The slump test, or slump cone test, is a quick check of fresh concrete consistency. While it seems simple, sample representativeness, equipment moisture, layered compaction, and execution speed can alter the obtained value and lead to wrong decisions in concrete reception or application.
What is the Test?
The slump test measures the difference between the initial height of the frustum-shaped mold and the height of the displaced center of the concrete after the mold is vertically removed. The result is expressed in millimeters and describes the deformation of fresh concrete under its own weight.
The test is applicable to plastic hydraulic-cement concrete. ASTM C143/C143M indicates that the method is typically applicable to concrete with coarse aggregate up to 37.5 millimeters and is not appropriate for non-plastic or non-cohesive concrete [1]. When larger aggregate is present, the wet sieving operation specified for the sample must be followed.
This article adopts a specific angle of execution, limitations, and interpretation of consistency. It does not repeat the eight previous articles, dedicated to soils, aggregates, compaction, abrasion, and absorption. Here, the central problem is to correctly evaluate fresh concrete before relating the result to placement and construction control.
What is it for?
The slump test allows verifying if the consistency of the received concrete is within the range defined by the project, delivery ticket, or control plan. It can indicate that the mix is drier or more fluid than expected and alert to problems of batching, transport, water loss, unauthorized addition, or initial set.
Consistency influences the ease of transport, placement, spreading, and compaction. Concrete that is too dry can hinder filling and increase the risk of voids; concrete that is too fluid can exhibit segregation, bleeding, or loss of stability when not designed for that condition.
The value is not a direct measure of strength. ASTM warns that while in the laboratory slump may increase with water in a controlled mix, the relationship between slump and strength is not consistent under field conditions [1]. The result should be analyzed with temperature, time since mixing, air content, composition, and strength specimens.
Standard or Reference Standards
The main reference is ASTM C143/C143M, Standard Test Method for Slump of Hydraulic-Cement Concrete [1]. AASHTO T 119 is used in many road and infrastructure projects. The public WSDOT procedure consulted presents the AASHTO T 119-23 version and confirms the steps of sampling, filling, compaction, and measurement [2].
The official ASTM page consulted presents C143/C143M-20 as a historical document and lists an active C143/C143M-26A version. This difference should be verified before contractual use. The edition indicated in the project prevails, including tolerances, equipment, base, sampling method, and acceptance criteria.
The test must be performed on a sample obtained according to the fresh concrete sampling procedure, such as ASTM C172 or AASHTO R 60, when applicable. A result based on a segregated portion or collected at the beginning of discharge does not necessarily represent the load.
Sample Preparation
Identify the job site, element, truck, batch, loading time, arrival time, discharge location, and operator. Record the specified consistency, concrete type, maximum nominal aggregate size, and any on-site additions.
The sample must be representative of the batch. Collect portions from different moments of discharge, according to the sampling procedure, and mix them enough to homogenize without causing paste loss or segregation. Do not only take the first discharge or choose a visually more fluid area.
The test should begin quickly after obtaining the sample. WSDOT indicates that the start should occur within five minutes and that filling to mold removal should be completed within approximately 2.5 minutes [2]. The time between sampling, slump, temperature, air, and specimen molding must be coordinated.
If the concrete contains aggregate retained on the 37.5 millimeter sieve, remove this fraction by wet sieving according to the applicable procedure. Record the occurrence and do not directly compare the result with concrete whose sample has another maximum size without confirming the basis of comparison.
Equipment and Materials
Key equipment includes:
Rigid, smooth, undeformed frustum-shaped mold with handles and footrests.
Flat, rigid, non-absorbent, and moistened base plate.
Tamping rod with 16 millimeters diameter and hemispherical tip.
Shovel or scoop of adequate size to introduce representative increments.
Rule or graduated tape with 5 millimeter readability.
Container for sample collection and mixing.
Cloth, brush, water, and cleaning equipment.
Stopwatch, test sheet, and batch identification.
Gloves, safety glasses, boots, and protective clothing.
The mold must be clean, moist, and free of adhering mortar. Dents, dings, protruding seams, or loss of shape can prevent uniform lifting. The base must be stable and vibration-free. The rod must have a rounded tip and no burrs.
Fresh concrete is alkaline and can cause irritation or chemical burns. ASTM C143 emphasizes that it is the user's responsibility to establish appropriate safety practices [1].
Step-by-Step Procedure
1. Prepare the Area and Equipment
Moisten the inside of the mold, the base, and the rod, removing free water. Place the base on a level, firm, and non-absorbent surface. Position the mold and hold it by the footrests or place your feet on the hold-down pieces, without displacing it.
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2. Fill the First Layer
Introduce representative concrete up to approximately one-third of the mold's volume, about 67 millimeters deep. Distribute the material with the scoop instead of letting it fall in a single spot.
Compact with 25 strokes of the tamping rod, distributed over the entire section. In the first layer, penetrate its full depth and use inclined strokes near the perimeter, then moving to the center. Avoid violently hitting the base.
3. Fill and Compact the Second Layer
Add concrete up to about two-thirds of the volume, approximately 155 millimeters deep. Apply 25 uniformly distributed strokes, penetrating about 25 millimeters into the lower layer. Penetration integrates the layers and reduces the formation of separation planes.
4. Fill the Third Layer
Fill the mold until it overflows. During the application of the 25 strokes, always maintain some excess above the rim. If the level drops, add concrete and complete the total number of strokes. Penetrate about 25 millimeters into the second layer and distribute the strokes over the entire section.
5. Finish the Surface
After compaction, scrape and roll the tamping rod over the top to level the surface. Clean any concrete that remained near the base without allowing the mold to move.
6. Lift the Mold
Remove the mold in a vertical, firm, and continuous motion, without tilting, twisting, or lateral displacement. WSDOT specifies lifting about 300 millimeters in 5 ± 2 seconds [2]. The time must be controlled without interruption from the start of filling to removal.
7. Measure the Slump
Invert the mold and place it next to the specimen without disturbing it. Rest the tamping rod across the mold and measure the distance between the bottom of the rod and the original displaced center of the concrete's top. Record the value to the nearest 5 millimeters.
If pronounced shear or sliding of a portion of the specimen occurs, discard the result and repeat on another portion of the sample. If two consecutive tests show shear, the concrete may not have sufficient plasticity or cohesion for the method [2].
Calculations and Results Processing
The slump calculation is straightforward:
Slump = initial mold height minus the height of the displaced center of the specimen.
The initial height corresponds to the height of the mold in the test position. The final height should be measured at the center originally occupied by the top of the concrete. The lowest or highest zone of an irregular cone should not be measured.
The result should be recorded in millimeters, with the resolution required by the standard. The report should contain sample identification, collection time, start time, measurement time, value, specimen condition, and observations on shear, collapse, segregation, or bleeding.
Do not correct the value based on a visual impression. If the project specifies a range, compare the rounded result according to the standard and record the compliance decision. The addition of water or admixture to the truck should only occur with authorization, quantity recording, and confirmation of the mixing procedure.
Interpretation of Results
A low slump practically indicates a stiffer mix at the time of testing. It can result from less effective water, higher aggregate absorption, moisture loss during transport, altered batching, waiting time, or initial hydration.
High slump indicates greater fluidity but does not alone prove excess water. It can result from superplasticizer admixture, temperature, aggregate gradation, entrained air, or aggregate moisture variation. The cause should be verified in the delivery ticket, batch plant, and batch records.
The type of deformation also matters. A uniform slump is different from a lateral shear or a collapse. Shear suggests lack of cohesion or improper procedure; collapse may indicate concrete that is too fluid for the test application. In these cases, the numerical value should not be interpreted as a normal measurement.
ASTM indicates that very low or very high values may not have adequate significance for this method, especially when the concrete is not sufficiently plastic or cohesive [1]. Use complementary tests for temperature, air content, unit weight, and strength when the decision requires more than an indication of consistency.
Common Errors to Avoid
Using a non-representative sample of the batch.
Delaying the start or completion of the test.
Working on an absorbent, inclined, or unstable base.
Allowing the mold to move during filling.
Filling layers of unequal thickness.
Applying concentrated blows in one area.
Not penetrating previous layers.
Leaving the top without excess during final compaction.
Lifting the mold with inclination or twisting.
Measuring a zone different from the displaced center.
Accepting a specimen with shear or collapse without observation.
Adding water without authorization or record.
Directly associating slump with strength.
QA/QC Tip
Use a control sheet that links the slump to the truck, batch, structural element, collection time, and complementary tests. Record the condition of the mold, base, and rod before the test. Photograph an abnormal specimen, when permitted, and describe the form of rupture or deformation.
Perform periodic checks of the mold's dimensions and integrity. Confirm that the rod has a hemispherical tip and that the tape measures with the expected resolution. Train operators to maintain the same blow distribution and lifting time.
When the result is out of range, do not perform a second test on the already used specimen. Take a new representative portion, verify the procedure, and record the repetition. The decision should consider the load trend and not just the value that seems best.
Practical Application on Site
Slump supports concrete reception for foundations, columns, beams, slabs, walls, pavements, and precast elements. A low result may require confirmation of workability before placement; a high result may require investigation of water, admixture, segregation, and stability.