Proper management and caution are essential in adjusting the slump of concrete during the concreting process. One reason for variation in concrete slump from one transit mixer to another is the moisture content of the aggregate, as well as factors such as concrete temperature, mixing process, and the time between delivery, waiting, and unloading.
In some cases, disputes may arise between parties involved in construction when there are variations in concrete slump at the site. Therefore, it is crucial to clarify the reasons for the need for concrete slump adjustment and the appropriate methods for doing so in the field.
If the concrete slump falls outside the specified range, adjustments are necessary. For low slump values, introducing water or superplasticizer to the concrete in the mixer can address the problem.
However, if the slump value is high, other measures must be taken to adjust the slump. These include adding cement to fresh concrete in the transit mixer, additional revolutions of the concrete in the mixer, or introducing silica fume. It is important to note that slump adjustment is both time-consuming and expensive, and may not always be effective.
Why is Concrete Slump Adjustment Needed?
The laboratory mix design may require adjustments in response to variations in field conditions. Several factors can impact the slump of concrete, including poor quality control, material variations, and other variables such as batching tolerances, aggregate moisture content, concrete temperature, and the mixing process.
Concrete slump refers to the measure of the consistency and workability of concrete. It can be affected by several factors, such as the moisture content of the aggregates used in the mix, the temperature of the concrete, and the process used to mix the concrete. Additionally, poor quality control and variations in materials can also have a significant impact on the slump of the concrete.
Given these considerations, it’s important to monitor and adjust the laboratory mix design in response to changes in field conditions. This may involve modifying the composition of the concrete mix or adjusting the mixing process to achieve the desired slump. Ultimately, attention to these factors can help ensure that the concrete meets the necessary specifications for strength, durability, and workability.
1. Batching Tolerance
The variation in concrete batching tolerances can have an impact on the slump of the concrete. These tolerances refer to the acceptable percentage of deviations allowed for each mix ingredient in individual batches. The American Concrete Institute’s standard document ACI 304R-00 provides a set of typical batching tolerances, which are listed in Table-1.
Table-2 illustrates how each tolerance affects the batch weight of a typical concrete mix with a strength of 24 MPa. However, the impact of batching tolerances on concrete slump can be reduced to 1.27 cm by ensuring that the ingredients are mixed uniformly.
It is important to note that there is a tolerance of ±3% (4.95 liters/m3) for the total mixed water, which includes added water, residual mixer wash water, ice, and free moisture from aggregate. If this 3% tolerance for total water is met, the water-cement ratio will vary by 0.02, resulting in an increase in concrete slump by 2.54 cm.
To account for the possibility of adding a small amount of water on-site, most concrete batching plants add a limited quantity of water to the mixture. In some cases, the concrete batching plant may add 5% less water to the concrete mix. This approach helps to ensure that the concrete does not become too wet, which can have a negative impact on its strength and durability.
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 slump of concrete can be significantly impacted by the aggregate moisture content. Typically, the moisture content of the aggregate is highest in the morning and decreases as the day progresses due to the heat from sunlight. It is important to note that it is not possible to fully control the measurement of moisture in the aggregate. Consequently, variations in concrete slump cannot be entirely eliminated.
3. Concrete Temperature
Concrete is a widely used construction material due to its strength, durability, and versatility. However, concrete properties can be affected by various factors, including temperature. It has been observed that an increase in concrete temperature can lead to a decrease in the slump of the concrete.
The slump of concrete refers to its consistency or fluidity, which is measured by the distance that the concrete slumps or settles after being poured into a standard cone. The slump value of concrete is an important parameter as it affects the workability of concrete and its ability to be placed and compacted in the desired shape.
According to studies, an increase in concrete temperature by 12°C can cause a decrease in the slump of concrete. This means that the concrete becomes less workable and more difficult to handle and shape. To maintain the same slump value of the concrete at a higher temperature, it is necessary to add extra water.
In fact, it has been estimated that to maintain the same slump value of concrete at a higher temperature, an additional 3.78 liters of water is required. This extra water is necessary to compensate for the decrease in workability caused by the increase in temperature. It is important to note that adding too much water can have adverse effects on the strength and durability of the concrete, so it is essential to carefully control the amount of water added.
4. Mixing Concrete Ingredients
Proper mixing is a crucial step in producing uniform and consistent concrete batches. To achieve this, a recommended 70-100 revolutions of mixing is necessary. This process ensures that the ingredients are thoroughly mixed and the final product has a uniform texture and strength.
It is also important to consider the volume of mixed concrete in relation to the drum volume. The American Society for Testing and Materials (ASTM) specifies that the volume of mixed concrete should not exceed 63% of the drum volume. Going beyond this limit can result in an uneven mixture, compromising the strength and quality of the final product.
After transportation, it is recommended to provide an additional 30 revolutions at a normal mixing speed. This extra step helps to decrease segregation that may have occurred during the transportation process. Segregation can lead to an uneven distribution of the components, resulting in a lower quality final product. By taking this additional step, the concrete batch can be further homogenized, ensuring that the mixture is consistent and of high quality.
5. Delivery, Waiting, and Unloading Time
According to ASTM C94, it is necessary to maintain the required slump of concrete for 30 minutes, starting from the time it arrives at the construction site or after the initial adjustment of slump, whichever is later. However, this requirement is not applicable for the first and last 1/4 m3 of concrete discharge.
It is important for the contractor to be mindful of the potential effects of prolonged transportation, waiting, and unloading on the concrete slump. These factors have the potential to reduce the concrete slump, which can have a negative impact on the quality of the finished product.
Furthermore, it is the responsibility of the contractor to ensure that the concrete slump does not decrease due to prolonged delivery, waiting, or unloading. If the slump does decrease as a result of these factors, the contractor will be held accountable for any negative effects on the final product.
How to Adjust Concrete Slump?
1. Increase Concrete Slump
1.1 Water Addition
Concrete slump is a critical factor in ensuring that the concrete mix has the appropriate consistency for the intended application. ASTM C94 allows for the addition of water to increase the slump if it falls below the minimum specified range upon arrival on-site. However, it is important to note that the maximum acceptable water-cement ratio must not be exceeded.
When adding water to increase the slump, it is crucial to add it to the full load of concrete and avoid adding it to the middle or end of the batch. The water should be added through the head section of the drum or both head and discharge sections of the drum. It is not recommended to use a hose to spray additional water on the concrete.
After adding water, it is essential to provide 30 extra revolutions at the normal mixing speed to ensure that a homogenous mixture is attained. According to ASTM C64, 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 should be finalized within 15 minutes.
The amount of water added to the concrete significantly affects its properties. For example, adding approximately 2 liters/m3 of water increases the concrete slump by 10 mm, but this may vary depending on other conditions such as temperature and air content. It is important to note that introducing nearly 3.8 liters of water leads to a slump increase of 2.54 cm, a reduction of concrete strength by 1 MPa to 1.38 MPa, a waste of a quarter of a cement bag, and an increased shrinkage by 10%. Therefore, it is crucial to carefully consider the amount of water added to ensure that the desired properties of the concrete are maintained.
1.2 Addition of Superplasticizer
Adding a superplasticizer to a concrete mix is a popular method to increase its slump. This technique enables the concrete to become more fluid, making it easier to pour and work with, without affecting its properties. Superplasticizers are commonly used when the maximum allowable water-to-cement ratio (w/c) has already been used in the concrete mix.
By adding a superplasticizer, the concrete’s pumpability is improved without altering the w/c ratio. This is particularly useful in scenarios where the concrete needs to be transported over long distances or to higher elevations. Additionally, superplasticizers can help reduce the amount of water required in the mix, which can lead to improved concrete strength and durability.
Overall, the use of a superplasticizer is an effective technique for enhancing the slump of a concrete mix without compromising its properties. This method is widely used in the construction industry, particularly in large-scale projects where pumpability is essential, and the maximum w/c ratio has already been reached.
2 Decrease Concrete Slump
2.1 Addition of Cement
When the slump of concrete exceeds the maximum acceptable range, it is important to bring it down. One way to achieve this is by adding cement to the mixture. This addition of cement can effectively reduce the slump of the concrete.
The slump of concrete refers to the measure of its consistency and workability. It is determined by the amount of water present in the mixture. When the slump is too high, the concrete becomes more fluid, making it difficult to handle and increasing the risk of segregation and bleeding.
To avoid such problems, it is crucial to maintain the slump within the acceptable range. In cases where the slump exceeds this limit, the addition of cement can help reduce it. Cement has a binding effect on the water present in the mixture, causing it to thicken and reducing the slump.
It is worth noting that while adding cement can effectively reduce the slump of concrete, it can also affect the other properties of the mixture. Therefore, it is important to carefully consider the amount of cement to be added to achieve the desired consistency without compromising on other essential properties of the concrete.
2.2 Additional Revolution of Mixer Drum
The effectiveness of reducing the slump of concrete through increased heat and grinding action of additional revolutions is questionable and the process requires a long time.
The slump of concrete refers to the degree of consistency or fluidity of freshly mixed concrete. It is an important property that affects the workability and strength of concrete. However, if the slump is too high, it can lead to segregation and bleeding of the concrete, which can compromise its quality and durability.
To reduce the slump of concrete, some construction workers may resort to increasing the heat and grinding action of additional revolutions. However, this approach has its limitations. The effectiveness of this method is uncertain, and it may require a long time to achieve the desired result. Moreover, this method may not be suitable for all types of concrete mixes, and it may have adverse effects on the overall quality and durability of the concrete.
In summary, while increasing the heat and grinding action of additional revolutions may seem like a quick fix to reduce the slump of concrete, its effectiveness is questionable and may require a long time. It is important to carefully consider the potential drawbacks and limitations of this approach before implementing it in a construction project.
2.3 Use of Air Detainer
An air detainer is a substance that can be added to concrete in order to decrease the amount of air present in it, which in turn can lead to a reduction in its slump. The relationship between the reduction in air content and slump is such that a decrease of 1% in air content can result in a decrease of 1.27 cm in slump. It is important to note that concrete with high air content is typically not desirable and may be rejected and sent back to the concrete batching plant for further processing. By using an air detainer, it is possible to ensure that the concrete being produced meets the desired specifications and is of high quality.
2.4 Addition of Silica Fume
Adding silica fume to concrete is a technique that can be used to decrease the slump of the material. However, it’s not always the most suitable option, as it has the potential to darken the color of the concrete. This may not be acceptable to the client, as it could affect the overall aesthetic of the finished product. Therefore, it’s important to carefully consider whether or not the addition of silica fume is appropriate for a given project, taking into account factors such as the client’s preferences and the desired appearance of the final result. While it may be an effective way to address certain issues with concrete, the potential impact on color should not be overlooked.
FAQs
What are the causes of variations of concrete slump from one truck to another?
Concrete slump, the consistency or fluidity of freshly mixed concrete, can vary from one truck to another due to several factors. One of the significant factors affecting the concrete slump is the moisture content of the aggregates used in the mixture. If the aggregate has a higher moisture content, it can affect the water-cement ratio and increase the slump of the concrete.
Another factor that affects the concrete slump is the temperature of the concrete mix. If the concrete mix is too hot or too cold, it can affect the chemical reactions that occur during the curing process and alter the slump of the concrete.
The mixing process also plays a crucial role in determining the concrete slump. If the mixing process is not carried out correctly or for the appropriate duration, it can affect the consistency of the concrete mix, leading to variations in the slump from one truck to another.
Moreover, the time of delivery, waiting, and unloading of the concrete can also impact its slump. During transportation and delivery, the concrete can experience changes in temperature, humidity, and other environmental factors that can affect its consistency. Waiting too long to unload the concrete can also cause the slump to decrease, resulting in a stiffer mixture.
In conclusion, concrete slump variations from one truck to another can be attributed to several factors, including aggregate moisture content, concrete temperature, mixing process, and the time of delivery, waiting, and unloading. It is essential to take these factors into account when mixing, transporting, and unloading concrete to ensure consistent and high-quality results.
What happens if the concrete slump is high?
When the slump value of concrete is high, it is more likely to experience issues with segregation during placement. This means that the heavier particles in the mixture tend to settle at the bottom, while the lighter particles rise to the top, creating an uneven distribution. This can result in weaker and less durable concrete.
Moreover, when additional water is added to the mixture to increase the slump value, it can have negative effects on the strength and durability of the concrete. The water can weaken the bonds between the cement and other materials, leading to a decrease in overall strength. In addition, excess water can cause the concrete to crack and deteriorate over time, reducing its longevity and durability. Therefore, it is important to carefully control the water-to-cement ratio to maintain the desired slump value without compromising the strength and durability of the concrete.
What happens if the concrete slump is low?
Concrete is a popular building material used in the construction industry for various purposes. However, placing low workable concrete can be a challenging task. This is because it is not easy to shape and mold it into the desired form. As a result, professionals often require additional measures to improve its workability.
One of the common methods used to improve the workability of low workable concrete is by adding water. Water acts as a lubricant, making it easier to manipulate the concrete mix and achieve the desired shape and form. However, adding too much water can weaken the concrete and affect its strength and durability.
Another method used to enhance the workability of low workable concrete is by adding superplasticizer. Superplasticizer is a chemical admixture that reduces the amount of water required to achieve the desired workability of the concrete. It works by dispersing the cement particles, allowing them to flow and fill the gaps in the mixture easily. This results in a more workable and easier-to-handle concrete mixture.
In conclusion, the placement of low workable concrete can be a challenging task, but with the right techniques, it can be made more manageable. The addition of water or superplasticizer can help improve the workability of the concrete, making it easier to shape and mold to the desired form. It is essential to use these methods carefully to ensure that the concrete’s strength and durability are not compromised.
How do contractors reduce the slump of concrete on job site?
Concrete slump is a term used to describe the consistency and workability of freshly mixed concrete. When concrete has a high slump, it is easier to work with and can be poured more easily. However, sometimes the slump can be too high, which can lead to problems during construction. To reduce the slump of concrete, there are several methods that can be used.
One way to reduce the slump of concrete is to add more cement to the mix. Cement is a binding agent that helps to hold the concrete together, and adding more of it can make the mixture thicker and less workable. This can be useful in situations where the concrete needs to be more rigid or have a lower water content.
Another method to reduce the slump of concrete is to increase the number of revolutions of the drum during the mixing process. This helps to ensure that the ingredients are more thoroughly mixed together, and can result in a more consistent mixture overall. By increasing the number of revolutions, the concrete can become thicker and less likely to slump.
Finally, adding silica fume to the concrete mixture can also help to reduce the slump. Silica fume is a byproduct of the production of silicon metal and is often used as a pozzolan in concrete. It can help to increase the strength and durability of concrete, while also reducing the slump and making it easier to work with. Silica fume can be added to the concrete mix in small amounts to achieve the desired consistency.
How does superplasticizer affect the slump of concrete?
Superplasticizers are an essential tool in the construction industry for transforming stiff, low slump concrete into a more flowing and easily placed material. By using superplasticizers, the properties of the concrete are not affected, and it becomes more workable, making it easier to finish. This results in faster construction times and increased strength of the final product.
Superplasticizers have several benefits, including the ability to conserve cement, which is a significant expense in the construction process. Additionally, these additives can reduce shrinkage and thermal cracking, which can lead to significant structural problems down the line.
Overall, superplasticizers are a valuable addition to any construction project, as they can improve workability, speed up finishing times, increase strength, conserve resources, and reduce the risk of structural issues. By transforming stiff, low slump concrete into a more flowing material, construction professionals can achieve better results and complete projects more efficiently.