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  

 

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