Understanding the Phenomenon: The Comprehensive Guide to Bulking of Sand

Bulking of sand, a key phenomenon in construction, involves the increase in sand volume due to moisture content, significantly impacting sustainability and engineering practices. Recognizing the extent of volume change is crucial, especially since the finer the sand, the greater the volume increase, influencing concrete quality and the overall construction process.

Understanding and managing bulking of sand ensures accuracy in construction projects, from concrete mixing to backfilling, making testing and remedial measures critical components of engineering and laboratory tests. The comprehensive guide aims to delve into the causes, significance, and practical approaches to manage bulking, enhancing the sustainability and efficacy of construction efforts.

What Causes Bulking of Sand?

Bulking of sand, a critical factor in construction and engineering, is influenced by several key elements:

• Moisture Content: The presence of moisture forms thin films around sand particles. This leads to the particles exerting pressure and moving away from each other, thus increasing the volume of the sand.

• Fineness of the Aggregates: The degree of fineness of the sand particles plays a significant role. Finer sand particles result in a greater increase in volume when exposed to moisture compared to coarser sand.

• Percentage Moisture Content: The effect of bulking increases with the moisture content up to a certain limit. Beyond this limit, particularly when the sand becomes fully saturated, the bulking effect diminishes and can even reverse, leading to a decrease in volume.

It's noteworthy that the bulking of sand is most pronounced when the moisture content is around 4-6%. At this level, a mere 5 to 8 percent increase in moisture can cause the volume of dry sand to expand by 20 to 40 percent. This phenomenon is particularly significant in finer sand, which exhibits a more pronounced increase in volume compared to coarse sand.

Bulking of sand can be expressed in a graphical way as shown in Figure

Graph showing the percentage increase in volume to the percentage by weight of moisture

WHY TO DETERMINE PERCENTAGE OF BULKING?

When constructing concrete mixes, if sand is measured by volume without considering bulking, the damp sand will take up a much higher volume compared to the dry sand, resulting in a richer mix. This phenomenon will result in a reduced amount of concrete per bag of cement.

As an illustration, in the scenario where the bulking of sand is 10% and the mix ratio is 1:2:4, the resulting volume of sand utilized will be 1.1 * 2 = 2.2, as opposed to the conventional value of 2 per unit volume of cement.

If this adjustment is not implemented, the actual dry sand content in the concrete will be (1/1.1) *2= 1.82 per unit volume of cement, instead of 2. The resulting mix percentage would be 1:1.82:4, instead than 1:2:4. This suggests a decrease in the manufacturing of concrete. Additionally, it is possible for segregation, honeycombing, and a decrease in the production of concrete to occur.

Significance of Bulking of Sand in Construction

The significance of understanding and managing the bulking of sand in construction cannot be overstated, as it directly impacts the accuracy of volumetric measurements and the quality of concrete mixes. Here are key points to consider:

1.     Cost Implications and Concrete Quality:

o    Accurate volumetric proportioning requires accounting for the bulking of sand to avoid unnecessary increases in the cost of concrete and to ensure the desired quality of the mix.

o    Failure to consider the bulking effect can lead to an incorrect quantity of fine aggregate in the mix, complicating concrete placement or pumping.

2.     Enhancing Concrete Properties:

o    Bulking of sand plays a crucial role in reducing pores within the sand and compacting it, which in turn maintains workability, facilitates the placing of concrete, minimizes segregation, and contributes to a uniform concrete structure.

o    In the context of concrete preparation, sand is instrumental in reducing segregation and filling out the pores between cement and coarse aggregates, thereby enhancing the overall integrity of the concrete.

3.     Practical Considerations in Construction:

o    When calculating sand quantities for construction projects, it's essential to include bulking values to ensure the total volume of the concrete mixture meets expectations, thus maintaining the quality and strength of the concrete.

o    Ignoring the bulking effect can lead to inconsistencies in concrete mixes, wastage of materials, incorrect proportions, and ultimately, compromised strength and durability of the concrete structure.

o    In mortars, while bulking of sand does not directly affect strength, it leads to overconsumption of cement, thereby increasing construction costs.

Understanding and accurately accounting for the bulking of sand are critical for achieving precise material proportions, which is fundamental to ensuring the quality, reliability, and cost-effectiveness of construction projects.

Testing for Bulking of Sand: Procedures and Importance

Testing for bulking of sand involves both field and laboratory methods to measure the extent of volume increase due to moisture. These procedures are crucial for accurate construction and concrete mixing practices.

Field Method:

·        Step 1: Fill a container with a known volume (A) of moist sand, using the manner as filling the mixer hopper.

·            Step 2: The height of sand in the container is measured.

·        Step 3: The sand is then taken out the container carefully, ensuring no sand is wasted during this transaction.

·        Step 4: The sand is thereafter subjected to either a drying process and subsequently reintroduced into the gauge box, or alternatively, the container is filled with water and the moist sand is added to displace the water. Whichever technique is selected, the new depth of aggregate in the container gives the un-bulked volume (B).

·        Step 5: Then percentage bulking expressed as a

percentage of the dry volume = [(A-B)/B] *100

The Laboratory Test

·        Sufficient quantity of oven dried sand is filled in graduated cylinder up to a certain fixed mark. The sand is emptied in a container and the weight of dry sand is determined 1% water is added in the sand mixed thoroughly and filled in the graduated cylinder. The volume of sand will be found to increase. The process is repeated by increasing the percentage of water in steps of 1% each time till a decrease in the volume of sand is observed. Still this addition of water is continued till the volume of sand comes back to the original volume (fully saturated).

·        A graph is plotted between moisture content and percentage increase in volume. The percentage bulking is given by,

·        B = [(bulked volume - dry volume)/dry volume] *100

Necessary Adjustment for Bulking of Fine Aggregate

Method I

1.     Initial Setup: Sufficient quantity of the sand is put loosely into a container until it is about two third full.

2.     Preparation: The sand is levelled off and a steel rule is pushed vertically down through the sand at the middle to the bottom and the height is measured (say h mm).

3.     Measurement: The sand is transferred into another container. The first container is half filled with water and about half the sand is put back and rammed with a steel rod (about 6 mm in diameter) so that its volume is reduced to a minimum. Then the remainder of the sand is added and rammed in the same way. The depth is measured at the middle with the steel rule (say h’ mm).

4.     Calculation: The percentage of bulking of the sand due to moisture is calculated from the formula:

Percentage bulking = [(h/h^’)-1] *100

5.     Repetition: For accuracy, repeat the process twice and calculate the average value.

Method II

• Apparatus Needed: 250ml measuring cylinder up to the 200 ml mark, weighing balance, fine aggregate.

• Procedure: the cylinder is filled with water and the sand is stirred well. The sand surface will be found to be below its original level. Supposing the surface is at the mark y ml.
•  Calculations: The percentage bulking of sand is calculated as:

Percentage bulking = [(200/y)-1] *100

Importance of Testing:

• Accuracy in Volume: Testing for bulking ensures that the volume of sand used in construction projects is accurate, preventing errors in concrete mixing ratios.

• Quality of Concrete: By accounting for the bulking factor, the quality and strength of the concrete are maintained, avoiding potential structural issues.

• Cost Efficiency: Properly accounting for bulking can prevent the overuse of cement, saving on construction costs.

These testing procedures underscore the significance of bulking in construction, emphasizing the need for accurate measurement and consideration in project planning and execution.

Impact of Bulking on Concrete Preparation and Construction

The impact of bulking of sand on concrete preparation and construction is multifaceted, influencing both the physical properties of concrete and the efficiency of construction processes.

• Volume and Quality of Concrete:

• If unaccounted, bulking can lead to a shortfall in the total volume of concrete, adversely affecting its quality.

• Proper consideration of bulking ensures the maintenance of the desired concrete properties by reducing pores and enhancing compactness, workability, and uniformity.

• Material Proportioning and Cost Efficiency:

• Bulking increases sand volume by 20 to 40 percent with a 5 to 8 percent moisture increase, necessitating volume correction during batching for accurate ingredient proportions.

• Accurate proportioning based on corrected sand volume optimizes material usage, reducing costs and ensuring the concrete mix meets specific standards and guidelines.

• Water-Cement Ratio and Workability:

• The bulking effect requires more water to achieve specific levels of workability, making the correct water-cement ratio crucial for consistent workability and ease of concrete placement.

• By accounting for bulking, the density, strength, and durability of construction materials such as concrete are increased.

In summary, understanding and accounting for the bulking of sand are essential for ensuring the quality, reliability, and cost-effectiveness of concrete in construction projects.

Remedial Measures to Counteract Bulking of Sand

To effectively counteract the bulking of sand and ensure the accuracy and sustainability of construction projects, several remedial measures can be employed:

• Preventive Storage and Handling:

• Store sand in dry conditions to minimize moisture absorption, thereby reducing the potential for bulking.

• Slightly pre-wet sand before use to control on-site water absorption and manage the bulking phenomenon more effectively.

• Optimized Concrete Preparation:

• Gradually add water during concrete preparation to avoid excess moisture, which can exacerbate bulking.

• Use bulk sand with less sand and water while mixing to maintain the correct proportions and prevent over-saturation.

• In case of excess water or deficiency, adjust by adding more sand to achieve the desired consistency and moisture level.

• Quality Control and Testing:

• Conduct thorough sand quality checks focusing on particle size and shape for better compaction and stability, and assess for the presence of silt and clay by feeling the sand. If it sticks to the hand, there is clay present, which requires action.

• Implement site testing methods such as physical inspection for sand type, clay, and silt presence. For silt detection, use a measuring jar with water and salt; after 24 hours, the fine particles will settle at the top, indicating the need for washing the sand before use.

These measures, when properly implemented, can significantly mitigate the effects of bulking, ensuring that construction projects proceed with accuracy, efficiency, and reduced costs.

FAQs

What exactly is the bulking of sand and why is it important?

The bulking of sand, also known as the increase in volume of fine aggregate, occurs when moisture content rises, creating thin films around the particles. This causes the particles to exert pressure on one another and move apart, resulting in an increased volume. This phenomenon is significant as it affects the sand's behavior and properties in various applications, including construction.

Can you describe the process that occurs during the bulking of sand?

During the bulking of sand, a layer of water forms around each particle, pushing them apart and increasing the overall volume of the sand. This happens because moisture causes the particles to repel each other, leading to an expansion of the sand.

Why does sand bulk, and what impact does this have on making concrete?

Sand bulks because moisture creates a film around the particles, causing them to separate and the volume to increase. In concrete manufacturing, the bulking phenomenon can influence the mix proportions and the final strength of the concrete. The volume increase continues with additional moisture until a certain point, after which the volume starts to decrease as particles begin to pack closer together.

What steps should be taken to handle sand bulking correctly?

When dealing with bulking of sand, it is crucial to mix water and sand grains thoroughly and uniformly. The sand sample should not be compressed while being placed in the measuring jar, and it must be poured slowly and gradually from the top to avoid errors. Accurate measurement of the increase in sand volume due to bulking is essential for proper handling and application in construction projects.

Common Misconceptions About Bulking of Sand

One common misconception is that bulking only occurs in wet conditions. In reality, even dry sand can experience bulking due to capillary forces. Dispelling these myths is essential for accurate project planning.

Are there any alternatives to sand with lower bulking properties?

Alternative materials like crushed stone or manufactured sand have lower bulking properties. Evaluating the suitability of these materials based on project requirements is essential for optimal construction outcomes.