What to do if your delivered concrete has a slump that is too high

If delivered concrete has a slump that is significantly higher than specified, you should reject the mix or take immediate action to bring it within the desired range. Concrete that is too wet becomes weaker and more prone to cracking.

If delivered concrete has a slump that is too high, you must reject the load or adjust it with approved admixtures from the supplier to avoid weak, cracked concrete.

If you receive a concrete delivery that tests with a slump significantly higher than what was ordered for your job, your first action should be to either reject the concrete or implement measures to adjust it to the desired range, because concrete that is too wet becomes weaker and more susceptible to cracking. A higher slump indicates an excessive amount of water, which compromises the mix's strength and durability. For critical projects, especially those requiring specific compressive strengths (e.g., 2500 PSI at 28 days for general concrete, or higher for structural elements), maintaining the correct slump is crucial.

Why High Slump is a Problem

Concrete with an overly high slump has too much water, which weakens it and makes it more vulnerable to cracking. This happens because the excess water creates voids as it evaporates, reducing the density and strength of the cured concrete. In certain situations, particularly when high-range water reducers (HRWRs) are added to a normal mix to create a flowing mixture, the concrete can also segregate, where heavier aggregates settle, and bleed, where water rises to the surface.

Impact on Concrete Performance

Beyond just strength, high slump can lead to several other issues:

• Reduced Durability: Weaker concrete is less resistant to wear, weathering, and chemical attacks.
• Increased Shrinkage and Cracking: The higher water content means more evaporation, which increases drying shrinkage and the likelihood of random cracking.
• Poor Finishability: An overly wet mix can be difficult to finish properly, leading to a weak surface that may dust or scale.
• Segregation and Bleeding: As noted, if the mix is not properly designed, high slump can cause aggregates to separate and water to rise, creating an inconsistent final product.

If you're dealing with issues related to slump, understanding the Consequences of Concrete Slump Being Too High or Too Low can provide more context.

What Actions to Take

Your primary goal is to ensure the concrete placed meets the project's specifications for strength and durability. This often means working with the ready-mix producer.

1. Test the Slump Immediately

Before placing any concrete, a slump test should be performed on a representative sample of the batch. This test, often a modification of standard methods like those referenced by California Test 556, helps determine the concrete's workability. If the slump is too high (e.g., above 9 inches), the concrete may not be cohesive enough for the test to be significant, indicating a severe issue.

2. Communicate with the Ready-Mix Producer

If the slump is too high, immediately notify the ready-mix truck driver and your concrete supplier. They may be able to add approved admixtures, such as certain types of water reducers, to adjust the mix on site. However, adding water is generally not an acceptable solution, as it further reduces strength.

> Pro Tip: In Southern California's often dry conditions, it's easy for concrete to dry out too quickly. However, never add water to a high-slump mix yourself, as this can severely compromise its structural integrity. Always consult with the ready-mix supplier for approved adjustments.

3. Consider Rejecting the Load

If the slump cannot be brought within the specified range, or if the mix shows signs of severe segregation or bleeding, rejecting the load may be the safest option for your project. Placing compromised concrete can lead to costly repairs down the line. A new, properly mixed batch will be necessary.

Preventing High Slump Issues

To minimize the chance of receiving an overly wet mix:

• Specify Clearly: Provide clear and precise specifications for your concrete mix to the ready-mix supplier, including desired slump range and PSI (e.g., Recommended PSI for Driveways and Structural Concrete Components).
• On-Site Verification: Be prepared to test the slump upon delivery for each truckload.
• Plan for Conditions: Account for weather conditions; hot, dry Southern California days can affect slump and setting times, but this generally makes concrete stiffen, not become overly wet.

Redesigning the concrete mix, such as increasing fine aggregate or adding a pozzolanic material, is usually required to prevent segregation and bleeding in high-slump mixes that incorporate HRWRs. This is typically a task for the ready-mix plant, not a field adjustment.

Planning for a concrete project in Southern California involves many details beyond mix design. For help estimating quantities and understanding project needs, check out our Concrete Project Calculator. Western Concrete prioritizes delivering the right mix for your project's longevity and performance.