What is Quality Aggregate

Introduction

Since at least 75% of the volume of concrete consists of aggregate, its quality is of considerable importance. Not only may the aggregate limit the strength of concrete, as aggregate with undesirable properties cannot produce strong concrete, but the properties of aggregate greatly affect durability and structural performance of concrete.

Definition and Terms

Conformance – An affirmative indication or judgment that a product or service has met the requirements of the relevant specifications, contract, or regulation.
Nonconformity – The non-fulfillment of specified requirements.
ASTM – American Society of Testing and Materials.
BSI – British Standards Institution.

Deleterious Substances

There are two broad categories of deleterious substances that may be found in aggregates:
Impurities which interfere with the process of cement hydration.
Coatings preventing the development of good bonds between aggregate and hydrated cement paste.

Organic Matter

If aggregate contains organic impurities it may not be suitable for inclusion in concrete. Organic impurities refer to water soluble organic compounds derived from decaying vegetation (tannic acid and its derivatives) capable of retarding the setting and hardening of a concrete. Impurities are more likely to be found in fine (sand) aggregate.

Clay

Clay contamination may either be present as a film on all the aggregate particles or may consist of various sized nodules. Dispersed clay platelets or an aggregate will have the effect of increasing the water/cement ratio for a given workability and those platelets still adhering to the aggregate may decrease the bond between aggregate and cement paste.

Chlorides

Excessive chloride in aggregate can lead to reduced setting times, increased shrinkage and the corrosion of steel reinforcement.

Silt

Silt is a waterborne deposit consisting of very fine particles formed by attrition of parent material. Due to it’s high specific surface area this will increase water demand, thereby increasing the water cement ratio.

Particle Shape and Texture

Aggregate can be used in economical concrete mixes, impervious, durable asphalts, macadam and road bases. Particle size, shape and texture can have many effects on various design properties and are generally specified within certain limits. Rounded aggregate can lead to instability in a bituminous mixture yet is ideal as a concrete aggregate where good workability of the mix is essential for placing and compaction.

Definition of a Quarry

Within recent years throughout many industrial countries, the escalation in the numbers and scales of quarrying activity has led the authorities to define the nature and characteristics of a quarry. Four (4) definitions have emerged from this exercise. Van Shalkwyk, 1981, viewed historically three (3) definitions over a forty (40) year period, as follows :

Ricketts (1943), defined a quarry as
“Open pit, mine or excavation, where stone, sand and gravel or minerals are obtained.”
Webster (1961), concluded that a quarry was
“A stone mine and is distinguished from other types of mines by the fact
that usually it is open at the top and front.”
Nichols (1962), reviewing the engineering aspects of quarrying noted that
“It is an opencast mine in rock, chosen for physical rather than
chemical characteristics.”
Van Shalkwyk (1981), simply quoted a quarry as being a source of hard rock which has to be removed by blasting.

The Mines and Quarries Act of the U.K. (1954) Section 180 quotes that:- a quarry is an excavation or system of excavations made for the purpose of, or in connection with, the getting of minerals, or products of minerals.

The Minerals Act No. 61 of 2000 of Trinidad & Tobago does not give a definition of a quarry, and interprets that, mining means excavating or quarrying.

However a mine is described as includes any place, excavation or working in, on which operations connected with mining is carried on, together with all buildings, premises, erection and appliances belonging or appertaining thereto, above or below ground, for the purpose of winning, treating or preparing minerals, obtaining or extracting any mineral or metal by any mode or method.