Moisture Damage in Fibre-Reinforced Bituminous Road Materials

Ibrahim Kamaruddin and Madzlan Napiah

Universiti Teknologi PETRONAS, Seri Iskandar, Perak, Malaysia

Abstract
Water related damage in bituminous pavements is a major distress form in many South-east
Asian countries like Malaysia. Highway engineers acknowledge that stripping or loss of
adhesion between the aggregate surface and the bitumen film coating the aggregates and other
forms of moisture damage in bituminous pavements are shortening the pavement life as a result
of this damage. There is thus a need to evaluate the damage caused by water to pavements. The
use of the retained strength ratios obtained from laboratory moisture damage tests is a useful tool
in making quantitative predictions of the related damage caused by water. This study involved
laboratory work on the effect of water on the performance of bituminous mixtures. Comparisons
are made between the performance of Hot-rolled Asphalt (HRA) bituminous mixtures containing
a base bitumen of 50pen grade to that of a polymer-fibre reinforced HRA mixtures.

Keywords

Fibre-reinforced bituminous mix, stripping, tensile test, saturation, voids and porosity. 1. Introduction

The damaging effects of water on the physical properties and mechanical behaviour of
bituminous mixtures have been the focus of many studies. Many laboratory tests have been

The resulting damage brought about by water can be assessed quantitatively by mechanical tests.
Properties such as loss of tensile strength or decrease of resilient and stiffness moduli have been
measured. Water induced damage is usually easy to identify when stripping is evident. Where a
loss of pavement stiffness or moduli occurs without the visual evidence of stripping, the cause of
the problem is less easily recognised.

There are a number of laboratory methods and tests that are cited in the literatures that enable the
determination of the moisture susceptibility and thus the proneness of bituminous mixtures to
stripping or de-bonding. Based on previous work (Tunnicliff and Root, 1983 and Gilmore et. al,
1983), it appears that vacuum saturation of the bituminous specimens followed by conditioning
and testing using the Indirect Tensile test show promise for introducing moisture into the
specimens and measuring their strength to predict the moisture susceptibility of the mixtures.
Specimens were moisture conditioned before the determination of their structural strengths as
those mixtures most susceptible to moisture damage would have the lowest structural strengths
following conditioning. The retained strength is determined by comparing the dry tensile
strength to the wet conditioned tensile strength of the bituminous mixtures. 3. Adhesion Properties of Bituminous Mixtures

Loss of adhesion is especially common in bituminous mixes which utilises hydrophilic
aggregates i.e. aggregates which have an affinity for water. An example of this type of aggregate
is granite which is widely used in road pavement construction in Malaysia. Ishai and Craus
(1977) listed two ways in which modifications are possible should the use of hydrophilic
aggregates prove to be unavoidable:
1. modification of the adhesion properties of the bitumen by additives
2. modification of the surface properties of the aggregates by treating them with cement-
water solution or hydrated lime-water solution.

Ishai and Craus (1977) also acknowledged that of the two modifications, the first is more useful

Specification
Coarse
Aggregate
35 2.75 0.47 BS 594: Part
1: 1992 Table
3, Type F
Wearing
Course
designation
30/14

Sand

55

2.65

1.37
Filler
(Ordinary
Portland
Cement)

10

3.15

Penetration
(0.1 mm)
Softening

(°
°°
°C)
Polypropylene
(PP)
0.91 6 6 22* 160
Polyester (POL) 1.41 3 6 17* 250
*Values obtained from 20 readings using a light microscope at 400X magnifications.

The preparation of the bituminous samples in the laboratory is an energy intensive process. In
order to maintain thermal stability when using the polypropylene fibres, it was decided that the

mixing temperature when preparing the Hot Rolled Asphalt (HRA) mixture will not exceed
140°C and compaction be done at 130°C.

5. Wet-Dry Indirect Tensile Test

The wet-dry indirect tensile test was adopted as a principal measure of the bituminous mix
response to water damage. Most evaluations of water damage have been assessed quantitatively
by mechanical tests in which such properties as loss of tensile strength or decrease of resilient
and stiffness moduli have been measured. These are then given in the form of a tensile-strength
ratio and a modulus of elasticity ratio, for which the tensile strength and modulus of the dry
specimens served as references. The tensile strength ratio (TSR) and modulus of elasticity ratio
(MER) are dimensionless numbers used to represent the portion of tensile strength and modulus
retained following conditioning. Low values indicate high moisture damage. These ratios are
given as:

Tensile Strength Ratio (TSR)

dry

process. Static soaking seems to provide ideal condition for stripping to occur while both
pressure and vacuum saturation procedures may create damage to the specimens. For practical
laboratory purposes however, static soaking may require too much time. Saturation by partial
vacuuming for short period of time has therefore been used in moisture damage studies on

bituminous mixtures. If the volume of absorbed water exceeds the volume of voids, the specimen
has been supersaturated and damage.

A number of researchers have come up with various regimes for vacuuming and saturating the
bituminous specimens. Lottman (1982) for example used 26-inches of mercury to vacuum the
specimens while lower levels of vacuuming (4 and 15-inches of mercury) have been cited in the
literature (Gilmore et. al, 1983, Graf, 1986). In addition to vacuuming, Lottman (1982) also
subjected the specimens to an advanced moisture conditioning in which thermal cycles or a cycle
of freezing-soaking was carried out. Ishai and Nesichi (1988) subjected the specimens to hot
water immersion (at 60°C) for up to 14 days and testing the specimens at different immersion
period to determine the retained strength of the specimens while Kennedy et. al. (1983) subjected
the specimens to the boiling test, a freeze-thaw cycle before conducting the indirect tensile test
on dry and conditioned specimens.

7. Void Structure in Bituminous Mixtures

The moisture conditioning process attempts to allow water to penetrate and occupy the air voids
in the specimen. An appreciation of the void structure in bituminous mixtures is thus very vital.
Kumar and Goetz (1977) conducted a laboratory study to examine the influence of asphalt film
thickness, voids and permeability on asphalt hardening in asphalt mixtures and came up with a
hypothetical model of the air voids system in a compacted bituminous mixture. Different water
saturation techniques were employed in their study that included a 24 hours soaking and
vacuuming at different absolute pressures. The model divides the air voids system into three
categories; through passage accessible air voids, dead end accessible air voids and non-
accessible air voids. The 24 hour soaking allows water to only occupy the through passage