1.0. INTRODUCTION AND CLASSIFICATION OF CONCRETE

Concrete Mix as a System – Concrete is by far the most widely used man-made construction material and studies indicate that it will continue to be so in the years and decades to come'. Such versatility of concrete is due to the fact that from the common ingredients, namely, cement, aggregate and water (and sometimes admixtures).

The advances in concrete technology has paved the way to make the best use of locally available materials by judicious mix proportioning and proper workmanship, so as to result in a concrete satisfying the performance requirements.

The various aspects covered are the materials, mix proportioning, elements of workmanship (for example: placing, compaction and curing), methods of testing and relevant statistical approach to quality control and special precautions needed in extreme weather concreting.

Classification of Concrete Mixes Concrete mixes are classified in a number of ways, often depending upon the type of specifications, which are broadly of two types;

1)   The 'prescriptive' specifications where the proportions of the ingredients and their characteristics (namely, type of cement, maximum size of aggregate, etc.) are specified, with the hope that adherence to such prescriptive specification will result in satisfactory performance.

2)   The 'performance' oriented specification can be used wherein the requirements of the desirable properties of concrete are specified (example - strength, workability or any other property).

Concrete is accepted on the basis of these requirements being satisfied and the choice of materials and mix proportions is with the producer.

Based on the above considerations, concrete can be classified either as 'nominal mix' concrete or 'designed mix' concrete.

IS : 456  had classified concrete into 'controlled' concrete and 'ordinary' concrete, depending upon the levels of control exercised in the works and the method of proportioning concrete mixes.

According to this, where the mix proportions were fixed by designing the concrete mixes with preliminary tests were called 'controlled concrete'; whereas 'ordinary concrete' was one where 'nominal' concrete mixes were adopted by Volumetric.

This might have inadvertently led to a feeling that no quality control was necessary in case of nominal mixes.

However realizing that mix proportioning is only one aspect of quality control of concrete and that quality control really includes many other aspects like choice of appropriate concrete materials after proper tests, proper workmanship in batching, mixing, transportation, placing, compaction and curing, coupled with necessary checks and tests for quality acceptance and quality control.

Concrete can be classified in many other ways in special situations;

1.   Based on its Density - Light weight, Normal weight or Heavy weight concrete

2.   Based on its Workability - Flowing / Flowable, Pumpable concrete.

3.   Based on its Durability  - Sulphate Resistance and Fire Resistance Concrete.

(in specific environments)

GRADE OF CONCRETE - Among the many properties of concrete, its compressive strength is the most important and has been held as an index of its overall quality. Many other engineering properties of concrete appear to be generally related to its compressive strength. Concrete is, therefore mostly graded according to its compressive strength.

Grades of concrete lower than M 15 are not to be used in reinforced concrete works and grades of concrete lower than M 30 are not to be used for pre-stressed concrete works.

Similar grading of concrete on the basis of 28 days characteristic strength has also been adopted by IS and most of the other codes of practices.

TABLE.: I (GRADES OF CONCRETE)

GROUP	GRADE DESIGNATION	SPECIFIED CHARACTERISTIC COMPRESSIVE STRENGTH AT 28 DAYS (N/mm2)
Ordinary Concrete	M05	05
”	M10	10
”	M15	15
”	M20	20
Standard Concrete	M25	25
”	M30	30
”	M35	35
”	M40	40
”	M45	45
”	M50	50
”	M55	55
High Strength Concrete	M60	60
”	M65	65
”	M70	70
”	M75	75
”	M80	80

Note – 1: The designation of a concrete mix, letter M refers to the Mix and the number to the specified 28 days compressive strength of 15 cm x 15 cm x 15 cm cube, it is expressed in N/mm 2.

Note – 2:  M 05 and M 7.5 grades of concrete may be used for lean concrete bases and simple foundations for masonry walls.

Note – 3: Grades of concrete lower than M20 shall not be used in reinforced concrete.

Note – 4: Grades of concrete lower than M 30 are not to be used for pre-stressed concrete works.