Etapa administrativa ante el SII a) Consideraciones previas

Failure mechanisms dene the underlying cause of failure by isolating the components responsible for the defect in the functionality of the seal. These mechanisms are adhesive failure, cohesive failure and embedment as illustrated in Figure 2.3.

Figure 2.3: Seal failure mechanisms.

Adhesive failure

Adhesion refers to the bond at the binder-aggregate interface. This bond is the component responsible for adhesive failure. Weak bonds are due to faulty binders, too low binder application rates, cold temperatures and excessive delay in aggregate placing (TRH3, 2007). The sensitivity of this bond depends on the aggregate size, aggregate spread and the position of the binders. A single seal with an open spread and minimum binder application is highly sensitive in comparison to the same seal with a close or interlocked spread. Even less sensitive is the same interlocked single seal with a fog spray on top. The fog spray creates a shoulder bond that contributes to the adhesive bond.

Figure 2.4: Shoulder bond created by fog spray application (TRH3, 2007).

Separation of this bond in the presence of water is known as stripping. According to Bagampadde (2005), stripping is inuenced by aggregate surface characteristics like chemical stability, pore size distribution, polarity and surface energy. Stripping is thus chemical of nature and leads to stone loss under trac. Mo (2010) indicates that although non mechanical eects such as moisture, aging, chemical dierences etc. may enhance the development of ravelling, ravelling remains directly related to

2. LITERATURE REVIEW

fatigue damage of the adhesive bond. Ravelling is thus dened as stone loss due to fatigue of the binder-aggregate interface bond when subjected trac.

In practice stone loss occurs in the period after construction when the road opens for trac. The adhesive failure mechanism is thus associated with an early service life failure. In exceptional circumstances, ravelling can also occur at an advanced stage of a seal's service life, but this would be unusual.

Cohesive failure

Mo (2010) denes cohesive failure as cracks that initiate within the binder material. Practical experience as documented in the TRH3 (2007), categorises seal cracks as active cracks and passive cracks. Active cracks occur as reection cracks on the surface and do not originate within the binder material. Passive cracks in the form of crocodile cracks and secondary cracks are due to binder fatigue. These cracks are situated within the binder material while the underlying support structure is still in good condition. Repetitive mechanical loading, binder hardening (aging) and temperature changes are the driving factors behind binder fatigue which results cohesive failure. As bituminous materials oxidise they become harder, less ductile and more prone to cracking. Another factor of fatigue cracking was observed by Rowe et al. (2014) whilst conducting low temperature bitumen ductility tests. The development of non-mechanical loading i.e. thermal related cracking was reported. Cohesive failure is therefore associated with mechanical and environmental fatigue cracking and can be described as an end of service life phenomenon.

Embedment

Embedment is probably best explained in the context of surface texture depth loss. Texture loss consists of four phases which occur simultaneously during the service life of the seal. These phases include stone orientation, embedment, binder rise and aggregate wear as illustrated in Figure 2.5.

Stone orientation is the repositioning of the aggregates within the binder in an at- tempt to come at rest in its ALD. Although most orientation occurs beneath the rollers during proper construction, in some cases this phase continues for a couple of weeks when subject to trac. When the aggregates orientate the binder height increases which results in texture loss.

Embedment is the process where the seal aggregate permanently deforms the un- derlying substrate by penetrating into it when subject to load. As the aggregates embed, the relative depth of the binder with respect to aggregate asperities reduces and results in texture loss. In some cases the aggregate penetrates through the tack coat lm as it embeds into the base. This phenomenon is known as punching.

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Binder rise is a phenomenon resulting from the upsurge of existing moisture in the substrate. This upsurge of moisture that in some cases accumulates into a vapour beneath the seal, forces the binder upward, resulting in binder rise and texture loss. This phenomenon occurs particularly in soft and unoxidised binders.

Figure 2.5: Schematic representation of the surface texture loss phases.

Aggregate wear is simply a reduction in the aggregate vertical dimension due to the abrasive eect of trac, which leads to texture loss. The net eect of the four phases at any given time, results in the total loss of surface texture depth and is a mea- surable component which constitutes the criteria for embedment failure. The target minimum surface texture required for adequate skid resistance in wet conditions is set to 0.7 mm (TRH3, 2007). Thus, the initial surface texture after construction will be greater than the target minimum and gradually reduce towards the design minimum as embedment takes place (van Zyl, 2007). If the surface texture reduces below the design minimum due to embedment it will result in bleeding. Bleeding is the heaving of binder onto the surface due to the diminishing of voids in the seal structure as a result of texture loss, o which embedment is a major contributor. Surface texture decay is non-linear with time. Woodward et al. (2005) indicates that there are three phases of decay:

a) Rapid decay of texture. Mechanisms at work are stone orientation, embedment and abrasion of aggregate asperities until a stable mosaic is formed.

b) Slow decay of texture. Seal structure is stable with gradual loss of macro texture due to embedment.

c) Erratic behaviour where texture continues to decay but at variable rates. Factors such as seal aggregate size, base support and temperature aect the em- bedment. Laboratory testing by Woodward et al. (2005) indicated that larger seal aggregates have greater percentage of texture loss than smaller aggregates given the

2. LITERATURE REVIEW

same base and under the same loading conditions. This correlates with the TRH3 (2007) that suggests the construction of an inverted double seal on soft bases which are susceptible to embedment. The smaller stone acts as an armouring layer in this particular situation.

Embedment susceptibility can be detected with the Ball Penetration Test. The test was developed in an attempt to quantify the expected embedment and thereby the expected reduction of voids in the seal system (Milne, 2004). The Ball Penetration Test is used extensively in seal construction and consists of a standard Marshall hammer, dropped from a specied height onto a 19 mm steel ball. Temperature adjustments are made to the Ball Penetration readings depending on the base type. Base types that include bituminous materials show embedment increase with tem- perature as was found by Yaacob et al. (2007) on soft sand asphalt bases.

Embedment as a failure mechanism is considered to occur within the rst phase as described by Woodward et al. (2005). Milne (2004) documented that aggregate orientation occurs under the rollers (construction) and that surface texture depths stabilises within a couple of months after construction. Embedment failure is there- fore associated with early service life failure.