer, mounted on skids, wheels or a tractor hitch. The ingredients (including as much as two bags of cement) are dumped into a hopper, and mixed in a revolving drum, which is powered by an electric motor, a small gasoline engine, or a tractor. In post-RoF Germany, one can imagine the drum being turned as a result of animals trudging in a turnstile, or even hand-cranking.

An even more primitive method of hand-mixing is to lay the sand, cement and gravel in a "mud box," or a wheelbarrow, mix them up with a hoe-like instrument, and then slowly add water. (Ahrens 55).

In 2000, hand-mixing of concrete was facilitated by buying dry mix concrete, that is, a mix containing all of the ingredients except water. This isn't available in the seventeenth century, of course, but it isn't much of a step to go from bagging cement to bagging dry mix. The real problem with dry mix after RoF will be transportation costs; it will be cheaper to just have cement delivered to you, and mix it with local sand and gravel, then to pay for delivery of a much greater weight of dry mix.

For a big job, a mixing plant can be built on site. It may be possible to come up with a "set up, use, and knock down" plant design, primarily of lightweight (wood) construction, so that the plant can "hop" from one major construction site to another.

 

Placing Concrete

 

Formwork. The plasticity of newly formed concrete is both blessing and curse. Blessing, because that is what gives it its characteristic versatility of form. Curse, because it must be poured into a "form" (a kind of mold) to hold it until it has hardened enough to support its own weight. That typically takes 1-2 days in summer and 4-7 days in winter (Ahrens 62).

That means, in turn, that the preparation of the site includes building, erecting and bracing the forms, which can be made of wood, metal, or, when they become available, plastic or fiberglass. Handling formwork is probably the most time-consuming aspect of concrete construction in the modern age. (Of course, in the modern age, contractors don't have to worry about making their own cement. )

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Placing. Once the forms are ready, the freshly mixed concrete is placed where it needs to go. Time is of the essence, since the initial set takes place in 30-60 minutes, and moving the concrete after it occurs will reduce strength. That means that the labor on hand for placing the concrete has to be adequate for the amount being handled.

Concrete is compacted to reduce air voids. If the mix is high in water, this should be done with a hand tool. But vibrators allow the use of "stiff" (relatively low water content) mixes that can't be placed properly by hand. They can reduce the air content from, say, 1.5% to less than 0.5% in perhaps two minutes. (Used on a water-rich mix, they have the undesirable effect of segregating the water from the aggregate.)

Vibrators are gas- or electric-powered, and can be internal (placed inside the concrete), attached to the form, or placed on the surface. It is reasonable to expect the general contractors in Grantville to have a few concrete vibrators. (Ahrens 67-9).

An alternative is to formulate self-consolidating (self-compacting) concrete, which can be placed by its own weight, without vibration. This was introduced in the 1980s, and uses water-reducing and superplasticizing agents. We will have to rediscover the additives before we can make the concrete behave this way.

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Pumping was developed in the 1930s, if not earlier. Pumping comes in handy for reaching the upper floors of a building. There are piston, pneumatic and squeeze pump designs. Concrete can be pumped 500 feet vertically and more than 1500 feet horizontally. The concrete mix may need to be adjusted (e.g., restrict maximum aggregate size) to render it suitable for pumping. (Camp). A special concrete mix, "shotcrete," may be sprayed onto the interior of a fabric balloon to make a thin shell monolithic dome.

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The old Roman concrete (really, a mortar) was not plastic; it had a very low ("zero slump") water content. The Romans laid the coarse aggregate by hand and pounded the mortar into the gaps. The modern equivalent is roller-compacted concrete. It is placed with an asphalt paver and then compacted with a vibratory roller. (Moore). RCC is used mostly for low traffic pavement, and for dams (less heat generation).

 

Setting and Curing of Concrete

 

It is important to recognize that the setting (loss of plasticity) and curing (hardening and strengthening) of concrete is not the result of drying out by evaporation. Alite and belite react chemically with water (so-called "hydraulic reaction") to form calcium silicate hydrates (C-S-H), which are the effective cementing agents. The more reactive alite contributes to early strength and belite to late strength. (If pozzolans are present, there is also the reaction of silica and lime to form C-S-H.)

Once concrete has dried out, it stops getting harder and stronger. Fresh concrete has, initially, enough water for the concrete to reach full strength, but unfortunately it will lose water as a result of evaporation. Hence it is customary to moist-cure it for a period of time, either by sealing the surface (e.g., covering it with plastic or waterproof paper sheets, or spraying it with a sealant), or by supplying additional water during the curing period. Water can be supplied by covering the surface with wet material (wet burlap, canvas, sand, shaving, straw) or by spraying, flooding or ponding. (Ahrens 83).

Ideally, the concrete is moist-cured for 28 days (the standard); the usual recommendation is at least five days if the temperature is at least 70 deg. F, and at least seven days if it is 50-70. Fully moist-cured Type I concrete will reach 50% of its 28-day strength about 5 days after it is laid, and 75% after perhaps 12 days. Between three and six months after it's laid, its strength will be 125% of standard. In contrast, if the concrete is in air the entire time, its strength will plateau at a little over 50% of standard. If the concrete is moist-cured for 3 days and then air-cured, it will level off at about 80% of standard. And if it is moist-cured for 7 days before air-curing, it will reach the standard strength but not exceed it. (Ahrens 82).

Moist-curing not only improves strength, but also wearability and water-tightness. A 7-day moist-cure is sufficient for complete watertightness.

The curing reaction is slower if the temperature is lowered; it takes three times as long to reach a given strength at 33 deg. F. as at 70 deg. F. There is permanent damage if the concrete is frozen during the first 24 hours after it is laid. Hence, in modern practice, if the temperatures are near freezing; it is customary to heat the sand, gravel and water before they are mixed with the cement. Care must be taken not to heat the materials so much as to cause "flash" (rapid premature) setting of the concrete. The concrete, once placed, can be covered with insulating material such as canvas, straw or hay. The forms may need to be warmed up; it is probably better to use wood rather than steel. Another possible cold weather expedient is to build a temporary enclosure for the concreting area and heat it while the concrete is being laid and cured.

The danger in hot weather is that the heat causes greater evaporation of the water in the concrete. Evaporation at 90 deg. F. is quadruple that at 50 deg. F. and double that at 70 deg. F. There is also a further loss of strength at high temperatures, beyond that attributable to water loss. Often, the concrete will have a higher early strength but a lower ultimate strength.

The usual hot weather concreting expedient is to keep the aggregate cool by shading and sprinkling, and to cool the water by adding ice or refrigerating it. It is also possible to dampen the forms.

The rate of evaporation is also dependent on humidity; it increases five times when relative humidity decreases from 90% (Washington DC in the summer) to 50%. And on wind; the rate is four times higher in a 10 mph breeze than in a calm.

 

Concrete Testing

 

Because the raw materials used to make concrete are heavy, they usually are obtained locally. Consequently, concrete made in one area will not be quite the same as concrete made somewhere else even if they use the same nominal mix of cement, aggregate and water.

So, to be sure the concrete will perform as expected, you need to test it. Even a "how-to" manual will explain how to carry out a "slump" test, which measures the stiffness of the mix. Mixes with small slumps are difficult to work and those with big slumps won't be particularly strong or durable.

Those manuals also talk about compression tests. Unfortunately, they just tell how to make the sample cylinder; it is then sent off-site to a testing lab. It is possible that the Tech Center has a test machine which can be used post-RoF. If not, then we will have to wait until the machine shops can create one. In essence, it hydraulically applies increasing pressure to the sample cylinder until it fails.

Compression tests are typically carried out 1, 3, 7 and 28 days after the concrete is laid. Unfortunately, the one and three day strengths are not necessarily well correlated with the 28 day strength, because they are sensitive to certain factors which are operative in the short term. To get a better idea of what the 28-day strength is going to be, we need to accelerate the curing time. In general, this is done by using warm or even boiling water to moist-cure the concrete. (Camp).

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Don't expect that early post-RoF concrete will be the equal of modern concrete. For example, in spring 2000, students from Villanova University designed and constructed a reinforced concrete cross for a Catholic orphanage in Honduras. the Honduran-made concrete only had a compressive strength of 1800 psi, rather than the U.S. norm of 4,000 psi. (Dinehart). This was almost certainly attributable to want of quality control in a Third World country. We are likely to experience even more acute quality control problems in the seventeenth century.

 

Precast Concrete

 

The concrete doesn't have to be cast and cured in place. Instead, concrete elements can be precast, and then assembled. While precasting can be done at a building site, it is more common for it to be done at a precasting plant. Such a plant will often use steel forms because they can be reused many times. At the building site, the precast elements are erected and joined.

A special case of a precast element is a concrete block. This is intended to be used like masonry; that is, the blocks are mortared together at the building site. Block mixes usually contain less cement and less water than the mixes used to make structural concrete, and often contain lightweight aggregate. The blocks are usually steam-cured, at normal or even high pressure, at the plant.

Precasting plants won't be practical until the transportation network is capable of accommodating heavy traffic.

 

CONCRETE STRUCTURES

Roads

 

In Flint, 1633, Chapter 27, Jesse tells Jim, "Next time you're in Magdeburg, go talk to Mr. Simpson. I understand he's got plans for producing some sort of paving material. Find out what it is, concrete, macadam, whatever, and what it will take to get it down here to the field."

Jesse, of course, is thinking about aircraft runways, but concrete provides an excellent road surface for land vehicles, too. Details are given in Cooper, "All Roads Lead . . ." (Grantville Gazette, Volume 10). Concrete is durable; the first concrete street in the United States was laid by George Bartholomew in Bellefontaine, Ohio in 1891, and is still in use (Snell).

It is worth noting that roads can be made either of reinforced or unreinforced concrete. Given the shortage of steel in the early post-RoF period, I expect that unreinforced concrete will be preferred.

 

Apartment and office buildings

 

 

The Romans built insulae (tenements), usually not higher than six stories, using masonry or unreinforced concrete. (Idorn 38).

The first reinforced concrete building (a two story servant's cottage) was built by plasterer William Wilkinson in 1854. More upscale homes were built using reinforced concrete in the 1870s, but they were "made to resemble masonry to be socially acceptable." The single most important reason for concrete construction was fear of fire. (Camp).

It is worth noting that while concrete itself is fire resistant, the furnishings of a building are likely to be flammable. Hence, as concrete buildings became taller, it became important to assure that one could easily escape the upper floors in case of a fire, and also that firefighters could direct water against an upper-story fire.

Edison wanted to mass-produce concrete houses for the betterment of the poor. The whole project proved to be a marketing disaster. While the first two-story homes were put on the market in 1917 for a mere $1,200 apiece, none sold in the first month. "No one wanted to live in a house that had been described as 'the salvation of the slum dweller.'" (Peterson) So the fledgling concrete industry in Grantville should be careful how it markets its product.

The first concrete skyscraper (16 stories, 210 feet high, 50 x 100 feet base) was built in Cincinnati in 1904 (the Ingalls building). The columns were built first, then the walls, girders, joists and floor slabs.

Concrete ingredients were brought to the site and stored in the basement until needed. Mixing was done on-site, using a powered mixer (invented in the 1880s). One hundred cubic yards of concrete were produced on each 10-hour shift. The concrete was one part Portland cement, two parts sand, and four parts pebbles or crushed limestone. The compressive strength of the concrete isn't stated, but I would expect it to have been at least 2,200 p.s.i. in that period (and the steel to have a tensile strength of 80,000 p.s.i.). The total consumption of concrete was about 4000 cubic yards (Newby 274).

The total staff of workmen dealing with the concrete were 28 men, of whom nine wheeled cement, sand and stone, one attended to the mixer and ground hoist, two more to the hoist on the upper floor, four wheeled concrete on the upper floor, and twelve placed the concrete.

Three sets of forms (molds) were used; that is, while concrete was poured into one set, previously poured concrete was curing in the other two. The "floor cycle" was nearly three stories a month, with ten days to erect the molds for each story and two days to place the concrete. Molds were kept in position for about fourteen days after the concrete set, and, after they were removed, temporary struts were used to provide partial support for another thirty days while the concrete increased further in strength. The Ingalls Building was completed in eight months.

The principal floor panels were five inches thick. Supporting columns were 30x34 inches for the first ten floors and 10x10 for the remaining ones. The principal girders were 32 feet long, column to column, 27 inches deep, and 16-20 inches wide. The cross girders were 16 feet long, 18 inches deep, 9-12 inches wide. The exterior walls were eight inches thick. After the second floor, the floor height was 12'6", which was a foot less than what would have been required by steel girder construction and hence effectuated a saving in construction costs. (Ali, "Ingalls"; Twelvetrees Bldgs., 101, 312-37; Taylor 611-12) .

The height achievable with reinforced concrete is largely a function of the strength of the concrete. The stronger the concrete, the slenderer the supporting columns on the lowest floors can be. In the 1950s, 5000 psi was considered high strength. (Prairie Material) . With 6000 psi concrete, Place Victoria in Montreal reached a high of 624 feet, and in 1970, One Shell Plaza in Houston ascended to 714 feet. (Camp). By the 1960s, 7,500 psi was feasible; such concrete was used in 1968 to raise Lake Point Towers, in Chicago, to 645 feet (seventy stories).

In the early 1970s, builders had access to 9000 psi concrete. The 859 foot Water Tower Place, the tallest concrete building in the world from 1975-1990, used 3000 psi concrete for the slabs and 9000 psi for the columns (with some assist from superplasticizers) (ConcreteContractor.com).

The taller the building, the greater the lateral wind force which it must resist. The "structural system" must be suitable. The classic frame (column-beam) construction is good only to about twenty office stories. A shear wall construction (1940) is appropriate up to perhaps forty stories. And so on (Ali).

For early high-rises, the concrete was hoisted to the working floor in buckets. In the 1960s, it became possible to pump the concrete to some floors. However, the higher the floor, the greater the pumping pressure required, and so, once the building reaches a certain height, it's back to buckets.

 

Bridges and Dams

 

In the United States, the first concrete bridge was built in New York in 1871, and the first reinforced concrete bridge was the Alvord Lake Bridge in California (1889). It survived the San Francisco Earthquake. The first concrete dam was built in 1887 in California, and the first reinforced concrete one in 1899 (Prentice 18).

 

Special Structures

 

The tallest unreinforced concrete structure in the world is an obelisk, the 351-foot Jefferson Davis Monument in Kentucky. It is 8.5 feet thick at the base and tapers to 2.5 feet at the apex. The Pantheon is still the largest unreinforced solid concrete dome in the world (43.4 meter diameter). It was constructed in seven years; the similar sized dome of St. Peter's in Rome took fifty years to build with stone (Davidovits).

Reinforced concrete can be used in the construction of monumental public structures characterized by long, open spans suitable for large public gatherings. These are often based on thin shells with load resistant shapes, singly curved (cylinders, cones) or doubly curved (spheres, hyperbolic paraboloids). Special structures have been used as stadiums, performance and exhibition halls, churches and factories. (Bradshaw).

 

Fortifications

 

 

The first concrete fortification was probably the Aurelian Wall, 12.5 miles long, and built in 271-275 A.D. using brick-faced plain concrete. The walls were initially 11.5 feet thick and 26.2 feet high. ("Aurelian Walls," Wikipedia). When concrete was rediscovered, and more particularly when reinforced concrete was invented, the world military powers took note.

Initially, any concrete fortifications we build will face just solid shot. However, the USE navy used explosive shells in the Baltic War, and this will inevitably be copied by other governments.

Gillette (185) reports that the amount of concrete required for two 12 inch gun emplacements at Staten Island, NY was 5609 cubic yards, and cost $5.50/cubic yard.

I have a bit of data on the ability of unreinforced concrete to resist artillery fire:

—In an 1881 experiment, a Woolworth rifled cannon, of ten inch caliber, fired a 408 pound projectile at a range of 145 yards. It struck unreinforced concrete at a velocity of 1424 feet/second, and penetrated 13-17 feet. Under essentially the same conditions, the penetration into earth was 34.5 feet. (Mahan's Permanent Fortifications 163).

—One meter of concrete masonry is the equivalent of two meters of brick, or three of earth, when it comes to resisting a direct (flat) shot from a 1912 vintage field howitzer. (Fiebeger, A Textbook on Field Fortification ).

—Heavy guns (e.g., an 80-ton gun firing a 1700 pound cast iron projectile with a striking velocity of 1580 feet per second) could expect to penetrate 32 feet of Portland cement concrete or twelve inches of steel-faced wrought iron. (Abbot, Course of Lectures Upon the Defence of the Sea-coast of the U.S, 147 )

—In 1897, six feet of hard concrete resisted shells of the eight inch B.L. howitzer, and at Port Arthur, 4.5 feet concrete proved sufficient to resist single shells. (Sydenham, Fortification 128).

Reinforced concrete is of course capable of offering greater resistance. The German World War II "Verstärkt Feltmessig" (Vf) were bunkers with three foot thick walls and ceiling, intended to protect the troops from a 50 kg bomb or a 105 mm artillery shell. The next level up were the Ständige Anlage (St), and in the Baustarke B (build strength B), they used 6.5 feet of concrete to hold of a half-ton bomb or a 220 mm artillery shell. (Regelbau).

A USMC Staff Officer's Manual (Hyperwar) compares the resistance of plain concrete, reinforced concrete and other materials to attacks by modern weapons ranging from small arms fire to 88-mm artillery. In general, you need 20-30% less thickness of plain concrete, or 20-50% less reinforced concrete, than brick masonry, to resist the fire. The difference is most pronounced for the less powerful attacks.

A material known as "very high strength concrete" (high silica content; low water/cement ratio, steel fiber reinforcement; steam cured), with compressive strength four times that of conventional plain concrete, and tensile strength almost three times, reduces penetration about 50% (Cargile).

Concrete fortifications can be used, not only to protect troops or guns, but also to channel the enemy. For example, there were the "dragon's teeth" of World War II, three or four feet tall pyramids of reinforced concrete, spaced so that tanks couldn't drive through.

 

Concrete Ships and Other Floating Structures

 

 

Yes, you can sail a concrete ship. This shouldn't come as a surprise, since all metal ships ply the seas, and steel is more than three times denser than concrete. One secret is displacement; the ship displaces a volume of water whose weight is greater than the weight of the ship, creating buoyancy, and that is possible because the hull encloses a lot of empty space.

Displacement is not the only secret. Since the 1960s, engineering schools have raced concrete canoes in competition (NCCC). Formal regional competitions began in the 1970s, and the first national competition was held in 1988.

At the competition, the teams must swamp the canoe (fully submerge it) and then release it. To qualify, the canoe must float up and break the water surface. That means that the concrete itself must be able to float. The modern concrete canoes use concrete mixes which weigh 35-50 pounds/cubic foot, whereas the density of water is about 62 pounds/cubic foot(Bie).

The cement is usually a blend of Portland cement and a pozzolan (fly ash, metakaolin, slag, rice hull ash, silica fume). Latex polymers may be added. The concrete is usually reinforced with metal, carbon, fiberglass or plastic fiber. The lightweight coarse aggregates used in these concrete canoes include glass and ceramic beads, epoxy coated styrofoam beads, perlite, and vermiculite, The concrete must be air-entrained (at least 6%). The mix is usually low in water (maximum water cement ratio is 0.5:1), to keep strength high, and so superplasticizers are likely to be used to improve workability. (OSU, NCCC 2007 Rules).

Both West Virginia University and Fairmont State have participated in concrete canoe competitions since at least 1998 (their participation is mentioned in a 1998 issue of Mountaineer Spirit.) The American Society of Civil Engineers (ASCE) Virginias Conference (Virginia and West Virginia) was held at WVU on April 3-4, 1998, and WVU competed against teams from Old Dominion University, Virginia Polytechnic Institute, University of Virginia, Virginia Military Academy, and Fairmont State College. Virginia Tech won the regionals, but came in tied for last in the nationals, which were won by University of Alabama in Huntsville. Fairmont State has made it to the nationals four times, but all after RoF. So far as I know, WVU has never made it that far. The "winningest" team in the Virginias Regional Conference is certainly Virginia Tech, which made it to the nationals fourteen times (including 1988-95 and 1998-2000).

Which leads to the logical question: Are there any “Hokies” among Grantville's civil engineers?

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The first known concrete ship was a small ferrocement boat built by Joseph Louis Lambot (1848) and exhibited at the 1855 World's Fair. The first ocean-going concrete steamer, N.K. Fougner's 84-foot, 600 ton Namsenfjord, was launched in August, 1917. Fougner went on to build two more, the Patent and the Concrete. There was also W. Leslie Comyn's 5,000 ton Faith, launched in May 1918. Her first cargo was 4300 tons of salt and copper ore, which she carried from San Francisco to Vancouver. (Thomas; Bender)

During the First World War, steel scarcities prompted the construction of twelve other concrete ships under government auspices, at a total cost of $50 million. However, they didn't see any wartime use. One of these ships, the 420 foot oil tanker S.S. Peralta (launched 1921), is still afloat (as part of a breakwater for a paper mill in British Columbia).

Steel was again hard to come by during the Second World War, and at least twenty more concrete ships were built. All weighed 4690 tons and were 102.5 meters long; they were typically used as storeships. Some still survive as wharves and breakwaters. The closest ones to Grantville are the nine used to form a breakwater for a ferry landing at Kiptopeke Beach, Virginia.

So far as I know, since World War II, there have been no large merchant ships constructed using concrete. However, there have been quite a few amateur built "ferro-cement" sailboats. In 1984, Peter Freeman circumnavigated the world in a 32 footer, in the process setting the then record for fastest singlehanded nonstop circumnavigation ...