Verifi Concrete Quality Blog

Verifi Features

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.

Does Adding Superplasticizer After Batching Increase Entrained Air Content?

by Verifi

This question comes up frequently when we discuss superplasticizer additions in transit with Verifi.

Increasing the slump of a load of concrete usually increases the entrained air content. That is true whether the slump is increased with water or admixture; whether slump is increased at the plant, in transit, or at the jobsite; and whether slump is increased gradually or with a one-time addition.

Likewise, a decrease in slump when concrete is in the mixer is usually associated with a decrease in air content.

This first graph shows an air entrained concrete mix delivered without Verifi. The slump and air content decrease over time. Then, water is added just before discharge to increase slump, which results in an increase in entrained air content.

The next graph shows the same scenario, but polycarboxylate-based superplasticizer is added at the jobsite. Similarly, the air content increases as the slump increases.

This last graph shows a Verifi-managed delivery.  The slump and entrained air content remain essentially constant.

entrained air content verifi

© Verifi LLC

In conclusion, maintaining a consistent slump—with water or admixture—helps ensure consistent entrained air content. In addition, minimizing variation in other factors such as water content and mixing also help reduce variation in entrained air content. As always, it is important to test with the materials in question and to select the air entraining admixture best suited for the available materials and expected conditions.

 

Load of the Week | Slump Increase with Superplasticzer

by Verifi

It is common for the contractor to request a slump increase at the jobsite.  Concrete with higher slump is easier to pour, pump, and finish.

The best time to request this higher slump is during ordering. This saves time for adjustment at the jobsite and allows the ready mix concrete producer to select the most suitable mix design for the higher slump.  However, contractors make the request at the jobsite for a variety of reasons.

Without Verifi, the increase in slump at the jobsite is usually made by adding water.  The addition of water reduces strength and durability and delays setting time.

Verifi enables concrete producers to increase slump by adding superplasticizer, which does not negatively affect strength and durability.

In today’s Load of the Week, the original order was for a 5-inch slump.  At the jobsite, the contractor requested a slump increase to 7 inches.  The driver entered this new slump target on the Verifi in-cab interface.  Verifi then automatically added superplasticizer to reach the new 7-inch target slump.

As a rule of thumb, a 2-inch slump increase with water would have decreased the compressive strength by about 400 psi and delayed setting time 20 to 30 minutes.  This effect was avoided by the use of superplasticizer to increase slump.

concrete superplasticizer

[LP = leave plant, AS = arrive site, BP = begin pour, LS = leave site, RP = return plant]

Nominate a future Load of the Week by emailing the ticket number and ready mix concrete supplier name to feedback@verificoncrete.com.

Verifi Feature: Ticketed Slump Override

by Verifi

Is it OK for the slump of the concrete to not match the slump on the batch ticket?

Yes…in some cases.

For example, on a pumped job, the ticket may indicate the slump at the point of placement; that is, the end of the pump line. Due to slump loss during pumping, the concrete should be discharged from the truck at a slump greater than shown on the ticket.  In this example, Verifi would need to manage the slump to a target greater than the ticketed slump.

In another example, the contractor orders one slump but after the first load quickly realizes he needs a higher slump.  The concrete producer needs to override the slump on loads that are already ticketed so Verifi can start managing to the higher slump target.

Verifi can accommodate these cases with the Ticketed Slump Override feature. With this feature, the concrete producer enters the order number, date, location, mix code, and new slump for the override. Verifi will then use the new target slump, even if different from what is shown on the ticket. Verifi typically receives the ticketed slump through an interface with the batch plant software.

concrete slump ticket

A new override has to be entered each day, as job conditions affecting the override can change daily. The override can be turned on or off at any time.

Verifi keeps a record of all ticketed slump overrides. Access can be restricted to Verifi users with authority to change the slump.

ticketed slump override

The Ticketed Slump Override feature enables Verifi to adjust to the new slump in transit, which helps save time at the jobsite and avoids imprecise manual adjustments of slump.

Verifi customers can contact their Business Analyst to get started using the Ticketed Slump Override.

Load of the Week | Water & Superplasticizer in Transit

by Verifi

In today’s Load of the Week, Verifi automatically added both water and superplasticizer to reach the contractor’s desired slump.

Here’s what transpired:

1) The concrete was batched below the target initial slump of 5 inches.

2) Verifi automatically added water up to the maximum allowed for the load, then switched to superplasticizer to reach the target slump. This ensured the maximum water/cement ratio was not exceeded and the design concrete strength and durability were not compromised.

3) The target slump was changed from 5 to 8 inches. This was done at the request of the contractor. Verifi enables concrete producers to change the slump target at the request of the contractor, provided the change is within the design parameters of the mix. Note that the change was requested before the truck arrived on site. Therefore, the concrete producer could override the target slump and Verifi could adjust slump to the new target before the truck arrived on site.

4) Verifi continued to add superplasticizer to reach and maintain the new target slump of 8 inches.  All additions were automatic and did not require driver input.

5) The contractor poured the concrete shortly after arrival.

 

Concrete Superplasticizer

 [LP = leave plant, AS = arrive site, BP = begin pour, LS = leave site, RP = return plant]

Nominate a future Load of the Week by emailing the ticket number and ready mix concrete supplier name to feedback@verificoncrete.com.

Choosing The Right Superplasticizer for Verifi

by Verifi

Verifi can automatically add superplasticizer to concrete in the truck. Superplasticizers are among the most potent ingredients in concrete. Thus, it is important to choose the right superplasticizer to ensure consistent quality concrete. But what superplasticizer should you choose for use in Verifi?

The primary purpose of dosing superplasticizer with Verifi is adjusting slump. So choose an superplasticizer that controls slump with minimal effect on other properties.

Specifically, choose superplasticizers that are:

1.       Set neutral.

2.       Strength neutral.

3.       Air neutral.

4.       Fast mixing.

5.       Short to moderate slump life.

6.       High solids.

Admixture Tank Verifi
[Verifi Admixture Tank]

Verifi recommends using polycarboxylate-based superplasticizers. These are sold under trade names including ADVA®, Glenium®, and Viscocrete®.

However, not all polycarboxylate-based superplasticizers are suitable. Some are formulated to delay setting or promote rapid strength development. Superplasticizers formulated for long slump retention are also no longer needed because Verifi can maintain slump more precisely. Such admixtures tend to be sold at a premium price, which offers a potential for cost savings with Verifi.

Naphthalene-, melamine-, and lignosulfonate-based admixtures increase setting time with dose. So, increasing the slump, which is desired, will also delay setting, which is probably not desired.

Verifi recommends starting with the polycarboxylate-based superplasticizer the concrete producer is currently using, when possible. This simplifies the transition to dosing superplasticizer on the truck. However, testing is advised to confirm if the admixture is suitable. If not, Verifi will work with customers to select an alternate admixture.

By selecting the correct admixture, the consistency of concrete quality—including setting time, finishability, pumpability, air content, and strength development—can be improved dramatically.

 

Load of the Week | Automatic Superplasticizer

by Verifi

In today’s Load of the Week, Verifi adjusted the slump of the concrete by making automatic superplasticizer additions.

Verifi made three automatic superplasticizer additions to adjust the slump from 4 inches after batching to the 8-inch target. Instead of one large addition, Verifi’s unique algorithm monitored the superplasticizer performance in this particular load and precisely adjusted the three additions to reach but not exceed the target slump. Verifi used the time from batching to arrival on site to make this slump adjustment so that the concrete was ready to pour upon arrival, even though the contractor was not ready.

 

automatic superplasticizer

[LP = leave plant, AS = arrive site, BP = begin pour, LS = leave site, RP = return plant]

Nominate a future Load of the Week by emailing the ticket number and ready mix concrete supplier name to feedback@verificoncrete.com.

Load of the Week | Batching Near Target Slump

by Verifi

In a previous post, we showed a load batched 3.25 in. below the target slump.  Despite this large difference, Verifi successfully adjusted the slump to reach and maintain the target slump.

However, Verifi actually recommends batching at or slightly below the target slump.  This ensures effective mixing and consistent concrete admixture performance.  Verifi’s Batch Assistant feature helps batch plant operators batch closer to target.

Today’s Load of the Week was batched 0.75 in. below target slump. Verifi made one water addition just before the truck left the batch plant and two more additions in transit. The slump was at target at arrive site and during pouring.

 

Untitled

[LP = leave plant, AS = arrive site, BP = begin pour, LS = leave site, RP = return plant]

Nominate a future Load of the Week by emailing the ticket number and ready mix concrete supplier name to feedback@verificoncrete.com.

Load of the Week | Superplasticizer in Transit

by Verifi

In today’s Load of the Week, Verifi added superplasticizer in transit to achieve the target concrete slump. Verifi made three additions of a polycarboxylate-based superplasticizer  to increase the slump of the concrete. The use of multiple, small superplasticizer additions enables the slump to be targeted more precisely than with one large addition.

In most cases, some of the water is withheld (or “trimmed“) during batching so that it can be added after batching: at the plant, in transit or at the site. In the case of the Load of the Week, all water was added during batching and no water was available to be added by Verifi. Instead, superplasticizer was withheld at batching so that it was available to be added after batching.

 

Superplasticizer in Transit

[LP = leave plant, AS = arrive site, BP = begin pour, LS = leave site, RP = return plant]

Nominate a future Load of the Week by emailing the ticket number and ready mix concrete supplier name to feedback@verificoncrete.com.

Effect of Concrete Slump on Worker Safety

by Verifi

When choosing a slump for a project, do you consider the effect of concrete slump on worker safety? According to a study from Build It Smart, slump significantly affects the amount of effort expended by workers.

They measured the following forces to pull concrete with a rake:

  • 1 in. slump: 46 lbs
  • 3 in. slump: 27 lbs
  • 6 in. slump: 20 lbs

Effect of Concrete Slump on Worker Safety

The study recommends pouring concrete at the highest slump possible:

“Always use concrete with the highest slump that will ensure quality and strength while at the same time, reduce unnecessary work.”

In contrast, ACI 211.1-91, the American Concrete Institute’s guide to proportioning concrete mix designs, recommends using the lowest slump possible:

“Mixes of the stiffest consistency that can be placed efficiently should be used.”

It’s important to remember that a lower slump does not correspond to higher strength, as we wrote about earlier in “Is Slump an Indicator of Concrete Water Content?”. The use of admixtures–whether at the plant or in transit–enables concrete to be poured at high slump without compromising concrete quality.