Adjusting the slump of concrete is a part of the concreting process, which the contractors should manage properly and cautiously. The concrete slump variation from one transit mixer to another is generally due to aggregate moisture content, concrete temperature, mixing process; and time of delivery, waiting, and unloading.
Sometimes, the concrete slump variations at site raises disputes between the parties involved in the construction process. Therefore, it is crucial to clarify the reason behind the need for concrete slump adjustment and the way it should be done in the field.
If the concrete slump falls outside the specified range, the slump needs to be adjusted. If the slump is low, introducing water or superplasticizer to the concrete in the mixer can solve the problem.
However, if the slump value is high, adding cement to fresh concrete in the transit mixer, extra revolutions of concrete in the mixer, or introducing silica fume can tackle the problem. The slump adjustment is both expensive and time-consuming, and it may not always be effective.
Why is Concrete Slump Adjustment Needed?
Variations in field conditions demand the need for changes in the laboratory mix design. Apart from poor quality control and material variations, other factors like batching tolerances, aggregate moisture content, concrete temperature, and mixing process affect the slump of concrete.
1. Batching Tolerance
Concrete batching tolerances can vary the slump of concrete. The batching tolerance is an acceptable percentage of variations for individual batching of each mix ingredient. ACI 304R-00 provides typical batching tolerance, which is presented in Table-1.
Table-2 shows how each tolerance influences the batch weight of a typical 24 MPa concrete mix. The influence of batching tolerances on concrete slump can be minimized to 1.27 cm through uniform mixing of concrete ingredients.
A tolerance for total mixed water (added water, residual mixer wash water, ice, and free moisture from aggregate) is ±3%, 4.95 liters/m3. If the 3% tolerance for total water is met, the water-cement ratio will vary by 0.02, and the slump of concrete increases by 2.54 cm.
That is why most concrete batching plants add a limited quantity of water to the mixture to make room for a slight addition of water in the field. Sometimes, the concrete batching plant adds 5% less water to the concrete mix.
Table-1: Typical Batching Tolerances
| Material type | Tolerances, % | Effect of each tolerance on weight of 25 MPa concrete mix |
| Cement | ±1 of required mass | 2.96 kg/m3 |
| Aggregate | ±2 | 35.5 kg/m3 |
| Water (by volume or weight) | ±1 | 1.65 liter/m3 |
| Admixture (by volume or weight) | ±3 | - |
2. Aggregate Moisture Content
The aggregate moisture content can significantly influence the slump of the concrete. The aggregate moisture content is maximum in the morning and decreases as the day progresses due to heat from sunlight. So, it is clear that the measurement of moisture from aggregate cannot be controlled, and hence variations in concrete slump cannot be entirely avoided.
3. Concrete Temperature
The increase in concrete temperature leads to a decrease in the slump of concrete. The rise of concrete temperature by 12°C requires 3.78 liter extra water to maintain the same slump value that concrete could have at a lower temperature.
4. Mixing Concrete Ingredients
Proper mixing of concrete is essential to achieve a uniform concrete batch. 70-100 revolutions for mixing can help achieve a consistent batch.
The volume of mixed concrete should not exceed 63% of drum volume; otherwise, a uniform concrete batch cannot be achieved, as per ASTM C94.
It is recommended to provide 30 additional revolutions at a normal mixing speed to decrease segregation that may have occurred during transportation.
5. Delivery, Waiting, and Unloading Time
The required slump should be maintained for 30 minutes, beginning from the time of concrete arrival to the construction site or after initial slump adjustment, whichever is later, as per ASTM C94. This requirement is not applied for the first and last 1/4 m3 of concrete discharge.
The contractor should be aware that prolonged transportation, waiting, and unloading can reduce the concrete slump. The contractor would be held accountable if the concrete slump is reduced due to prolonged delivery, waiting, and unloading.
How to Adjust Concrete Slump?
1. Increase Concrete Slump
1.1 Water Addition
If the slump is less than the minimum specified range when the concrete arrives on-site, water can be added to increase the concrete slump, as per ASTM C94. Nonetheless, the maximum acceptable water-cement ratio must not be exceeded.
The water should be added to the full load of concrete. Avoid adding water to the middle or the end of the concrete batch. Add water through the head section of the drum or both head and discharge sections of the drum.
The use of a hose to spray additional water on concrete is not recommended. After the water is added to the concrete batch, provide 30 extra revolutions at the normal mixing speed to ensure that a homogenous mixture is attained.
ASTM C64 also states that “water cannot be added to the mixer if the combined revolutions from a long transportation or waiting time exceed 270”. The process of adding water to concrete in the transit mixer should be finalized within 15 minutes.
It is observed that the addition of 2 liters/m3 of water increases the concrete slump by 10 mm. However, this is influenced by other conditions such as temperature and air content.
The addition of water changes the concrete properties. For instance, introducing nearly 3.8 liters of water leads to the increase of concrete slump by 2.54 cm, reduction of concrete strength by 1 MPa to 1.38 MPa, waste of a quarter of a cement bag, and increased shrinkage by 10%.
1.2 Addition of Superplasticizer
The addition of a superplasticizer is another method to increase concrete slump. Superplasticizer increases concrete slump without affecting the concrete properties.
That is why it becomes the only option when the maximum permissible w/c ratio has been used in the concrete mix. The addition of superplasticizer improves concrete pumpability without changing the w/c ratio.
2 Decrease Concrete Slump
2.1 Addition of Cement
When the slump of concrete is higher than the maximum acceptable range, it would be necessary to decrease it. The addition of cement can reduce the slump of concrete.
2.2 Additional Revolution of Mixer Drum
The slump of concrete can be reduced through increased heat and grinding action of additional revolutions, but its effectiveness is questionable and requires a long time.
2.3 Use of Air Detainer
An air detainer may be used to decrease air content and hence decrease the concrete slump. It is reported that a reduction of the air content by 1% leads to a decrease of slump by 1.27 cm. One should be aware that concrete with high air content is usually sent back to the concrete batching plant.
2.4 Addition of Silica Fume
The addition of silica fume is another way of reducing the slump of concrete, but it may not be the right choice since silica fume darkens concrete color, which may not be acceptable to the client.
FAQs
The variation of concrete slump from one truck to another is due to aggregate moisture content, concrete temperature, mixing process; and time of delivery, waiting, and, unloading.
Concrete with high slump value may suffer from segregation during placement. Additionally, the concrete may lose its strength and durability when the slump has been increased due to the accidental addition of water to the mixture.
The placement of low workable concrete is difficult and cannot be shaped easily. That is why the addition of water or superplasticizer may be required to improve concrete workability.
The slump of concrete can be reduced by the addition of cement, extra revolutions of drum, and the addition of silica fume.
Superplasticizers can transform stiff, low slump concrete to flowing, pouring, and easily placed concrete without affecting its properties. They can improve workability, speed up finishing, increase strength, conserve cement, and reduce shrinkage and thermal cracking.
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