Verifi Concrete Quality Blog

Posts Tagged ‘slump test’

Test Concrete Slump at Point of Discharge or Point of Placement?

by Verifi

Slump can be tested for acceptance at the point of discharge or point of placement. This small detail can have large implications. So where should you test concrete slump?

The point of discharge is at the end of the truck chute. The point of placement is at the final location of the concrete, typically at the end of the pump. The slump, air, and temperature of the concrete can change during pumping. Therefore, concrete producers may need to deliver concrete that is out of specification (slump and air too high, for example) so that it will be in specification after pumping.

end of concrete pump
[End of concrete pump]

end of truck chute

[End of truck chute]

The concrete producer would generally prefer to test at the point of discharge. After the concrete leaves the truck, it is usually out of the producer’s control. Each job has different placement conditions, so the need to custom adjust concrete for each job can create a significant workload. In addition, the point of placement can be difficult to access, making sampling and testing difficult.

The contractor is generally concerned with both the point of discharge and point of placement. The point of placement is where concrete will be finished, which can dramatically affect contractor productivity. However, the contractor should also be concerned with point of discharge so that concrete can be pumped quickly and without blockage. The contractor needs the concrete producer to take changes during pumping into consideration when proportioning a mixture, something that can be achieved by testing for acceptance at the point of placement. The American Society of Concrete Contractors (ASCC) Position Statement #20 recommends testing for acceptance at the point of discharge, with additional samples at the point of placement to establish the effect of placement on concrete properties.

Engineers are similarly more likely to be concerned with properties at point of placement. Slump at the jobsite is not an indicator of strength, water content, or segregation resistance and should be a concern primarily of the contractor. Properties like temperature and air content should be the consideration of the engineer.

In reality, both point of placement and point of discharge are important. The engineer, contractor, and producer should closely coordinate throughout construction to ensure properties are correct at both locations.

Testing at the point of placement can create significantly more work for the concrete producer. Most dispatch and batch software programs have only one field for one slump, and no way to indicate whether that slump is at point of placement or point of discharge. (It’s usually point of placement.)  Therefore, adjusting the concrete to account for point of placement is often a manual process that occurs outside of these software applications. The producer must have an internal process in place to manage each project to account for point of placement testing. Dispatchers, sales, and QC must closely coordinate with contractors to determine the properties needed at both point of discharge and point of placement. Changes to target properties at discharge over the course of the project are not uncommon.

Verifi can help by ensuring the concrete is consistent and fully documented at the point of discharge. Contractors should closely watch the properties of concrete indicated by Verifi at discharge and immediately contact their concrete producer if they need changes.

The three concretes you shouldn’t slump test (manually)

by Verifi

The slump test is the most widely used test for workability worldwide. However, the slump test is not applicable to all concrete. ASTM C143, which is the standard for slump, defines three conditions where the slump test should not be used.

Non-plastic concrete: When slump is less than ½ in (15 mm), concrete may not be adequately plastic for the slump test. The slump test is not effective in distinguishing these concrete mixtures: two concretes with a zero slump can have drastically different workability, especially in their response to vibration.

Non-cohesive concrete: When slump is greater than 9 in (230 mm), concrete may not be adequately cohesive for the slump test. Non-cohesive concrete is highly susceptible to segregation and should be redesigned, such as by adding a viscosity modifying admixture or adjusting aggregate grading. Not all concrete with slump greater than 9 in is non-cohesive. For example, self-consolidating concrete can flow under its own mass with adequate cohesion to resist segregation.

Shear slump concrete: If during the slump test, a portion of the concrete shears from the rest of the concrete, the slump cannot be evaluated. It is very difficult to find the true displaced center of the slump specimen. ASTM C143 advises that two consecutive shear slumps indicate the concrete should not be evaluated with the slump test.

 slump test Verifi

Verifi measures, manages, and records slumps from 0 to 10 in (250 mm) in the truck mixing drum. Verifi can distinguish between workability levels even if the slump test cannot. This is the advantage of measuring the entire specimen of concrete in the mixer. However, when comparing Verifi to a manual slump reading, it’s important to ensure the manual reading is of a valid slump. If the manual slump test indicates the concrete is non-plastic, non-cohesive, or results in a shear slump, the manual slump test results should not be used.

Do You Know Slump?

by Verifi

Can you tell the slump just by looking?  Without Verifi, slump is more often than not estimated visually. See how well you do with this short quiz.

joe truck

Click here to start.

How Accurate is the Slump Test?

by Verifi

Concrete can be accepted or rejected on the basis of slump.  NRMCA estimates that 43% of concrete rejected at the jobsite is due to slump.  So, how accurate is this test?

How much should two tests vary?

According to ASTM C143, when the same person is testing the same concrete, results from two “properly conducted” tests can vary as much as 1.13 in. for a concrete with a slump of 6.5 in.  When two people perform the test, the range can be as high as 1.5 in.

 

Slump test precision and bias

Slump Test Precision and Bias in ASTM C143

(click for larger image)

 

Is the concrete sampled properly?

Further variation can be introduced when sampling from the truck to the wheelbarrow to the slump cone.  ASTM C172 requires that concrete be sampled from the middle portion of the discharge.  When evaluating concrete for acceptance, ASTM C94 allows samples for slump and air to be taken after ¼ yd3 or ¼ m3 is discharged.  The first portion of concrete can misrepresent the remaining portion if the concrete is not fully mixed, segregated, or the top of the drum and chute are not clean and damp.

 

What about improperly conducted tests?

The number and vigor of stokes for compaction (3 layers, 25 strokes per layer), the time to perform the test (less than 2.5 min), and the time to lift the cone (5+-/2 s) all affect test results.  The test should be conducted on a flat, rigid, level, and moist surface.

 

Is slump even applicable?

ASTM C143 indicates that slump may not be applicable for concrete with slump less than ½ in. or greater than 9 in.  If a portion of the concrete shears off from the sample, the test is considered not applicable.

 

How does Verifi help?

Verifi automates the determination of slump by using sophisticated algorithms to process hydraulic pressure and drum speed sensor data from the truck. See how.

Verifi determines slump

These measurements are made on the full load in the drum to avoid sampling errors.  Human error in performing the test is eliminated.  Verifi  typically measures slump 95% of the time within ASTM limits.   Much of this difference is due to sampling and testing errors with the manual slump test.