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

 

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Dec 13

United Equipment Sales Saves The Day in El Salvador!

We all know that building tunnels can be challenging! But making sure they don’t leak can turn into a real nightmare.

We were recently approached by a construction company in El Salvador that had built a 4000 feet tunnel but ran into a big problem when ground water started running down the hill and eventually leak into the tunnel through the rocks.

Tunnel Repair Engineering Drawing

The company first hired a contractor who ended up charging them 1 million USD but failed to fix the problem. Then, they contacted us to see if we could help them find a solution to their problem and work within their budget and source the appropriate equipment for the job.  After a series of meetings with their team of engineers going over the site, dimensions, challenges, limitations, we were able to together design a plan that not only would incorporate a permanent fix, work within their budget but also save them over $150K! This was achieved not only with the equipment we sold them but with the actual solution we came up with. In today’s economy, any savings goes a long way, but $150K, for any construction company is pretty extraordinary!

Worthington 650 CFM compressor

One of the reason the previous contractor didn’t succeed in fixing the problem was that he didn’t use a large air compressor. We suggested and sold them a Worthington 650 CFM compressor that would do the job. The compressor will be placed at the end of the tunnel connected to 4000 feet of heavy duty concrete pipes to be able to cover the length of the tunnel.

Concrete Pipes

So we prepared 400, 3”x 10” feet sections and equipped them with clamps.

Then, we devised a solution for shotcreting the tunnel wall. It required both a Blastcreeter 5000 and a Reed C50HP pump to get the job done.  To shotcrete the seams of the rocks, they would need to mix the concrete with a Bobcat at the end of the tunnel, then dump it into a mixer to mix it up, then dump it into the high pressure concrete pump’s hopper.

Reed C50 HP

The Reed C50HP can shotcrete 1000 to 2000 feet with a robot arm.  The pump is also equipped with a shotcrete nozzle 650 CFM, using air pressure to blow wet concrete between the rocks. The pump would be moved 1000 feet at a time through the tunnel until the job is completed.

Blastcreeter 5000

So how were we able to not only solve their problem but also save $150K? We will do the math for you. A Blastcreeter 5000 costs about $60K new, we were able to sell them a refurbed one for $30K. A Reed C50HP, goes for $106K brand new, we had a used one on our lot for $42K. That’s almost $100K of savings, right there. The solution we came up with also saved them an additional $60-90K, because we worked it out so that they only had to purchase 2 pumps instead of 4.

This is a great example showing how combining real expertise on the ground, with in-depth knowledge of equipment performance with the ability to source discounted dealer certified refurbed equipment, can make the difference in a bid won and a project executed in time and within budget.

Do you have a tricky concrete job or a problem to solve? Check out our Troubleshooting article, our special deals on shotcrete equipment   or call us at 503- 283-2105, we can work with you on any job you have. Give us a chance and we will show you how you too can save and deliver quality results!
Or visit us at: Unitedequipmentsales.com to check our latest deals.