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Engineers should make a proper decision when an on-site slump test fails. The decision could be an outright rejection of the complete batch of concrete or the suspension of the work due to the contractor's failure to follow project requirements. Alternatively, the client may permit the contractor to continue the work and wait to see whether concrete strength reaches the designated level.
Chances are that you are reading this article while your client is looking at the failed slump cone test in an unamused manner.
Several measures are adopted by engineers to solve problems associated with on-site slump test, for instance, adding water or superplasticizer to concrete if the slump value is low. However, when a slump value is high, solving the problem could be more challenging. High slump value denotes a high water-cement ratio, which thereby reduces the durability and strength of the concrete.
For concrete with a high slump value, the test may be repeated since improper test procedure may have caused the failure, or the concrete in the mixer should be allowed to revolve for a maximum of 10 minutes to see whether the concrete falls within the specified slump range. The slump test is a measure of concrete consistency from batch to batch and commonly ranges from 25 mm to 75 mm for concrete compacted with vibration.
Slump Failure Criteria
If the slump value does not fall into the specified slump range, a check test should be conducted on another portion of the same sample. If the second sample also fails the test, the concrete is assumed to have failed the project's requirements as per standard specification for ready mixed concrete (ASTM C94).
ASTM C143 "Standard test method for slump of hydraulic-cement concrete" also states that if two consecutive slump tests fail, the concrete is considered not to have the necessary cohesiveness and plasticity.
What Should Engineer Do If On-site Slump Test Fails?
- Generally, slump value ranges from 25 mm to 175 mm and depends on how much workability is required.
- Start slump test 5 minutes after the production of fresh concrete sample. Prepare the strength specimens 15 minutes after the production of new concrete batch.
- If the on-site slump test fails, the site engineer may use his/her contractual power and reject the concrete and suspend the concreting process.
- The engineer may argue that it is better to refuse the concrete and prevent future repair works and possibly demolish the structure if the concrete fails to reach the designated strength.
- However, it is believed that a slump test is just a measure of concrete consistency, and it may not cause the reduction of concrete strength. So, the site engineer can allow the contractor to continue concreting work, but proper slump adjustments should be made.
- If the concreting process is permitted to be finished, the contractor still needs to prove that the finished work complies with other contractual requirements, such as a compression test.
- Engineers should be aware that the workability is related to both strength and durability of concrete. If the concrete attains the designated strength, the durability may still get affected.
- When the concrete slump is more than the maximum limit, and concrete durability is of high importance, it is better to reject the concrete.
- If the slump value is low, add water according to the guidelines in Table-1, or superplasticizer, in measured amounts, to make concrete more workable and reach the designated range of slump value. After that, place the concrete into its position.
- The addition of water to the concrete mix should not increase the water-cement ratio above the maximum permitted limit as per specifications.
- If the concrete slump value is too high, the site engineer may allow the mixer to revolve for 8 to 10 minutes to reduce the slump. If the test still fails, one may add cement to the concrete mix to minimize slump value.
- After adding water or cement to the concrete mix, remix the batch for 30 revolutions to make sure the additional material is correctly blended with the fresh concrete.
- Sometimes, the site engineer may decide to mix a concrete batch with a very high slump value with the concrete batches that have acceptable slump value during concrete pouring to reduce the former batch's detrimental effect.
- If the concrete fails to reach the designated compressive strength, the contractor should consider suitable rehabilitation techniques or demolish and reconstruct the works. This increases the cost of construction as well as the construction period.
- Sometimes, the concrete slump value, which is less than the minimum limit, is the indicator for the late concrete placement after its arrival on the construction site.
Table-1 Guidelines for the Approximate Changes in Water Content, in Percent, for Various Changes in Slump
|From a slump of, mm||To a slump of, mm||To a slump of, mm||To a slump of, mm||To a slump of, mm||To a slump of, mm|
A concrete slump test or slump cone test is used to determine the workability or consistency of concrete mix prepared at the laboratory or the construction site during the progress of the work.
A concrete slump test is carried out from batch to batch to check the uniform quality of concrete during construction. To read more about the slump test of concrete, please click here.
If the slump value does not fall into the specified slump range, a check test should be carried out on another portion of the same sample. If the second test fails as well, the concrete is assumed to have failed the project's requirements, as per standard specification for ready mixed concrete (ASTM C94).
High slump value means the water-cement ratio of concrete is high. Hence, both the strength and durability of concrete may get reduced.
Slump and Compacting Factor Test Values and Uses in Concrete Workability