Here’s a great article found at Equipment and Contracting Magazine about the differences between concrete, shotcrete and gunite:

For those embarking on a construction project involving the use of concrete, there may be some question about the best method of application.  There are three primary methods that can be utilized, each with its own advantages and disadvantages: traditional poured concrete, shotcrete, and gunite.  The best choice for the job will depend on a number of factors, including budget, where the concrete will be applied, and the skill of the workers applying the concrete.

Concrete is the most commonly used man-made material on earth.  It is a composite material, made of three elements: Portland cement, water and aggregates.  The Portland cement acts as a binding agent when mixed with water and aggregates.  Water chemically reacts with the cement and makes the concrete workable. Aggregates in a concrete mixture may be fine (sand) or coarse (rock or gravel).  When mixed, these three components can be applied and then will harden into a durable material.

Traditionally, concrete has been poured from a ready mix truck onto a project site.  It is either placed on the ground or into forms, and then vibrated to flush out air and to make sure that it is compact.  Because it typically requires the use of forms and vibration for the concrete to be compacted, it is often more expensive and time-consuming than other forms of concrete application.  However, there are significant advantages of poured concrete, such as its ability to  be used in larger areas, like in a building foundation.

With shotcrete, the concrete is projected at high velocity, typically onto a vertical or overhead surface.  The force of this application method consolidates the concrete, resulting in an outstanding bond with most substrates.  It is often more cost-efficient than traditional concrete placement methods, as it requires less formwork.  Shotcrete involves applying the concrete after it is already mixed.

Gunite is similar to shotcrete, in that it is a method of applying concrete involving a high pressure hose.  However, with gunite, the concrete is loaded dry, and mixed with water at the nozzle when it is sprayed.  It is generally less expensive than shotcrete, and allows builders more work time to complete a concrete project.

All three methods result in a water-resistant surface that is less susceptible to deterioration over time than other building materials, such as steel or wood. In addition, because concrete can last in higher temperatures without compromising its structure, any of these methods can be used to fireproof steel.  The finished products also share the other positive attributes of concrete, including strength, low maintenance and relative cost-effectiveness compared to materials.  Before concrete hardens, it is also very pliable and can be easily shaped.

The traditional poured concrete method has several advantages.  First, it is incredibly versatile; contractors can use this method for a variety of projects by building precise, customizable forms.  Second, poured concrete results in a strong, smooth surface.  Third, the application of concrete in this method is common, so that it doesn’t require the specialized skills of shotcrete and gunite.  Fourth, it is generally the most appropriate method for large scale building projects, as it is more time and cost-effective than the other processes.

However, poured concrete is the most expensive of the three methods.  It has a much higher labor cost, because it required forms to be built and then removed once the concrete has hardened.  In addition, the concrete must be vibrated to flush out air and to compact the material.   It is also challenging to form shapes with poured concrete or to make joints.

As a concrete application process, shotcrete has several benefits.  Operators who apply concrete using the shotcrete method must be skilled, but need not be experts in mixing concrete because it is premixed and loaded into the hopper.  Shotcrete takes less time than pouring concrete, and it forms a strong and consistent coating. It also is an inexpensive way to apply concrete in curves and irregular shapes, which are often difficult to impossible to achieve with convention concrete methods.  Shotcrete also does not need a complex system of forms as with poured concrete, and generally needs only a one-sided form or no forms if the soil is compacted.  Finally, it does not require vibration or compaction after placement.

Yet there are drawbacks to using the shotcrete process.  Because it is premixed, once the process is started, the concrete must be applied quickly, without any stops.  If too much water is added to the mix to keep it from hardening, cracks may form in the concrete.  In addition, shotcrete — while less expensive than poured concrete — is more expensive than gunite.  Finally, shotcrete takes a significant amount of time as compared to poured concrete, making it a less than ideal choice for large foundations, structural piers or other large projects.

There are many advantages to choosing gunite as an application method, particularly if you are working with skilled operators who can expertly mix the concrete. The gunite method allows operators to stop and start projects without creating what are known as “cold joints.”  These are areas that form from two separate pours when operators attempt to blend a new pour into an old pour. This cold joint will not only look different;  it will also be susceptible to cracking.  With gunite, an operator can stop and start the process without creating a cold joint, because the velocity of the application allows the materials to bond together.  You will also have more time to work with the concrete, because the concrete will be mixed on site and the process can be stopped and started as needed.  Gunite is generally a less expensive process than shotcrete and concrete pouring, with most of the same advantages as shotcrete.

However, the gunite process is not without disadvantages.  It does require a skilled operator who understands how to achieve the proper ratios when mixing concrete.  Otherwise, the quality of the concrete could be compromised.  The dry mixture may also clog the hose pipe, causing problems on the construction site.  Gunite also has the tendency to produce over-spray (rebound), which is both messy and wasteful.

Ultimately, the concrete application method that works best will depend on the specific needs of the project. Understanding the benefits and drawbacks of each process can help you make an informed choice as you move forward with your concrete construction project.

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Read article at equipment and contracting.com

From the Portland Cement Association:

Read article on Cement.org

Here’s a great article in ICF Builder magazine with tips on keeping your crew safe while working with concrete pumper trucks.

Truck-mounted boom pumps are by far the most common way to fill ICF walls and footings. They’re efficient, effective, and affordable. The average ICF residential project, for instance, will see at least three visits from a pump truck. On commercial jobs, it’s not uncommon to have as many as a dozen different trips to the jobsite, with two pumps working simultaneously.

These pours, though common, can be dangerous. The combination of scaffolding heights, concrete weight, pipeline pressures, and visibility constraints present ample opportunity for injury.

A few years ago, the American Concrete Pumping Association (ACPA) published a booklet specifically for the ICF industry to ensure pour day goes as smoothly as possible. It contains the safety information needed by co-workers to keep them out of harm’s way throughout the course of the day’s work. You should make an effort to see that all persons involved with your pour have access to this information, even if it must be verbally transmitted or translated.

Note that the pump manufacturer’s recommendations supercede any information provided by the ACPA.

On the Ground
Workers near the pump truck must know how to stop the pump and boom. Ask the operator to show you the locations of the emergency stop switches. Personal protective equipment—goggles, hard hat, ear protection, and rubber gloves—are important when working on any concrete pour, and are especially important when working near the concrete pump. Be sure the boots and gloves can resist cement lime burns.

Never put your hands, feet, or any other body part into the water box, concrete valve, or hopper when the hydraulic system is operational or ready to operate.

Avoid standing between the ready-mix truck and the pump truck at all times. Instead, stand to the side, where the driver can see you. When backing in ready-mix trucks, use clear and concise hand signals.

Watch the Boom: Keep an eye on the movements of the boom, even when there are no electrical wires nearby. Alert the operator if he is nearing any obstruction or hazard. If electrical lines are in the vicinity of the area to be poured, it is the position of the ACPA that a spotter shall be employed whose only duty is to observe the movement of the boom and warn the operator if the boom approaches within 20 feet of an electrical wire. This distance is increased to 50 feet if power lines are mounted on steel towers. If the pump or boom were to come into contact with the lines and become energized, anyone in contact with any part of the truck or even near the unit, is at risk of electrocution.

Hopper Levels: Wait until the pump operator gives the “okay” before allowing the ready mix driver to put concrete in the pump hopper. Filling the hopper early can cause the pump to plug.

Do not let the concrete level in the hopper become low enough that you can see the top of the valve mechanism. If air is allowed into the cylinder, it will be compressed, and pressurized air in the line always poses a hazard as it is expelled from the hopper or the delivery pipeline. (See Air in the Line below.)

Never put any solid object in the hopper when the pump is in operation. If foreign material that could create a blockage somehow gets into the hopper, alert the operator to stop the pump. If you can’t get the operator’s attention, push the emergency stop. Do not attempt to remove the material from the hopper or grate while the hydraulic system is ready to work.

Also, make sure the ready-mix driver doesn’t clean his equipment out into the hopper. The extra water will separate the cement and fine sand from the coarse aggregate, which severely weakens the concrete, and also can create a blockage in the pump.

Air in the Line
If air is taken into the material cylinders, take the following steps to minimize the hazard:

1. Stop the pump immediately. Hit the emergency stop button if that is the quickest way to stop the pump. There may be an expulsion of compressed air the next time the concrete valve shifts, which can be safely absorbed by filling the hopper with concrete.
2. Alert the operator of the problem. It is his job to know the procedures for safely bleeding the compressed air from the delivery system. These procedures may include pumping in reverse for a couple of strokes.
3. Persons standing at the discharge end of the delivery line must be warned to move “a prudent and reasonable distance” beyond the reach of the hose until all of the air has been purged and the operator gives you the OK.
4. If workers are positioned in high or precarious places, warn them to expect a loud sound as the air escapes the pipeline. Warn them even if they are well away from the discharge to prevent them from falling as a result of being startled by the noise. If possible, allow enough time for them to get to ground level.
5. When the pump is restarted, the slowest possible speed should be used until all the air is removed from the pipeline. Don’t assume that the first little air bubble is the end of the compressed air.

Note that air will also be present in the line during startup, after moving, when adding or removing hoses, whenever the line has been taken apart or opened for any reason, when waiting for concrete, and when concrete is allowed to drain from the discharge hose.

Similarly, never intentionally introduce compressed air to the system in an attempt to clear a blockage. It is not only unsafe, but futile as well. If pump pressure can’t move the blockage, air pressure won’t either. The operator should know safe blockage removal procedures.

Whenever there is a blockage, the pump must be run in reverse for at least two strokes or the remaining pressure bled off at a safety valve. Use extra caution to ensure the pump and controls are completely powered off and the line is depressurized before opening the pipeline.

On the Scaffold
Workers on scaffolding are exposed to nearly all of the same dangers as the ground crew, with the additional risk factors involved with working on narrow runs of scaffolding.

Slips, trips, and falls that would be minor on the ground can create serious, life-altering injuries when they occur at heights. To minimize these risks, workers on the scaffold are required to wear the same personal protective equipment as the ground crew, which should be designed to withstand the caustic effects of concrete.

Guardrails are mandated by OSHA, and easy to install on all major turnbuckle bracing systems. Stay inside the guardrail or use another form of OSHA-approved fall protection.

Workers should also be constantly aware of the boom’s location. Remember, if any part of the pump or boom becomes energized, everyone with any contact with any part of the truck is at risk of electrocution.

Beware of Pressure: In addition to slips, trips, and falls, “hose whip” is another fairly common cause of accidents. This occurs when air is caught in the line, compressed and then suddenly released at the opening, causing the hose tip to whip violently or suddenly spray concrete. Never position any body part between the end of the delivery system and a fixed object (like a hand between the hose tip and the wall forms). Similarly, never allow workers to position themselves between the boom hose and any fixed object like a wall or column.

The chance of a sudden expulsion of concrete (accompanied by chunks of gravel and wet concrete) is why personal protective equipment such as hard hats and eye protection are mandatory.

As already mentioned, air can be introduced into the delivery system in many different ways, but most commonly when restarting the pump.

Also, keep in mind that pipelines wear with each stroke of the pump, and failure of a pipe, clamp, hose, or elbow is always possible. For this and a number of other reasons, spend as little time as possible standing under the boom. (For the same reason, never stand on, sit on, or straddle a pipeline while it’s pressurized.)

Set-Up: Never hang more weight on the boom than it has been designed to hold. If the speed of the concrete free-falling from the tip hose must be slowed, the ACPA recommends using a reducing hose, a smaller discharge hose or a boom configuration to reduce free-fall instead of metal devices attached to the discharge hose.

Specially designed flat hoses are available that are made specifically for eliminating free fall of the concrete and have been tested to burst-pressures sufficient for today’s high pressure concrete pump. Be sure the hose you use meets or exceeds the working pressure rating of the equipment used and is rated for a burst pressure of two times the pump working pressure when new.

In the field, a double elbow or “ram’s horn” is sometimes used to slow the speed of concrete. While the ACPA does not recommend their use, they point out that risks can be somewhat reduced by attaching them between the boom and the boom hose (not the end of the hose). If the hose should whip and hit a worker, any heavy metal device or metal hose end could cause serious injuries or even death.

The Hose man: The hose man should hold the hose securely with both hands, but not hug it. Never try to support the hose tip with the back or shoulders; instead, let the hose hang from the boom.

Try to keep the boom hose no less than two feet above the top of the forms. As the boom moves, the hose tip may come in contact with the forms and be blocked, which could create back pressure and cause the hose to whip.

Do not kink the end of the hose. Kinking will cause the pump to quickly reach maximum concrete pressure, which will likely unkink the hose by force.

The hose man should position himself so that he’s looking the same direction that the pour is progressing. This will allow him to see obstacles and avoid tripping.

The hose man should always stay inside the guardrail or secure himself from falling using some other method. Never tie off to the boom.

Communication: Today’s remote control boom pumps make communication easier than ever before. The operator has the flexibility to get into the best possible position to observe the pour. To avoid confusion and conflicting signals, only one person should signal the pump operator.

Before the pour begins, the hose man, the operator and the spotter must review and agree on the hand signals (See ACPA recommendations below). It is absolutely critical that the pump operator and hose man understand each other.

There’s typically no reason for anyone besides the operator to control the pumping equipment. Even if you are a trained operator and the regular operator has released the controls to you, there must not be more than one operator at a time. (This does not apply to emergency stopping of the pump or boom if there is a need to do so.)

Conclusions
Concrete pump technology is one of the factors that has allowed the ICF industry to enjoy the growth it has seen in the last decade. It’s an economical and efficient means of placing concrete with a few basic rules to ensure the safety of the crew.

1. Personal Protective Equipment is a must
2. Everyone should be aware of the boom’s location, especially if power lines are nearby.
3. Be aware of air pressure in the lines, and the possibility of “hose whip.”

Knowing these safety guidelines will help to ensure a successful and profitable pour on the jobsite.

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Read article at ICFMag.com

From the Portland Cement Association:

 

The terms curing and drying are frequently used interchangeably with regard to the moisture condition of new concrete slabs. The following definitions clarify these terms.

Curing

Curing of concrete is defined as providing adequate moisture, temperature, and time to allow the concrete to achieve the desired properties for its intended use. This would mean maintaining a relative humidity in the concrete of greater than 80 percent, a temperature greater than 50 degrees Fahrenheit, and for a time typically ranging from three to 14 days depending on the specific application. When these recommendations are properly specified and performed in the field, the final properties of the concrete mixture will be achieved.

Drying

Drying of concrete is defined as providing the proper conditions to allow the concrete to achieve a moisture condition appropriate for its intended use. The moisture condition of a concrete slab is of significant importance for the application of moisture sensitive floor finishes such as vinyl composition tile, linoleum, wood flooring, and non-breathable coating like epoxy. The moisture condition is specified as a maximum relative humidity by percent or a vapor transmission rate in lb/1000 ft²/24 hr. A typical value specified for relative humidity may be less than 75 to 80 percent to assure the successful application of the flooring materials, while a commonly specified value for vapor transmission rate may be 3 lb/1000 ft²/24 hr.

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Read article at cement.org

Denver Concrete Vibrator would like to thank all of the attendees to The Precast Show 2020 in Fort Worth, TX.  Your presence made this years show one of the best.   See you next year at The Precast Show 2021 in New Orleans, LA.

From Equipment and Contracting.com

The concrete mixture is critical for most concrete properties and these details need to be considered in the mix design. During this planning stage, various factors need to be evaluated: workability, durability, strength, economy, and placement conditions. The information in this article will help with the selection of concrete ingredients as well as the sequences of activities that are necessary for the mix design. Additionally, specific methods can be used to optimize the aggregates with the goal of achieving optimal grading.

Two common phases are used in the planning process:

• Mix Design: During this phase, the required properties of the concrete mixture need to be identified. The planning includes the evaluation of intended use, exposure conditions, and geometry.
• Mix Proportioning: In this step, the quantities of ingredients are determined to ensure the requirements are met. Usually, the goal is to maximize economy and the necessary properties at the same time.

Development Sequence for Concrete Mixtures

Before construction begins, the mixtures are planned to match the construction system, environment, and loading that are anticipated for the job site. Various factors can affect strength and durability, including the air entrainment, workability, temperature, and permeability. Keep in mind that durability isn’t necessarily a function of strength, which is why many factors need to be evaluated to include non-strength-related concrete properties.

Once a plan is created, then a series of trial batches can be designed to test the job-specific materials. These test batches should include possible placement temperatures as well as ingredients that will be added to the mix. If needed, adjustments can be implemented so that the workability in the field can be completed with confidence.

Next, the batch testing moves to a full-scale process in the field. These field trials can be done using portable plants. This batching process should match the anticipated paving operations, including the mix time and other factors that will affect the concrete. Multiple testing points for workability can be implemented to simulate transportation time.

Following this step-by-step process creates an opportunity to adjust the mixture while minimizing risk. When the batching process is in the field, air content and workability are the two concrete properties that can be manipulated at this point.

Optimization of Aggregate Grading

The concrete mixture can be enhanced by optimizing the grading with the combined aggregates. Generally, the aggregates are dimensionally and chemically stable, which is why it is preferred to maximize aggregate content and minimize the cement paste that has higher chemical reactivity. When the space is well-aggregated, it decreases the space that needs to be filled in with cement paste.

Shilstone uses a combined grading analysis for specifying and selecting the mix proportions. Three tools can be used to optimize the aggregate grading:

• Coarseness Factor Chart (CFC): Creates an overview of the mixture using mathematical calculations to separate three size groups (coarse particles, intermediate particles, and fine particles).
• 45 Power Chart: This chart is used to show trends and describe the ideal combine grading. The chart is created based on sieve sizes plotted on an X axis. Then, the aggregate is passed through the sieve and compared to the measurement on the 0.45 power chart. If the comparison deviates from the line, then it means that the project is deviating from the optimum.
• PARS – Percent of Aggregate Retained on Each Sieve: This graphic shows the percent of aggregate that is retained for the sieve size. It is common to find a deficiency in the retained particles. Two adjacent sieve sizes tend to be balanced, while three adjacent deficient sizes mean there is a problem that should be corrected.

Mixture Portion Calculations

Before the trial batches are completed, a starting point can be determined by using desktop calculations of the mix proportions. Not only is the math used in these calculations, but it can also be helpful to tap into historical data and field experience.

While several calculations can be used for mix proportions, the Absolute Volume Method is a common calculation to use. The Absolution Volume Method Includes 12 Steps:

1. Concrete Strength: Durability and structural requirements are evaluated to determine the specified strength of the concrete mix.
2. Water-Cementitious Material Ratio: This is known as the w/cm ratio, which is the mass of the water divided by the mass of the cementitious materials. Unlike a water-to-cement (w/c) calculation, w/cm calculates only the mass ratio of water with the supplementary cementitious materials that will be added to the concrete.
3. Aggregates: The proportion of the concrete mixtures are affected by the distribution, particle size, porosity, shape, and surface texture of the aggregate. These characteristics need to be considered because they affect the water demand and workability of the concrete.
4. Air Content: Air entrainment is important since the concrete will be exposed to cycles of thawing and freezing. The size of the aggregate and the severity of exposure will affect the amount of air required.
5. Workability/Slump: Job placement conditions will determine the necessary consistency, workability, and plasticity for the concrete. A slump test can be used for the measurement of concrete consistency.
6. Water Content: Various factors influence the amount of water required for the concrete mix, including the slump requirements, aggregate size and texture, air content, cementitious material, and temperature. Water-reducing admixtures can be incorporated if water content needs to be reduced.
7. Cementitious Materials Content: This content can be calculated when the water content is divided by the water-cementitious materials (w/cm) ratio.
8. Cementitious Materials Type: Some mix designers complete this step first since the selected materials can influence the previous steps. Special requirements need to be considered, such as low-heat requirements, alkali-silica reactivity, and sulfate resistance.
9. Admixtures: Admixture quantities are calculated to achieve the desired water-reducing and air effect. Chemical admixtures change the concrete air content and water requirement and need to be factored into the mix proportions.
10. Fine Aggregate: After the quantities of water, air, coarse aggregate, and cementitious materials are determined, then the fine aggregate amount is identified. The absolute volume method is used based on the relative density (specific gravity) of the material.
11. Moisture/Absorption Correction: Corrections are often required due to compensation for moisture on and in the aggregates. All aggregates will contain a measurable amount of moisture, which is why mixing water needs to be added to the batch based on the amount of free moisture that comes from the aggregates.
12. Trial Batches: In this final state, the batch weights are checked in both laboratory tests and field batches. It is important to mix enough concrete so that the necessary tests can be completed, including air and slump tests, compressive-strength tests, and flexural tests if required.

Adjusting Properties

The required concrete properties can be achieved, but it might require adjustments to the selected materials, proportions, or other concrete mixing factors. Here is an overview of adjustments that can be made to achieve desired properties:

• Workability: Including stability, compatibility, mobility, and freedom from segregation based on job Adjustments might include water content, the proportion of cement and aggregate, admixtures, aggregate properties, cement characteristics, time and temperature.
• Stiffening and Setting: These rates are critical since they affect the ability for the concrete to be placed, finished, and sawed. The goal is to avoid blemishes on the surface and cracking throughout the concrete. Setting and stiffening are affected by cementitious materials, chemical admixtures, aggregate moisture, temperature, and water-cementitious materials.
• Bleeding: This development of a water layer on concrete that is freshly placed happens because of the settlement of particles and upward movement of water. Bleeding can be prevented or minimized by reducing the slump, ratio of water-cementitious materials, and water content. An increase in the cement of supplementary cementitious materials in the mix can be beneficial. Also, the aggregate needs to be properly graded, and chemical admixtures can be used to reduce bleeding.
• Air-Void System: This fundamental aspect that affects concrete durability. These adjustments affect the control of the air-void system in a concrete mix: alkali content, fineness in cement, supplementary cementitious materials, aggregates, and workability.
• Density (Unit Weight): Density is used as an indicator of batching consolidation and uniformity. It is affected by the material density in the mixture (mostly from the coarse aggregate), air content, aggregate moisture content, and relative proportions of the materials and water.
• Strength: Requirements for strength need to match the intent of the design, and are influenced by the ratio of water-cementitious materials, cement chemistry, SCMs, chemical admixtures, aggregates, and temperature.
• Volume Stability: Volume changes (contraction and expansion) occur in concrete due to moisture and temperature variations. It is important to minimize the change in volume to reduce the risk of cracking. This can be achieved with the following considerations: paste content, aggregates, and curing.
• Permeability and Frost Resistance: Permeability affects the durability of the concrete mixture. Lower permeability can be achieved by increasing the content of cementitious materials, reducing the ratio of water-cementitious materials, using supplementary cementitious materials, using good curing practices, using materials that are resistant to the anticipated chemical attack, creating a satisfactory air-void system, and selective aggregates with a history of resistance to D-cracking.
• Abrasion Resistance: This factor is important to maintain the skid resistance of the pavement. Improvements include: choosing hard and dense aggregates, increasing the compressive strength, and increasing the curing time.
• Sulfate Resistance: If the concrete will be exposed to high sulfate compounds, then adjustment might be required: reducing permeability with w/cm ratio, choosing a sulfate-resistant cement, adding Class F fly ash, using 20 – 50% GGBF slag, or using more supplementary cementitious materials.
• Alkali-Silica Reaction: Avoid this problem with some of these adjustments: choosing aggregates with historical performance, using low-alkali cement, using supplementary cementitious materials, using lithium admixtures, blending non-reactive and reactive aggregate.

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From the Portland Cement Association:

Mixing, transporting, and handling of concrete should be carefully coordinated with placing and finishing operations. Concrete should not be deposited more rapidly than it can be spread, struck off, consolidated, and bullfloated and  deposited continuously as near as possible to its final position.

In slab construction, placing should be started along the perimeter at one end of the work with each batch placed against previously dispatched concrete. Do not dump the concrete  in separate piles and then level and work them together; nor should it be deposited in large piles and moved horizontally into final position.

Consolidation

In some types of construction, the concrete is placed in forms, and then consolidated. Consolidation compacts fresh concrete to mold it within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycomb, and entrapped air.

Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily destroyed and the concrete behaves like a liquid; it settles in the forms under the action of gravity and the large entrapped air voids rise more easily to the surface.

Finishing

Concrete that will be visible, such as driveways, highways, or patios, often needs finishing. Slabs can be finished in many ways, depending on the intended service use. Options include various colors and textures, such as exposed aggregate or a patterned-stamped surface. Some surfaces may require only strikeoff and screeding to proper contour and elevation, while for other surfaces a broomed, floated, or troweled finish may be specified.

Screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of the slab to proper grade. A straight edge is moved across the concrete with a sawing motion and advanced forward a short distance with each movement.

Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. This looks like a long-handled straight edge pulled across the concrete.

Jointing is required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the unhardened concrete. Sawcut joints can be made after the concrete is sufficiently hard or strong enough to prevent raveling.

After the jointing the concrete, it should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the surface; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations.

Where a smooth, hard, dense surface is desired, floating should be followed by steel troweling. Troweling should not be done on a surface that has not been floated; troweling after only bullfloating is not an adequate finish procedure. A slip-resistant surface can be produced by brooming before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression.

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Read article at Cement.org

From Equipment and Contracting.com.

Pumping concrete is not just for large, commercial construction projects. Even a homeowner building a small patio can save time and money pumping concrete instead of moving it in a wheelbarrow. There are several reasons a project manager should consider concrete pumping on their next job site. There are also many advantages of concrete pumping over other methods. First, we’ll give you some background on concrete pumping. Then we’ll show you how concrete pumping can benefit your bottom line.

What Is Concrete Pumping? 

I bet you think it’s just concrete coming out of a pump. True – but an oversimplification. The machine has two cylinders that are fitted with pistons. Liquid concrete is poured into a hopper attached to the machine. The first piston creates air pressure to draw the liquid concrete from the hopper into the first cylinder. At the same time, the other piston pushes the concrete out via a discharge pipe. Then the two pistons swap jobs. This allows the concrete to flow continuously. A valve is used to switch the cylinders between the hopper and the discharge pipe.

Big Business 

Concrete pumping is gaining in popularity. According to Trucking info’s September 2019 article Business is Booming for Mobile Concrete Pumps, the industry has grown 14% in the last two decades. 34% of concrete is now poured into place. (45% is still poured directly off trucks and the rest is moved manually by wheelbarrows and buckets.) Concrete pumping is a $1.75 billion business. It is expected to grow nearly 25% by 2021.

Types of Concrete Pumps 

Truck Mounted Pump 

As the name suggests, the pump is mounted on a truck. It is also known as a boom pump because an articulating robotic arm, called a boom, places the concrete. The boom is maneuvered by remote control. This type of concrete pump is often used on large construction projects. It can pour large volumes of concrete very quickly. The result is a faster, accurate pour. Also, the robotic arm can be used for other tasks, such as electrical and piping repairs.

Trailer, Line, or Stationary Pump 

With this style of pump, steel or rubber hoses are attached to the machine, which is mounted on a trailer. Multiple hoses are joined to extend the reach of the pump. Trailer pumps are best for smaller jobs that require the concrete to pumped at a lower volume. Also known as line or stationary pumps, they are used for projects like sidewalks and swimming pools.

Specialized Usage Pump 

Specialized equipment is always more expensive. But some job sites, like tunnels, require custom-made concrete pumps. One type of specialized usage pump, though rarely used, is rail-mounted.

 

5 Reasons to Use Concrete Pumping 

So Far Away 

You can’t always get the mixer close enough to where you need to pour the concrete. This is common in construction sites located in a city or residential neighborhood.

Labor Shortage 

The wheelbarrow is one of humankind’s greatest inventions. But it requires a lot of muscle to move wet concrete around. Even if saving money on labor is not your primary goal, it may be challenging to find enough workers in today’s economy.

Instability 

Even if you have enough workers to push around a bunch of wheelbarrows, they can’t readily wheel over uneven or rocky ground that may surround your pour site.

No Swinging Room 

You’ll need to use concrete pumping if your pour site is inside a building, at height, or underground.

Behind Schedule 

If your project is running over, concrete pumping is much faster and can get you back on schedule.

Benefits of Concrete Pumping 

There are many benefits to using concrete pumping, including:

• You can place the concrete farther away and at greater heights.
• You can place the concrete even in bad weather.
• It is easier to pour smaller amounts in multiple locations as some job sites may require.
• It takes fewer workers to pour in place.
• The concrete moves faster from the source to the pouring location.
• It can allow you to get to areas not accessible by crane.
• The time-savings may allow you to complete work on multiple job sites in a day, reducing costs.

Wrap Up 

There really is no job too big or too small that a concrete pump can’t handle. In most situations, the project manager will find that there are overall cost savings in using concrete pumping on their job sites. Even if concrete pumping is more expensive than other pouring methods, the savings in labor and time can more than offset the service. In some cases, concrete pumping may be the only reasonable solution for challenging job sites.

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