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CLOSE THIS BOOKStabilizers and Mortars ( for compressed earth blocks) (GTZ, 1994, 20 p.)
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENTAcknowledgements
VIEW THE DOCUMENTIntroduction
VIEW THE DOCUMENTStabilizers
VIEW THE DOCUMENTMortars
VIEW THE DOCUMENTNatural products
VIEW THE DOCUMENTLime
VIEW THE DOCUMENTPortland cement
VIEW THE DOCUMENTGypsum
VIEW THE DOCUMENTBitumen
VIEW THE DOCUMENTSynthetic products
VIEW THE DOCUMENTSpecialized Commercial Products
VIEW THE DOCUMENTSelect bibliography

Bitumen

I. INTRODUCTION

Description

Bitumen is a product made up of a filler and at least 40% heavy hydrocarbons. It should not be confused with asphalt, which denotes products containing less than 20% hydrocarbons, the remainder consisting of filler, sand or gravel. (A word of warning should be given about the American use of the word "asphalt", which denotes the product known as "bitumen" in Europe, while in the USA "bitumen" is a black binding agent such as distilled bitumen or coal tar).

In its natural state, bitumen is too thick to be usable. To make it usable, it is either heated, or a "cutback" or emulsion is prepared. For stabilization purposes, these two methods are used.

"Cutback" is obtained by mixing the bitumen with a volatile solvent such as diesel fuel, kerosene, or naphtha. Depending on the mix used, "cutbacks" take more or less time to dry. They cannot be used in the rain and are highly inflammable. Their viscosity is defined by an index (0 = very fluid; 3 = viscous).

Emulsions are obtained by dispersing bitumen in water with the help of an emulsifier, which is either of an anionic or a cationic kind. The emulsions obtained from the latter are more common and are compatible with many soil types. Emulsions are very fluid and are easily mixed with pre-moistened soils.

Conditions of use

The mixing of bitumen with the soil is crucial for the effectiveness of the stabilization. Too much mixing can increase water absorption after drying, as a result of a premature breakdown of the emulsion.

To ensure easy mixing, it is preferable to add the bitumen to a small quantity of soil and then to mix this small quantity into the remaining soil. If sand has to be added to the soil, the bitumen should be added to the sand and then the stabilized sand added to the remaining soil (especially for cutbacks). Emulsions should be diluted in the mixing water. These mixing methods are very important for low proportions (eg 2%) of bitumen.

How quickly the mix has to be used depends on the breakdown threshold of the product. Fast breakdown cutbacks and emulsions shorten drying times, hence such mixes must be used without delay. Bitumen-stabilized materials should preferably be dried out in dry air, rather than in a humid environment. A high temperature enhances the final effectiveness of the stabilization.

II. STABILIZER

Historical background

The use of bitumen as a stabilizer is very ancient. The Greek historian Herodutus describes how it was used in Babylon in the 5th century BC for making mortar to lay unfired moulded bricks. Even so, the use of bitumen throughout the course of history has been limited. Indeed bitumen as such was first produced on an industrial scale only a few decades ago, in the 1940s in the USA. Civil engineers have learnt to use bitumen in road construction. In Algeria, for example, nearly 28,000 kms of road have been built using this technique. Nowadays in the USA, stabilized earth blocks and bricks are sold under the name of "Bitudobe" or "Asphadobe". The material is also widely used in both Central and South America.

How stabilization occurs

Cutback and bituminous emulsions come in the form of microscopic droplets in suspension in a solvent or water. The stabilizer is mixed into the soil, and when the water or solvent evaporates the droplets of bitumen spread out to form strong, very thin films which adhere to and coat the soil particles. Bitumen improves the water-resistant properties of the soil (less absorption by clays) and can improve the cohesion of naturally non-cohesive soils, by acting as a binder.


CUTBACKS

EMULSIONS

CRACKING OR DRYING

EUROPE

USA (ASTM)

BITUMEN CONTENT (%)

ANIONIC (NEGATIVE) (ASTM)

CATIONIC (POSITIVE) (ASTM)

VISCOSITY


SC 0

45 - 50





SLOW

SC 1

SC 70

55-61

SS 1

CSS 1

FLUID


SC 2

SC 250

63 -70

SS 1 h

CSS 1h

VISCOUS


SC 3

70-75






MC 0

61 - 65





MEDIUM

MC 1

MC 70

68 -72

MS 2

CMS 2

FLUID


MC 2

MC 250

73 - 77

MS 2 h

CMS 2 h

VISCOUS


MC3

79 -82






RC 0

62 - 65





RAPID

RC 1

RC 70

70-73

RS 1

CRS 1

FLUID


RC 2

RC 250

74-78

RS 2

CRS 2

VISCOUS


RC 3

79 - 83





Suitability of different bitumens

To ensure an even distribution of the bitumen in the soil, the latter should preferably be in a plastic state. Using a bitumen cutback or emulsion is, therefore, a delicate operation when stabilizing compressed earth blocks that are compacted at a water content close to the optimum (OMC). On the other hand, it is easy to use this kind of product when stabilizing earth mortar renders. Its effect in this case consists mainly in increasing the impermeabilization capacity of the render.

Table 1 (on the front page) summarizes some important characteristics of the various kinds of bitumen that can be used for stabilization.

Proportions of bitumen to be used

Normally 2 to 3% bitumen cutback, or 4 to 6% bitumen emulsion is added, but can even be as high as 20%. Proportions vary in accordance with the grain size distribution of the soil, because bitumen stabilization involves the coating of the specific surface of the grains. The values given here are for the bitumen prior to its being diluted in a watery suspension or by a solvent. Bitumen has only a very slight effect on the colour of the material and has no particular smell once the stabilized products have dried out.

Soils that can be used

Although clayey soils have been successfully treated with cutback or emulsions, bitumen stabilization is more suitable for sandy or sandy-gravel soils, for soils lacking in cohesion, or when an impermeable finish is particularly desired. With extremely clean sandy soils, the low adhesion of the bitumen to the surface of the siliceous particles can lead to the separation of the bitumen under the action of water, with the result that the stabilizing effect of the bitumen on the soil is considerably reduced.

For emulsions, the following figures may be given:

High sand content soils: 4 to 6%
Low sand content soils: 7 to 12%
Clayey soils: 13 to 20%

These percentages are for the hydrocarbon itself and not for the liquid in which it is in suspension.

The correct type of bitumen emulsion can be selected with the aid of Table 2.

Effects of bitumen-stabilization

Some of the effects of bitumen-stabilizadon on soils are the following:

DRY DENSITY: Bitumen brings about a fall in density and increases the Optimum Moisture Content of water plus bitumen.

COMPRESSIVE STRENGTH: In a dry state, this increases with the proportion of bitumen up to a certain threshold value, after which it falls sharply, that is, once the ideal level of coating has been achieved. In a moist state, the strength rises steadily with the quantity of bitumen used, independently of dry strength.

ABSORPTION: This is a function of the moisture content during mixing and falls to very low levels after a certain threshold value has been reached. It is advisable to determine this value. After several days' drying, water absorption is stable over time.

Effects of certain products on bitumen-stabilization

ORGANIC MATTER AND SULPHATES: The presence of these in the soil hinders the efficiency of bitumen stabilization as their adhesion to the "rains prevents the adhesion of the bitumen. Acid organic matter (eg forest soils) are very harmful. The neutral and basic organic matter found in arid and semi-arid regions are not particularly harmful.

SALTS: Mineral salts are very harmful. They can be neutralized by adding 1 to 2% by weight of cement. When bitumen-stabilizadon is carried out on an industrial scale, salt contents of more than 0.2 % are not accepted, but sometimes up to 6 %sodium chloride (NaCI) is permissible.

LIME: When the soil is too clayey, 1 to 2% lime can be added to flocculate the soil.

III. MORTAR

Bonding mortar

Bitumen mixes easily with soils to form a plastic or soft paste. It is therefore very easy to use as a bonding mortar and is especially suitable for bitumen-stabilized block masonry.

Plasters and renders

Bitumen-stabilized soils used for plasters and renders should be neither too clayey, nor too sandy and dusty. The quantity of bitumen used ranges from 2 to 6%. They are usually cutbacks; however, where bituminous emulsions are used, the mixture must be made slowly in order to avoid any breakdown of the emulsion. Bitumen-stabilization for renders is particularly effective on soils which have already been reinforced with straw or even with dung. The bitumen is added only at the end, 2 to 3 hours before the render is applied. Mixtures of asphalt, awn arabic and caustic soda solution are also highly effective. The support should be properly prepared, brushed and moistened.

HOW TO CHOOSE THE RIGHT EMULSION

Fraction with grains of < 0.08 mm

Moist soil with more than 5 % water

Dry soil with less than 5 % water

0 to 5%

SS-1h

CMS-2h


(CSS -1 h)

(SS -1 h*)

5 to 15%

SS-1h

CMS-2h


(CSS -1 h)

(SS -1 h*)

15 to 20%

SS -1 h

CMS -2h


(CSS -1 h)


* the soil must be moistened in advance

Explanation of the abbreviations in the two charts:

EMULSIONS



SS

Slow Setting

CUTBACKS


MS

Medium Setting


RS

Rapid Setting

SC

Slow Curing

CSS

Cationic Slow Setting

MC

Medium Curing

CMS

Cationic Medium Setting

RC

Rapid Curing

CRS

Cationic Rapid Setting

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