Failed slump test: Should engineers allow the contractor to continue concreting if the on-site slump test fails? This is a pretty traditional issue that many graduate engineers pose. Here, we are going to spot some light on the answer.
When an on-site slump test fails, engineers must make an appropriate conclusion. The choice may be to reject the whole batch of concrete outright or to suspend construction due to the contractor’s inability to adhere to project specifications. Alternatively, the engineer may instruct the contractor to proceed with the construction and monitor the concrete strength to ensure it reaches the specified level.
Engineers use a variety of techniques to address issues connected with on-site slump testing, including adding water or superplasticizer to concrete if the slump result is low. However, when the slump value is large, resolving the issue may be more difficult. A high slump value indicates a high water-cement ratio, which affects the concrete’s durability and strength.
For concrete with a high slump, the test may be redone since an incorrect test process may have resulted in the failure, or the concrete in the mixer should be left to rotate for a maximum of 10 minutes to determine if it falls within the required slump range. The slump test is used to determine the consistency of concrete from batch to batch and typically varies between 25 and 75 mm for vibration-compacted concrete.
Table of Contents
Slump Failure
ANSI/ASTM C 94, “”If the observed slump or air content exceeds the stated limitations, a check test on another piece of the same sample should be conducted immediately.” In the case of a second failure, the concrete shall be deemed to have violated the specification’s standards.” ASTM C 94 is referred to on the majority of projects, since it is integrated into ACI 318 and other national building regulations. ASTM C 143, “Standard Test Method for Slump of Hydraulic Cement Concrete,” likewise mandates the conduct of two slump tests but prohibits the use of either test to determine the slump. The requirement is to monitor the displacement of concrete when the slump cone is removed. “If a determined falling away or shearing off of the concrete from one side or section of the mass occurs, discard the test and do another on another area of the sample,” ASTM C 143 reads. Additionally, it notes that “if two successive tests on a sample of concrete demonstrate a piece of the concrete sliding away or shearing away from the bulk of the specimen, the concrete generally lacks the essential flexibility and cohesion to pass the slump test.” Thus, doing two slump or air tests on the concrete prior to rejecting it is the proper method.
Slump Test Failed, What Now?
- If the slump test conducted on-site fails, the engineer may use his or her contractual authority to reject the concrete and halt the concreting operation.
- The engineer may argue that it is preferable to reject the concrete and avoid future repair work, as well as the possibility of demolishing the building if the concrete does not exceed the specified strength.
- However, it is considered that a slump test is only a measure of the consistency of the concrete and may not result in a loss of strength. Thus, the site engineer may let the contractor to continue concreting, although slump corrections should be made as necessary.
- If the contractor is authorized to complete the concreting process, he or she must still demonstrate that the completed work complies with additional contractual criteria, such as a compression test.
- Engineers should be aware that the workability of concrete is directly connected to its strength and durability. Even if the concrete reaches the specified strength, its durability may be compromised.
- When the concrete slump exceeds the maximum allowable value and the concrete’s endurance is critical, it is preferable to reject the concrete.
- If the slump value is too low, add water or superplasticizer in regulated quantities to make the concrete more workable and bring it within the required slump range. After that, position the concrete in its final location.
- Water added to the concrete mix should not exceed the maximum permissible limit specified in the specifications.
- If the slump value of the concrete is excessive, the site engineer may let the mixer to rotate for eight to ten minutes to lessen the slump. If the test continues to fail, cement may be added to the concrete mix to reduce the slump value.
- Remix the batch for 30 revolutions after adding water or cement to ensure that the new material is properly integrated with the fresh concrete.
- Occasionally, the site engineer may elect to combine a concrete batch with an extremely high slump value with batches with an acceptable slump value during concrete pouring to mitigate the negative impact of the former batch.
- If the concrete does not exceed the specified compressive strength, the contractor should explore appropriate rehabilitation measures or destroy and rebuild the structure. This raises both the cost and duration of building.
Table-1 Approximate Changes in Water Content, in Percentage, for Various Slump Changes
| 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 |
| – | 50 | 75 | 100 | 125 | 150 |
| 25 | +6.5% | +11.3% | +17.9% | +17.5% | +19.9% |
| 50 | – | +4.5% | +7.9% | +10.3% | +12.6% |
| 75 | – | – | +3.2% | +5.6% | +7.7% |
| 100 | – | – | – | +2.3% | +4.4% |
| 125 | – | – | – | – | +2% |
Summary
If slump test failed, the are two schools of opinions:
The first school of thinking is very straightforward: the contractor violates contractual obligations, and so the engineer may order suspension of the Works according to G. C. C. Clause 54 (2)(c). The contractor is not entitled to any expense claims under the terms of G. C. C. Clause 54(2)(a) – (d), which is the primary worry of the majority of engineers. This is the contractual authority granted to the Engineer in the event of a failure to perform testing needed by the contract, despite the fact that some engineers say that slump tests are not as critical as other tests such as compression tests.
The second school of thinking is to allow the contractor to continue working on the concrete and then compel the contractor to demonstrate that the completed work complies with additional contractual criteria, such as compression testing. This is based on the notion that workability is primarily necessary to obtain the desired compression strength of the concrete. If the compression test also fails, the contractor is responsible for demolishing and reconstructing the works. Indeed, this is a somewhat passive approach to building and is not advised for the following reasons:
I The workability of newly laid concrete is linked to both its strength and durability. Even if the future compression test passes, a failure in the slump test implies that the final concrete structure’s durability may be compromised. (ii) If the compression test fails, the contractor must allocate additional time and money to demolish and rebuild the work, considerably slowing the development of the project. Thus, in light of such a high possibility of occurrence, why shouldn’t the Engineer use his authority to halt the contractor and save this additional time and expense?
Read More