Apr 09

Repairing Our Aging Bridges, a Concrete Dilemma!

Aging bridges

Aging bridges

In 2013, our nation’s bridges were rated and we got a C+ on our scorecard!  With the majority of our bridges approaching the ripe age of 42 years, United States is officially dealing with issues caused by an aging road infrastructure. We have over 607,380 bridges spread across the nation’s 102 largest metropolitan areas (where most of the traffic takes place with commuters and freight vehicles) and one out of nine bridges has been declared structurally deficient. The Federal Highway Administration has its work cut out for itself! The current annual budget of $12.8 billion won’t address the existing backlog by 2028! An additional $8 billion a year is needed to repair them appropriately, an additional budget that states and counties need to find to speed this process.

So what are the main issues with our aging bridges? They are either deficient, structurally deficient and/or functionally obsolete. So what’s does this mean? Well, a bridge that is considered structurally deficient will require significant maintenance, repair or even replacement and annual inspections. A deficient bridge is both structurally deficient and functionally obsolete.
A functionally obsolete bridge does not meet the current standards like load carrying capacity and width of lanes.

States that top the list of structural deficient bridges (with over 20%) are Pennsylvania followed by Iowa and Oklahoma. However, when you combined the number of bridges that have structural deficiency and are functionally obsolete, the District of Columbia leads all 50 states with 77%!

Concrete repair

Highway repair ahead!

The first signs of aging bridges are potholes, cracks, missing concrete chunks, posted signs with load restrictions and can extend in extreme cases to the closure of the bridge. The right bridge lane usually shows signs of damage faster that the left lane used for accelerating and decelerating, the shoulder lanes. To the naked eye, spalls or chips of concrete appear, they are usually caused by thermal strain due to rapid freeze thaw weathering. Dangerous explosive spalling can occur of refractory concrete and render the structure not usable as a result. Delamination or blisters is another common problem. When excess water and air in the mix are trapped under the surface mortar, voids are created along with weakened areas just below the surface that will come apart in the future. This problem stems from starting the finishing phase before the bleeding process is complete or when concrete is placed on cold substrates when ground temperatures are below 40F.

PROBLEMS:

Corrosion of steel reinforcements in concrete
This occurs when chloride ion that is found in sea water, ice melt water and deicer salts (made of sodium chloride and calcium chloride) starts corroding the concrete-steel contact surface. This can happen through drying shrinkage, cracking or the concrete’s pore water. The higher the temperature, the faster the corrosion! So regions like the Florida coast encounter constant and rapid degradation due to its warm temperatures and exposure to sea water. The corrosion produces rust which causes internal pressure and creates cracks, potholes…

Rust must be removed from the exposed reinforcing steel before repairing or the corrosion will continue under repaired patches. A layer of corrosion inhibitor must also be applied for the repair to last.

Unprotected concrete elements

Some bridges were built using bare concrete elements with black steel. Others used reinforced steel without epoxy or galvanized coating or a polymer concrete overlay. Others were made with a low slump dense concrete that results in low permeability concrete and allows chloride ions to do corrode the structure.

REPAIRS

Deck patching: for temporary partial depth repairs, bitumous concrete, quick-set hydraulic concrete, polymer mortar can be used. For full-depth patch repairs, Portland cement is the choice material.

Deck overlays:
In this method, repairs are done without removal of the chloride contaminated concrete.
Material used for these temporary repair methods are: latex-modified concrete (LMC), low slump dense concrete (LSDC) and hot mix asphaltic concrete with a preformed membrane.

Patching with Cast-in-Place PCC for Superstructure and Substructure:
This method requires the removal of loose concrete identified by sounding with a hammer. Formwork may be needed, which excludes the application of bonding agents but requires keeping forms cool by providing a cover during the curing phase.

shotcrete pump

Reed B50 Shotcrete pump

SchwingBPA500

Schwing BPA 500 Shotcrete Pump

Patching with shotcrete:
Removal of lose concrete is required. Repairs on superstructure and substructures tend to use dry-mix mortar. No bonding agent should be used. A single layer of shotcrete should be applied to avoid cold layers. A bottom up application technique should be followed to fill vertical cavities. Overhead surface may require multiple layers, 1 to 2 in. thick so that sagging doesn’t occur. Moist curing for 7 days should be provided using a cover or sprinkling system.

Encasement and Jacketing:
When column and piers have greatly deteriorated over time, concrete can be place to fill cavities, providing a new encasement for the element. In worse cases, a concrete jacket can be added after the damaged concrete has been removed.

EXPERIMENTAL  METHODS
For deck, some of the techniques involve Microsilica concrete overlays, corrosion inhibitor overlays, polymer impregnation and more.

For Superstructure and substructure elements, patching with Corrosion inhibitors is mostly used.

PREVENTION

Anything that will prevent chloride ions from entering and diffusing into concrete will work. It needs to be breathable so that water vapor can pass but not liquid water.

Deck sealers are a good option, they are either solvent or water based. Penetrating sealers are the only via options for the job (a combination of silanes and siloxanes). However, they should not be used on structure with active corrosion or high chloride contaminated concrete.

In general, large scale bridges in urban areas should be repaired first since the demand on them is higher than in rural areas. So if you are looking for a good shotcrete machine at a competitive price, give us a call at: 503-283-2105.

Further Reading:

Infrastructure Report Card (2013)

Concrete Bridge Protection Repair & Rehabilitation Plan from the Strategic Highway Research Program

 

 

 

 

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Mar 15

The Art of Vertical Concrete Pumping

Vertical concrete pumping

High rise construction

With the world tallest building (160 floors) completed in 2008 in Dubai (the Burj Dubai Tower)
reaching 1988.19 Ft (606 meters), the high-rise building industry has come a long way. From its humble beginning hand carrying small concrete buckets, to large buckets being lifted by cranes, the ever growing needs of long distance line pumping has pushed concrete pumps manufacturers to reinvent its technology to respond the ever growing new demands.

Some of the challenges:

When the city of Taipei decided to build in 1998 its Taipei 101 Tower/Financial Center (1,667 feet), the project was even more challenging than Dubai because of the known risks of typhoons, high winds and earthquakes. As a result, the building is able to withstand winds up to 133 miles per hour and 7-point earthquakes! This required concrete filled steel mega columns, mega-truss, a truss structure at the core, a Tuned Mass Dumper System and a 800-ton spherical steel ball suspended like a pendulum! Not your average hi-rise engineering design!

The challenge in long-distance pumping isn’t only the pumping but it is the logistics. How much pipe, hose, elbows, thrusts blocks, anchors ? How many guys you need on-site? Who will set up what by when?

When building structures reach over 1900 feet, concrete has to flow and remain longer in the delivery line (anywhere around 35 minutes). This means that the entire content of the pump chamber has to move through with each motion (piston or peristaltic) otherwise any concrete residue can harden, wear out moving parts and cause serious damage.

Projects placing higher concrete volumes require additional planning and design for pump lines. You will need 1.1 pounds of pressure, when using a standard 5-inch standpipe to move concrete one vertical foot. So, if you need to pump 1000 vertical feet, you will need at least 1.1 pounds of pressure. There is nothing better to understand the complexity involved in vertical pumping the projects than see it. So we have compiled a short series of videos showing long-distance concrete pumping on various projects around the world.

Vertical pumping also requires thorough testing of concrete performance at the batching plant, measuring concrete obtain with various pump types, output and the inner pressure of the pipe. In addition core tests and porosity tests were conducted. In the case of the Taipei financial center, a Schwing BP 8000 HDR pump was selected for consistent output and quality of concrete pumped. For the Burj Dubai Tower, 3 Putzmeister 14000 SHP D super high pressure pumps pumped a total of 165,000 m3 of high-strength concrete over a period of 32 months! Now, that’s durability! The frame, hopper, S-transfer tube and bearings have been enhanced to handle the tremendous force.

Tips From the Pros:

As a rule of thumb, the pipe’s diameter should be minimum 3-4 times the size of the largest aggregate size. When pumping concrete against gravity (vertically), a diameter smaller than the usual 125 mm pipeline should be used. It is absolutely necessary to anchor riser pipelines to the structure for the duration of the project. Pipe bends can be secured by casting them through concrete “thrust blocks”.

Be careful and always top the feeder hopper. NEVER let air into the line, especially at the beginning of the pouring job. Hose whipping is the number one cause of injuries and death on the job. When pumping at great heights, this danger is multiplied. To find out how to avoid hose whipping on the job, you can read this article.

Slump loss isn’t usually a common problem when pumping long-distance especially when the aggregate absorption is low. The key is to keep the aggregate wet during the project. This can easily be accomplished in hot regions with sprinklers watering aggregate piles.

New technologies

The country of Saudi Arabia is developing various hi-strength high-rise concrete mixes (Saudi Readymix Concrete) to be used in conjunction with advanced pumping techniques for the exploding construction market in this region of the world. Their high alumina cement content is heat resistant (up to 1500 C), ideal for building in the Middle East.

Schwing-SP1000X

Schwing-SP1000X

At United Equipment Sales, we carry all major pump brands like Schwing, Putzmeister, Reed, Mayco, Olin and more that can handle just about any job size. Below are some examples. Give us a call at (503)283-2105 if you have any questions on pumps, hose and accessories.

See our latest inventory here.

Vertical concrete pumping

Putzmeister TK30

 

 

 

Reed concrete line pump

Reed 4050

concrete line pump

Olin 565 Concrete line pump

Jan 09

Tips for Cold Weather Concreting

Record low temperatures

Record low temperatures

With news of polar arctic weather sweeping through the Midwest and Eastern part of the country, one can’t help thinking about the movie The Day After and wondering if this is just a passing thing or something more serious! With record low temperatures (-20!) bringing many cities to a grinding halt, we thought it would be interesting to find out what are some of the challenges of pouring concrete in cold weather and recommended mitigation methods.

So how cold is cold? According to the American Concrete Institute (ACI) the definition of cold-weather concreting, as stated in ACI 306 is, “a period when for more than three successive days the average daily air temperature drops below 40 degrees Fahrenheit and stays below 50 degrees Fahrenheit for more than one-half of any 24 hour period.”  Based on this information and this week’s nationwide temperatures, no construction is happening in most of the country! Or is it?

SPECIAL PLANNING FOR LOW TEMPERATURES

To ensure that concrete placed in cold weather will last a long time, proper planning is required. This includes: selection of cement mix, proper mixing, placing, curing time, finishing techniques and overall protection of the site and concrete. In cold weather, the curing period takes longer because the lower rate of strength gain. Make sure to take into account the transportation time from the plant to the point of placement as it can have a major impact on the temperature of the mix.

If at the time of pouring, temperatures drop below 40-50°F, you can heat the water or the aggregates to reach the desired temperature. Heating cement is not an option!

Concrete operators can also use a low slump concrete for flatwork and lower water/cement ratio mixes because it will reduce the setting time and bleeding water. Selecting a concrete mix that contains admixtures that accelerate or a Type III Portland cement, Hi-Early cement that will greatly help reducing the protection time from freezing. It is especially important to increase the quantity of accelerated admixtures (PolarSet®, DCI®, Daraccel®, Gilco® or Lubricon®) towards the end of the pour to produce a more consistent set and avoid the results of cooler mixing water. It will also speed up the setting time of the last concrete batches so that the entire concrete area can set at the same time.

THE MAIN RISK: PREVENT THERMAL CRACKING

Freezing

Freezing

It is essential that every surface that will enter in contact with the concrete is free of frost, ice and snow. Whether it is reinforcement, embeds, forms, fillers, or ground

The first 24 hours after the concrete is poured is the most critical period because it is the usual amount of time that is required to reach its minimal strength of 500 pounds per square inch. If freezing occurs while the concrete is fresh or has not reached this minimal strength, ice will form in the frozen water altering the cement mix which in turn will damage its overall strength. So at 500 psi, the cement is able to resist the expansion caused by freezing water and be safely removed from the forms. As a rule of thumb, an 18°F (10°C) drop in concrete temperature will double the setting time and make it more vulnerable to freezing. Use the maturity method to verify that the concrete has reached the proper strengthening level.

PROTECTIONS TO KEEP TEMPERATURES ABOVE 50F

Insulated blanket

Insulated blanket

Some of the most commonly used methods to keep temperatures above 50° Fahrenheit are: evaporation reducers, curing compounds, polyethylene sheeting and insulating blankets. Cover protruding rebars and make sure that they don’t blow away at night when the temperatures are even lower. If the temperatures are very low, you can use a combination of electric heated blankets and insulated blankets. If the site is subject to cold winds, a 6-foot wind breakers is recommended to reduce evaporation and drop in temperature. A more costly method involves heated enclosures that can be made of wood, canvas tarpaulins, or polyethylene. Heaters can be direct-fire, indirect-fire or hydronic systems (use glycol/water solution to produce heat in a closed system of pipes). Adequate venting to the outside must be included to protect workers from inhaling carbon monoxide gas. Operators should make sure that the heaters are properly fueled to last through the night.

Two elements will affect the quality of curing: moisture and temperature. In cold weather, little to no moisture is required for curing in cold weather conditions. It is recommended to keep the concrete temperature above 40° degrees Fahrenheit for three to seven days.

WHAT IF TEMPERATURES DROP BELOW 50F?
If during the three to seven days that followed the pouring, the temperatures drop below 50 degrees Fahrenheit or are near freezing, then the time has come for a good curing compound to create a  protective liquid membrane (usually made of a water based hydrocarbon resin).

MAINTENANCE DO’s AND DON’Ts

Do make sure that no extra water or bleed water is on the surface of the concrete.
Do prevent ice from forming at all costs (this will cause an immediate drop in hydration and strength).
Do watch for hardened concrete area and make sure that they don’t occur.
Do leave forms in place as long as possible because they distribute heat more evenly during the setting phase.

Don’t overworked areas that seem to be setting more slowing.
Don’t seal freshly placed concrete or if bleed water is visible.
Don’t turn off heat too quickly as the difference of temperature between the enclosed area and the outside could cause thermal cracking (days and weeks may be required in case of large structures)

Have more questions about cold weather concrete or concrete pumping? Call Dick at (503)283-2105.

United Equipment Sales

 

Further reading:

Additional information can be found in ACI 306.1, Standard Specification for Cold Weather Concreting, ACI 306R, Cold Weather Concreting

Technical Bulletin
Role of Concrete Pouring

 

Jan 02

Transform Old Concrete With Cement Stain

Boring concrete

Let’s be honest, while concrete is a great economical and lasting option, its look over time can turn a garage, a sun room, a basement, a porch or a patio into a plain eyesore! But do not despair, there are a number of options that can completely transform the look of old boring concrete at a fraction of the cost of replacing or pouring new concrete. One of this option consists in applying cement acid-stain, available in various colors, to give a brand new look and protective layer.

One of the advantages of using cement stain over paint, is that cement stain is permanent, won’t fade or peel. It creates marbled variations of color on cured concrete by chemical reaction with the minerals in the concrete. Concrete resurfacing results will vary greatly due to variations in the underlying concrete.

Concrete stain flooring

Concrete stain flooring

MATERIAL NEEDED:

Paint tray
Paint brush, spray bottle and/or pad
Paint roller
Tarp or plastic roll
Concrete stain (comes in solid and translucent colors)
Concrete sealer

PROCESS

STEP 1: CLEAN
As for all painting job, the first step is to clean the surface of oil, dirt, mud, dust and residues. Use coca cola to remove oil stain on garage floors. Use a broom, a brush for the areas that need the most work. You can also power wash the area but make sure that it has plenty of time to completely dry.  Skipping the drying process will prevent proper adhesion of the stain. This step is really important because stain is essentially translucent color that will not only show the underlying defects or colors variations but can also make them even more visible.

STEP 2: TEST STAIN
Always test the color you choose in a small area to make sure that the result is acceptable. Don’t just rely on the manufacturer’s color chart because stain reacts differently to different types of surfaces. This is especially important if you are working on an intricate pattern with various colors not just applying one color over the entire area.

STEP 3: APPLICATION TECHNIQUES
Apply too little and there won’t be enough color to etch the concrete and last. Spray too much and you will end up with a puddling problem and too dark effect. Practice makes perfect so either test a small area or hire a professional if you are going for a large area and complex design. This requires practice, proper timing.  Generally, you want to apply a first coat using one of the techniques below and finish with a light brush to soften the edges and  make sure the stain is evenly applied before moving to the next section. Some of the techniques are:

Paint with a brush: always start at the perimeter (edge of a wall or building)
Scrubbing: if you are using this technique make sure to maintain a n
atural stroke to avoid streaks
Spraying: this is great when creating flowing patterns that requires loose applications.
Rolling: Great for intricate geometrical patterns like diamond shapes.
Cut lines with a circular saw (1/8” deep) if you are making repeated patterns of different colors. Adding some blue tape in the grooves will help prevent stain of different colors from bleeding into other areas.
Use stencils to create beautiful forms
Use tape to create square tiles for a stone work look
Stamp concrete to create wood board patterns
Simulate bricks patterns and much more…

STEP 4: FINISHING
It is recommended to fully wet and neutralize the area to avoid accidental foot prints or marks. Let it dry fully then add a concrete sealant finish coat. This can be done either spraying, using a cross-hatch rolling, or even buffing if you are going for a wax finish look. Wait at least 4 hours before stepping or moving furniture back.

Artistic Concrete design

Artistic Concrete design

PRO TIPS:
If you have an area that can’t be cleaned, has some defect, you may have to be creative and turn defects or color variations into unique patterns and designs by applying more stain in these areas.

If you have rust stains or have filled cracks, the stain will appear darker in these areas, so plan accordingly.
The best time to apply stain is on a cloudy day. Avoid direct sun on a summer day. High heat will speed up the drying time and make it very difficult to smoothly blend one area into the next without creating color variations and streaks.

So if you have a concrete patio or a slab that is in need of some TLC, try out acid stain. Tackle it yourself or hire a professional and enjoy the results. Check out amazing diy projects and pictures on Pinterest to see how others have transformed their boring floors and patios into gorgeous creations!

Happy New Year!

Dec 03

Hot Weather Concreting

Hot weather concreteWith rising world temperatures due to global warming, the job of concrete operators in hot, arid and tropical climates just got more challenging. Concrete placement in hot summer months or in regions of the world with constant high temperatures like Central & South America, the Middle East, Africa, South East Asia and Australia requires comprehensive planning, precise rapid execution and close monitoring during the finishing and curing phases.

High Temperature

High Temperature

What temperature range can be considered hot for fresh concrete pouring? Whenever the temperature reaches above 77 or 90 degrees Fahrenheit, a plan and specific measures should be put in place to remedy the damaging effect of such temperature. Consider that an increase of 20 degrees Fahrenheit can shrink the setting time to about 50 percent!

Placing fresh concrete in high temperatures impacts just about every step of the process, from concrete handling, the placing, setting and curing time. In addition, the high temperature also affect the cement mixture itself, the concrete equipment used to batch and transport the concrete. All of the above factors must be considered and addressed when designing a plan for the job.

Some of the main risks are accelerated cement hydration (which can cause plastic shrinkage and cracking), higher water-cement ratio that could cause reduce strength and slump loss.

Below are some measures that can be taken to reduce risks of plastic cracking and lower strength:

PLANNING PHASE

  • Hire a crew experienced in rapid concrete placing in these hot conditions.
  • Check forecast temperatures (day highs and night lows) and plan the job on the best days.
  • Make sure you have adequate water availability on the site.
  • Order or rent sunshades from the sun prior to pouring to reduce the surface temperature.
  • Schedule wind screens in areas prone to high winds conditions.
  • Rent misting or fogging equipment.
  • Schedule your crew and pouring at the coolest times of the day (early morning and night time).
  • Select the proper cool concrete mixture and set retarders for the job.
  • In temperatures above 90° F the use of liquid nitrogen on the batching truck.

PLACEMENT

  • Mist cool water on subgrade forms and steel reinforcement prior to placing.
  • Have chilled water on hand to adjust the slump when needed.
  • Check the rate of evaporation, the relative humidity and the concrete temperature with the help of a Nomogram. This is optional when operating in temperatures below 80° F but a must for temperatures above 90° F.
  • Poor softer concrete so that it can be placed more rapidly but don’t add too much water to the mix.
  • Choose proper curing methods (set retarders and water reducers) and apply when the concrete finishing is done.

SETTING, FINISHING & CURING

  • Use retardant admixtures during the setting time.
  • Make sure finishing is complete (when the surface sheen has disappeared) before starting the curing process (minimum 3 days).
  • Never add water to concrete during the 3 days of finishing!
  • Install sunshades, wind screen, misters or foggers to protect the concrete during the curing process.

United Equipment SalesFollowing the above recommendations, with proper planning and careful execution, placing concrete in hot weather conditions can be successfully achieved. Have any questions on concrete mix or equipment? Give us a call at: (503)283-2105

FURTHER READING

Hot weather concrete construction
Hot weather concrete tipsHot Weather Concreting

Oct 30

Avoid Common Mistakes When Pouring Concrete

Concrete PouringHandling concrete is both an art and a science! In this article we will focus more on the science part and the required steps that should be followed no matter what your budget or time frame is. In other words, don’t mess with the basic recipe or your cake will suffer!

PREPARATION: before any concrete is poured, you must make sure a few things are in place:

1)      Know if the base is ready
Starting with a solid base that has been properly drained is essential. Skipping this important step could cause soft areas to later produce cracks over time and under the pressure of heavy vehicles. The best way to do this is pour between 2-4 inches of sand or small gravel and pack it down. This is especially important if the area is known to not have good drainage.

2)      Use reinforcement
Once the forms are built, you should reinforce the concrete with rebars (3/8 or ½ inch). Avoid steel mesh or even plastic fibers because they do not provide additional strength. They are helpful in reducing cracking.

3)      Know how much water to add
While it may be tempting to add more water to the mix to ease its flow down the chute and speed the pouring process into forms, this will also result in a weaker concrete.
So resist the temptation and instead keep the cement mix as dry as possible. It may be a little more work to push it into the form, in the corners and the edges but this will pay in the short, medium and long-term.

There are a few things to keep in mind after the concrete has been poured:

Smoothing concrete1)      Know when to begin smoothing the surface
This step requires precise timing, having everyone ready and all the required tools nearby. What happens if you start trowelling too soon? You risk bringing the cement paste to the surface and weakening it. One easy way to know when it is time to start troweling is when the water starts to disappear from the surface. This signals the perfect time to start smoothing quickly as concrete is known to harden fast. Depending on the job you are doing, you may need a rebar cutter, a plate compactor, a bull float (for large slabs), a square-edge shovel, one or more finishing trowels. For the finishing details, you will need as a minimum, a groover, an edger and a stiff square broom.

2)      Cutting down control joints
Once the concrete has been poured, it is necessary to cut the control joints down to 25% of the depth of the slab. So if you poured a 4” slab, your joints should be 1” deep. This is done to accommodate for the shrinkage and cracking that occur during the curing process (about 1/8″ per 10 feet of length). The cutting can be done while the cement is still fresh using grooving tools or with an early-entry dry-cut lightweight saw (1” to 3” deep). On hot days, you may need to cut joints within 6-12 hours or risk cracks. The important thing, if you decide to wait and use a saw blade, is that the edges of concrete won’t chip during the cut.  So proceed carefully.

3)      Slow down the curing process
Good curing means evaporation should be prevented or reduced. Slowing down the drying process is the key to strong concrete. The strength is the direct result of the lime hydrating during the curing process. One simple way to do this is to cover the slab with wet burlap, wet blankets or a plastic sheet for a week or more. In hot conditions, above 80 degrees, watering the slab frequently with a garden hose or a light sprinkler is necessary to keep it moist and let it reach it maximum strength. Concrete will reach approximately 80% of its strength in 7 days and almost 100% after 28 days. Using curing compounds (retarders, air entraining agents, plasticizers etc.) will also help sealing the moisture. Find out more about ad mixers in this article http://excavatorheavyequipment.com/blog/2013/07/01/why-use-concrete-ad-mixers/

4)      Test concrete
As a minimum, you should conduct a slump test. Depending on your situation, you could also do a flow table test, or a ball penetration test.

5.      Wait to build
Concrete reaches a high percentage of its final strength after 28 days, so construction should wait for at least 5 days to start.

By following these important steps you will avoid some of the most common mistakes that are learned the hard way. They will save you time, money, reputation and give you more time to focus on the art of concrete and its many forms.

More questions? Call us at (503)283-2105 or visit our Youtube channel. We always like to hear from our readers.

Aug 05

Architectural & Decorative Concrete

The concrete industry has come a long way and keeps reinventing itself to answer the needs of builders, architects and even decorators. With more and more home owners attracted to a greener and more sustainable lifestyle, we have seen, in more recent years, a new use of concrete, blending innovation and artistry to create truly unique surfaces for interior floors, patios and kitchen counters. Here are some of the coolest new possibilities:

Concrete Countertops in the kitchen:

Granite Counter top

When you think kitchen countertop, a few options usually come up. Depending on your budget, you can get high end and more expensive products like: polished granite, marble, quartz, soapstone, stainless steel and glass.  On the cheaper end and ideal for DIY afficionados, you have tile, wood and laminate. Other options that are becoming more popular are concrete and recycled concrete. The later one being one of the most eco-friendly choice that allows you to combine post-consumer products like glass, paper, composite and plastic with concrete for a truly unique finished product.

Plain concrete can be used in countertops, landscaping and customized with various textures and finishes (stain color). A really nice feature of concrete is its ability to be energy efficient. Whenever the temperature rises, the concrete traps the heat and releases it when the temperature cools down.

White Portland cement
To transform what may be viewed as boring cold grey concrete into bright, elegant aesthetic surfaces requires a combination of special ingredients, forms and finishing technique. The magical ingredient in this case is white Portland cement.  It comes plain or pigmented and is available in wide array of colors, from pastel to saturated colors. It can be used indoor or outdoor and has the same durability properties as grey cement but contains more metal oxides like iron and manganese, giving it its whiteness. White cement concrete is used in exposed architectural concrete, landscaping, building accents and more. It provides greater energy efficiency and safety because of its light color.

Some design ideas

Stone finishing

Concrete can be transformed to look like natural stone. To create this effect, you will need to cast the concrete in an integral color then apply multiple layers of acid stain to the surface until the desired result is obtained. TIP: Use the veins produced by the marbelizing effect to hide seams on your countertop.

Wood grained concrete texture

 

Create a wood-grained finish to any surface by applying a technique called “board forming”. To create this unique texture, you will need to cast the concrete in forms that have been previously lined with unfinished wood that will imprint the wood texture into the concrete surface. You can also create board lines by cutting straight lines with a saw after it is cast. Next is creating a realistic color using both integral color with topical stains.

To create a smooth and elegant look to your surface, you can polish or sand your concrete. It is recommended that the counter top be poured in one piece and not show any seam. To create the look of marble, you will need to use a titanium-white concrete mix that contains white silica sand. You can also hand sand the surface lightly for a less shiny and more natural look.

Exposed Aggregate Finish
This is where the use of post-consumer products like glass, stones, bricks, even shells can be recycled and added to concrete to make truly unique finished surface. There are 3-4 steps to create this type of finish: first you pour the concrete, then you insert pieces of material of your choice (broken glass, decorative stone, shells etc.) Once the concrete has set  you grind the surface to expose the aggregate. Finish with a high gloss sealer to protect your newly created surface.
TIP: for spectacular results it is best to select pieces that will create the most contrast with the concrete color.

If you are looking for a good deal on a concrete pump or want to make sure you have the right pump for the job, give us a call at: (503)283-2105 or visit us at: www.Unitedequipmentsales.com

Related reading
Milestones in the history of concrete construction
Photos of concrete counter tops
Applications for architectural concrete

Aug 02

How to Minimize Dust Exposure in Shotcrete Placement

Dust generation may seem to be part and parcel of any construction job and nothing to worry about but long-time exposure and inhalation of dust can lead to damaging health effects especially when working in tunnels and mining jobs.

Let’s first look into factors that cause excess dust and how we can avoid or reduce them. Take for instance, the process of building tunnels. It requires going through a series of steps that generate huge amounts of dust: drilling, namely: blasting, crushing, extracting and shotcrete. While the first four processes have to control the dust generated and remove it from the atmosphere through special ventilation, water sprays and dust collectors, shotcrete placement doesn’t.  However, hiring a shotcrete crew that is experienced in working underground will minimize dust generation because they will know how to control it at the source.

Silica

Where is silica found:
Silica is found in asphalt, brick, cement, concrete, drywall, grout, mortar, stone, sand, and tile. OSHA regulation 29 CFR 1926.55(a) requires that exposures must be below a Permissible Exposure Level (PEL) of 0.1 mg/m3. The National Institute for Occupational Safety and Health has a lower Recommended Exposure Level of 0.05 mg/m3.

Health risks from exposure to Silica:
Repeated exposure to cement dust will lead to irritation to eye, nose, throat and upper respiratory system. When skin is directly exposed to cement, further irritation can occur and skin cracking can result from chemical burns. Rinse eyes or skin with water and soap if it comes into contact with cement dust and if the burning continues consult a doctor.

Another serious risk for workers is lung injuries from Silica exposure. This can lead to chronic obstructive pulmonary disease (COPD), lung cancer, tuberculosis and Silicosis. There are about 3,600 to 7,300 new cases of silicosis every year. Note: This is a progressive illness that will continue to worsen even after exposure to silica has stopped. While it may take years for these diseases to show up, workers should be on the lookout for certain symptoms like a chronic dry cough and shortness of breath. These disabling, irreversible and at times fatal diseases occur when silica particles are inhaled by workers. As a result, nodules start growing and can become so large that they impede normal breathing.

concrete safety

Dust Control Checklist

How to protect your crew:
1. Prevent dust from being released in the air by using water or vacuums at the source.
2. When water and vacuums are not available use a respirator.
3. Replace sand with aluminum oxide for blasting.
4. Use a wet abrasive blaster that mixes water with media before leaving the nozzle.
5. Avoid eating, drinking and smoking in dusty area to reduce, wash face and hands first.
6. Use disposable clothing or wash work clothes at the work site.
7. Shower and change into clean clothes before leaving the job site to avoid contamination.
8. Do a periodic lung screening.

With over 40 years of experience in the industry, we know the importance of safety at work. We also know that using the right pump for the job means even greater safety, better results and more savings in the long run. So if you are looking for a great deal on concrete or plaster pumps, give us a call at (503)283-2105  or visit our website.

Additional reading:
Concrete Safety procedures
Maintenance Tips for gunite
Work Safely with Silica 

Jul 17

TESTING FRESH CONCRETE – PART 3: Ball Penetration Test

In our previous article, we looked into Flow test as another common test method to ensure that fresh poured concrete has the right consistency and workability. As in all testing methods, always keep in mind that some limitationsoften apply. Today we will focus on a third method called Ball Penetration Test (ASTM C360) and better understand main causes of segregation and bleeding in concrete.

Kelly Ball

THE BALL PENETRATION TEST (a.k.a Kelly Ball Test) can be conducted on site by measuring the indentation made by a Kelly Ball into plastic concrete. The device consists of a 6 inches steel cylinder (15cm) in diameter with a semi spherical shaped bottom and 4 5/8 inches in height. It also comes with a frame that guides a vertical handle marked in ¼” increments on one side and half inches on the other.

Advantages: this test can be conducted directly on the concrete in site (in a form, a hopper, wheelbarrow), there is no need for filling and rodding a container. This means that the test can be completed faster than with a slump test and will yield more precise results.

California Case Study:
The California Division of Highways used to perform a slump cone method but in 1953, they adopted the Kelly Ball Test. In addition to the ASTM and AASHO requirements, they looked for a minimum depth of 6 inches on concrete and a minimum clearance of 9 inches. If the test fails to show 1 inch of slump (1/2 inch of penetration), more reading must be taken until 3 successive readings within 1 inch are obtained. They average the first three valid penetration readings. ASTM and AASHO test methods are to the nearest ¼ inch while with the California method each inch of penetration.is read as 2 inches on the ball shaft.

WORKABILITY
DEGREE

SLUMP/PENETRATION (mm)

USES

Very low

0-25

Roads with power operated machines

Low

25-50

Road with hand operated machines

Medium

50-100

Flat slabs with crushed aggregates

High

100-175

For congested reinforcement (not for vibration)

Limitations:
There are several known sources of variances of the Kelly Ball Test, namely: sampling, testing, differences in moisture content, inadequate mixing of materials, variation in mixing time and types of aggregates.

To better master concrete workability, one must study and understand  what causes segregation and bleeding in fresh concrete. Below is some useful information that should help concrete operators on the job.

CONCRETE SEGREGATION
This phenomenon occurs when the elements of the cement paste separate creating an uneven distribution. It can happen when the concrete is mixed, transported, placed or compacted.

Types of segregation
When coarse particles are mixed with finer particles they can settle more because they are heavier. This is the first form of segregation also called coarse segregation.  The mix will have a low asphalt content, low density, lots of air voids. It is the main cause for segregation.
The second type of segregation can happen in wet mixes, when cement and water separate from the mix. It is called Fine segregation and produces cement with high asphalt content with low density, concrete deformation and more.

Factors causing segregation:

  • Too much coarse aggregate in the concrete mix (in coarse segregation)
  • Too much fine aggregate in the mix (in fine segregation)
  • Excessive vibration
  • Overworking and flowing of concrete along the form through a chute
  • Pouring concrete from considerable height
  • Sudden change of direction through a chute

CONCRETE BLEEDING

This happens when the solid elements of the mix do not properly hold to the water when they are poured and some of the water rises to the surface of the fresh concrete. This being said, most freshly placed concrete will result in some level of bleeding. The amount of bleeding is proportional to the depth of concrete poured. Bleeding can create pores or “wormhole channels” in the interior and weaken the concrete. Do not attempt to remix the bleeding water during the finishing steps as this will result in a weak surface, non-durable concrete vulnerable to freezing and thawing and rebar corrosion.

What can cause bleeding
Too much water in the mix. Whatever time you thing you may have saved during placement will be lost waiting for the water to evaporate! Excessive compaction can also push up water to the surface of the fresh concrete.

Solution:

  • Wait for the bleeding water to evaporate before completing the finishing of the top surface.
  • Never trowel concrete while bleedwater is still on the surface!
  • Use more finely ground cement.
  • Add calcium chloride to cement and increase the fineness of cement.
  • Add fly ash, pozzolans or aluminium powder in the concrete.
  • Select a proper finishing method to protect the slab surface.
  • For air-entrained concrete, use an air-entraining agent to lower the amount of water needed to achieve the desired slump.

Having the right concrete pump for the job is also important to minimize risks of segregation or bleeding. Not sure about which pump to buy? Why guess when you can just give us a call us at (503)281-2105? We always love a challenge and are ready to help you save money on your next job.

SUGGESTED FURTHER READING

Testing Fresh Concrete Part 1: Slump Test
Testing Fresh Concrete Part 2: Flow Test
Report from the California Division of Highways
More info on Concrete workability

 

Jun 30

TESTING FRESH CONCRETE – PART 2

In our previous article, we investigated slump testing as one of the most commonly used test method by site engineers to ensure that fresh poured concrete has the right consistency and workability. We also learned that this method is not optimal for all forms of concrete. There are 2 other methods for testing consistency, they are Flow test (using vibration) and Ball penetration test.

Fresh Concrete Testing with a Flow Table

Flow test:
Also known as Flow Table Test is often performed when fresh, unhardened self-consolidating concrete arrives on site and the operator needs to monitor its consistency before pouring can start. It is simple, inexpensive and can be easily reproduced and conducted on any site.

How does it works:  Viscosity is what determines the rate of spread.
1. Make sure the flow table is perfectly flat on the ground.
2. Clean the surface of the flow table.
3. Place the cone in the center of the flow table in between your feet and proceed as you would for a standard slump test (fill it with fresh concrete applying 2 equal layers.that each have been tamped down a dozen times). Level the top of the mold removing any excess. Clean up the table of any extra concrete.
4.Wait 30 seconds, then lift the cone.
5. Raise up the tabletop 40 mm and drop it 15 times forcing the concrete to flow.
6. You can now measure the diameter with a rule in both directions to the nearest 10 mm. Watch a video of a flow table test being conducted.

Tools for performing a flow test

Best applications:
This testing methods works best for high concrete workability using coarse aggregates no larger than 25 mm (1 in.) and with a slump of more than 175 mm.The reason why is that anything larger will not produce flowable and nonsegregating self-consolidating concrete.

Limitations: 
Flow test results can be difficult to interpret, so they are used primarily as a qualitative index of workability. The good news is that there are several other tests relying on vibration such as compaction test (Waltz test), Vebe consistometer, Thaulow tester etc. that can also measure the rheological properties of cement mixtures.

Understanding the results:
If the concrete is too pasty, it which will cause cavities and corrosion of the rebar in the medium term and weaken the concrete’s ability to resist stress.
A mix that tends to segregate will produce a non-circular pool of concrete.
If a ring of clear water appears after a few minutes, the mix indicates a bleeding problem.
NOTE: concrete slump and final mortar spread correlate linearly when the concrete slump is greater than sever inches.

Diameter of flow (cm)-25
———————————— X 100 = FLOW %
               25

% of Flow

0-20%

20-60%

60-100%

100-120%

120%-150%

Consistency

Dry

Stiff

Plastic

Wet

Sloppy

Dick Hibbard

Combining a slump test with a flow table test will ensure that your mix has the right consistency and workability on the job. So don’t cut corners and do the right thing, it will save you money and time. And if you are looking for a great deal on a concrete pump, give us a call at: (503)283-2105. We would love to hear from you!
SUGGESTED FURTHER READING
ASTM Standards
Sampling & testing fresh concrete (UK)
Flow Test Evaluation (Georgia Department of Transportation)