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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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 

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)

Jun 20

HOW TO TEST FRESH CONCRETE – Part 1

Fresh Concrete Testing

Considering the many factors that can affect the quality of fresh concrete, it is understandable that dozens of tests methods are available and being used today. However, we will focus today on one of the most prevalent test method called Slump test.

What are the main properties of fresh concrete that should be measured?
Primarily its strength and durability. Both factors are highly dependable on the degree of compaction and affected by transportation, placement and curing. So creating and maintaining the consistency of the mix are essential to a successful job and regular testing will help achieve the desired results.

There are 4 main areas that need to be tested:
1. Consistency
2. Workability (compacting factor test, WeBe Time test)
3. Segregation
4. Bleeding water test

Consistency or fluidity of concrete:
Think of it as the degree of wetness but beware, the wetter the mix doesn’t equate to greater workability.  Too much water can lead to segregation, too much bleeding, sand streaking on the surface. Too dry of a mix and holes and cracks (another form of segregation) will form as a sign of low plasticity.

What kinds of Test should be done
:
The most commonly used method is called Slump test. It can easily be performed and doesn’t require complicated equipment. It measures the behavior of an inverted cone of concrete (focusing on workability, consistency & wetness).

What does the process entail: Use a lightly moisten slump cone (metallic mould open at both ends with a handle). Fill it in three successive layers that have been each temped +20 times. It is recommended to hold the mold with feet to make sure that it doesn’t move during the pouring of the concrete. Quickly lift the mold right after it has been filled to the top, place the mold next to it. You can now measure the decrease in height (usually in increments of ¼ in).

Main Slump test results

Types of Slump
Collapse Slump
: it indicates a mix that is too wet or that the high workability mix you need shouldn’t be tested with this method.

Shear Slump: either the top or half of the cone tapers off. You should repeat the test. If the same result keeps happening, your mix needs more cohesion.

True Slump: most of the original shape remains, indicating a dry mix with stiff consistency.
Note: Slump test is not reliable for lean mixes (lower cement content to liquid ratio used for base layers).

Action to take:
Any variation in slump results is a signal to the mixer operator that he needs to make a change. If you experience an increase in slump, your moisture level could have suddenly increased or you may not be adding enough sand to your mix without you realizing it.

WORKABILITY
Level

SLUMP (mm)

COMPACTING
Factor

SUITABLE USES

Very low

0-25

0.78

Super dry mixes for making road with power operated vibrated machines

Low

25-50

0.85

For foundations with reinforcement and road with hand operated machines

Medium

50-100

0.92

Manually compacted flat slabs and reinforced concrete

High

100-175

0.95

great for void filling, underwater applications, pumping over long distances, large flat areas, not appropriate for vibration

When to perform a Slump Test?
On site on a day-to-day or even hour-to-hour basis when material is being brought into the mixer.

Is there an easier way to conduct the test?
You can use a K-Slump tester (best for in-situ measurement and in form testing). It looks like a syringe.

There are digital Slump Meters that use sensors and controls. They will save operators time to clean, pour, temper and measure the slump throughout the job.

There are other more sophisticated systems that combine devices and software to monitor the consistent quality of concrete from the plant and during transportation in the ready mix truck (concrete process control)

Are there any limitations to the slump test?
This test is best for testing medium to high workability (between 5-260 mm). It should only be performed on concrete using 1.5 inch aggregates or less and not be used for stiff mixes with zero slump or for wet mixes which resulted with a collapsed slump.

If the on-site slump test fails, should engineers allow the contractor continue the concreting work?
In some cases, they can order to stop the job. Depending on which school of thought they follow, some engineers may trust compression tests more than slump tests. If this is the case, they will let the contractor continue their work but demand that the finished work complies with the agreed upon compression strength. However, if the compression test fail, the work will have to be entirely demolished and redone. This approach is both costly in terms of time, resources and will break the budget. So why risk it when you can do it right from the beginning?

Using the right concrete pump for the job is also essential. With over 40 years in the concrete pumping industry, United Equipment Sales stand by its products, can help you solve challenges on the job and give you great deals on your next machine. Call Dick at (503)283-2105

Suggested Further Reading:
Fresh Concrete Properties and Its Standard Tests
The Importance of Testing Concrete
Slump Test  

 

Jun 08

AVOID CONCRETE HOSE WHIPPING – Tips from the Pros

Hose Whipping

Hose whipping is one of the most common accident that occurs when operating a concrete pump. If you have never seen it happen on the job, check Youtube and you will see that this is no joke. The good news is that it can easily be prevented if you understand what can cause it and what simple measures to take.

So, what is hose whipping? Simply put, it is the uncontrolled and rapid motion of a flexible rubber hose on the end of a concrete placement boom or other type of concrete delivery line.

 

What causes it?
Most of the time, it happens, when air enters the line and finds a blockage in the line. The pressure increases and whatever blocked the line shoots out violently. It can also be caused by the ready mix driver that lets the hopper go dry, or the pump operator who doesn’t realize the hopper has gone dry or has a plug in the line.

However, the risk can occur way before the concrete comes out of the system. If, when the system starts up, the air comes into the line and the operator pushes a concrete mix that contains too much rocks, the pipe and hose will be dry and not yet primed. Some loose components are separated from the rest of the mix by air, creating pressure and a block. When the boom is connected to one or more lay-down rubber hoses, the danger is even greater.

Another common situation that has most operators tend to ignore is whenever you stop pumping and restart. It accounts for about 50% of these accidents because the reality on the job is that pumps stops dozens of times a day.  If after stopping, a small amount of concrete come out, it means that the line is lubricated.  However, since it is rare that the air in the line can trigger a hose whip, workers tend to downplay the risk and not stay away from the end of the hose when restarting pumping. Big mistake and again so easily preventable!

When does hose whipping happen?
Either during priming, pumping operation or when cleaning out the system. In other words, anytime the pump is on!

Dangers for the crew
Workers can either be directly hit by the hose, knocked over by it or injured by material that is ejected. It all happens really quickly and violently.

What Should Be Done to ensure that hose whipping never happens?
Here are some practical tips from the pros that have seen just about everything on the job site:

 

Always Wear Safety Gear on the Job!

ALWAYS…

  • Use properly trained professional pump operators.
  • Keep workers away from the hose at least in the first one-half yard of pumping.
  • Use quality air clamp on hose end.
  • Limit reductions as much as possible. Use a steel reducer, if you have to.
  • Have a person monitor the hopper and shut the pump in the event air is sucked into the line (it isn’t the delivery truck driver’s responsibility!)
  • Pump a little slower so that you can catch when your turn your strokes down or put it in reverse to expel the air out.
  • Use smaller rocks on smaller hoses to reduce block and releases in reducer.
  • Use safety gear: glasses, helmet, gloves etc.

NEVER…

  • Stand near the hose until the concrete is flowing freely.
  • Stand within 20 feet of the hose when starting, priming and restarting.
  • Use a metal fittings on end of delivery hoses
  • Stretch the delivery hose if it doesn’t quite reach the pouring area. Instead, move the concrete pump closer or use a larger boom.
  • Allow concrete to flow out of the hose after stopping, this can let air into it. Instead fold the hose.
  • Allow concrete to harden in the line, this will result in blockages and damage to your equipment.
  • Allow an untrained worker near the hose on the job site.

AVOID…

  • Reducers, reduction hoses, always use a steel reducer, if you have to reduce.
  • Keep the work area clean to avoid falls and tripping.

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

Additional Recommended Reading:

How to Avoid Blockages
Hose Whip Safety Alert
Examples of unsafe practices

Aug 16

5 TIPS TO SAVE MONEY ON CONCRETE PUMPS

Putzmeister-TK303 concrete line pump

In difficult economic times, we all need to learn to do more with less. This often means extending the life of our equipment by treating it with a little extra TLC. Prevention is the key.

Here are a few tips everyone should follow to do just that with concrete pumps:

 1.      Grease your machine

Main parts: lubrication of pump’s pivot points
Frequency: approximately every 2 hours of use

Note: Failure to grease your pump could result in potential failure of bearings and bushings, breaking down of equipment on the job, loss of productivity, costly repairs…

2.      Check fluid levels

Minimum: hydraulic oil, engine oil & slosh box
Frequency: Daily before starting the job.
Since it only takes a minute, don’t skip this step. Start making it part of your job routine.

3. Clean the pump

Areas: Pump water through the pump and the hose simultaneously.
Frequency: End of job/daily

Note: This isn’t something you can postpone, depending on the temperatures, concrete can harden in the pump and hoses anywhere from 0-2hrs!

4. Hire an experienced operator

There are many advantages to have an experienced operator.
Here are a just a few:

a. Safety on the job:  Knowing how to quickly react to pumping problems is paramount. It can save lives.

b. Proper hose connection (many deadly accidents have happened because of this).

c. Overall inspection of pump and hoses prior to starting the job.

5. Follow the manual’s procedure

Take a moment to scan through the manual that came with your pump. Only dealers will provide you with a manual when purchasing a used pump (brokers don’t always do) so if you are serious about your work, contact a dealer first. They will stand by their products and will help you make a more educated decision.

For product or brand specific information on used concrete pumps (Putzmeister, Mayco, Schwing, Olin and Reed ) or to find a great deal, visit: http://unitedequipmentsales.com/UsedConcreteLinePumps.php