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)

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