Verifi Concrete Quality Blog

Better Concrete

Should You Specify Slump?

by Verifi

Some specifications and guides for concrete still set a maximum slump. Should you specify slump?

Historically, maximum slump was used by engineers to control water content. However, the wide variety of materials used in concrete means that slump is not a direct indication of water content. Admixtures, SCMs, and aggregates dramatically affect slump. Also, differences in site conditions, such as transit time and concrete temperature, affect the amount of water needed to reach a certain slump at the site.

Slump is primarily the concern of the contractor.  The engineer typically focuses on water content and hardened properties. As long as the water content, hardened properties, and all workability parameters, such as segregation and bleeding, are well controlled, the contractor should not be needlessly limited by maximum slump. Yet many specifications and guides still limit slump in an effort to control water content.

The following are examples in American Concrete Institute (ACI) documents.

ACI 211: Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete

ACI 211 has recommendations in Table 6.3.1 for slump of concrete without chemical admixture.  The footnote suggests the slump limits are in place to control water. Since nearly all concrete contains chemical admixture, these requirements are largely irrelevant.

maximum slump aci

ACI 301: Specification for Structural Concrete

ACI 301 requires that “unless otherwise specified or permitted, concrete shall have, at the point of delivery, a slump of 4 in.” The maximum slump can be increased to 8 in. when high-range water reducing admixture is used and 6 in. when mid-range water reducing admixture is used. Verifi data indicate that concrete is often placed at higher slumps without issue.

ACI 302.1R: Guide for Concrete Slab and Floor Construction

ACI 302 recommends maximum slumps for floors. However, Verifi data indicate that contractors prefer higher slumps for floors. ACI Maximum Slump

ACI 304.2R: Placing Concrete by Pumping Methods

ACI 304 recommends slump of 2 to 6 in. for pumping, but states that slumps greater than 6 in. can be pumped without difficulty. In most cases, pump operators would prefer higher slump to reduce pumping pressure and increase throughput. Recommending a lower slump has little to do with pumpability.

Conclusion

Verifi data indicate that concrete is often poured successfully at higher slumps than recommended in ACI documents. As long as the mixture proportions are within design parameters and hardened concrete properties are achieved, the slump can be adjusted to meet the contractor’s needs for efficient concrete construction.

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.

Concrete Batch Tickets—Explained

by Verifi

Every load of concrete comes with a concrete batch ticket. Here’s what to look for to understand what’s in your concrete at the time of batching.

Let’s start with this example ticket.  Since there is no required format, tickets may contain more or less information.

concrete batch ticket verifi

© Verifi LLC

Tickets show basic information about the purchase, such truck number, ticket number, batch time and date, mix design identity, customer, and job location.

Some tickets show batch weights. Concrete producers invest significantly to create proprietary mixture proportions, so they may choose to not provide batch weights. This ticket shows the design quantity of each material based on saturated surface dry aggregates (SSD), followed by the amount required for 10 yd3 based on aggregates in their actual moisture condition.

Most batch software programs, including the one used here, use apparent aggregate moisture content (total moisture minus absorption capacity), which is simpler but less accurate than using the total moisture content. In most cases, the error is inconsequential.  In this case, the aggregates have greater moisture than the SSD condition, so the amount batched must be increased.

The required water content is reduced by all other sources of water; that is, the free moisture in the aggregates as batched, the water estimated to be remaining in the drum before loading, and the amount of water trimmed by the batch operator. Note that water can be shown in gallons or pounds depending on whether it is measured by volume or weight. In this load, only 64% of the design water was added directly in the batch plant.

The actual amount batched varied slightly from the required quantity. This can occur, for example, due to aggregate bin gates or water valves closing too soon or late.

The amount of water is summarized at the end.  In this case, 30.9 gal of water was not added at batching and is available to be added on the truck. This water is available intentionally to account for slump loss to the jobsite.

Verifi interfaces with the batch software to obtain the actual amount of water batched and the amount remaining to be added after batching. Verifi can automatically add this water to maintain the target slump in transit. By using this number, Verifi can ensure the maximum design water content is not exceeded.

 

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.

 

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.

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.

 

Adding Water to Concrete in Transit: Is More or Less Used?

by Verifi

To compensate for slump loss during transportation, extra water is typically added to concrete at the plant or at the site to increase slump. Verifi enables this water to be added in transit.

So, does adding water to concrete in transit increase or decrease the amount of water added to concrete? According to a recent study from Missouri University of Science & Technology (MS&T) sponsored by the Missouri Department of Transportation (MoDOT), equal or less water is used when added in transit instead of at the jobsite.

The researchers considered two scenarios using Verifi-equipped trucks. In the first scenario, Verifi was set to add water every time the slump decreased by 0.5 in below target (“In transit water addition”). In the second scenario, slump decreased over 90 minutes and water was added at the end (“Single water addition at end”). This second scenario represents conventional industry practice. All mixes started at the target slump for the mix. 

Figure 28 from the report shows that equal or less water was typically added to the concrete when the water was added in transit.

Water Addition to Concrete

Figure 29 shows that compressive strength was higher when water was added in transit (denoted “Automatic Water addition” in the chart) compared to mixes with water added at the end of the delivery.  This was expected due to less water added.Concrete Compressive Strength

(Note that the mix design should be adjusted to account for this added water–regardless of where it is added–so that the maximum water/cementitious materials ratio is not exceeded. Verifi tracks water additions and can be set to stop water additions after the maximum water content for the load is reached.)

Types of Water Reducing Admixtures

by Verifi

Verifi can automatically add water-reducing admixtures to concrete in the truck to maintain slump.

 Choosing the right types of water reducing admixtures is important. A wide array of water-reducers is commercially available and can be categorized as follows.

Performance

ASTM C494, the standard specification for chemical admixtures for concrete, defines water reducers based on the amount of water reduction and corresponding effects on setting time, compressive strength, flexural strength, shrinkage, and freeze-thaw durability.

  • Water Reducing Admixture: water reduction of 5% or greater

  • Mid-Range : Although not defined in ASTM C494, most chemical admixture companies market this category

  • High-Range: water reduction of 12% or greater

 The terms superplasticizer and high-range water reducing admixture (HRWRA) can be used interchangeably.

Chemistry

The most common types of chemistry used in water reducers include:

  • Lignosulfonate

  • Sugars/Sugar Acids, such as molasses, corn syrup, and gluconate

  • Melamine Sulfonate Formaldehyde Condensate (MSFC)

  • Naphthalene Sulfonate Formaldehyde Condensate (NSFC)

  • Polycarboxylate Ether (PCE)

 Normal-RangeMid-RangeHigh-Range
LignosulfonateXX
Sugars/Sugar AcidsXX
MelamineX
NaphthaleneX
PolycarboxylateXX

Most commercially available admixtures are a formulation of multiple chemical ingredients. The above types of chemistries can be blended together or with other chemical ingredients to achieve performance characteristics such as delayed or accelerated setting time, controlled air content, increased finishability, or enhanced rheology.

 Brand Name

Admixtures suppliers market admixtures under different brands, each representing different performance categories. Several products may be sold under each brand, each tailored to different applications.

Grace: WRDA®, Zyla®, Mira®, Daracem®, ADVA®
BASF: Pozzolith®, Polyheed®, Rheobuild®, Glenium®
Sika: Plastiment®, Sikament®, Viscocrete®

 Selecting Admixtures for Verifi

Verifi recommends the use of polycarboxylate-based superplasticizers. These are the latest generation of superplasticizers and can be engineered at the molecular level for specific performance, such as effects on setting time and early age strength.

When the correct polycarboxylate-based superplasticizer is chosen, the dose can be varied as conditions change to maintain a consistent slump at discharge, while also ensuring consistent setting time, air content, and strength development.

 

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.

Admixture in Transit: What Engineers Need to Know

by Verifi

Concrete producers can use Verifi to add concrete admixtures such as superplasticizer to the truck to maintain slump from batching to discharge.  Here are a few things to keep in mind when specifying and evaluating concrete with superplasticizer additions through Verifi.

  1. Superplasticizers increase slump without reducing concrete quality.  Contractors can get the slump they want without reducing strength.  Entrained air content, setting time, durability, and other important properties are typically more consistent.

  2. Admixture dose can vary.  Longer haul time, hotter temperatures, higher slumps, and variations in raw materials such as cement all can increase superplasticizer dose.   Superplasticizers are designed to work over a range of doses.  Admixture suppliers’ recommendations are a starting point for determining the acceptable range of doses.  Mix submittals should have the range of acceptable doses.

  3. Do not use slump as a means to control water.  Superplasticizer can be used to achieve any slump the contractor wants.  A slump above target does not mean that concrete quality is compromised, nor that concrete should be rejected.  Even before superplasticizer addition, slump at the jobsite is not indicator of water content.  Do not use slump as an acceptance test to control water content, before or after superplasticizer addition.

  4. Superplasticizer can be fully mixed in the truck, even at agitating drum speed.  Verifi research shows that each superplasticizer requires a certain number of revolutions to be fully mixed into the concrete.  This is true regardless of drum speed, even at agitating drum speed.  Verifi software monitors slump and drum revolutions in real time and notifies the driver when mixing is complete and concrete is ready to pour.

  5. Superplasticizer additions are automatic and fully recorded.  Verifi ensures the highest level of control possible for concrete quality.

Concrete admixtures in transit Verifi

© Verifi LLC

[Slump vs. Target, Concrete Admixture Additions]

Learn more about how concrete admixture added in transit maintains slump until discharge without reducing concrete quality here.