Rounded coarse aggregate Rounded coarse aggregate have minimum surface area for given volume, hence they require least amount of cement paste.
- 0.1 Which coarse aggregates are best suited for concrete?
- 0.2 Which of the following coarse aggregate is generally used?
- 1 What is 20mm aggregate used for?
- 2 Which type of aggregate produces minimum void in concrete?
- 3 What type of aggregate is used for concrete?
Which type of aggregate has minimum voids?
Answer (Detailed Solution Below) – Option 1 : Rounded Free General Studies Chapter Test 1 15 Questions 15 Marks 27 Mins Explanation: Rounded Aggregate: (i) The aggregate with rounded shape has the minimum percentage of voids ranging from 32 to 33%, (ii) It gives minimum ratio of surface area to given volume and hence requires minimum water for lubrication.
Iii) It gives good workability for the given amount of water and hence needs less cement for a given water-cement ratio. (iv) The only disadvantage is that the interlocking between its particles is less and hence the development of bonds is poor. This is why rounded aggregate is not suitable for high-strength concrete and for pavements subjected to tension.
Irregular aggregate (voids produce 36%): (i) They require more cement paste as compared to rounded aggregate. (ii) Due to irregularity in shape they developed good bond and are suitable for making ordinary concrete. Angular aggregate (Voids produce up to 40%): (i) These have sharp, angular and rough particles.
- Ii) Angular aggregate provides very good bond than then earlier two, is most suitable for high strength concrete and pavements however r equirement of cement paste is relatively more.
- Flaky and Elongated aggregates: (i) Both of these influence the concrete properties adversely.
- Ii) The least lateral dimension of flaky aggregate (thickness) should be less than 0.6 times the mean dimension.
This test is not applicable to sizes smaller than 6.3 mm. (iii) Elongated aggregate are those aggregate whose length is 1.8 times its mean dimension. Also, this test is not applicable to sizes smaller than 6.3 mm. Last updated on Jan 1, 0001 BPSC AE Answer Key released.
Which coarse aggregates are best suited for concrete?
The Effect of Aggregate Properties on Concrete The Effect of Aggregate Properties on Concrete Concrete is a mixture of cementious material, aggregate, and water. Aggregate is commonly considered inert filler, which accounts for 60 to 80 percent of the volume and 70 to 85 percent of the weight of concrete. Although aggregate is considered inert filler, it is a necessary component that defines the concrete’s thermal and elastic properties and dimensional stability.
- Aggregate is classified as two different types, coarse and fine.
- Coarse aggregate is usually greater than 4.75 mm (retained on a No.4 sieve), while fine aggregate is less than 4.75 mm (passing the No.4 sieve).
- The compressive aggregate strength is an important factor in the selection of aggregate.
- When determining the strength of normal concrete, most concrete aggregates are several times stronger than the other components in concrete and therefore not a factor in the strength of normal strength concrete.
Lightweight aggregate concrete may be more influenced by the compressive strength of the aggregates. Other physical and mineralogical properties of aggregate must be known before mixing concrete to obtain a desirable mixture. These properties include shape and texture, size gradation, moisture conten t, specific gravity, reactivity, soundness and bulk unit weight.
- These properties along with the water/cementitious material ratio determine the, and of concrete.
- The shape and texture of aggregate affects the properties of fresh concrete more than hardened concrete.
- Concrete is more workable when smooth and rounded aggregate is used instead of rough angular or elongated aggregate.
Most natural sands and gravel from riverbeds or seashores are smooth and rounded and are excellent aggregates. Crushed stone produces much more angular and elongated aggregates, which have a higher surface-to-volume ratio, better bond characteristics but require more cement paste to produce a workable mixture.
- The surface texture of aggregate can be either smooth or rough.
- A smooth surface can improve workability, yet a rougher surface generates a stronger bond between the paste and the aggregate creating a higher strength.
- The grading or size distribution of aggregate is an important characteristic because it determines the paste requirement for workable concrete.
This paste requirement is the factor controlling the cost, since cement is the most expensive component. It is therefore desirable to minimize the amount of paste consistent with the production of concrete that can be handled, compacted, and finished while providing the necessary strength and durability.
The required amount of cement paste is dependent upon the amount of void space that must be filled and the total surface area that must be covered. When the particles are of uniform size the spacing is the greatest, but when a range of sizes is used the void spaces are filled and the paste requirement is lowered.
The more these voids are filled, the less workable the concrete becomes, therefore, a compromise between workability and economy is necessary. Theof an aggregate is an important factor when developing the proper water/cementitious material ratio. All aggregates contain some moisture based on the porosity of the particles and the moisture condition of the storage area.
The moisture content can range from less than one percent in gravel to up to 40 percent in very porous sandstone and expanded shale. Aggregate can be found in four different moisture states that include oven-dry (OD), air-dry (AD), saturated-surface dry (SSD) and wet. Of these four states, only OD and SSD correspond to a specific moisture state and can be used as reference states for calculating moisture content.
In order to calculate the quantity of water that aggregate will either add or subtract to the paste, the following three quantities must be calculated: absorption capacity, effective absorption, and surface moisture. Most stockpiled coarse aggregate is in the AD state with an absorption of less than one percent, but most fine aggregate is often in the wet state with surface moisture up to five percent.
This surface moisture on the fine aggregate creates a thick film over the surface of the particles pushing them apart and increasing the apparent volume. This is commonly known as bulking and can cause significant errors in proportioning volume. The density of the aggregates is required in mixture proportioning to establish weight-volume relationships.
Specific gravity is easily calculated by determining the densities by the displacement of water. All aggregates contain some porosity, and the specific gravity value depends on whether these pores are included in the measurement. There are two terms that are used to distinguish this measurement; absolute specific gravity and bulk specific gravity.
Absolute specific gravity (ASG) refers to the solid material excluding the pores, and bulk specific gravity (BSG), sometimes called apparent specific gravity, includes the volume of the pores. For the purpose of mixture proportioning, it is important to know the space occupied by the aggregate particles, including the pores within the particles.
The BSG of an aggregate is not directly related to its performance in concrete, although, the specification of BSG is often done to meet minimum density requirements. For mixture proportioning, the bulk unit weight (a.k.a. bulk density) is required. The bulk density measures the volume that the graded aggregate will occupy in concrete, including the solid aggregate particles and the voids between them.
Since the weight of the aggregate is dependent on the moisture content of the aggregate, a constant moisture content is required. This is achieved by using OD aggregate. Additionally, the bulk density is required for the volume method of mixture proportioning. The most common on the basis of bulk specific gravity is lightweight, normal-weight, and heavyweight aggregates.
In normal concrete the aggregate weighs 1,520 – 1,680 kg/m 3, but occasionally designs require either lightweight or heavyweight concrete. Lightweight concrete contains aggregate that is natural or synthetic which weighs less than 1,100 kg/m 3 and heavyweight concrete contains aggregates that are natural or synthetic which weigh more than 2080 kg/m 3,
- Although aggregates are most commonly known to be inert filler in concrete, the different properties of aggregate have a large impact on the strength, durability, workability, and economy of concrete.
- These different properties of aggregate allow designers and contractors the most flexibility to meet their design and construction requirements.
References 1. Mehta and Monteiro. (1993) Concrete Structure, Properties, and Materials, Prentice-Hall, Inc., Englewood Cliffs, NJ
Mindess and Young (1981) Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ Kosmatka and Panarese (1994) Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, Illinois
: The Effect of Aggregate Properties on Concrete
Which of the following coarse aggregate is generally used?
Coarse Aggregate Size Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder.
What is 20mm aggregate used for?
20mm aggregate – IS 383:2016 Aggregates are inert granular materials such as sand that, along with water and portland cement, are an essential ingredient in concrete.20 mm Aggregates are mainly used in ready mix concrete, asphalt/bitumen/concrete roads base and sub-base course, pavements, separately or mixed with other aggregates depending on the application.
Which type of aggregate produces minimum void in concrete?
Free ISRO Scientist Civil 2020 Paper 80 Questions 240 Marks 90 Mins Cubical aggregate has maximum strength in concrete as it has good packing and strength in all direction. Rounded aggregate is not suitable for concrete. Flaky means have less thickness, elongated means having more length. These aggregate can be easily crushed and having a minimum strength. Reasons:
- Generally, in normal concrete loads are taken by aggregates only and cement acts as a binder, therefore, a normal concrete can have maximum strength till the aggregates are not broken.
- If the aggregates fail under a load before failure of cement sand matrix. The concrete produced with that aggregates will not achieve the desired strength.
- So using flaky and elongated aggregates might lead to failure of concrete and hence should be avoided.
Important Points Classification of aggregates on basis of shape –
- Rounded aggregates / spherical – Rounded aggregates result the minimum percentage of voids (32 – 33%) hence gives more workability. They require lesser amount of water-cement ratio. They are not considered for high strength concrete because of poor interlocking behaviour and weak bond strength.
- Irregular or partly rounded aggregates – Irregular aggregates may result 35- 37% of voids. These will give lesser workability when compared to rounded aggregates.
- Angular aggregates – Angular aggregates result maximum percentage of voids (38-45%) hence gives less workability
- Flaky aggregates – When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate. Or in the other, when the least dimension of aggregate is less than the 60% of its mean dimension then it is said to be flaky aggregate.
- Elongated aggregates – When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension.
- Flaky and elongated aggregates – When the aggregate length is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates. The above 3 types of aggregates are not suitable for concrete mixing
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What is aggregate cement ratio?
Effect of Aggregate Cement Ratio on Workability of Concrete – Aggregate cement ratio is the ratio of weights of aggregate to the weight of cement. If this ratio is more, that implies aggregates are more and cement is less and if this ratio is less, that implies weight of aggregate is less and weight of cement is more (relatively).
- When the weight of cement is more, i.e.
- Aggregate cement ratio is less, the concrete has more cement paste required to coat aggregates and fill the voids between them.
- This more cement paste makes the concrete relatively easy to mix, place and compact as it reduces the friction between aggregates and allows its smooth movement.
Thus the workability of concrete increases. When the weight of cement is less, i.e. aggregate cement ratio is more, then there will be very less cement paste to coat aggregate surfaces and fill the voids, thus, mixing, placing and compacting of concrete will be more harder than previous case.
How do you choose coarse aggregate?
How to Choose Quality Coarse Aggregate for Concrete Construction? – In order to select good quality coarse aggregates, engineers should consider the following crucial points:
The typical coarse aggregate size ranges from 4.75 mm to 18 mm.Gravel or broken stone aggregates with rough and non-glassy texture are the best aggregates because they create a good bond with the cement paste.Aggregates need to be hard, strong, chemically inert, and non-porous.Organic substances and dirt coating compromise the strength and durability of concrete.Coarse aggregates should not absorb moisture by more than 10% of their weight after submersion in water for 24 hours.If aggregates absorb water by more than 10% of their weight, they are considered as porous aggregate that are undesirable for concrete production.Aggregates should no contain flaky and elongated particles by more than 35% of their total quantity.Angular and fairly cubical coarse aggregate particles are desirable.Coarse aggregate should contain various fraction sizes to ensure adequate compaction of particles, hence resulting in increased density and reduced voids in concrete.In the majority of concrete works, aggregate sizes of 20 mm or smaller are used.To ensure good coarse aggregate compaction and higher concrete density, it is recommended to mix 20 mm and 10 mm coarse aggregates in the ratio of 70:30 or 60:40.It is recommended to stack different aggregate sizes to use them efficiently. Prevent the use of friable and chert aggregates. The former is susceptible to splitting, whereas the latter has low resistance against weathering and is likely to cause popouts.
Figure-1: Undesirable Aggregate Shape Figure-2: Desirable Coarse Aggregate Shape
What type of aggregate is used for concrete?
CEMEX offers a wide range of aggregate materials for concrete. Aggregates are granular materials that are used with a cementing medium to form concrete or hydraulic mortar. They are key ingredients in the manufacture of concrete, mortar, and other construction materials, and are used in the construction and maintenance of structures such as highways, walkways, parking lots, airport runways and railways.
Concrete aggregates are composed of geological materials such as gravel, sand and crushed rock. The size of the particles determines whether it is a coarse aggregate (e.g. gravel) or a fine aggregate (e.g. sand). The resulting concrete can be used in its natural state or crushed, according to its use and application.
Aggregate materials help to make concrete mixes more compact. They also decrease the consumption of cement and water and contribute to the mechanical strength of the concrete, making them an indispensable ingredient in the construction and maintenance of rigid structures.
Where is 40mm aggregate used?
40mm aggregate is used for PCC in foundation or base course in rigid pavement.
What is the size of aggregate?
Key points – • ‘Aggregate’ is a term for any particulate material. It includes gravel, crushed stone, sand, slag, recycled concrete and geosynthetic aggregates. Aggregate may be natural, manufactured or recycled. • Aggregates make up some 60 -80% of the concrete mix.
- They provide compressive strength and bulk to concrete.
- Aggregates in any particular mix of concrete are selected for their durability, strength, workability and ability to receive finishes.
- For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete.
• Aggregates are divided into either ‘ coarse ‘ or ‘ fine ‘ categories. – Coarse aggregates are particulates that are greater than 4.75mm. The usual range employed is between 9.5mm and 37.5mm in diameter. – Fine aggregates are usually sand or crushed stone that are less than 9.55mm in diameter. 20 mm aggregate
Which type of aggregate gives more strength?
For maximum strength an angular shape aggregate is suitable because it provides friction and interlocking both.