Concrete Slump Testing and Adjustment Guide

Introduction

This guide provides comprehensive information on measuring concrete slump and addressing slump deviations when ready-mix concrete is delivered to your job site. Proper slump testing and adjustment are critical for ensuring concrete meets specified workability requirements while maintaining structural integrity.

Part 1: Measuring Concrete Slump

Standard Test Method

The slump test measures the consistency and workability of fresh concrete. This procedure is based on ASTM C143 standard test method, which is the industry-recognized standard for slump testing.

Required Equipment

·      Slump cone: 12 inches tall, with an 8-inch diameter base and 4-inch diameter top

·      Tamping rod: 5/8-inch diameter, 24 inches long, with a hemispherical (rounded) tip

·      Base plate: Flat, rigid, and non-absorbent surface

·      Measuring device: Ruler or tape measure (graduated in 1/4-inch increments minimum)

·      Scoop or shovel: For handling concrete sample

Preparation Steps

    1.         Clean all equipment: Ensure the cone, tamping rod, and base plate are clean and free of hardened concrete from previous tests.

    2.         Dampen the equipment: Moisten the inside of the cone and the base plate with water, but remove any standing water. This prevents the concrete from sticking and moisture absorption.

    3.         Position the cone: Place the cone on the flat base plate. Stand firmly on the foot pieces to hold the cone securely in place during filling.

    4.         Obtain a representative sample: Take the concrete sample from the middle portion of the discharge, not from the first or last concrete discharged from the truck. The sample should be representative of the entire batch.

Filling and Rodding Procedure

First Layer (Bottom Third)

·      Fill the cone approximately one-third full (approximately 2.5 to 3 inches deep)

·      Rod the layer 25 times with the tamping rod

·      Distribute the strokes evenly across the cross-section

·      Allow the rod to penetrate the full depth of the layer

·      For each stroke, lift the rod slightly and allow it to drop with its own weight plus slight additional pressure

Second Layer (Middle Third)

·      Fill the cone to approximately two-thirds full (approximately 6 inches from the base)

·      Rod this layer 25 times

·      Penetrate approximately 1 inch into the underlying first layer

·      Distribute strokes evenly across the cross-section

Third Layer (Top Third)

·      Fill the cone to slightly overflowing above the top rim

·      Rod this layer 25 times

·      Penetrate approximately 1 inch into the underlying second layer

·      Distribute strokes evenly across the cross-section

Completing the Test

    1.         Strike off excess concrete: After rodding the final layer, remove excess concrete by rolling and sliding the tamping rod across the top of the cone with a sawing motion. The top surface should be flush with the rim.

    2.         Remove the cone: Immediately lift the cone straight upward in a steady motion taking 5 to 6 seconds. Do not use any twisting or lateral motion. Lift vertically and evenly to avoid disturbing the concrete.

    3.         Time requirement: The entire test from the start of filling to removal of the cone should be completed in 2 to 2.5 minutes without interruption.

Measuring the Slump

    1.         Position the cone: Place the empty cone upside down beside the slumped concrete pile as a reference.

    2.         Set up measurement: Lay the tamping rod horizontally across the top of the inverted cone, extending over the highest point of the slumped concrete.

    3.         Measure vertically: Measure the vertical distance from the bottom of the tamping rod to the displaced original center (highest point) of the concrete specimen.

    4.         Record the slump: Record the measurement to the nearest 1/4 inch. For example, if the concrete has settled 4 inches from its original height, this is a "4-inch slump."

Evaluating the Test Results

Valid Test Indicators:

·      The concrete subsides evenly and maintains a relatively symmetrical shape

·      The concrete slumps gradually without violent collapse

Invalid Test Indicators (Retest Required):

·      Shear failure: One-half of the cone slides along a plane and separates from the other half

·      Collapse: The concrete falls apart or spreads excessively to one side

·      If either occurs, discard the sample and perform a new test on a fresh sample

Important Testing Notes

·      Perform the test immediately after obtaining the sample to prevent moisture loss

·      Conduct tests away from vibration sources that could affect results

·      Protect the sample from rapid moisture loss (wind, sun, low humidity)

·      The temperature of the concrete can affect slump; note temperature if specified

·      Always follow project specifications, which may have additional requirements beyond the basic standard

Part 2: Adjusting Concrete Slump

When Slump is Too Low (Stiffer Than Specified)

A slump lower than specified indicates the concrete is too stiff and may be difficult to place, consolidate, and finish properly.

Option 1: Add Water

Advantages:

·      Quickest and most common solution

·      Readily available

·      Increases workability immediately

Procedure:

·      Add water only if the maximum water-cement ratio has not been exceeded

·      Add in small increments (typically 1 to 2 gallons per cubic yard at a time)

·      Rotate the drum at mixing speed for a minimum of 30 revolutions or 1 minute after each addition

·      Retest slump after thorough mixing

·      Repeat if necessary until desired slump is achieved

Critical Warnings:

·      Excessive water reduces concrete strength significantly

·      Each additional gallon of water per cubic yard can reduce compressive strength by 200-300 psi

·      Adding water increases the water-cement ratio, which may violate mix design specifications

·      Document all water additions in delivery tickets and quality control records

·      Some specifications prohibit field addition of water; verify project requirements first

·      Never add water without proper authorization from the responsible engineer or inspector

Option 2: Add Water-Reducing Admixture (Plasticizer)

Advantages:

·      Increases workability without compromising strength

·      Does not increase water-cement ratio

·      Maintains concrete quality and durability

·      Preferred method when available

Procedure:

·      Requires appropriate admixture to be available on-site or carried by the ready-mix truck

·      Follow manufacturer's dosage recommendations

·      Mix thoroughly after addition (minimum 30 revolutions)

·      Retest slump after adequate mixing

·      Document admixture type, amount, and time of addition

Limitations:

·      Not always available at the job site

·      May require advance planning with the concrete supplier

·      Has limits on maximum dosage

·      May have time-dependent effects on workability

Option 3: Reject the Load

When to consider:

·      Slump is significantly below specification

·      Project specifications prohibit field adjustments

·      Maximum water-cement ratio would be exceeded

·      Structural integrity requirements are strict

When Slump is Too High (Wetter Than Specified)

A slump higher than specified indicates excess water, which compromises strength, durability, and increases shrinkage and cracking potential.

Option 1: Reject the Load (Recommended)

Why rejection is often the best choice:

·      Excess water has already compromised the concrete quality

·      Strength will be reduced and cannot be easily restored

·      Durability issues including increased permeability and freeze-thaw susceptibility

·      Excessive shrinkage leading to cracking

·      The concrete does not meet specified requirements

·      Adding cement or dry materials in the field is generally not permitted and impractical

Procedure:

·      Document the actual slump measurement

·      Note the delivery ticket number and time

·      Photograph the slump test if possible

·      Contact the concrete supplier immediately

·      Request a replacement load meeting specifications

·      Most reputable suppliers will replace non-conforming loads at no charge

Option 2: Limited Use for Non-Critical Applications

Only if rejection is not feasible:

·      Use for non-structural elements (sidewalks, driveways, non-bearing slabs)

·      Reassign to areas with lower performance requirements

·      Must have approval from engineer or owner

·      Fully document the deviation and adjusted placement location

·      Adjust construction schedule accordingly

Option 3: Allow Partial Stiffening (Rarely Practical)

Limitations:

·      Requires significant time for moisture evaporation

·      Impractical given typical delivery timeframes and setting time

·      Uneven stiffening may occur

·      Generally not recommended for quality work

Part 3: Best Practices and Quality Control

Before Concrete Arrival

·      Confirm specified slump with ready-mix supplier on the order

·      Ensure slump testing equipment is on-site, clean, and ready

·      Designate qualified personnel to perform slump tests

·      Review project specifications for acceptable slump range and adjustment procedures

·      Prepare placement areas to minimize delays

Upon Concrete Arrival

·      Test immediately upon arrival, before any concrete is discharged

·      Perform the slump test on the first truck and periodically on subsequent trucks

·      Check delivery ticket for load information (mix design, time of batching, water added)

·      Document ambient temperature and weather conditions

·      Verify the concrete has not exceeded maximum time from batching (typically 90 minutes)

Documentation Requirements

Maintain complete records including:

·      Date, time, and location of placement

·      Delivery ticket number

·      Tested slump value

·      Any adjustments made (water added, admixtures, etc.)

·      Weather conditions

·      Names of personnel performing tests

·      Cylinder samples taken for strength testing

Communication Protocol

·      Establish clear communication with the concrete supplier before delivery

·      Know who has authority to accept, reject, or authorize adjustments

·      Have contact information for the batch plant dispatcher

·      Coordinate with on-site engineer or inspector for non-conforming loads

Rights and Responsibilities

Your rights:

·      Reject any load that does not meet specifications

·      Request replacement loads at no additional cost if specifications are not met

·      Test concrete at any time during delivery

·      Documentation of all concrete properties

Your responsibilities:

·      Provide accurate information when ordering concrete

·      Have placement area ready when concrete arrives

·      Perform timely slump tests

·      Minimize delays in unloading to prevent concrete from stiffening in the truck

·      Document all tests and adjustments

Conclusion

Proper slump testing and adjustment procedures are essential for ensuring concrete quality and structural performance. When concrete arrives with incorrect slump, immediate testing and appropriate corrective action protect your project from potential defects and failures. Always prioritize following specifications and maintaining proper documentation. When in doubt about whether to adjust or reject concrete, consult with the project engineer or a qualified concrete technologist.

Remember: It is always better to reject a load than to place concrete that does not meet specifications. The cost of removing and replacing defective concrete far exceeds the cost of a rejected load.

Additional Resources

For detailed specifications and additional information, consult:

·      ASTM C143: Standard Test Method for Slump of Hydraulic-Cement Concrete

·      ACI 318: Building Code Requirements for Structural Concrete

·      ACI 301: Specifications for Structural Concrete

·      Your project specifications and quality control plan

·      Local building codes and requirements

Disclaimer: This guide provides general information based on industry standards. Always follow your specific project specifications, local codes, and engineer's requirements. When in doubt, consult with qualified concrete professionals or testing laboratories.