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Strength of Cement
Strength of cement is the most important of all the cement properties. Grades mentioned in the cement bags as 53/43 grade OPC/PPC in fact represent the strength of the cement. 53 Grade OPC Cement simply means that 28-days compressive strength of the cement-mortar cubes prepared out of that cement in a standard manner will be of 53 MPa.
Strength test of cement is carried out on the cubes of hardened Cement-Sand mortar; not on a neat cement paste. Strength of cement is defined in three ways: Compressive, Tensile & flexural. Usually compressive test is carried out.
Strength of cement-sand paste can be affected by number of items including: water-cement ratio, cement-sand ratio, type and grading of sand, manner of mixing, size and shape of specimen, curing conditions, rate of loading and age of specimen.
ASTM C 109-92 prescribes a Cement-Sand mix with following specifications:
Cement / Sand - 1:2.75
Water / Cement - 0.485
Type of Sand - Ottawa (Illinois)
Cube Size - 51 mm (2 inches)
Loading conditions while carried out compression test of above prepared cubes should not be less than 20 secs. and no more than 80 secs.
Since cement gains strength over time, the time at which strength test is to be conducted must be specified. Typically times are 1 day (for high early strength cement), 3 days, 7 days, 28 days and 90 days (for low heat of hydration cement).
Also, it should be noted that cement-mortar strength is not directly related to concrete strength. Strength of concrete is dictated by many other factors. Cement-mortar strength is generally used as a quality control measure of cement itself..
Following presents the strength requirements for the OPC cement as per the ASTM & NS standard at various days;
ASTM C 150 Portland Cement Mortar Compressive Strength Specifications
in MPa
As per NS Standard Cement Mortar Compressive Strength Specifications
in MPa
Classification
of Fine Aggregate
Natural sand, finely crushed stone and crushed gravel are treated as
“Fine Aggregates”. Aggregates used for making concrete normally lies
between the range - with a maximum size of 80mm and a minimum of 150
micron. The maximum sizes of 80mm, 40mm, 20mm, 10mm, 4.75mm, 2.36mm,
600 micron, 300 micron and 150 micron are more common.
The aggregate fractions from 4.75mm to 150 micron are termed as “Sand” or “Fine Aggregate”. Also, those from 4.75mm to 80 mm are termed as “Gravel” or “Coarse Aggregate”. The Size 4.75mm is a common fraction appearing both in coarse & fine aggregate. These various sizes are usually divided by means of “Sieve Analysis”. Sieve Analysis is the operation of dividing a sample of aggregate into various fractions each consisting of particles of the same size. The Sieve Analysis is conducted to determine the particle size distribution in a sample of aggregate, which is also called “Gradation”.
Grading pattern of a sample of aggregates is assessed by sieving a sample successively through all the sieves mounted one over the other in the other of size, with larger sieve on the top. The material retained on each sieve after shaking represents the fraction of aggregate coarser than the corresponding sieve and finer than the sieve above.
A single factor computed from the sieve analysis is used; known as “Fineness Modulus”. Fineness Modulus is defined as the sum of the cumulative percentages retained on the sieves of the standard series, divided by 100. The standard series consists of sieves, each twice the size of the preceding one (ASTM No. 100, 50, 30, 16, 8, 4) and up to the largest sieve size present. Typical values of fineness modulus range from 2.2 to 3.2 with following classifications:
Fine Sand : 2.2 – 2.6
Medium Sand : 2.6 – 2.9
Coarse Sand : 2.9 – 3.2
The usefulness of the fineness modulus lies in detecting slight variations
in the fine aggregates from the same source, which would affect the
workability of fresh concrete.