GRAIN SIZE CHARACTERISTICS OF SOILS BASIC AND TUTORIALS

SOIL GRAIN SIZE CHARACTERISTICS BASIC INFORMATION

What Are The Grain Size Characteristics of Soils?

Large-grained materials such as cobbles and boulders are sometimes considered to be soil. The differentiation of cobbles and boulders depends somewhat on local practice, but boulders are generally taken to be particles larger than 200 to 300 mm or 9 to 12 in. 

The Unified Soil Classification System suggests that boulders be defined as particles that will not pass a 12-in. (300 mm) opening. Cobbles are smaller than boulders and range down to particles that are retained on a 3-inch (75 mm) sieve. 

Gravels and sands are classified as coarse-grained soils; silts and clays are fine-grained soils. For engineering purposes, gravel is defined as soil that passes a 3-inch (75 mm) sieve and is retained by a No. 4 sieve (4.75 mm or 0.187 in.) or No. 10 sieve (2.00 mm or 0.078 in.), depending on the classification system. 

Sand is defined as soil particles smaller than gravel but retained on a No. 200 sieve (0.075 mm or about 0.003 in.). Soils passing the No. 200 sieve may be silt or clay. Although grain-size criteria were used in some older classification systems to differentiate silt from clay, the two systems described herein make this differentiation based on plasticity rather than grain size.

The grain-size characteristics of soils that are predominantly coarse grained are evaluated by a sieve analysis. A nest of sieves is prepared by stacking sieves one above the other with the largest opening at the top followed by sieves of successively smaller openings and a catch pan at the bottom. 

Opening sizes of commonly used sieves are shown in Table 15.1. A sample of dry soil is poured onto the top sieve, the nest is covered, and it is then shaken by hand or mechanical shaker until each particle has dropped to a sieve with openings too small to pass, and the particle is retained.

The cumulative weight of all material larger than each sieve size is determined and divided by the total sample weight to obtain the percent retained for that sieve size, and this value is subtracted from 100% to obtain the percent passing that sieve size. 

Results are displayed by plotting the percent passing (on a linear scale) against the sieve opening

size (on a log scale) and connecting the plotted points with a smooth curve referred to as a grain-size

distribution curve.

A sample of some grain-size distribution curves is presented in Fig. 15.1. The notation Dxx refers to the size D , in mm, for which xx percent of the sample by weight passes a sieve with an opening equal to D . The D10 size, sometimes called the effective grain size, is the grain diameter for which 10% of the sample (by weight) is finer. 

It is determined from the grain-size distribution curve at the point where the curve crosses a horizontal line through the 10% passing value on the y axis. Other D sizes are found in a similar manner. The D 50 size, called the median grain size, is the grain diameter for which half the sample (by weight) is smaller and half is larger.

Two parameters are used to describe the general shape of the grain-size distribution curve. The coefficient of uniformity, Cu , is:

Cu = D60/D10

The coefficient of curvature, Cc, is:

Cc = (D30)^2/ D60 D10

CONCRETE TYPES ACCORDING TO GRADES BASICS AND TUTORIALS

CIVIL CONSTRUCTION CONCRETE TYPES AND GRADE
What Are The Types and Grades Of Concrete?


Lightweight Concrete
Structural lightweight concrete is usually made from aggregates conforming to ASTM C330 that are usually produced in a kiln, such as expanded clays and shales. Structural lightweight concrete has a density between 90 and 120 lb/ft3 (1440 to 1920 kg/m3).

Production of lightweight concrete is more difficult than normal-weight concrete because the aggregates vary in absorption of water, specific gravity, moisture content, and amount of grading of undersize. Slump and unit weight tests should be performed often to ensure uniformity of the mix.

During placing and finishing of the concrete, the aggregates may float to the surface. Workability can be improved by increasing the percentage of fines or by using an air-entraining admixture to incorporate 4 to 6% air. Dry aggregate should not be put into the mix because it will continue to absorb moisture and cause the concrete to harden before placement is completed.  Continuous water curing is important with lightweight concrete.

No-fines concrete is obtained by using pea gravel as the coarse aggregate and 20 to 30% entrained air instead of sand. It is used for low dead weight and insulation when strength is not important.

This concrete weighs from 105 to 118 lb/ft3 (1680 to 1890 kg/m3) and has a compressive strength from 200 to 1000 psi (1 to 7 MPa).

Aporous concretemade by gap grading or single-size aggregate grading is used for low conductivity or where drainage is needed.

Lightweight concrete can also be made with gas-forming of foaming agents which are used as admixtures. Foam concretes range in weight from20 to 110 lb/ft3 (320 to 1760 kg/m3). Themodulus of elasticity of lightweight concrete can be computed using the same formula as normal concrete. The shrinkage of lightweight concrete is similar to or slightly greater than for normal concrete.

Heavyweight Concrete
Heavyweight concretes are used primarily for shielding purposes against gamma and x-radiation in nuclear reactors and other structures. Barite, limonite and magnetite, steel punchings, and steel shot are typically used as aggregates. Heavyweight concretes weigh from 200 to 350 lb/ft3 (3200 to 5600 kg/m3) with strengths from 3200 to 6000 psi (22 to 41 MPa).

Gradings and mix proportions are similar to those for normal weight concrete. Heavyweight concretes usually do not have good resistance to weathering or abrasion.

High-Strength Concrete
Concretes with strengths in excess of 6000 psi (41 MPa) are referred to as high-strength concretes. Strengths up to 18,000 psi (124 MPa) have been used in buildings.

Admixtures such as superplasticizers, silica fume, and supplementary cementing materials such as fly ash improve the dispersion of cement in the mix and produce workable concretes with lower water-cement ratios, lower void ratios, and higher strength.  Coarse aggregates should be strong fine-grained gravel with rough surfaces.

For concrete strengths in excess of 6000 psi (41 MPa), the modulus of elasticity should be taken as
E = 40,000 √f'c + 1 x 10^6

where
f'c = compressive strength at 28 d, psi
The shrinkage of high-strength concrete is about the same as that for normal concrete.