Hi, I'm Chad Polan.

And I'm an engineer in the concrete industry.

And today we're going to walk you through how we make a piece of precast concrete rigid frame bridge similar to the one that you see here behind me.

We will go through the whole process starting with the reinforcing cage.

How to assemble that.

Formwork itself.

How to put everything together.

The actual pouring of the concrete.

The science of the concrete a little bit.

How we mix it and batch it and what goes into that.

And ultimately, the finished product.

We've use every tool that you can think of.

The basics that everybody's got to have all the way up to vibration equipment, rebar, cutting saws, grinders, steel fabrication tools.

Additionally, we do a lot of carpentry and specialty work.

So you'll see everything that a carpenter would use.

Table saws, chop saws, all the way up to batching concrete where you get big heavy equipment involved in there as far as crushers, bucket loaders, forklifts to help assemble the forms.

So you really get everything from the stuff you get off the shelf at a hardware store to the really expensive cool stuff.

[music playing] So what these guys are doing right now is assembling the reinforcing cage.

This is, essentially, what gives the finished concrete product its strength.

Each product is specific.

We design it to take into account the loading out in the field, Earth cover.

There's a number of parameters that we got to consider.

And that will dictate what size reinforcing bar and the spacing.

All of those are very important.

The spacing between the bars is important.

And then, once this gets into the form, the spacing from the bars to the edge of the form is very important as well.

So we have these charts.

It will tell them the exact size of the bar, the length of it, and the spacing in between.

And then, they'll come, they'll mark it out, they'll lay out each individual mat at a time.

You basically have two mats going across the top the deck.

Then, you'll have to mats going down the side of each leg.

The spacing in each one of those is going to be different.

The most critical bars for strength out in the field are these bottom bars, we'll call them the belly steel.

Concrete by its nature, it's strong in compression and not strong in tension.

So the steel actually takes up that load.

So the bottom bars, as it sits out in the field, you're going to have loading from the top.

And those bottom bars are basically what carries all that load.

Basically, what you'll see is concrete will crack.

And then, this steel catches it.

And that means it's doing its job.

To assemble the cages, we tie it with steel wire.

He's using what we call a tie gun right now.

It's basically just a spool of standard wire.

At every intersection of the reinforcing, you'll tie that together and it gives it rigidity it makes it strong and it makes it so we can move it over to the form.

This all used to be done by hand with what we call a yo yo, which you still have to do from time to time if you can't get into it.

But as you can see, they'll do something in an hour that used to take probably four hours.

These plastic pieces that you see throughout the cage are what we call spacers.

There's a couple of different varieties.

But that's what gives it its clearance to the form work.

Or as the product sets out in the field, that's what gives that bar the proper clearance to the bottom of the piece.

And again, that's very important.

The strength of the piece is developed depending on the distance from the bottom of this bar to the bottom of the piece, whether it be 1 inch, 2 inch, 3 inch, that's all part of the design.

You also have some on the end that he's hammering on.

That basically just keeps the cage clear on the end of the form.

But all these plastic parts stay in there.

They get cast right in the concrete.

And it basically makes this cage-- once it's set in the form in the right spot.

You can see over here, you have a piece of pipe that goes through that they tie in ahead of time.

That's what we call a weep hole So that allows water-- hydrostatic pressure from the outside of the bridge when this thing's in the ground just from ground water.

It allows it to weep through into the stream.

Most of these that we do are all stream crossings.

If you allow that hydrostatic pressure to weep through, then you don't have to put quite as much steel in.

And in the case of a stream, you can do that.

It's just going back into the stream.

If it was a road or a pedestrian tunnel underneath, you probably wouldn't want to do that.

So you'll see the guys-- despite the fact that were outside of the building right - will still wear some personal protective equipment.

That's very important.

The helmet always.

Glasses.

You can always get stuff in your eye.

The ends of all these bars are extremely sharp.

It's easy to catch yourself.

So they'll wear gloves and extra layer.

Boots you want.

Not necessarily protective-wise for safety, but for comfort, knee pads if you're getting down on the ground a lot.

If you're working in the building, it's your standard.

We have overhead cranes.

So you're wearing a helmet at all times.

Eyeglasses.

Hearing protection at times, depending on what project you're working on.

Everything we deal with is very heavy.

It's being suspended by cranes and by forklifts.

So just general-- being generally aware is very important.

Out in the yard, it's similar stuff.

You're always wearing steel toed boots, hard hat, generally glasses, generally earplugs if you need that when you're around loud stuff.

So this is one of our rigid frame bridge molds or forms.

This one, in particular, will do-- it's a segmental form.

It will do anything from an 8 foot span up to a 32 foot span.

At full capacity, it'll do an 8 foot tall leg.

In this particular case, we only need a three foot tall leg, which is where you'll have your carpenters build, basically, false floors on it right here.

That'll shorten that leg up.

Basically, every project we do is a different height or a different span.

So you're constantly building stuff out of wood to modify the form a little more.

So right now, they're opening a form back up.

It's preparing for the reinforcing cage to be brought over.

While the form is open and accessible, they will oil it and grease it so that the concrete strips easily out of the form again tomorrow.

They'll coat the entire form with it.

And anything that gets cast in any sort of insert.

You want to do this before the reinforcing cage goes in.

You do not want to get any of this on the reinforcing because you want the concrete to bar and to the reinforcing.

And it makes that composite structure and it'll give it strength.

[music playing] So right now, they're doing two things.

They're closing up the form, final fabrication of that.

And we're also doing the final QC check or quality control check.

The form had already been put together.

So he checked the actual dimensions prior to this stage.

But now that all the components are in the form and it's getting closed up, he'll do his final check.

There's really not much you can do with mistakes in this field.

You can grind it up, but it's labor intensive and mechanically intensive and it's-- you don't get much out of it.

[music playing] So these are our aggregate bins.

You've got two different types of stone on the sides and then sand in the middle.

Which ones they use depend on the mix of concrete.

As you can see, he'll load it up with a bucket loader.

Comes in, drops through these grates, and then goes down onto the conveyor belts.

Eventually into the truck to mix in with concrete.

These silos out here have the cement in them-- cement or cementitious material.

We also use fly ash and furnace slag.

Those will deposit that material right onto the belt and it will bring it up to the truck.

Behind you in the building is where we have the other aggregates, the stone and the sand.

Those will get deposited inside the building along with any other mixture that he needs to put in there.

Come up the belt, go into the truck with water, the truck will turn to make the concrete mix.

What you are seeing right there is calcium that the driver is actually putting in by the bag inside the building.

During winter months, we'll use calcium to try to help prevent against frozen chunks and ice and stuff like that.

It's all in sequencing.

What goes in and what order.

You've got to have some water in there to begin with, what we call head water.

And then, you start putting in your cement and your aggregates.

That allows for a thorough mix and that doesn't segregate.

If you're pouring concrete from one place and it's flowing 15 feet, you want the stone and the sand to flow 15 feet as well.

You don't want just the water to go 15 feet.

So you don't want it to segregate.

And you also want to avoid chunks, what we call meatballs, which they're not good.

As far as additional admixtures, you have-- with SEC concrete, which is what we use, you'll have water reducing admixtures.

That basically allows the concrete to be workable when you're setting it in the forms and it reduces the amount of water and it helps the concrete to not segregate.

There's also accelerating chemical mixtures and retarding chemical mixtures.

And those are what they sound like.

They'll either allow the concrete to set up quicker or slow it down.

If this truck right now is going two hours away, you might put a retarding agent in there and that will allow the concrete to stay plastic for longer before it sets up.

So this is kind of the control station for all our admixtures, all the chemicals that go into the mix.

It's all monitored, sensored, they can deliver up to ounces, basically.

It's all measured small quantities.

Only a little bit goes in each load.

So it's pretty technical.

It's all run from in that room.

The dispatcher will plug everything and what they need for quantities.

You can peak around in there.

Allie bites but not very hard.

That's the batching software.

That's where he puts it all together.

Puts the recipe together, the sand, the stone, any admixtures.

He's got a camera up here where he can monitor as it dispenses and keep an eye on it.

That's about all I know about that stuff.

[music playing] Before any concrete can be cast into the mold for the finished product, we have to run a number of tests.

He's going to check the temperature of the fresh concrete, he's going to take the unit weight, he's going to check the air content, then he's going to make cylinders that we can later break for strength.

Right now, he's checking the unit weight, and that's also the device that we'll use to check the air.

Those are all parameters that are based on the job itself and are dictated by if it's a DOT project or by the engineering design firm.

They'll give us the parameters that we have to meet.

In order to perform these tests for any government agency and for most projects, you have to be ACI certified as a level one field technician, which Mike is.

You have to take a test every five years, I believe it is, to renew your certification.

That's the procedure for testing the air.

You fill up that chamber, you'll seal it off, and then release the pressure in the chamber and it'll give a reading.

Concrete is porous and you want trained air or air bubbles inside that.

And what it does is, especially in cold weather climates, it allows for what they call a free [inaudible] process.

That came out as A5.

So assuming that passes, which it did, now we can go over and we'll start pouring.

He'll finish taking the rest of his tests, the strength cylinders that later we'll break to make sure it meets strength.

Additional to the piece, they'll cast six to eight of these.

These are how we determine the strength of the concrete in the piece after seven days, after 28 days, and after 56 days.

If you're working for the state of New Hampshire or any other Department of Transportation, they're going to require that you verify that pieces match certain strength.

That's how you do it.

You can't test the piece once it's cast.

But you break these cylinders.

The typical benchmark for concrete strength is 28 days.

That's generally what you're shooting for.

Because you want to reach that-- what it was designed for.

If it's 5,000 PSI, you need to hit that within 28 days.

Concrete will continue to develop strength for-- it varies for a lot of different reasons, but maybe up to six months before that hydration and the setting process is complete.

It's really like a living creature for a while.

And of course, the goal is to reach that strength as soon as possible.

A lot of times, we'll try to reach the desired strength overnight.

That way, we can strip it, put it right outside, and occasionally, it can go right to a job.

The port and play stuff you see out on the highway and on big bridges, they'll pour that and they'll let it sit without any traffic on it or anything for at least 28 days, generally.

This is our compression testing machine.

Those cylinders that you saw out there, you literally put them in here, he'll dial it in, and that large piston right there just keeps-- continuously puts more and more pressure on it.

You've got a dial readout up here.

And once that concrete cylinder breaks, that's when you know your PSI or your pounds per square inch that-- the strength of the concrete.

I don't suppose we can see that.

We probably could.

Yeah, we probably could.

I could see if Mike's got a spare one around.

So right now, he's just prepping the cylinder to break.

It's basically just cleaning off the ends.

Make sure there's no loose debris on it.

You wrap that little-- wrap around it in order to prevent a lot of shards from kind of falling off.

You set it in this compressor machine with those two plates on.

Those just even out the loading.

And once he gets it all set up, he'll close it up and start applying pressure to that piston up top.

And that'll apply even pressure down.

The value of how much will be up there on that readout.

You can see on the right hand side right now it's got about 2,700 pounds per square inch it's putting down there.

The total load of the pressure is on the left.

You heard the initial crack.

Sometimes, you can get a little more after the initial crack before the cylinder truly fails.

And there, that's where it failed.

So 39 37.

3,930 pounds per square inch.

That's what you use as your barometer for the strength of the concrete.

If we have not met it with this one, it needs to set for another day or another week.

And then, we'll break another cylinder to make sure we achieve the designed strength.

And that's a good failure.

You want to see a kind of break throughout the piece of concrete as opposed to just one chunk off the top.

[machinery noises] Fire away, bud.

So during the pour, some of the things that we want to pay attention to are anything moving while the concrete is coming down the chute.

It's heavy, so it has a tendency-- it can push stuff around.

They're watching that.

They want to make sure they get a nice even spread throughout the form.

The truck driver will move the chute around in order to do that.

And then, it's just pace.

They pour it at a pace that allows it to fill up evenly and not put too much pressure on weak points in the form.

After that, it's essentially just pouring up to the finish line.

And then, finishing the product of on the top.

[music playing] Most people don't realize how much stuff gets cast out of concrete and how much of it is out there.

But we produce everything from bridges like this that carry streams under roadways to manholes for a sewer, storm water treatment, highway barrier that you drive down and you see all over the highways in New England, sound wall posts and panels, tanks of every shape and size, water cisterns, head walls, precast reaches a lot of different-- all different places.

[music playing] So now that it's finished, the QC guy has done his final check to make sure everything hasn't moved.

And they have finished the product as far as the surface.

Now they tarp it.

And it'll cure overnight.

And tomorrow morning we'll strip that piece and start the whole thing over again.

Concrete's a pretty remarkable material.

For all intents and purposes, the recipe for it has been the same for at least 2,000 years.

The four main ingredients, stone, sand, water, and cement have been used since the Romans were making it.

They would use volcanic ash, seawater, limestone, seashells at times, and those buildings, they were built and still standing today.

I mean, some of those are built in the years when they had BC next to it.

It was the Colosseum, the Pantheon, Marcelo Theater, those are all 2,000 years old.

So the basic formula hasn't changed.

It's just some of the admixture and some of the technology has advanced it.

But for all intents and purposes, it's been a bulletproof product for a long time.

Major funding for I Build New Hampshire is provided by-- [music playing] With additional funding from-- [music playing]