Aggregate Contaminants and Their Effects
 

Introduction

From time to time, cases arise where aggregate contaminants cause serious serviceability problems, and these seem to have become more frequent in the Western and Eastern Cape Provinces.

This is possibly due to the depletion of historically sound aggregate sources and the opening of new sand pits and quarries.

Broadly speaking, aggregate contaminants can be classified as organic or inorganic in origin. Contamination may occur at the sand pit or quarry (i.e. intrinsic contaminants), during transportation, or on site.

Organic Materials

Organic materials, when present, are usually found in the sand or fine aggregate, but from time to time are also found in coarse aggregates.

The aggregate contaminants can be solid in origin, for example seeds or roots or decomposed vegetable matter; or liquid in origin, for example fertilizer, tree resin, and urine. Examples of contamination by sugar and coal are also on record elsewhere in South Africa.

Solid contaminants may be so small as to be almost un-noticeable to a casual visual check. They are also low density materials which tend to float to the surface of the concrete and this has caused serious problems with power floated concrete floors in particular.

Organic contaminants generally cause retardation of set, either generally or localised, as well as discolouration - as tabulated below:

Contaminant

Retardation

Discolouration

Softness

Other/Remarks

Local

General

Seeds

(esp. Port Jackson)

No

No

No

No

Surface popping, time of occurrence unpredictable and not possible to determine when the popping will stop.

Root particles

Yes

No

Local, straw to dark brown

Local

Aesthetics, stain tends to bleed through paint

Vegetable matter

Yes

No

Local, straw to dark brown

Local

Same as root particles

Tree resin

Yes

No

Straw to brown

Local with sticky resin

Aesthetics

Fertilizer

No

Yes

Sometimes

General

Retardation can last for days – in extreme cases concrete will not harden properly and will have to be demolished.

Sugar

No

Yes

Yes - dark grey

General

Same as fertilizer

Urine

No

Yes

Sometimes – dark grey

General

Same as fertilizer

 

Plaster Made With Sand

Fig 1: Example of plaster made with sand contaminated with loosely cemented decomposed vegetable matter

Inorganic Materials

Contamination by inorganic materials is rare and generally occurs on site or at the batch plant, although it is possible for contamination to occur while concrete aggregates are being transported.

These materials include pieces of steel or other metals, steel wire, and other aggregate contaminants. This type of contamination is almost always the result of poor housekeeping.

The effects of this type of contamination are normally aesthetic - for example, rust stains from accidentally embedded steel. There have been cases where contamination by unsound “aggregate” particles has caused severe damage through expansion and resultant ‘spalling’ of the cover concrete.

Remedial Measures

It is not possible to recommend general remedial measures. Each case must be evaluated individually.

In extreme cases of general retardation in parts of the project, the concrete will have to be demolished and replaced.

Local retardation is often repaired by drilling out the soft spot and patching the hole with a proprietary repair material - but the result is unsightly unless the concrete or plaster is then painted.

Prevention is definitely better than cure in these cases and concrete aggregate deliveries and stockpiles should be checked regularly - particularly if changing aggregate source, supplier or transporter.