marshall test

TITLE :

MARSHALL STABILITY TEST

INTRODUCTION :

Bituminous mixes (some times 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 coarse. The design of asphalt paving mix, as with the de sign of other engineering materials 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 defo rmation: 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 s hould 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 drain age 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 prepare standard specimens of asphalt concrete for measurement of stability and flow in the Marshall apparatus and to determine density, percentage air voids, and percentage of aggregates voids filled with binder.

THEORY : MARSHAL MIX DESIGN

The Marshall stability and Àow test provides the performance prediction measure for  the Marshall mix designmethod.The stability portion of the test measures the maximum load supported by the test specimen at aloading rate of 50.8 mm/minute.Load is applied to the specimen till failure, and the maximum load is designatedas stability.During the loading, an attached dial gauge measures the specimen¶s plastic Àow (deformation) dueto the loading.The Àow value is recorded in 0.25 mm (0.01 inch) increments at the same time when themaximum load is recorded.The important steps involved in marshal mix design are summarized next. 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 wei ghting 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 Gbcm = Wa / Wa-Ww where, Wa = weight of sample in air (g) Ww = weight of sample in water (g)

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 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. 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. It is a very important instrument ideal for carrying out Marshall stability test of  bituminous mixture. It is easy to operate, highly efficiency rate and has high degree of automation. The apparatus is suitable for conducting Marshall stability test of bituminous mixture. The apparatus can truly judge the fail point of bituminous mixtures to design the match ratio or check the construction quality of bitumen road surface. The Marshall stability of a mix is defined as a maximum load carried by a compacted specimen at a standard test temperature of 60º C. The test is applicable to hot mix designs using bitumen and aggregates upto a maximum size of 25 mm

The test is applicable to hot mix designs using bitumen and aggregates upto a maximum size of 25mm. In this method, the resistance to plastic deformation of  cylindrical specimen of bitumenous mixture is measured when the same is loaded at periphery at 5 cm per min. This test procedure is used in designing and evaluating bituminous paving mixes. The test procedure is extensively used in routine test programmes for paving jobs. There are two major features of the Marshall method of  designing mixes namely, a) Density ± voids analysis b) Stability ± flow tests. The marshall stability of mix is defined as a maximum load carried by a compacted specimen at a standard test temperature of 60ºC. The flow value is deformation the marshall test specimen under goes during the loading upto the maximum load, 0.25 mm units. In this test and attempt is made to determine optimum binder content for  the type of aggregate mix and traffic intensity.

APPAR ATUS :

1. 2. 3. 4.

Marshall compression machine Oven Weighing Scale Stop watch

PROCEDURE :

1. The aggregates,graded according to JKR/SPJ/1988 was dried at 105 -110°C and sufficient amount was weighed ( about 1200 g) to give a height of 63 ± 1.3 mm when compacted in the mould. 2. The required quantity of bitumen was weighed out and heated to temperatures, which will give a viscosity of 170 ± 20² mm/s.

3. The aggregate was heated in oven to a temperature not higher than 28°C above the binder temperature. 4. A crater was formed in the aggregate contained in a heated mixed bowl, the binder poured in and the aggregate was coated when the mixing was carried out. The mixing temperature shall be within the limit set for the binder  temperature.

5. A thoroughly cleaned mould was heated on a hot plate or in oven to a temperature between 93 and 149°C, the mou ld was 101.6 mm diameter by 76.2 high and provided with base plate and extension collar. 6. The mixture is then placed in a heated Marshall mould with a collar and base and the mixture is spaded around the sides of the mould. A filter paper is placed under the sample and on top of the sample.

7. The mould is placed in the Marshall compaction pedestal. 8. The material is compacted with 50 blows of the hammer (or as specified), and the sample is inverted and compacted in the the other face with same number  of blows.

9. After compaction, the mould is inverted. With collar on the bottom, the base is removed and the sample is extracted by pushing it out the extractor. 10.The sample is allowed to stand for the few hours to cool.

11.The mass of the sample in air and when submerged is used to measure the density of specimen, so as to allow, calculation of the void properties.