The Science Behind Rock Fragmentation on Construction Engineering: Exploring Aggregate Properties and Explosives in the Aggregate Industry

1. Introduction

Any substance used to construct a structure is referred to as a building material. The construction industry employs a variety of materials for construction depending on their structural performance. Building materials used in construction are governed by regional and national standards. On the other hand, architects select building materials based on cost and attractiveness. Choosing the right material for a project means extending and improving its lifespan [1]. Steel, concrete, wood, stone, and brick/masonry are the current construction materials employed. Aggregates are the main component of them and are utilized mostly for concrete structures. Aggregates with different particle size and properties are required in different construction engineering practices [2].

Aggregates can be created artificially or naturally. Larger rock formations are typically mined for natural aggregates using an open excavation (quarry). Usually, mechanical crushing is used to reduce extracted rock to sizes that can be utilized. Aggregate that has been manufactured artificially is frequently a byproduct of other manufacturing industries [3]. The excavated rock that has been processed into a usable product, such as aggregates or metals, is used almost entirely in the construction of buildings, bridges, roads, and railways. The primary raw material for the construction industry is rock aggregates. In 2021, 1.5 billion tons of crushed stone valued at more than $19 billion was produced by an estimated 1,410 companies operating 3,440 quarries and 180 sales and (or) distribution yards in 50 States in USA [4]. Worldwide demand for aggregates is highest among other minerals and is estimated to be rising by 4.7% annually [5]. Figure 01 illustrates the worldwide demand for minerals in the year 2021.

Figure 01: Worldwide demand for minerals in 2021

2         Aggregate Properties

2.1        Physical properties

2.1.1       Composition

It must be considered to never utilize aggregates made of materials that can chemically react with cement's alkalis and cause excessive expansion, cracking, and other damage to the concrete mix. As a result, it is required to test aggregates to determine whether or not they contain any of those parts [9].

2.1.2       Size

The strength and workability of concrete in both its fresh and hardened states can be significantly impacted by the size of the aggregate. Both fine and coarse aggregate are comparable in this regard. The maximum size of coarse aggregate that can be utilized shouldn't be more than 5 mm less than the minimum clear space between reinforcements in RCC structures or more than 1/4th of the minimum thickness of the structural member [10].

Utilizing aggregates of the largest possible size reduces the amount of cement used as a binding agent, the amount of water needed, and the amount of shrinkage and creeping that occurs as new concrete dries. On the contrary, using a greater proportion of larger aggregate may make concrete less workable when it is applied. Sand should be used as the fine aggregate in conventional concreting and a coarse aggregate with a nominal size of 20 mm should be used for better soundness, compactness, band, and workability of concrete [11].

2.1.3       Shape

The workability of concrete in its fresh state as well as its durability, strength, and bonding in its hardened stage can all be impacted by the shape of the aggregate grains. Visual inspection, along with other characteristics and indices like the roundness index, sphericity, angularity number, flakiness index, elongation index, etc., are used to determine the shape of the aggregates. Round, irregular, angular, flaky, elongated, and flaky elongated are the several shapes that can be categorized. Each category of aggregate has varied yields and potential requirements.

Rounded aggregates - need less cement content because of their reduced specific surface area and low angularity number, which results in a constant W/C ratio [11].

Angular aggregates - most recommended shape for aggregates in the concrete manufacturing process is one with high angularity numbers and higher specific surface area accessible for proper bonding between cement paste and aggregates as well as superior interlocking properties. However, when this number rises, the workability of the concrete declines [12].

Flaky and elongated aggregates - having greater specific surface areas yet due to their shape, lack adequate interlocking characteristics. They are also less workable than angular aggregates since they have a larger angularity number [13].

2.1.4       Surface texture

The degree to which a particle of aggregate's surface is smooth or rough, polished or dull, is measured by the surface texture of the aggregate. The parent source rock's pore properties, hardness, and grain size all affect a crushed aggregate's surface texture. This can be decided through visual inspection. The water content of the concrete mix and its workability in its fresh condition can both be significantly influenced by the surface texture of the fine and coarse aggregate. Due to the existence of voids and pores, rough texture aggregates are known to make stronger bonding with cement paste than other textured aggregates. Conversely, glass-like smooth aggregates absorb less water than coarse aggregates, which may affect the water requirement as previously mentioned [14].

2.1.5       Specific gravity

This ratio provides the aggregates' apparent specific gravity since it accounts for the aggregates' volume of impermeable pores (not capillary voids). Compared to absolute specific gravity, apparent specific gravity is more beneficial. Most natural aggregates have a specific gravity between 2.6 and 2.9. In the case of lightweight aggregates, it can be below this range. The appropriateness and quantity of the aggregates for the specified volume of concrete can be determined using specific gravity [15].

Aggregates having a greater specific gravity are often compact, solid, and robust. They also have fewer holes and a lower water absorption rate, making them desirable aggregates. Aggregates having a lower specific gravity, on the other hand, may be porous, frail, and mostly water absorbing [16].

2.1.6       Bulk density

Bulk density can be applied to transform volume batches from weight batches. Additionally, it is influenced by the particle's shape and size distribution. Similar-sized grains or particles produce more voids and have lower bulk densities, whereas aggregates with a range of particle sizes have a lower void content and a greater bulk density. The void ratio under typical conditions can be calculated using the bulk density under saturated and surface dry conditions. The amount of cement paste used in a concrete mix is impacted by this void ratio [17]. Similar to how rounded and angular aggregates produce less void than elongated and flaky ones. This raises the needed cement content. When the mixture contains 35–40% fine aggregate by the total mass of the aggregate mix, the bulk density of the aggregate is at its highest.

2.1.7       Porosity, water absorption and moisture content

The bond between aggregate and cement paste, durability, chemical stability of the concrete, resistance to abrasion, resistance to freezing and thawing, and a specific gravity of the concrete and aggregate itself are all significantly influenced by the porosity, permeability, and absorption of the aggregate.

In most cases, pores develop through the formation of rocks or other natural processes like the generation of fossils and decomposition. This pore may be on the surface of the rock or enclosed within it, and its sizes and shapes may vary from rock to rock. It is discovered to vary between 0 and 50% of the aggregate volume.

The volume, pace, and size of these pores influence how much and how quickly water permeates the aggregates, which in turn influences the aggregate's capacity to absorb water. The initial moisture content has an impact on the amount of water needed to mix concrete [18].

2.2        Thermal properties

The main thermal attributes that might influence how well concrete performs are the coefficient of thermal expansion, specific heat, and thermal conductivity of the aggregate. It has been discovered that the durability of the concrete may be considerably impacted when exposed to a freezing and thawing environment if the coefficient of thermal expansion of the aggregates differs by more than 5.5 x 10^-6 per °C from that of cement paste [19]. The insulation performance of the concrete structure may be impacted by the specific heat and thermal conductivity [20].

Hazardous substances, organic debris, clay, and other fine materials have an impact on the bonding properties and render concrete fragile and unreliable. Concrete that contains salt minerals will become hydrophobic, absorbing moisture from the air and producing efflorescence. Washing the aggregates with fresh water will prevent this effect [18].

2.3        Chemical properties

2.3.1       Chemical composition of aggregates

Natural aggregates are composed of a variety of substances, including clay minerals, iron oxide, silica minerals, feldspars, micaceous minerals, carbonate minerals, sulfate minerals, and iron sulfide minerals. To be utilized as aggregates for concreting or other applications, all of these mineral aggregates should have all the qualities of a good aggregate as described above. All components that can reduce the stability, toughness, and durability of concrete must be removed.

2.3.2       Alkali - aggregate reaction

Alkali aggregate reaction, or AAR, is a reaction that begins with the attack of alkaline hydroxides, which are made from alkaline oxides (K₂O, Na₂O) present in the cement, on reactive siliceous minerals or other minerals in aggregate, forming alkali silicate gel with unlimited swelling ability in the presence of water [21]. The integrity, strength, and durability of concrete are compromised by this swelling of leftovers. Adopting a lower W/C ratio, special low alkali cement, a good surface sealer, and sound aggregate itself can help prevent it [22].

2.4        Mechanical Properties

2.4.1       Strength

Strength of the aggregates simply refers to the aggregate's compressive or crushing strength where Aggregate Crushing Value (ACV) serves as a representation. ACV, which is often calculated for coarse aggregates, is the quantifiable resistance offered by aggregates to the gradually applied compressive/crushing loads. For surfaces other than the wearing surface, the limitation value for ACV is equivalent to 45% and 30%, respectively [21].

2.4.2       Toughness

The ability of an aggregate to withstand an abruptly imposed stress or an impact load is referred to as its toughness. Aggregate impact value, a measurement of an aggregate's resistance to impact load, serves as a cue. The aggregate impact value is determined by dividing the amount of material that passes through the 2.36 mm sieve by the sample's total weight. For wearing surface and other surfaces, limiting impact value is 30% and 45% respectively [16].

2.4.3       Hardness

The aggregate's hardness specifies how resistant the aggregate is to wear, abrasion, and attrition. It is measured using the aggregate's abrasion value, which measures how resistant the aggregate is to wear and abrasion. It can be determined using a variety of tests, including the Los Angeles, Deval, and Dorry abrasion tests. The percentage of aggregate passing through the IS sieve of 1.7mm to the total weight of the sample is used to calculate the aggregate abrasion value. For the wearing surface and other surfaces, the aggregate abrasion value limitation value is 30% and 50%, respectively [23].

2.4.4       Bond strength

The compressive and flexural strength and durability of the concrete are significantly influenced by the bond strength between the aggregates and cement paste. In order to create a concrete structure that is strong and enduring, a proper stronger bond between the aggregate and cement paste is required. Therefore, a number of elements exist, have an impact on how cement paste and aggregate are bound together. These include the aggregate's shape and surface roughness, the presence of harmful materials, the composition of the aggregate, etc. Aggregates and cement paste adhere better to each other when they are more rugged [24].

Similarly, angular aggregates have a more defined region for improved interlocking and higher bonding. The presence of harmful material may render aggregate surfaces unsuitable for bond formation or negatively impact cement hydration. The tests used to evaluate the bonds' quality are complex and expensive. As a result, it is calculated by looking at the large fragments of broken concrete left over from the crushing value test.

3         Aggregate Classification

Aggregates can be divided into several categories according to different properties, such as size, source, and unit weight [6]. Widely used classification is based on Aggregate size and can be classified as coarse aggregates and fine aggregates [7].

Coarse aggregate: Coarse aggregates are those that are typically retained on a No. 4 (4.75 mm) sieve. The typical size range for coarse aggregate is 5 to 150 mm. The maximum size of coarse aggregate for typical concrete used for structural elements like beams and columns is around 25 mm. The maximum size for mass concrete used for dams or deep foundations is 150 mm. Figure 02 shows some examples of coarse aggregates.

Figure 02: Examples of coarse aggregates

Fine aggregate (sand): Fine aggregate is defined as material that passes through a No. 4 (4.75 mm) sieve but is primarily retained on a No. 200 (75 mm) screen. The most widely utilized fine aggregate is river sand. Crushed rock fines are another option for fine aggregate. However, compared to river sand, the finish of concrete with crushed rock fines is not good.

The workability, durability, strength, weight, and shrinkage of the concrete are all significantly influenced by the composition, shape, and size of the aggregate. In precast concrete countertop mixes, aggregate plays a particularly crucial role in determining how the cast surface will resemble [8]. The aggregate sizes along with their applications are summarized in Table 01.

                                          Table 01: Aggregate sizes with their applications

4         Fragmentation requirements in aggregate industry

4.1        Rock Fragmentation requirements for construction industry

In the construction industry, rock fragmentation is an essential process for breaking down large rocks and boulders into smaller, more manageable pieces that can be used for construction purposes. There are several requirements for rock fragmentation in the construction industry, including:

Size: The rock fragments must be of a size that is appropriate for the construction project. This will depend on the specific application, but generally, smaller pieces are more desirable for easier handling and placement.

Shape: The shape of the rock fragments is also important. Irregular shapes can make it difficult to lay the fragments properly and can result in gaps between the pieces.

Consistency: The fragments should be uniform in size and shape to ensure consistency in the construction process. This is important for achieving a uniform strength and stability in the finished product.

Safety: Safety is paramount during rock fragmentation, and all necessary precautions must be taken to protect workers and equipment. This may include the use of protective gear, safety barriers, and monitoring equipment.

Environmental impact: The process of rock fragmentation can have a significant impact on the environment, particularly in terms of noise, dust, and vibration. It is important to minimize these impacts through the use of appropriate equipment and techniques.

Overall, the requirements for rock fragmentation in the construction industry are focused on producing rock fragments that are safe, consistent, and suitable for the intended construction application.

4.2        Importance of degree of rock fragmentation on construction engineering requirements

The degree of rock fragmentation is an important factor that can significantly affect the construction engineering requirements. Here are some reasons:

Material handling: The degree of fragmentation determines the size of the rock fragments, and this affects the equipment needed to handle them. If the rock is not fragmented enough, it may require larger equipment, such as excavators or bulldozers, to handle the larger pieces. On the other hand, if the rock is fragmented too much, it may require smaller equipment, such as loaders or trucks, to handle the smaller fragments.

Placement and compaction: The degree of rock fragmentation also affects how the fragments can be placed and compacted. If the fragments are too large, they may not fit together well, leaving gaps that can weaken the structure. If the fragments are too small, they may not be able to be compacted effectively, which can also lead to reduced strength.

Structural stability: The degree of fragmentation can affect the structural stability of the finished construction. The fragments need to be of a consistent size and shape to ensure that they fit together well and create a stable structure. If the fragments are too large or irregularly shaped, they may not fit together tightly, which can weaken the structure.

Time and cost: The degree of fragmentation can also affect the time and cost of the construction project. If the rock is not fragmented enough, it may take longer to break it down and handle it, which can increase the time and cost of the project. On the other hand, if the rock is fragmented too much, it may require additional processing to achieve the desired size and shape, which can also increase time and cost.

In summary, the degree of rock fragmentation is an important factor that affects various aspects of construction engineering requirements, including material handling, placement and compaction, structural stability, and time and cost. Therefore, it is essential to carefully consider the degree of fragmentation needed for a specific construction project to ensure that the finished product meets the desired requirements. To fragment the in-situ rocks in to a desired size, explosive selection play a major role.

5         Explosives used in aggregate industry.

Explosives are a crucial tool used in the aggregate industry for breaking and excavating rock, as well as for preparing construction materials. The selection of the right type of explosive is essential to ensure the best possible outcome. This section will cover the different types of explosives used in the aggregate industry and their properties, advantages, and disadvantages, with references to scientific studies and industry reports.

The two main types of explosives used in the aggregate industry are high explosives and low explosives. Figure 03 shows a diagram of a reacting cartridge of low explosive. If the reaction is stopped when the cartridge has been partially consumed and the archived pressure profile is examined, it can be observed that there is a steady rise in pressure at the reaction until the maximum pressure is reached. Low explosives only produce gas pressure during the combustion process. A high explosive detonates and exhibits a totally different pressure profile (Figure 03).

Figure 03: Pressure profiles of Low and High explosives

High explosives include dynamite, ammonium nitrate/fuel oil (ANFO), and emulsion explosives, while low explosives include black powder and safety fuse.

5.1        High explosives

Dynamite is a common type of high explosive used in the industry, which is a mixture of nitroglycerin, sorbents (such as powdered shells or clay), and stabilizers. According to a study by L. G. Hill et al. (1994), dynamite is commonly used for breaking down hard rock and produces a rapid and powerful shock wave, making it an effective explosive for the aggregate industry. However, the study notes that dynamite is sensitive to shock and friction, making it dangerous to handle and transport [25].

ANFO is another popular high explosive used in the aggregate industry, which is a mixture of ammonium nitrate and fuel oil. ANFO is cheaper and safer to use than dynamite but is less powerful, making it suitable for breaking down softer rocks. According to an industry report by the Mine Safety and Health Administration (MSHA) (2010), ANFO is commonly used in the aggregate industry because it is economical and has a lower risk of accidental detonation than dynamite [26].

Emulsion explosives are a type of high explosive that are made by mixing an oxidizer and fuel with water (very least ammonium nitrate, water, oils and a surfactant), producing a gel-like substance. Emulsion explosives are safer to handle and more stable than dynamite, but they are less powerful. Y. Liu et al. (2019) found that emulsion explosives are effective for rock breaking in the aggregate industry, with less damage to the surrounding environment and structures [27].

5.2        Low explosives

Black powder is a low explosive used for small-scale blasting operations and is made from a mixture of potassium nitrate, sulfur, and charcoal. According to the MSHA (2010) report, black powder is still used in some small-scale mining and quarrying operations. Safety fuse is another type of low explosive used in industry, which is a cord-like material that burns at a set rate, allowing workers to time the detonation of the explosive [28].

The use of explosives in the aggregate industry must follow strict regulations and guidelines to ensure the safety of workers and the public. Proper training, handling, storage, transportation, and disposal of explosives are crucial for minimizing the risk of accidents and damage to the environment. According to an industry report by the National Stone, Sand & Gravel Association (NSSGA) (2019), the industry is committed to following best practices for the safe use of explosives, including training and certification of workers, proper storage and transportation of explosives, and following federal and state regulations for blasting operations.

In conclusion, the use of explosives in the aggregate industry is necessary for efficient and productive rock excavation and preparation of construction materials. The selection of the right type of explosive is critical, considering the nature of the rock and the safety of workers and the public. Therefore, the industry must follow strict regulations and guidelines to ensure the safe and efficient use of explosives.

6         Conclusion

This article delves into the crucial role of rock fragmentation in the aggregate industry and its profound impact on construction engineering requirements. It begins by introducing rock aggregates and highlighting their significance in construction projects. The properties of aggregates are then discussed, emphasizing their influence on the overall quality and performance of construction materials.

Next, the article delves into fragmentation requirements specific to the aggregate industry. The importance of meeting fragmentation requirements is emphasized, as it directly affects factors like concrete strength, compaction, stability, and overall project success. The article further examines the explosives used in the aggregate industry to facilitate rock fragmentation. It highlights the types of explosives commonly employed, focusing on their safety considerations, performance characteristics, and environmental impact. The responsible usage of explosives is emphasized, ensuring that quarrying operations maintain a balance between efficiency and environmental sustainability.

Throughout the article, the intricate relationship between rock fragmentation and construction engineering requirements is underscored. The degree of rock fragmentation significantly influences the workability, strength, and durability of construction materials derived from aggregates. Understanding the science behind aggregate properties enables engineers and industry professionals to optimize construction outcomes and meet project specifications.

By shedding light on the crucial link between rock fragmentation, aggregate properties, and construction engineering requirements, this article provides valuable insights for professionals in the aggregate industry, engineers, and anyone involved in construction projects. Understanding and optimizing rock fragmentation can lead to enhanced construction outcomes, improved resource utilization, and sustainable practices in the aggregate industry.

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