Engineers are responsible for making important decisions when an on-site slump test fails. There are various options available, such as completely rejecting the batch of concrete or halting work due to the contractor’s failure to meet project requirements. On the other hand, the client may choose to allow the contractor to continue work and wait to see if the concrete reaches the designated strength level.
It’s possible that you’re reading this article because your client is unhappy with the failed slump cone test. Engineers have several methods for dealing with issues related to on-site slump tests. For example, if the slump value is low, water or superplasticizer may be added to the concrete. However, if the slump value is high, it can be more challenging to address the problem. High slump values indicate a high water-cement ratio, which can lead to reduced durability and strength of the concrete.
To solve problems with concrete that has a high slump value, engineers may repeat the test to ensure that the procedure was performed correctly. Alternatively, the concrete in the mixer can be allowed to revolve for a maximum of 10 minutes to see if it falls within the specified slump range. The slump test is an important measure of concrete consistency from batch to batch. Typically, concrete compacted with vibration has a slump range of 25 mm to 75 mm.
Slump Failure Criteria
The specified slump range is an important parameter in determining the consistency of hydraulic-cement concrete. If the slump value falls outside of the specified range, it is necessary to conduct a check test on another portion of the same sample. This is done to ensure that the original sample was not simply an anomaly.
If the second sample also fails to meet the specified slump range, the concrete is assumed to have failed the project’s requirements as per the standard specification for ready mixed concrete outlined in ASTM C94. This indicates that the concrete does not possess the necessary cohesiveness and plasticity required for the project.
According to the ASTM C143, if two consecutive slump tests fail, this further confirms that the concrete does not possess the required cohesiveness and plasticity. As a result, it is deemed that the concrete does not meet the necessary standards and cannot be used for the intended project.
What Should Engineer Do If On-site Slump Test Fails?
The slump value of fresh concrete is a crucial factor that affects the workability of the concrete mix. The range of slump value generally varies from 25 mm to 175 mm, depending on the required workability. To conduct a slump test, it is recommended to start the test five minutes after producing a fresh concrete sample, and strength specimens should be prepared 15 minutes after producing a new concrete batch. If the on-site slump test fails, the site engineer may use their contractual power to reject the concrete and suspend the concreting process to prevent future repair works or even demolition of the structure.
However, it is believed that a slump test is only a measure of concrete consistency and may not cause a reduction in concrete strength. Therefore, if the concreting process is permitted to continue, the contractor must make necessary adjustments to the concrete mix to achieve the desired slump value. Nevertheless, engineers should be aware that workability is related to both the strength and durability of the concrete, and if the concrete attains the designated strength, the durability may still get affected.
If the slump value is higher than the maximum limit and the concrete durability is of high importance, it is better to reject the concrete. On the other hand, if the slump value is too low, adding water or a measured amount of superplasticizer can increase the workability of the concrete mix to reach the designated slump range. However, the addition of water should not increase the water-cement ratio above the maximum limit. If the slump value is too high, allowing the mixer to revolve for 8 to 10 minutes can reduce the slump, and if necessary, adding cement to the concrete mix can also minimize the slump value. After making any adjustments to the concrete mix, the batch should be remixed for 30 revolutions to ensure that the additional materials are 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 detrimental effect of the former batch. However, if the concrete fails to reach the designated compressive strength, the contractor should consider suitable rehabilitation techniques or demolish and reconstruct the works, increasing the construction cost and period. Additionally, a slump value less than the minimum limit may indicate late concrete placement after arriving 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 |
| – | 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% |
FAQs
What is a concrete slump test?
The slump test, also known as the slump cone test, is a method used to assess the workability or consistency of a concrete mixture. This test can be conducted either at a laboratory or on-site during the construction process to ensure the uniform quality of the concrete batch.
The primary purpose of the slump test is to measure the consistency of the concrete and determine its suitability for its intended use. This is particularly important in construction projects where the concrete is used to create structures or foundations, as any irregularities or inconsistencies in the concrete mixture could compromise the strength and stability of the final product.
Conducting a slump test involves measuring the change in height of a concrete sample after it has been placed in a cone-shaped mold and then removed. By comparing the height of the sample before and after removal from the mold, the workability and consistency of the concrete can be assessed and any necessary adjustments to the mixture can be made.
Overall, the slump test is a crucial tool for ensuring the quality and consistency of concrete during construction projects. By conducting this test regularly and making adjustments to the concrete mixture as needed, construction teams can ensure that their structures and foundations are strong, stable, and built to last.
What are the slump test criteria?
According to the standard specification for ready mixed concrete (ASTM C94), if the slump value of a sample does not fall within the specified range, it is necessary to conduct a check test on another portion of the same sample. This second test is crucial in determining the accuracy of the initial result. If the slump value of the second test also falls outside the specified range, it can be concluded that the concrete does not meet the requirements for the project.
In other words, the second test serves as a confirmation test to ensure that the initial result is valid. If both tests yield a value outside the specified range, it means that the concrete does not have the necessary consistency and workability to meet the project’s requirements. Therefore, it is crucial to conduct these check tests to ensure the quality and integrity of the concrete used in construction projects.
What does high slump value indicate?
When making concrete, the amount of water added in relation to the amount of cement used is a crucial factor. The ratio of water to cement affects the workability of the mixture, which is often measured by the slump value. The slump value is a measure of the consistency of the concrete, and a high slump value means that the mixture is more fluid and easier to pour.
However, a high slump value can also indicate that the water-cement ratio is high. This means that more water has been added to the mixture than is necessary for the chemical reaction to occur between the cement and water. This excess water can weaken the concrete, reducing both its strength and durability.
In other words, while a high slump value may make it easier to work with concrete, it can come at a cost to its overall quality. It’s important to carefully consider the water-cement ratio and slump value when mixing concrete to ensure that the final product is strong and durable.