Concrete is a vital component in construction, and ensuring the right consistency is crucial. However, managing variations in concrete slump on construction sites can be a challenge. In this article, we’ll explore why concrete slump adjustment is necessary, delve into the factors causing variations, and discuss practical solutions with the help of a video demonstration.
Concrete slump, the measure of its consistency, can vary due to factors like aggregate moisture, temperature, mixing process, and delivery time. When the slump falls outside the specified range, adjustments are necessary. However, this process is not only costly but also time-consuming, with no guaranteed effectiveness.
Concrete batching tolerances play a significant role in slump variations. Table-1 illustrates typical batching tolerances, showing their impact on the batch weight of a 24 MPa concrete mix in Table-2. Uniform mixing helps minimize the influence of batching tolerances on concrete slump.
| Material Type | Tolerances (%) | Effect on 25 MPa Concrete Mix Weight |
|---|---|---|
| Cement | ±1 | 2.96 kg/m3 |
| Aggregate | ±2 | 35.5 kg/m3 |
| Water | ±1 | 1.65 liter/m3 |
| Admixture | ±3 | – |
The moisture content of aggregates significantly affects concrete slump. As moisture varies throughout the day, controlling aggregate moisture is challenging, leading to unavoidable slump variations.
Higher concrete temperatures result in decreased slump. A 12°C increase necessitates 3.78 liters of extra water to maintain the same slump value.
Proper mixing, with 70-100 revolutions, is essential to achieve a uniform batch. The drum volume should not exceed 63%, and an additional 30 revolutions help prevent segregation during transportation.
Maintaining the required slump for 30 minutes is crucial. Prolonged transportation and waiting can reduce slump, making contractors accountable.
Adding water to low-slump concrete is a common solution. It should be done within 15 minutes and not exceed 2 liters/m3 to prevent adverse effects on properties.
Superplasticizer is another option to increase slump without affecting concrete properties when the maximum w/c ratio is used.
For high-slump concrete, adding cement is effective in reducing slump.
Increasing revolutions can decrease slump, but its effectiveness is questionable and time-consuming.
Reducing air content with an air detainer decreases slump. A 1% reduction in air content leads to a 1.27 cm slump decrease.
While silica fume can reduce slump, its impact on concrete color may not be acceptable to clients.
In conclusion, understanding and managing concrete slump variations is crucial for a successful construction process. Watch the video tutorial for practical insights into these adjustment methods.