How to finish a large span of concrete

How to finish a large span of concrete

This diy step by step article is about how to finish concrete slab. Consequently, you need several tools which you can either buy or make by yourself in order to give concrete a nice looking finish. Generally speaking, there are a lot of possible finishes you can give to your concrete slab or foundation, but the easiest one is to use a large float made out of 2×4 boards. Nevertheless, make sure you finish the concrete slab or foundation as soon as possible, otherwise the concrete might dry out and you won’t be able to use the float to smoothen its surface. Finishing a large span of concrete is more complex than smaller projects, as you need a few friends to help you and a certain techniques, to get the job done properly.

Made from this plan

In order to finish concrete slab, you need the following:

Materials

  • Concrete
  • Wood boards for building the forms
  • 2×2 lumber for securing the forms

Tools

  • Safety gloves, safety glasses
  • Measuring tape
  • Bristle push broom for finishing the concrete slab
  • Wooden float to level the concrete slab
  • Use boots, as not to get your cloths and skin dirty with concrete
  • Clean thoroughly your tools after you have finished the project
  • Few hours up to a day, according to the surface

How to pour a concrete slab

How to finish a large span of concrete

Pouring concrete with hose

In order to pour a slab you need to level off the surface and build a basic formwork. The formwork is usually made of wood boards, but you have to make sure it is rigid, as the concrete exercise a high pressure after it has been poured in.

When building the formwork, you have to make sure it is aligned and level. It is essential to build the formward at the same level on all sides, as in most of the cases you have to use a screed (a long straight piece of lumber or metal) to keep the concrete surface flat.

If your yard is large, then the concrete truck can pour the concrete directly in the formwork. Nevertheless, there are cases in which the truck cannot reach a certain location, so you have to use a pomp and 4” hoses. If this is the case, make sure you pour the concrete as evenly as possible on the whole surface, otherwise you will find it difficult to finish the concrete slab.

How to finish a large span of concrete

How to pour a concrete slab

Remember that actually pouring concrete in the formwork with a hose, isn’t as easily as it might seem at the first glance. On the contrary, the person who is in charge with this operation has to be used to this, otherwise it could be even injured during the process.

While pouring the concrete slab, the hose will get really heavy, so it is a good idea to get a few friends to give you a hand. Make sure you use rubber boots, some old cloths and protective gloves, otherwise the concrete might affect your skin.

How to finish a concrete slab

How to finish a large span of concrete

Finishing concrete slab

Consequently, you could build a float by using a regular stick and a 2×4 wooden board. The board should be around 2 feet wide, as to be able to use it easily when finishing concrete slab. If the weather is very hot, you should hurry yourself, otherwise the concrete might dry out and you woundn’t be able to finish concrete slab.

How to finish a large span of concrete

How to finish concrete slab

Continue using the float over the whole surface making sure you give the concrete a nice finish. As you notice in the image, you still have to use rubber boots as to be able to finish concrete on the whole surface.

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More and more during recent years architect’s specifications are calling for finely finished concrete surfaces on walls, ceilings, and floors. At first, contractors used hand rubbing bricks, but finding this technique to be slow and costly for large areas, they turned to various sorts of powered tools. One of the first tools to be used for grinding concrete was the concrete vibrator. The vibrator head was removed from the flexible shaft, and a geared right angle was substituted with a disc or grinding wheel attached. This is still true to a great extant. There are basically two methods used to smooth off hardened concrete surfaces. They are dry grinding and wet rubbing. Dry grinding is done of dry concrete surfaces to remove fins and marks caused by forms. Formerly resinoid abrasive cup grinding wheels were used for this, but recently abrasive discs have been developed that are much lighter than the grinding wheels an do an excellent job. These discs are made up of multiple layer of abrasive-impregnated cotton fiber material bonded together under heat and pressure with strong resin bond. A silicion- carbide abrasive is used since this has been found to be the best abrasive for concrete. These discs are usually about one-fourth of an inch thick, and come in diameters of 7 to 9 inches. They can be applied to the work at an angle of 30 to 40 degrees using the weight of the tool for pressure. These discs can be attached to any type of portable tool and should be operated at from 3,000 to 4,000 rpm. Wet rubbing is accomplished on green concrete by a slow speed grinding wheel or disc, at the same time applying water. The water, plus the rubbing action, brings up a cement paste which results in a very smooth, plaster-like surface. The grinding wheel or disc is used with a right angle head, which has various gear reductions to reduce the grinding speed to 300 to 500 rpm. This wet angle head is provided with an attachment for a water hose so that water can run through the center of the grinding wheel onto the concrete.

Concrete can be finished in many ways. The most basic option is a smooth troweled surface, but this can be slippery when wet. Broom finished concrete solves this problem.

Find concrete contractors near you to help with your project.

WHAT IS BROOM FINISH CONCRETE?

Broom finished concrete is popular for patios, driveways and more. Sometimes called brushed concrete, a broom is used to create a slip-resistant texture on the surface of the concrete. Concrete finishers have been broom finishing for about as long as there has been concrete.

Broom finishes have been used for many years and provide a durable, high-traction surface. Concrete Solutions in San Diego, CA

Broom finished thin overlays add skid resistance and upgrade a deteriorating concrete surface. Concrete Solutions in San Diego, CA.

If you’re considering a broom finish for your concrete, here are the pros and cons:

  • Slip resistant
  • Very affordable
  • Easy to do
  • Durable
  • Can be too rough on bare feet
  • Not as decorative as other finishes
  • May trap dirt and debris
  • Can cause color variations

HOW LONG SHOULD YOU WAIT TO BROOM FINISH CONCRETE?

When it comes to brooming, timing is critical—brush too early and the surface will be weak, brush too late and there won’t be enough texture. It must be done while the slab is still soft, but after the bleed water has dissipated. Depending on the weather, this can take anywhere from 20 minutes to 4 hours.

Each contractor has a trick for determining when to broom concrete. Some wait until their finger doesn’t leave an imprint, others simply know it’s ready by how the broom feels when pulled across the surface.

HOW TO BROOM FINISH CONCRETE

The typical process for a broom finish is:

  • Pour the slab
  • Strike off with a screed
  • Bull float
  • Wait for the bleed water to evaporate
    Although with low water-cement ratio exterior concrete with the proper amount of air, there might not be much bleed water. Bleed water is a result of the wet concrete settling and with entrained air, it doesn’t settle much and therefore little water comes to the surface. The proper amount of air is always critical in any exterior concrete that will be exposed to free-thaw action. For concrete with ¾ or 1-inch aggregate, order the concrete with 6% entrained air (plus or minus 1%)-and make sure you are getting it, otherwise the surface will spall. For smaller aggregate you need more air-7% for ½ inch and 7.5% for 3/8 inch.
  • Trowel
    There’s some disagreement here. In many cases, today’s finishers won’t trowel a slab that’s getting a broom-finished surface, just bull float and broom. One veteran finisher, however, told me “I like to use a fresno to get the bull float lines out.” Bob Simonelli, with Structural Services Inc., says that some troweling is OK, “but be careful not to over-finish the surface and work some of the air out.” Advice in a 1996 edition of Concrete Construction‘s Problem Clinic, however, says you can trowel twice before brooming, but be sure to keep the trowel flat during the second troweling and begin brooming “immediately after the second troweling.” If you get the surface troweled hard, it will be difficult to get much texture. PCA’s Cement Mason’s Guide says to use a damp broom after troweling.
  • Broom the surface
    Run a concrete broom perpendicular to the slope, if there is one. On concrete that’s intended to drain, though, broom marks should be run towards the drain. One thing to note is that a broom-finished exterior surface is just as durable as a smooth finish.
  • Cure the concrete
    You can (and must) cure broom-finished concrete with sheets of polyethylene or by spraying on curing compound. For plain gray concrete, a curing agent with some color (typically white) in it helps you to see where it’s been applied. The color dissipates after a few weeks. For decorative concrete, use a cure & seal. Don’t forget the curing!

A good broom finish is something of an art. Dyes and stains can be applied very successfully to broomed finishes. You can even create decorative effects by running the broom texture in various directions. Typically the broom should be run from side to side of the concrete without stopping. With a standard broom, you should pull the broom towards you, then lift it and set it back on the far side to pull it across again.

WHAT TYPE OF BROOM IS BEST?

Brooms are available from a variety of sources. They come in various widths and the block that holds the bristles can be made from wood, aluminum, or plastic. Brooms tend to be wet a lot and the plastic blocks (high-density polyethylene) won’t rot or warp. Bristle materials can be horsehair, polypropylene, or nylon and come in various stiffnesses and sizes to produce different textures. For extreme textures, wire combs are available to produce tined finishes.

Marion Brush makes a brush (the Auto Glide) where the head automatically tilts to the correct angle, so you can get a good broom finish whether you are pushing or pulling the broom.

Concrete brooms come in different configurations,some including brushes that will provide different levels of texture.

Handleless brushes are good for very wide installations. Marion Brush Mfg Co.

Marion Brush Co. specializes in concrete brooms. Their president, Gary Bolden, says that “there are a lot of variables in getting a good broom finish: the slump of the concrete, the weather (sun, wind), and the timing. Every contractor has a tendency to do a slab in their own way. Some might broom sooner than others. Some may pour and broom finish and use a softer brush so they can get on the slab sooner.” Marion’s Chameleon concrete broom allows you to change out an insert to get different textures or to adjust to conditions. The color-coded inserts come in five textures from supersoft to rough.

Marion’s concrete brushes are made from 612 nylon, which Bolden admits is one of the most expensive materials used for brooms, but which he says will outwear other materials by 3 or 4 times. The other advantages of this nylon are that it stays cleaner during use and that it has a memory. No, it can’t remember your phone number, but if the bristles get bent, you can pour boiling water on them and they will return to their original shape.

A couple of other interesting kinds of brooms are handleless and brooms attached to bull floats or fresnos. The handleless brooms are pulled back and forth across the surface with ropes-Marion Brush and Cleform make these. This invention is especially handy on very wide pours where it would be difficult to push a broom all the way across and where the bull float handle begins to get so heavy that it pushes the broom too deeply into the concrete. Also available from Marion are brooms attached to bull floats or fresnos. This allows the float to support the weight of the handle rather than the broom. Bolden says that this positions the broom so that the finish is made by the sides of the bristles rather than the tips, which provides a more uniform finish.

SLIP RESISTANT BROOM FINISHED OVERLAYS

Another way to provide slip resistance is to overlay the concrete and broom finish or texture the overlay. There are several products specifically designed for this application. For example, Concrete Solutions’ Ultra Surface is a polymer concrete that can go down as thin as 1/16 inch, on a properly prepared surface (typically pressure washed or sandblasted, since the sealers need to be removed). Mapei also makes Concrete Renew, which is similar. These products contain polymers for strength and bonding and should be applied with a squeegee and broom-finished immediately after placement.

How to finish a large span of concrete

Concrete joints are used to compensate when concrete expands or shrinks with changes in temperature. Concrete joints are normally used to prevent cracks when the concrete shrinks by creating forming, tooling, sawing, and placing joint formers. The pre-planned cracks will provide a better finish concrete product and will be formed in specific locations where those cracks could be monitored. Sometimes because of the material and width or span of the concrete joints are required to improve the performance of the material and allow the materials to expand/contract or move without damaging other structures.

Concrete Contraction Joints

Intended to create a weakened area in the concrete and regulate where cracks will occur, normally in a straight line. Contraction joints should be placed to produce panels that are as square as possible and never exceed a length to width ratio of 1 ½ to 1. Joints are commonly spaced at distances equal to 24 to 30 times the slab thickness.

Joint spacing that is greater than 15 feet requires the use of load transfer devices. Contraction joints may be tooled into the concrete surface at the time of placement. Joints may also be sawed into the hardened concrete surface. It is important to understand that the longer sawing is delayed, the higher the potential for cracks to establish themselves before sawing is complete.

Concrete Expansion Joints

Concrete expansion joints are used to separate slabs and concrete from other parts of the structure. Expansion joints allow independent movement between adjoining structural members, minimizing cracking when such movements are restrained. It allows for thermal expansion and contraction without inducing stress into the system.

Why Do We Need Them?

Construction joints are used in circumstances where two consecutive placements of concrete meet. Construction joints are normally placed at the end of the day or when concrete pour has been stopped for longer than the initial setting time of concrete. Construction joints should be designed and specified by a structural engineer. You can also achieve bond and continue reinforcement through a construction joint. If enough PCC is available at the end of the day, the construction joint can be placed at a planned transverse contraction joint.

Placing Tips

The following recommended tips should be observed:

  • Maximum Joint Spacing should be 24 to 36 times the thickness of the slab.
  • Joints should be spaced about 10 feet and a maximum of 15 feet.
  • When using joint groove for contraction joints, the joint should be a minimum depth of ¼ thickness of the slab.
  • Saw-cut joints should be done within 4 to 12 hours after the concrete has been finished.
  • Keyed joints are not recommended for industrial floors.
  • Dry-cut joints should be made between 1 to 4 hours after completion of finishing.
  • The pre-molded joint filler should be used to separate slabs from building walls or footings. Place 2 inches of sand over the top of a footing to prevent bond to the footing.
  • If the slab contains wire mesh, it is recommended to discontinue the mesh across contraction joints.
  • Joint spacing should also be chosen, so that concrete sections are approximately square.
  • It is recommended to have concrete joints along column lines, either sawed or keyed.
  • Metal dowels should be used in slabs that carry heavy loads.
  • Plan exact locations of all joints, including the timing of saw cutting.
  • Use isolation joints between slabs and columns, walls and footings and where curbs or sidewalks meet other concrete structures.
  • Materials used on concrete joints must be flexible enough to absorb or deform as needed and then being able to restore back to their original state.
  • Always look for materials that are permeable, and that can bond to the concrete.

Tools

The following tools are normally used to create concrete joints, although these tools might vary depending on the size and scope of the project. Here are the most commonly used tools:

  • Hand groover or walking groover: depending on how large the slab is, you might want to choose one of these two.
  • Cordless control joint tools: ideal for use on small to medium size projects.
  • Concrete saw: ideal for saw cutting concrete, but be sure to know how deep that cut needs to be. Otherwise, you might be creating other problems.

A die grinder with a masonry blade can be used in place of a concrete saw to cut joints in dry concrete.

Updated on 3/5/2021

More retail, restaurant, warehouse, fleet management, and office facilities are opting for polished concrete flooring as an alternative to marble, granite, tile, VCT, wood, linoleum, or coated concrete. Even homeowners are catching on to the appeal of these smooth, high-luster floors, which can be stained to replicate the look of polished stone.

Considering Polished Concrete Floor or looking for a trustable contractor to provide Concrete Polishing Services? Here is your must-read POLISHED CONCRETE COMPLETE GUIDE.

How to finish a large span of concrete

POLISHED CONCRETE: THE COMPLETE GUIDE

CHAPTERS

1. Polished Concrete Fundamentals

What is Polished Concrete?

Polished concrete is concrete that has been processed through a series of mechanically ground “polishing/grinding” steps utilizing professional equipment designed for concrete polishing. This process also includes the use of a concrete densifier/hardener which penetrates into the concrete and creates a chemical reaction to help harden and dustproof the surface. During concrete polishing, the surface is processed through a series of steps (in general a minimum of 6-8 grinding steps of processing is considered polished concrete) utilizing progressively finer grinding tools. The grinding tools are progressive grits of industrial diamonds in a bonded material such as metal/hybrid/resin often referred to as diamond polishing pads.

Concrete is not considered well polished before 800 grit, and it is normally finished to either the 800, 1500, or 3000 grit level. Dyes designed for concrete polishing are often applied to add color to polished concrete as well as other options such as scoring, creating radial lines, grids, bands, borders, and other designs.

2. Why Polished Concrete

Simply put, a polished concrete floor system transforms a porous concrete floor into a tightened surface, dense enough to keep water, oil, and other contaminants from penetrating the surface.

Elegance and Attractive

Very Strong and Durable

Less Maintenance

Extended Long Life-Span

Highly Cost Effective

Hygiene Friendly

Minimum Downtime

Better Ambient Lighting

3. Concrete Polishing Process

How to finish a large span of concrete

Currently, the industry breaks down the process of polished concrete into grinding and polishing. Some simply use the word “polishing” for the whole process. Each phase is then broken down into multiple steps, consisting of consecutively finer grit abrasives. Every step is refined to its purest possible form on a microscopic level from one progressively finer abrasive to the next until the desired level of ‘polishing’ is achieved.

During the processes, a densifier/hardener is applied that is absorbed into the concrete creating a chemical reaction that makes the concrete more dense and hard.

Recent advances in polishing equipment and techniques have allowed concrete to be ground, honed, and polished to a high polish with clarity of reflection and depth. The process is performed with large planetary head machines where the main head rotates in one direction and smaller satellite heads spin in the opposite direction.

The result is a beautiful, durable and efficient surface that eliminates the need for carpets or tile that requires expensive replacement, maintenance, and use of harsh cleaning chemicals.

IMPORTANT TIPS

Maximum “clarity of reflection” and “durability” of the surface is achieved by strictly following the procedures, with which a contractor executes the grit sequence. Even if the desired result is not clarity of reflection or a glass-like finish, each grit must be fully refined to obtain the maximum durability and a clean crisp look of the finished surface. It is imperative not to skip grits in the processing sequence.

The process of concrete polishing is very technical in nature and requires real craftsmanship.

4. Options & Choices

There are many options available for different Polished Concrete designs. Different Aggregate Exposure, different Clarity of Reflection, and different dyed Color will change the visual aspects and physical characteristics of your polished concrete floor significantly. So know your options and potential outcomes before you make choices.

4.1 Aggregate Exposure

The Aggregate Exposure of the floor represents the amount of aggregate that will be exposed at the surface. Consider the needs and functions of your building when deciding whether aggregate exposure is appropriate. Grinding the concrete to expose aggregate may be considered attractive for a commercial space, but it can be unsuitable for a manufacturing space where dropped fasteners need to be easily spotted on the floor.

Below are the four most common aggregate exposure levels for polished concrete floor finishes.

Light Sand / Cream Finish
The light sand exposure is a ground and polished surface, typically exposing only the sand particles in the concrete floor. This finish is considered a “creamy looking” surface and is the most popular choice for Architectural Ground & Polished floors.

Salt & Pepper Finish
The Salt & Pepper exposure is a ground and polished surface, typically exposing a spattering of fine aggregate in the concrete floor. This finish is most often chosen to give the appearance of an aged surface. Approximate surface cut depth 1/16 inch.

Medium Aggregate Finish
Medium Aggregate exposure typically exposes the greatest amount of medium aggregate in the concrete floor with little or no large aggregate exposure at random locations. Approximate surface cut depth 1/8 inch.

Large Aggregate Finish
Large Aggregate exposure is a more heavily “cut” surface, typically exposing the greatest amount of larger aggregate in the concrete floor. The finish is the most often chosen when the substrate has been seeded with custom aggregate. On some floors including restoration of older concrete, there may be limitations to how deeply the floor can be cut. Approximate surface cut depth 1/4 inch.

How to finish a large span of concrete

Correctly laid out joints. Note: Inside corners, where cracks would typically occur, have correctly placed joints.

It is important to be active in deciding where control joints are placed. Often, jointing is not taken seriously enough and the “sawcutter” comes to your job and puts the cuts where he feels they belong or where it is convenient for him. And, most plans don’t have joint spacing marked on them. So don’t leave this important part of concrete construction to chance.

WHAT ARE CONTROL JOINTS?

Control joints are planned cracks which allow for movements caused by temperature changes and drying shrinkage. In other words, if the concrete does crack-you want to have an active role in deciding where it will crack and that it will crack in a straight line instead of randomly.

WHEN TO CUT CONTROL JOINTS

Make sure you are cutting joints soon enough. In hot weather, concrete might crack if joints are not cut within 6-12 hours after finishing concrete. In this condition, if you don’t want to use a grooving tool to cut joints, there are early-entry dry-cut lightweight saws that can be used almost immediately after finishing. These saws cut 1″ to 3″ deep, depending on the model.

CONTROL JOINT SPACING

Space joints (in feet) no more than 2-3 times the slab thickness (in inches). A 4″ slab should have joints 8-12 feet apart.

When arranging joints, skilled contractors will often use them to create an attractive diamond pattern. If your concrete will be stamped, ask about the best ways to avoid interrupting the pattern with control joints. Watch this video for a good solution: Pouring Concrete with Zip Strips.

Concrete Groover Video (Placing Control Joints in Fresh Concrete)
Time: 03:11

MORE JOINTING TIPS

  • Cut joints deep enough
    Cut joints 25% of the depth of the slab. A 4″ thick slab should have joints 1″ deep.
  • How to cut joints
    Groover tools cut joints in fresh concrete. Saw cutting cuts joints as soon as the concrete is hard enough that the edges abutting the cut don’t chip from the saw blade.
  • Place joints under walls or under carpet areas
    Under walls they won’t be seen. Under carpet areas the joints won’t have a chance to telegraph through vinyl areas.
  • Avoid re-entrant corners
    Planning the joint pattern can sometimes eliminate re-entrant corners.

CONTROL JOINT VS. EXPANSION JOINT

Control joints are meant to control cracking, while expansion joints are meant to allow for movement. Expansion joints, or isolation joints, are used between two different concrete pours, or where concrete meets with another material or even a structure. Expansion joints are more common on big commercial projects and often aren’t required when pouring residential slabs. These joints usually require filling, especially if the concrete is going to be polished or finished with a coating (see Sealing Joints in a Concrete Slab).

How to finish a large span of concrete

Correctly laid out joints. Note: Inside corners, where cracks would typically occur, have correctly placed joints.

It is important to be active in deciding where control joints are placed. Often, jointing is not taken seriously enough and the “sawcutter” comes to your job and puts the cuts where he feels they belong or where it is convenient for him. And, most plans don’t have joint spacing marked on them. So don’t leave this important part of concrete construction to chance.

WHAT ARE CONTROL JOINTS?

Control joints are planned cracks which allow for movements caused by temperature changes and drying shrinkage. In other words, if the concrete does crack-you want to have an active role in deciding where it will crack and that it will crack in a straight line instead of randomly.

WHEN TO CUT CONTROL JOINTS

Make sure you are cutting joints soon enough. In hot weather, concrete might crack if joints are not cut within 6-12 hours after finishing concrete. In this condition, if you don’t want to use a grooving tool to cut joints, there are early-entry dry-cut lightweight saws that can be used almost immediately after finishing. These saws cut 1″ to 3″ deep, depending on the model.

CONTROL JOINT SPACING

Space joints (in feet) no more than 2-3 times the slab thickness (in inches). A 4″ slab should have joints 8-12 feet apart.

When arranging joints, skilled contractors will often use them to create an attractive diamond pattern. If your concrete will be stamped, ask about the best ways to avoid interrupting the pattern with control joints. Watch this video for a good solution: Pouring Concrete with Zip Strips.

Concrete Groover Video (Placing Control Joints in Fresh Concrete)
Time: 03:11

MORE JOINTING TIPS

  • Cut joints deep enough
    Cut joints 25% of the depth of the slab. A 4″ thick slab should have joints 1″ deep.
  • How to cut joints
    Groover tools cut joints in fresh concrete. Saw cutting cuts joints as soon as the concrete is hard enough that the edges abutting the cut don’t chip from the saw blade.
  • Place joints under walls or under carpet areas
    Under walls they won’t be seen. Under carpet areas the joints won’t have a chance to telegraph through vinyl areas.
  • Avoid re-entrant corners
    Planning the joint pattern can sometimes eliminate re-entrant corners.

CONTROL JOINT VS. EXPANSION JOINT

Control joints are meant to control cracking, while expansion joints are meant to allow for movement. Expansion joints, or isolation joints, are used between two different concrete pours, or where concrete meets with another material or even a structure. Expansion joints are more common on big commercial projects and often aren’t required when pouring residential slabs. These joints usually require filling, especially if the concrete is going to be polished or finished with a coating (see Sealing Joints in a Concrete Slab).

How to finish a large span of concreteInstalled quickly with less labor, maintenance-free precast concrete box culvert bridges deliver a long service life. These eight box culvert designs demonstrate why precast is the best choice for a variety of site conditions.

By Gary K. Munkelt, P.E.

Maintenance and replacement of bridges over small streams are ongoing concerns for transportation agencies responsible for their upkeep. In recent years, however, infrastructure funding has not kept up with maintenance needs, and when bridge maintenance is not performed, the structures fall into disrepair.

A decision must be made as to when a bridge needs to be replaced. Should the bridge be replaced in kind, or are there better alternatives? The decision will be based on considerations such as cost, complexity of design, available materials and amount of time the highway is closed to traffic. Future maintenance should always be a part of the decision-making process.

How to finish a large span of concreteMaintenance-free service and ASTM design assurance

Several systems are available for consideration before replacing a bridge. One of the alternative bridge systems to consider is precast concrete box culverts. They offer a range of sizes and configurations to fit specific site conditions. When properly installed, a precast concrete bridge replacement can provide maintenance-free service for many years. There are concrete structures in operation today that are 100 years old, and many of these structures don’t require an annual maintenance budget. The concrete sits in place year after year and does its job.

Design of box culverts is not difficult and is normally performed by the precast manufacturer. Standards such as ASTM C1577–11a1 provide guidelines to ensure that the product design is adequate. By producing the same product repeatedly, the precast concrete industry can offer reliable, high-quality products. Precast manufacturers are located in most areas of the country, making the product readily available. Competition between producers helps to promote economy of cost.

How to finish a large span of concreteAdvantages of precast bridges over CIP installations

Replacing a bridge over a stream using a conventional cast-in-place (CIP) installation can close a road for 10 to 12 months due to the time required for curing concrete on site. For a typical CIP job, footings must be installed first. After they cure, the pedestals formwork can be made, followed by another concrete pour. After that, more curing time is required to form and place the concrete riding surface. Waiting three to four weeks between pours significantly extends the time required to finish a project.

Less time and labor: A precast concrete box culvert can be installed much faster than CIP construction, because the three- to four-week curing time is spent at the fabricator’s plant. Precast box culverts are often manufactured before a project is started. In many cases, preparation at the site takes less than one week. Installation of the finished precast box culverts is complete in a matter of days. There are box culvert installations on county roads where the road is closed for only two to three weeks. Fast installations provide an added advantage in cost savings, because labor hours are kept at a minimum.

How to finish a large span of concreteDesign agencies should consult with precasters in the project area to determine their capabilities. Using product sizes that are standard for a producer will normally enable them to provide more efficient pricing for quotes.
Design flexibility: The box culvert concept has been modified over the years to solve many job-site problems. Standard rectangular boxes are produced in many combinations of height and width. ASTM C1577 provides a table for sizes ranging from 3 ft wide by 2 ft high to 12 ft wide by 12 ft high. Other sizes are available and are limited only by the ability of local precasters to adapt their forms.

Eight designs meet a variety of site conditions

Many precast concrete box culvert configurations may be used to span crossings that vary from narrow rivulets to fairly wide streams. Advantages of each these structures are usually unique to the job site, installation contractor or precast manufacturer. Here are eight precast box culvert designs:

1. Single box culvert: One structure, 3 ft to 12 ft wide with wing walls, is installed (end-to-end as needed for road width) above narrow streams. The height of the box culvert is dependent on amount of flow and site conditions.

How to finish a large span of concrete2. Double box culvert: Two 12-ft-wide box culverts are used together under highways that span medium-wide streams.

3. Triple box culvert: Three 12-ft-wide box culverts are set side by side for larger streams. The length of the culverts depends on the width of the highway.

4. Bottom slab trough: Some designers require a modified bottom slab to provide a small trough for low flows during periods of drought. This modification consists of a second pour of concrete that can be formed to any desired shape.

5. Clamshell design: An innovative method of reducing the weight of each component is to utilize a “clamshell” design, where the box is made in two pieces. The advantage to the contractor is that product weight is cut in half, requiring lighter, less-expensive equipment during installation.

How to finish a large span of concrete6. U-shaped design: Some manufacturers make a box by using a “U” shaped piece and putting a flat slab on top. Or they use a flat slab for the bottom and set a U-shaped piece upside-down on the top.

7. Three-sided arch design: This is an adaptation of the box culvert concept that has no bottom slab. It is popular where there are environmental issues or a desire to avoid disturbance to a stream bed. This structure can sit on a CIP footing situated on opposite sides of a stream. The three-sided arch spans the stream without adversely affecting the stream’s natural state.

8. Wildlife crossings: Single, clamshell, U-shaped or three-sided arch box culvert designs may be used to provide a safe crossing for wildlife under roadways.

How to finish a large span of concrete1 ASTM C1577-11a, “Standard Specification for Precast Reinforced Concrete Monolithic Box Sections for Culverts, Storm Drains, and Sewers Designed According to AASHTO LFRD”