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Don't Put Mistakes `In Concrete'

Article-Don't Put Mistakes `In Concrete'

You don't want to put a $100,000 building on top of a mistake," says Jay Harmon, Iowa State University agricultural engineer. While it's not necessary to become a concrete expert before starting construction, producers should become familiar with the basics of choosing quality concrete for their needs."If you buy good-quality concrete, it should last longer than the building does," Harmon states.

You don't want to put a $100,000 building on top of a mistake," says Jay Harmon, Iowa State University agricultural engineer. While it's not necessary to become a concrete expert before starting construction, producers should become familiar with the basics of choosing quality concrete for their needs.

"If you buy good-quality concrete, it should last longer than the building does," Harmon states. Talk to the ready-mix concrete producer. Jay Runestad, executive director of the North Central Cement Promotion Association, Boone, IA, suggests discussing the following questions:

* What will the concrete be used for?

* How much concrete will be needed?

* How strong does the concrete need to be?

* What are the durability requirements?

* What are the concrete mix requirements needed to achieve these properties?

* What workability is desired?

Robert Mensch, consulting agricultural engineer, Fairmont, MN, says he tells clients to be proactive and involved when dealing with both concrete producers and their construction projects. "Farmers sometimes have too much faith that their contractors will do a good job," Mensch says. "Producers can't just turn everything over to the contractor and assume he will do everything right. Producers need to be on the job site every day. They also need to educate themselves on the points they should be looking for."

Larry Jacobson, University of Minnesota agricultural engineer, says the single most important thing for producers to remember about working with concrete is that shortcuts during installation can compromise the strength and durability of the finished product.

What Is Concrete? Concrete is a mixture of Portland cement, water, air and aggregates. According to Runestad, concrete is made up of 66-78% aggregates. These aggregates are some type of rocks, such as sand and gravel, which provide volume at low cost.

Cement and water form a paste that hardens and "glues" the aggregates together. Quality and durability of the finished concrete depends on the binding qualities of the cement paste. "The concrete is only going to be as good as the "glue" that holds it together," Runestad says.

Water-Cement Ratio The water-cement ratio refers to the number of gallons of water mixed per bag of cement. "Enough water is needed for full curing, but excess water leaves voids when it evaporates," says Harmon. Table 1 lists several examples of water-cement ratios for various uses.

Concrete should be placed with the stiffest consistency possible, while remaining reasonable to work with and finish. "Stiff" concrete will shrink and crack less.

Harmon says if a mix is too stiff to handle well, producers should either reject the batch or add both cement and water, (not just water) and then reduce the amount of aggregates in subsequent batches.

Slump is a measure of concrete consistency and workability. A higher slump rating means the concrete is wetter and more workable. A lower slump rating indicates stiffer concrete.

"The amount and gradation of the aggregate is a major factor in determining the consistency or workability (slump) in a mix with a given water-cement ratio," says Runestad.

Maximum slump ratings for specific types of concrete are shown in Table 2. Runestad suggests asking a consulting engineer or ready-mix producer for directions regarding measuring slump.

Aggregate Considerations The aggregate used in the concrete must be free of silt and organic matter. Both large and small aggregate are used in a good concrete mixture. The small aggregate fills in the spaces left by the larger aggregate. Harmon says the maximum size of coarse aggregate depends on the sizes and shapes of concrete members, the amount of reinforcing steel used, and the distribution of reinforcing steel. "Generally, the maximum size of aggregate should not exceed one-fifth the minimum dimension of the concrete member, or three-fourths of the size of the clear space between reinforcing bars, or between reinforcement and forms," Harmon says.

The size of aggregate should not exceed one-third the slab thickness in un-reinforced slabs. "Smaller sizes may be used when economics or availability require them," Harmon says. "Larger aggregate means less air entrainment is necessary."

Air Entrainment Air entrainment refers to the intentional adding of many small air bubbles to the concrete mix. "These air bubbles make the concrete more durable because they allow a place for freezing water to expand in cold weather. They also make the concrete more workable," Runestad says. The air entrainment will slightly reduce the strength of concrete.

The air bubbles should not be visible in the concrete. "If you see air bubbles in your concrete, you are not seeing entrained air. Visible bubbles would be entrapped air, which is not a good thing," Runestad cautions.

Preparing the Subgrade Before a slab of concrete is placed in an area, all organic material (whether it is grass, or manure) needs to be removed from the area. The site should be graded so there is uniform support for the concrete slab. There should be no unusually soft or hard spots. Reinforcement rods may help bridge small soft spots located below the poured concrete.

Just before the concrete is placed, Runestad says it is a good idea to make sure the ground is damp, otherwise the dry soil will pull too much water out of the bottom of the concrete slab.

Don't Over-Work Concrete Concrete should be placed very close to its final position. "You should not be placing large quantities of the wet concrete at a given point and working it over long distances in the form," Runestad says. "Over-working the wet concrete leads to segregation of the mix. Mortar tends to flow out ahead of the coarser material, and the aggregate settles out of the mix."

Position the truck and use a movable chute to place the concrete as nearly as possible to its final position to avoid having to "over-work" the concrete, Runestad suggests.

Harmon says the concrete should be leveled with either a vibrating screed or a straight 2x4-in. board. "Some screeds are effective to a concrete depth of about 12 in.," Harmon explains. "Additional consolidation of concrete along bulkheads often is achieved with internal vibrators. However, avoid allowing screeds to vibrate after the concrete has been consolidated because this tends to bring excessive fine material to the surface."

Harmon says it is best to keep the vibrating screed moving consistently. If the vibrating screed is started and stopped repeatedly, the movement removes entrained air.

Harmon says immediately after screeding, work a bullfloat over the slab at right angles to the direction of screeding. The bullfloat removes small ridges left by the screed and slightly presses down the larger aggregates.

Inadequate vibration or compaction of concrete leads to honeycombs in the concrete, Mensch cautions.

Harmon suggests using some type of pattern to roughen the concrete surfaces for hogs. Concrete surfaces that will be used for the breeding herd, for example, should have a 4- to 6-in. diamond pattern added to the surface with grooved saw cuts, Harmon says. Cuts should be 11/44 to 31/48 in. deep and 11/44 to 11/42 in. wide.

Curing The experts agree, you don't want concrete to "dry," you want it to "cure." In order to cure properly, the concrete needs to be kept wet. Allowing the concrete to dry out before it is cured can ruin the surface and lead to future problems.

Weather Effects Harmon says little curing occurs when the concrete temperature is below 40 degrees F because the chemical reaction stops. "If the temperature is 70 degrees F. or more, the concrete needs to cure at least three days. Or if it is a low as 50 degrees F., it needs to cure for about five days," Runestad says. "Do not allow it to freeze for the first four days to prevent damage." Concrete should not be placed on frozen ground.

Wind and low humidity can dry concrete too rapidly and weaken it. Fresh concrete should be kept damp for at least five days during curing. Evaporation can be slowed down by covering concrete with 4 mil. plastic sheeting, Runestad suggests.

Concrete should be kept as cool as is practical in hot weather. Time concrete placement to take advantage of shade and try to reduce evaporation with a windbreak.

Sometimes in cold weather, calcium chloride accelerators may be used to hasten set-up of the concrete. "However, in any type of concrete that will contain steel reinforcing, you should not use calcium chloride accelerators because they can cause corrosion,"

Runestad cautions. "There are non-chloride accelerators available, but they may be more costly."

Mensch often encourages his producer clients to test the concrete they are receiving at a job site. He says tests on concrete used for a finishing building can run between $300-$500 to obtain information on the compressive strength of the material, for example.

Runestad suggests producers test un-hardened concrete for slump, air content and temperature. Slump measures the workability of the concrete. There are tests that look at the size of the bubbles in the air entrained concrete, and how the bubbles are spaced. "By measuring the temperature of the concrete in-place, you can estimate how much time you will have to finish the project," Runestad says.

Even if samples are taken, what happens to the concrete after testing can still make all the difference. "Sampling gives an indication that the correct concrete was delivered to the site, but water can be added after the sample is taken, or the concrete could be improperly placed, or improperly vibrated," Mensch cautions.

Finding a Contractor The experts agree, finding the right concrete contractor to do the job can make all the difference in the success of the construction project. Harmon suggests producers do background checking to find the best contractor for a particular job. "Find out the contractor's answer to questions such as, 'What do you do to cure concrete during weather extremes, such as very hot or very cold conditions,'" Harmon says. "Self defense helps you prevent future problems."

The concrete contractor has to know what he is doing and practice good quality control methods, says Larry Jacobson, University of Minnesota agricultural engineer. "Whether or not the concrete in the truck is 3500 psi or 4000 psi is probably of little consequence if the contractor waters it down or takes shortcuts with installation," he says. "It is crucial to take the time to find a quality concrete contractor."

Producers don't need to be experts who understand every detail of what goes into a concrete mix, Runestad says. "But producers should educate themselves so they can talk to the ready-mix concrete producer and ask the right questions to have a good idea about the quality of the product they are getting," he adds.

Jay Runestad, executive director of the North Central Cement Promotion Association, Boone, IA, has a list of the top mistakes pork producers often make that impact concrete projects. The most important first step, he says, is to make sure producers know they are seeking the right concrete for their specific construction job.

You Get What You Pay For Trying to get the absolute lowest cost without regard to the quality of the concrete is the most common mistake Runestad sees pork producers making. "You have to determine, either by doing research, or talking to the concrete producer, what concrete you need to do the job right," Runestad says. "Once you determine what you need, then you can compare prices on the same product. It is important to be aware that the price reflects the quality and type of concrete that is best for the job."

Don't Add Water Order the concrete the way you want to use it rather than adding water, Runestad says. "When you add water, you are changing the water-cement ratio," he says. "Remember, the water and the cement are the glue that hold the rocks (aggregate) together. You have large rocks and small rocks and something has to hold them together. If you dilute the paste, then you aren't going to have a strong 'glue.' Concrete that has been diluted with water may look the same as stronger concrete when it is cured, but the decreased durability will be a problem later."

Cure Concrete Properly The water and cement mixed together acts as a "glue" of sorts. But Runestad points out, concrete isn't like some glues that can be allowed to dry out right away. The cement "glue" is slow-acting. Runestad says the concrete needs to be kept wet in order to keep the moisture level right for curing.

Concrete doesn't "dry," Runestad explains, it "cures." You need to cure concrete 3-7 days. Consequently, that means concrete needs to be kept wet for 3-7 days. "You could continuously sprinkle the concrete with a garden sprinkler once the concrete is set up," Runestad suggests. "Another option would be to cover the concrete with something waterproof, like plastic."

Curing Some curing compounds are available to keep water in curing concrete, Runestad says. These spray-on compounds last for quite a long time, but may be a more expensive alternative.

How Long Should Concrete Last? Runestad says concrete away from feeders and waterers could last a long time. "If a building is properly ventilated, and humidity is kept at manageable levels, concrete could last indefinitely," Runestad says.

Areas around feeders and waterers are where problems are likely to show up. If milk products are used in the feed, milk mixed with water can result in lactic acid formation. Lactic acid slowly attacks concrete and can lead to some deterioration.

The last thing a producer wants to hear is the sickening splash of pigs falling into the manure pit. Don't cut corners on slats, Jay Harmon warns. "You don't want slats to be too thin," he says. "The last thing you want to have to do is empty both a barn full of pigs and a manure pit to replace sagging slats that weren't made thick enough," he says.

Slats need to be designed for the type of livestock to be housed in the building. A slat thickness table is available in the MidWest Plan Service (MPS-36). Harmon suggests the rated load of 125 lb./ft. for finishing pigs, and 170 lb./ft. for sows. "The thickness will depend on concrete strength, span and bar size," he says. "Generally, a 10-ft. slat for finishing pigs that uses 4500 psi concrete and a No. 5 rebar will be 5 in. thick or more. Concrete with a different strength may be a different thickness."

Jay Runestad, executive director of the North Central Cement Promotion Association, Boone, IA, says good slats should be expected to last 20 years or more.

"You should not allow any company to place cracked slats in your buildings," Runestad says. "Sometimes gang slats may get hairline cracks during handling. There should be no cracks in slats the day they are placed. All cracked slats should be discarded."

Once slats are in place, it is a good idea to conduct regular inspections, Runestad suggests. Scaling signals problems on the top side of slats when larger aggregate and concrete are exposed.

"When slats fail structurally, they will fail from the bottom side," Runestad says. If the concrete was too porous, or the cover over the steel was too thin, or a little crack develops in the slat, water vapor and manure gases attack the steel reinforcing rods. When that happens, an oxide develops. Resulting expansion breaks the concrete off of the steel."

Steel doesn't last long when it is exposed to moisture and gases. Consequently, the slat doesn't last long once the steel is corroded and weakened. "Telltale signs of trouble would be a crack down the middle of the bottom of the slat, or cracks down one or both sides of the slat," Runestad warns. "These cracks are on the underside of the slat, where they can't be seen. The steel bar within the slats is located within 1 in. of the bottom of the slat, so the crack would appear below the steel or on the sides."

Runestad says if there is any reason for a producer to suspect a slat problem, some type of checking device should be rigged up with a mirror and light, for example. "It just isn't safe to get down in the pit to check the slats," Runestad warns.