Monthly Archives: June 2014

Jun 30

TESTING FRESH CONCRETE – PART 2

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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)

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Jun 20

HOW TO TEST FRESH CONCRETE – Part 1

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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  

 

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Jun 08

AVOID CONCRETE HOSE WHIPPING – Tips from the Pros

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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

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