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.
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.
|Roads with power operated machines|
|Road with hand operated machines|
|Flat slabs with crushed aggregates|
|For congested reinforcement (not for vibration)|
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.
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
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.
- 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.
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