Civil Engineering: Marshall test and ProForma Download

MARSHALL METHOD OF ASPHALT-CONCRETE MIX DESIGN

(You can download Pro-Forma link given in the end of this page)
you can also read Marshall Mix design Graphs

INTRODUCTION:-

Bituminous mixes (sometimes called asphalt mixes) are used in the surface layer of road and airfield pavements. The mix is composed usually of aggregate and asphalt cements. Some types of bituminous mixes are also used in base course. The design of asphalt paving mix, as with the design of other engineering materials is largely a matter of selecting and proportioning constituent materials to obtain the desired properties in the finished pavement structure.

The desirable properties of Asphalt mixes are:

1. Resistance to permanent deformation: The mix should not distort or be displaced when subjected to traffic loads. The resistance to permanent deformation is more important at high temperatures.

2. Fatigue resistance: the mix should not crack when subjected to repeated loads over a period of time.

3. Resistance to low temperature cracking. This mix property is important in cold regions.

4. Durability: the mix should contain sufficient asphalt cement to ensure an adequate film thickness around the aggregate particles. The compacted mix should not have very high air voids, which accelerates the aging process.

5. Resistance to moisture-induced damage.

6. Skid resistance.

7. Workability: the mix must be capable of being placed and compacted with reasonable effort.

8. Low noise and good drainage properties: If the mix is to be used for the surface (wearing) layer of the pavement structure.

Marshall Stability and Hveem stabilometer tests are largely used for the routine testing. Criteria for the suitable mix design have been specified by the Asphalt Institute.

OBJECTIVE:- To design the Asphalt concrete mix using Marshall method.

MARSHALL METHOD OF MIX DESIGN:-

In this method, the resistance to plastic deformation of a compacted cylindrical specimen of bituminous mixture is measured when the specimen is loaded diametrically at a deformation rate

Of 50 mm per minute. There are two major features of the Marshall method of mix design. (i) density-voids analysis and (ii) stability-flow tests. The Marshall stability of the mix is defined as the maximum load carried by the specimen at a standard test temperature of 60°C. The flow value is the deformation that the test specimen undergoes during loading upto the maximum load. Flow is measured in 0.25 mm units. In this test, an attempt is made to obtain optimum binder content for the type of aggregate mix used and the expected traffic intensity.

STEPS OF DESIGN:-

1. Select aggregate grading to be used (Table 11.2)

2. Determine the proportion of each aggregate size required to produce the design grading.

3. Determine the specific gravity of the aggregate combination and asphalt cement.

4. Prepare the trial specimens with varying asphalt contents.

5. Determine the specific gravity of each compacted specimen.

6. Perform stability tests on the specimens.

7. Calculate the percentage of voids, and percent voids filled with Bitumen in each specimen.

8. Select the optimum binder content from the data obtained.

9. Evaluate the design with the design requirements.

APPARATUS:-

1. Mold Assembly: cylindrical moulds of 10 cm diameter and 7.5 cm height consisting of a base plate and collar extension

2. Sample Extractor: for extruding the compacted specimen from the mould

3. Compaction pedestal and hammer.

4. Breaking head.

5. Loading machine

6. Flow meter, water bath, thermometers

Marshall test

PROCEDURE:-

In the Marshall test method of mix design three compacted samples are prepared for each binder content. At least four binder contents are to be tested to get the optimum binder content. All the compacted specimens are subject to the following tests:

• Bulk density determination.

• Stability and flow test.

• Density and voids analysis.

Preparation of test specimens:-

The coarse aggregate, fine aggregate, and the filler material should be proportioned so as to fulfill the requirements of the relevant standards. The required quantity of the mix is taken so as to produce compacted bituminous mix specimens of thickness 63.5 mm approximately. 1200 gm of aggregates and filler are required to produce the desired thickness. The aggregates are heated to a temperature of 175° to 190°C the compaction mould assembly and rammer are cleaned and kept pre-heated to a temperature of 100°C to 145°C. The bitumen is heated to a temperature of 121°C to 138°C and the required amount of first trial of bitumen is added to the heated aggregate and thoroughly mixed. The mix is placed in a mould and compacted with number of blows specified. The sample is taken out of the mould after few minutes using sample extractor.

Bulk density of the compacted specimen:-

The bulk density of the sample is usually determined by weighting the sample in air and in water. It may be necessary to coat samples with paraffin before determining density. The specific gravity Gbcm of the specimen is given by

Stability test:-

In conducting the stability test, the specimen is immersed in a bath of water at a temperature of 60° ± 1°C for a period of 30 minutes. It is then placed in the Marshall stability testing machine (Fig. 7.1) and loaded at a constant rate of deformation of 5 mm per minute until failure. The total maximum in kN (that causes failure of the specimen) is taken as Marshall Stability. The stability value so obtained is corrected for volume (Table 11.1). The total amount of deformation is units of 0.25 mm that occurs at maximum load is recorded as Flow Value. The total time between removing the specimen from the bath and completion of the test should not exceed 30 seconds.

RESULTS AND CALCULATIONS:-

Following results and analysis is performed on the data obtained from the experiments

Bulk specific gravity of aggregate (Gbam):-

Since the aggregate mixture consists of different fractions of coarse aggregate, fine aggregate, and mineral filler with different specific gravities, the bulk specific gravity of the total aggregate in the paving mixture is given as

Maximu specific gravity of aggregate mixture (Gbam):-

The maximum specific gravity of aggregate mixture should be obtained as per ASTM D2041, however because of the difficulty in conducting this experiment an alternative procedure could be utilized to obtain the maximum specific gravity using the following equation:

Percent voids in compacted mineral aggregate (VMA):-

The percent voids in mineral aggregate (VMA) is the percentage of void spaces between the granular particles in the compacted paving mixture, including the air voids and the volume occupied by the effective asphalt content.

Percent air voids in compacted mixture (Pav):-

Percent air voids is the ratio (expressed as a percentage) between the volume of the air voids between the coated particles and the total volume of the mixture.

DETERMINATION OF OPTIMUM BINDER CONTENT:-

Five separate smooth curves are drawn with percent of asphalt on x-axis and the following on y-axis

• unit weight

• Marshall stability

• Flow

• VMA

• Voids in total mix (Pav)

Optimum binder content is selected as the average binder content for maximum density, maximum stability and specified percent air voids in the total mix. Thus

EVALUATION AND ADJUSTMENT OF MIX DESIGN:-

The overall objective of the mix design is to determine an optimum blend of different components that will satisfy the requirements of the given specifications ,This mixture should have:

1. Adequate amount of asphalt to ensure a durable pavement.

2. Adequate mix stability to prevent unacceptable distortion and displacement when traffic load is applied.

3. Adequate voids in the total compacted mixture to permit a small amount of compaction when traffic load is applied without bleeding and loss of stability.

4. Adequate workability to facilitate placement of the mix without segregation.

If the mix design for the optimum binder content does not satisfy all the requirements of specifications, It is necessary to adjust the original blend of aggregates. The trial mixes can be adjusted by using the following guidelines.

1. If low voids : The voids can be increased by adding more coarse aggregates.

2. If high voids : Increase the amount of mineral filler in the mix.

3. If low stability: This condition suggests low quality of aggregates. The quality of aggregates should be improved. (use different aggregate or use cement coated aggregate).