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.

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MARSHALL STABILITY TEST.(ASTM – D – 1559 & MS-2)

 MARSHALL STABILITY TEST.(ASTM – D – 1559 & MS-2)

INTRODUCTION:

Bruce Marshall, a former Bituminous Engineer with the Mississippi State Highway Department, formulated the concepts of the Marshall method of designing paving mixtures. The U.S.Army Crops of Engineers, through extensive research and correlation studies, improved and added certain features to Marshall’s test procedure, and ultimately developed mix design criteria. The original Marshall method is applicable only to hot-mix asphalt paving mixtures containing aggregates with maximum sizes of 25mm or less. The aggregate size more than 25mm use the Modified Marshall method.  

This method covers the measurement of the resistance to plastic flow of cylindrical specimens of bituminous paving mixture loaded on the lateral surface by means of the Marshall apparatus.

OBJECTIVE:

To determine the stability, flow, voids, voids in mineral aggregates, voids filled with asphalt and density of the asphalt mixture by Marshall stability test.

APPARATUS:

a) Specimen Mould Assembly – Mould cylinders 101.6mm(4 in.) in diameter by 75mm(3 in.) in height, base plates, and extension collars.

b) Specimen Extractor – Steel disk with a diameter 100mm, and 12.7mm thick for extracting the compacting specimen from the specimen mould with the use of the mould collar. A suitable bar is required to transfer the load from the proving ring adapter to the extension collar while extracting the specimen.

c) Compaction Hammer – The compaction hammer shall have a flat, circular tamping face and a 4.5kg(10 lb) sliding weight with a free fall of 457mm (18 in.). Two compaction hammers are recommended.

d) Compaction Pedestal – The compaction pedestal shall consist of 200X200X460mm(8X8X18 in.) wooden post capped with a 305X305X25mm(12X12X1 in.) steel plate. The pedestal should be installed on concrete slab so that the post is plumb and the cap is level. Mould holder provided consisting of spring tension device designed to hold compaction mould centered in place on compaction pedestal.

e) Breaking Head – It consists of upper and lower cylindrical segments or test heads having an inside radius of curvature of 50 mm. The lower segment is mounted on a base having two vertical guide rods, which facilitate insertion in the holes of upper test head.   

f) Loading Machine – The loading machine is provided with a gear system to lift the base in upward direction./ on the upper end of the machine, a calibrated proving ring of 5 tonne capacity is fixed. In between the base and the proving ring, the specimen contained in test head is placed. The loading machine produces a movement at the rate of 50mm per minute. Machine is capable of reversing its movement downward also.

g) Flow meter – One dial gauge fixed to the guide rods of a testing machine can serve the purpose. Least count of 0.25mm(0.01 in.) is adequate.

h) Oven or hot plates

i) Mixing apparatus.

j) Thermostatically control water bath.

k) Thermometers of range 0 – 3600C with 10C sensitivity.

PROCEDURE:

In the Marshall method each compacted test specimen is subjected to the following tests and analysis in the order listed below:

i) Bulk density determination

ii) Stability and flow test

iii) Density and voids analysis

At least three samples are prepared for each binder content.

PREPARATION OF TEST SPECIMENS:

The coarse aggregates, fine aggregates and the filler material should be proportioned and mixed in such a way that final mix after blending has the gradation with in the specified range. The aggregates and filler are mixed together in the desired proportion as per the design requirements and fulfilling the specified gradation. The required quantity of the mix is taken so as to produce a compacted bituminous mix specimen of thickness 63.5mm, approximately. 

Preparation of Mixtures: Weigh into separate pans for each test specimen the amount of each size fraction required to produce a batch that will result in a compacted specimen 63.5 +/- 1.27mm(2.5 +/-0.05 in.) in height. This will normally be about 1200gm(2.7 lb.). It is generally to prepare a trial specimen prior to preparing the aggregate batches. If the trial specimen height falls outside the limits, the amount of aggregate used for the specimen may be adjusted using:

                                                         63.5 (mass of aggregate used)

Adjusted mass of aggregate    =

                                                       Specimen height (mm) obtained

Take the sample as mentioned above, and heated to a temperature of 1750 to 1900C. The compaction mould assembly and hammer are cleaned and kept pre-heated to a temperature of 1000C to 1450C. The bitumen is heated to temperature of 1210 to 1380C and the required quantity of first trial percentage of bitumen (say, 3.5% by weight of mineral aggregates) is added to the heated aggregate and thoroughly mixed using a mechanical mixer or by hand mixing with trowel. The mixing temperature may be 1530 to 1600C. The mix is placed in a mould and compacted by hammer, with 75 blows on either side (for light compaction it is 50 blows). The compaction temperature may be 1380 to 1490C. The compacted specimen should have a thickness of 63.5 +/- 3.0mm. Three specimens should be prepared at each trial bitumen content, which may be varied at 0.5 percent increments up to about 7.5 or 8.0 percent.

Marshall Stability and Flow values: The specimens to be tested are kept immersed under water in a thermostatically controlled water bath maintained at 600 +/- 10C for 30 to 40 minutes. The specimen are taken out one by one, placed in the Marshall test head and the Marshall Stability value (maximum load carried in kg. before failure) and the flow value (the deformation the specimen undergoes during loading up to the maximum load in 0.25mm units) are noted. The corrected Marshall stability value of each specimen is determined by applying the appropriate correction factor.

The following tests are determined first, to find out the density, voids, VMA and VFB.

TESTS & CALCULATIONS:

The specific gravity and apparent specific gravity values of the different aggregates, filler and bitumen used are determined first.

i) Bulk specific gravity of aggregate ‘Gsb’ is given by:

                             P1 + P2 +. ……+ Pn

Gsb   =      

                     P1/G1 + P2/G2 +. ……+ Pn/Gn

Where,        Gsb             = Bulk specific gravity for the total aggregate.

          P1, P2, Pn           = Individual percentages by weight of aggregate. 

                  G1, G2, Gn = Individual bulk specific gravities of aggregate.

ii) Effective specific gravity of aggregate ‘Gse’ is given by:

Gse   = (100-Pb) / ((Pmm/Gmm)-(Pb/Gb))

Where, Gse          = Effective sp.gravity of aggregate, constant for all at 5% bitumen content.

             Gmm       = Maximum sp.gravity of paving mixture determine by Vacuum pump test                                     (ASTM – D – 2041).

              Pb           = Bitumen content, percent by total weight of mixture.

             Gb           = Specific gravity of Bitumen.

         

 iii) Maximum specific gravity of mixture ‘Gmm’ is given by:

                                   100

Gmm          =

                         Ps/Gse + Pb/Gb     

Where, Gmm       = Maximum specific gravity of paving mixture (no air voids)

              Ps           = Aggregate content, percent by total weight of mixture

              Pb           = Bitumen content, percent by total weight of mixture

             Gse          = Effective specific gravity of aggregate

             Gb           = Specific gravity of bitumen

iv) Bitumen absorption ‘Pba’ is given by:

                             Gse - Gsb   

Pba   =       100                      Gb

                             Gse Gsb

Where,       Pba   = Absorbed bitumen, percent by weight of aggregate

                    Gse   = Effective specific gravity of aggregate

                   Gsb   = Bulk specific gravity of aggregate

                   Gb     = Specific gravity of bitumen

v) Effective bitumen content of a paving mixture ‘Pbe’ is given by:

                            Pba

Pbe   =          Pb     -     Ps

                            100            

vi) Voids in mineral aggregate in compacted paving mixture ‘VMA’ is given by: 

                              Gmb Ps

VMA     =  100  -

                                 Gsb

Where, VMA       = Voids in mineral aggregate, percent of bulk volume

              Gsb         = Bulk specific gravity of total aggregate

              Gmb       = Bulk specific gravity of compacted mixture

              Ps           = Aggregate content, percent by total weight of mixture

vii) Air voids in compacted mixture ‘Va’ is given by:

                           Gmm  -  Gmb

Va     =  100 X

                                  Gmm

Where,       Va     = Air voids in compacted mixture, percent of total volume

                  Gmm  = Maximum specific gravity of paving mixture

                   Gmb  = Bulk specific gravity of compacted mixture

viii) Voids filled with bitumen in compacted mixture ‘VFB’ is given by:

                 100(VMA – Va)            

VFB =

                               VMA

Where,       VFB = Voids filled with bitumen, percent of VMA

                  VMA  = Voids in mineral aggregate, percent of bulk volume

                  Va      = Air voids in compacted mixture, percent of total volume

DETERMINATION OF OPTIMUM BITUMEN CONTENT:

Six graphs are plotted with values of bitumen content against the values of:

a) Density ‘Gmb’ g/cc, 

b) Marshall Stability, S kg, 

c) Voids in total mix, Va %, 

d) Flow value, F (0.25mm units), 

e) Voids filled with bitumen, VFB % & 

f) Voids in mineral aggregate, VMA %.

Let the bitumen contents corresponding to maximum density be B1, corresponding to maximum stability be B2 and that corresponding to the specified voids content Va (4.0% in the case of dense AC mix) be B3. Then the Optimum Bitumen Content is given by:

Optimum Bitumen Content (OBC) = (B1 + B2 + B3)/3

The values of flow and VFB are found from the graphs, corresponding to bitumen content OBC. All the design values of Marshall stability, flow, voids and VFB are checked at the Optimum Bitumen Content, with the specified design requirements of the mix.

The highest possible Marshall stability values in the mix should be aimed at consistent with the other four requirements mentioned above. In case the mix designed does not fulfill any one or more of the design requirements, the gradation of the aggregates or filler content or bitumen content or combination of these are altered and the design tests are repeated till all the requirements are simultaneously fulfilled.

JOB MIX FORMULA:

The proportions in which the different aggregates, filler and bitumen are to be mixed are specified by weight or by volume for implementation during construction.

Caution: Mixes with high Marshall stability values and very low Flow values are not desirable as the pavements of such mixes may be brittle and are likely to crack under heavy traffic.

Correction Factors for Std. Marshall

Correction Factors for Modified Marshall

Note:

i) Water Sensitivity: The loss of stability on immersion in water at 60oC for 24 hrs. The allowable limit is max 25% for Base Course and 20% for Wearing Course of retained strength.

ii) Marshall Quotient (Stiffness): is the ratio of stability and flow. Allowable limits

for base course = 400

for wearing surfaces = 500.

MARSHALL CURVES:

Bitumen(%) ’vs’ Stability(kg)

Bitumen(%) ‘vs’ Density (g/cc

Bitumen(%) ‘vs’ Flow(mm) 

Bitumen (%) ‘vs’ Voids(%)

Bitumen(%) ‘vs’ VFB(%)

Bitumen(%) ‘vs’ VMA(%)

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Bin Hafeez General Contracting Site Plant

 

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