BENIFITS OF HYDRATED LIME IN HOT MIX ASPHALT

 BENIFITS OF HYDRATED LIME IN HOT MIX ASPHALT

Hydrated lime in hot mix asphalt (HMA) creates multiple benefits. A considerable amount of information exists in the current literature on hydrated lime’s ability to control water sensitivity and its well-accepted ability as an antistrip to inhibit moisture damage. However, recent studies demonstrate that lime also generates other effects in HMA. Specifically, lime acts as an active filler, anti-oxidant, and as an additive that reacts with clay fines in HMA. These mechanisms create multiple benefits for pavements:

1. Hydrated lime reduces stripping.

2. It acts as a mineral filler, stiffening the asphalt binder and HMA.

3. It improves resistance to fracture growth (i.e., it improves fracture toughness) at low temperatures.

4. It favorably alters oxidation kinetics and interacts with products of oxidation to reduce their deleterious effects.

5. It alters the plastic properties of clay fines to improve moisture stability and durability. \

The ability of lime to improve the resistance of HMA mixtures to moisture damage, reduce oxidative aging, improve the mechanical properties, and improve resistance to fatigue and rutting, has led to observed improvements in the field performance of lime-treated HMA pavements. Life cycle cost analyses have shown that using lime results in approximate savings of $20/ton of HMA mix while field performance data showed an increase of 38% in the expected pavement life.

Several highway agencies have proven the effectiveness of lime with cold-in-place recycled mixtures. Lime treatment of the CIR mixtures increases their initial stability which allows the early opening of the facility to traffic and improves their resistance to moisture damage which significantly extends the useful life of the pavement. 

Various methods are used to add hydrated lime to HMA. They range from adding dry lime to the drum mixer at the point of asphalt binder entry, to adding lime to aggregate followed by “marination” forseveral days. This report summarizes studies evaluating different modes of application. Because different methods have been used successfully, preferred modes of application vary from state to state. In 2003, the NLA produced an overview of how to add lime to HMA mixtures based on site visits (http://www.lime.org/howtoadd.pdf).

Hydrated lime is an additive that increases pavement life and performance through multiple mechanisms. This document consolidates recent studies and updates previous literature compilations on hydrated lime’s multiple benefits.

BACKGROUND

DEFINITIONS AND MECHANISMS

Stripping is commonly defined as "loss of adhesion between the aggregate surface and asphalt cement binder in the presence of moisture." HMA may experience loss of strength in the presence of moisture without visible evidence of debonding because water may affect the cohesive strength of the asphalt binder. Thus, the terms "water susceptibility" and "water sensitivity" are often used to designate the loss of strength or other properties of HMA in the presence of moisture.

The water susceptibility of HMA is controlled by:

• Aggregate properties

• Asphalt cement binder properties

• Mixture characteristics

• Climate

• Traffic

• Construction practices

• Pavement design considerations

It is usually the aggregate properties that dominate the water susceptibility properties of an HMA. Although asphalt cement properties may also affect water susceptibility, generally an aggregate related water susceptibility problem cannot be overcome by selecting an unmodified asphalt cement binder with superior ant stripping properties. 

Problem pavements under high traffic levels normally experience more rapid premature distress than similar pavements under low traffic loading. Compacted mixtures with high air voids are generally more likely to experience stripping than pavements that are compacted to low air void contents.

The hot and wet climates of the southern United States and the cold and relatively dry climates of the western United States experience the most dramatic stripping problems. In the southeastern states, the combination of high temperatures (low asphalt viscosity) and wet weather (in the summer months) cause stripping. The mountain and high desert areas of the west experience severe stripping problems due to moisture, freeze-thaw cycles (up to 230 air freeze-thaw cycles annually), and aggregates that have poor adhesion to asphalt in the presence of moisture. Most other regions also experience moisture problems that can manifest themselves through incompatibility between binders and aggregates and/or loss of cohesion in the bitumen due to moisture penetration.