What are aggregates? What are the basic properties of the aggregates used in concrete?

Civil Engineering - Interview Questions

Aggregate : Aggregate is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined materials in the world. Aggregates are a component of composite materials such as concrete and asphalt concrete; the aggregate serves as reinforcement to add strength to the overall composite material.

Basic properties of aggregates used in concrete :

1. Composition - Aggregates consisting of materials that can react with alkalies in cement and cause excessive expansion, cracking and deterioration of concrete mix should never be used. Therefore it is required to test aggregates to know whether there is presence of any such constituents in aggregate or not.

2. Size and shape - The size and shape of the aggregate particles greatly influence the quantity of cement required in concrete mix and hence ultimately economy of concrete. For the preparation of economical concrete mix on should use largest coarse aggregates feasible for the structure. IS-456 suggests following recommendation to decide the maximum size of coarse aggregate to be used in P.C.C & R.C.C mix.

Maximum size of aggregate should be less than-

One-fourth of the minimum dimension of the concrete member.

One-fifth of the minimum dimension of the reinforced concrete member.

The minimum clear spacing between reinforced bars or 5 mm less than the minimum cover between the reinforced bars and form, whichever is smaller for heavily reinforced concrete members such as the ribs of the main bars.

Remember that the size & shape of aggregate particles influence the properties of freshly mixed concrete more as compared to those of hardened concrete.

3. Surface texture - The development of hard bond strength between aggregate particles and cement paste depends upon the surface texture, surface roughness and surface porosity of the aggregate particles.

If the surface is rough but porous, maximum bond strength develops. In porous surface aggregates, the bond strength increases due to setting of cement paste in the pores.

4. Specific gravity - The ratio of weight of oven dried aggregates maintained for 24 hours at a temperature of 100 to 1100C, to the weight of equal volume of water displaced by saturated dry surface aggregate is known as specific gravity of aggregates.

Specific gravities are primarily of two types.

Apparent specific gravity

Bulk specific gravity

Specific gravity is a mean to decide the suitability of the aggregate. Low specific gravity generally indicates porous, weak and absorptive materials, whereas high specific gravity indicates materials of good quality. Specific gravity of major aggregates falls within the range of 2.6 to 2.9.

Specific gravity values are also used while designing concrete mix.

5. Bulk density- It is defined as the weight of the aggregate required to fill a container of unit volume. It is generally expressed in kg/litre.

Bulk density of aggregates depends upon the following 3 factors.

Degree of compaction

Grading of aggregates

Shape of aggregate particles

6. Voids - The empty spaces between the aggregate particles are known as voids. The volume of void equals the difference between the gross volume of the aggregate mass and the volume occupied by the particles alone.

7. Porosity and absorption - The minute holes formed in rocks during solidification of the molten magma, due to air bubbles, are known as pores. Rocks containing pores are called porous rocks.

Water absorption may be defined as the difference between the weight of very dry aggregates and the weight of the saturated aggregates with surface dry conditions.

Depending upon the amount of moisture content in aggregates, it can exist in any of the 4 conditions.

Very dry aggregate ( having no moisture)

Dry aggregate (contain some moisture in its pores)

Saturated surface dry aggregate (pores completely filled with moisture but no moisture on surface)

Moist or wet aggregates (pores are filled with moisture and also having moisture on surface)

8. Bulking of sand - It can be defined as in increase in the bulk volume of the quantity of sand (i.e. fine aggregate) in a moist condition over the volume of the same quantity of dry or completely saturated sand. The ratio of the volume of moist sand due to the volume of sand when dry, is called bulking factor.

Fine sands bulk more than coarse sand

When water is added to dry and loose sand, a thin film of water is formed around the sand particles. Interlocking of air in between the sand particles and the film of water tends to push the particles apart due to surface tension and thus increase the volume. But in case of fully saturated sand the water films are broken and the volume becomes equal to that of dry sand.

9. Fineness modulus - Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 micron and dividing this sum by 100.

Fineness modulus is generally used to get an idea of how coarse or fine the aggregate is. More fineness modulus value indicates that the aggregate is coarser and small value of fineness modulus indicates that the aggregate is finer.

10. Specific surface - The surface area per unit weight of the material is termed as specific surface. This is an indirect measure of the aggregate grading. Specific surface increases with the reduction in the size of aggregate particle. The specific surface area of the fine aggregate is very much more than that of coarse aggregate.

11. Crushing value - The aggregates crushing value gives a relative measure of resistance of an aggregate to crushing under gradually applied compressive load. The aggregate crushing strength value is a useful factor to know the behavior of aggregates when subjected to compressive loads.

12. Impact value - The aggregate impact value gives a relative measure of the resistance of an aggregate to sudden shock or impact. The impact value of an aggregate is sometime used as an alternative to its crushing value.

13. Abrasion value - The abrasion value gives a relative measure of resistance of an aggregate to wear when it is rotated in a cylinder along with some abrasive charge.

Recently Updated Interview Questions in Civil Engineering

What are the advantages and disadvantages of using concrete as a building material?

Concrete is a widely used building material with several advantages and disadvantages. Its popularity stems from its versatility, strength, and durability. However, like any material, it has limitations and potential drawbacks. Here are the advantages and disadvantages of using concrete as a building material:

Advantages :

* Strength and Durability : Concrete is renowned for its high compressive strength, making it suitable for supporting heavy loads and resisting structural stresses. It can withstand a wide range of environmental conditions, including fire, wind, and moisture, without significant deterioration.

* Versatility : Concrete can be molded into various shapes and sizes to accommodate diverse architectural designs. It can be used for walls, slabs, columns, beams, and more, allowing for flexible construction possibilities.

* Longevity : Properly designed and maintained concrete structures can have a long service life, often exceeding 50 years or more. This longevity reduces the need for frequent repairs or replacements.

* Fire Resistance : Concrete is inherently fire-resistant and does not combust. It can provide valuable fire protection in buildings, helping to contain fires and limit their spread.

* Thermal Mass : Concrete's thermal mass properties allow it to absorb, store, and slowly release heat, contributing to energy efficiency in buildings. This can help regulate indoor temperatures and reduce heating and cooling costs.

* Sound Insulation : Concrete's density makes it an effective sound barrier, reducing the transmission of noise between rooms or from the exterior. It is commonly used in multi-unit residential and commercial buildings for this reason.

* Low Maintenance : Compared to some other materials, concrete requires relatively low maintenance. It does not rot or decay, and minor surface repairs are typically manageable.

Disadvantages :

* Weight : Concrete is heavy, which can pose challenges in certain applications. The weight of concrete structures may require more robust foundations and structural support systems, adding to construction costs.

* Cracking : Concrete is susceptible to cracking, especially if not properly designed, mixed, or cured. These cracks can compromise the structural integrity and aesthetics of a building. Control joints and reinforcement are used to mitigate cracking.

* Environmental Impact : The production of cement, a key component of concrete, generates a significant amount of carbon dioxide (CO2) emissions, contributing to environmental concerns. Efforts are being made to reduce the carbon footprint of concrete through alternative materials and manufacturing processes.

* Cost : High-quality concrete and skilled labor for formwork, pouring, and finishing can be expensive, particularly in comparison to some alternative building materials.

* Limited Insulation : Uninsulated concrete walls can have poor thermal insulation properties, which may necessitate additional insulation layers to meet modern energy efficiency standards.

* Curing Time : Concrete typically requires time to cure and reach its full strength. This can affect construction schedules, leading to longer project timelines.

* Aesthetics : Concrete is often considered less visually appealing than some other materials, although decorative finishes and treatments can enhance its appearance.

Metal and Steel Structures

Q1: What are the main types of steel used in construction?

A1: The main types of steel used in construction are carbon steel, alloy steel, stainless steel, and galvanized steel.

Q2: What are the advantages of using steel in building construction?

A2: Advantages of using steel in building construction include high strength-to-weight ratio, durability, speed of construction, adaptability, recyclability, and resistance to fire, pests, and rot.

Q3: What is the difference between hot-rolled and cold-formed steel sections?

A3: Hot-rolled steel sections are produced by heating steel to high temperatures and rolling it into the desired shape, while cold-formed steel sections are made by bending or rolling steel at room temperature without the need for heating.

Q4: What are the common types of connections used in steel structures?

A4: Common types of connections used in steel structures include bolted connections, welded connections, and riveted connections.

Q5: What factors affect the design of steel structures?

A5: Factors affecting the design of steel structures include the type and magnitude of loads, the material properties, the structural system and member sizes, the type of connections, the corrosion protection methods, and the applicable building codes and standards.