MARSHALL STABILITY AND FLOW VALUES OF BITUMINOUS MIX SPECIMEN

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 specimens 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 0.01 mm units) are noted. The corrected Marshall stability value of each specimen is determined by applying the appropriate correction factor, if the average height of the specimen is not exactly 63.5 mm. The correction factors are given in Table.

Correction Factors for Stability and Flow Value

Volume of Specimen  in Cubic Centimeters	Approximate Thickness of Specimen in mm	Correction Factors
457-470	57.1	1.19
471-482	58.7	1.14
483-495	60.3	1.09
496-508	61.9	1.04
509-522	63.5	1.00
523-535	65.1	0.96
536-546	66.7	0.93
547-559	68.3	0.89
560-573	69.9	0.86

DENSITY AND VOIDS ANALYSIS OF BITUMINOUS MIX SPECIMEN

Soon after the compacted bituminous mix specimens have cooled to room temperature, and then the weight, average thickness and diameter of the specimen are noted. The specimens are also weighed in air and then in water. The bulk density value G_b of the specimen is calculated from the weight and volume. The voids analyses are made as given below:

 

Where,

            V_v = air voids in the mix, %

                        V_b = volume of bitumen

                        VMA = voids in mineral aggregates, %

                        VFB = voids filled with bitumen %

PREPARATION OF TEST SPECIMEN FOR MARSHALL METHOD

The coarse aggregates, fine aggregates and the filter materials should be proportioned and mixed specified gradation of mineral aggregates and bitumen binder as per IRG: 29–1968 are given in Table. The aggregates and filler are mix 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.

Approximately 1200g of aggregates and filler are taken and heated to a temperature of 175⁰ to 190⁰C. The compaction mould assembly and rammer are cleaned and kept pre-heated to a temperature of 100⁰C to 145⁰C. The bitumen is heated to temperature of 121⁰C 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 for 80/100grade bitumen may be around 154⁰C and that for 60/70grade, about 160⁰C.The mix is placed in a mould and compacted by rammer, with 50 blows on either side. The compacting temperatures may be about 138⁰C for 80/100grade bitumen and 149⁰C for 60/70grade. The compacted specimen should have thickness of 63.5mm. The weight of the aggregate taken may be may be suitably altered to obtain a thickness of 63.5±3.0 mm.  At least two specimens, but preferably three or four 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.

 

Specified Grading of Aggregates for Bituminous Concrete

Passing Sieve size	Retained Sieve size	Weight of Aggregate (gm)
20 mm	12.5 mm	120
12.5 mm	9.5 mm	120
9.5 mm	4.75 mm	240
4.75 mm	2.36 mm	216
2.36 mm	600 micron	216
600 micron	300 micron	72
300 micron	150 micron	72
150 micron	75 micron	84
75 micron	Filler	60
Total =		1200 gm

FLASH AND FIRE POINT TEST OF BITUMINOUS MATERIAL

Bituminous materials leave out volatiles at high temperatures depending upon their grade. These volatile vapors catch fire causing a flash. This condition is very hazardous and it is therefore essential to determine this temperature for each bitumen grade, so that the paving engineers may restrict the mixing or application temperatures well within the limits. The flash point is the lowest temperatures at which flash occurs due to the ignition of the volatile vapors when a small flame is brought in con tact with the vapors a bituminous product, gradually heated under standardized conditions. As mentioned above, this test is an indication of the critical temperature at and above which suitable precautions should be taken to prevent fire hazards during application. When the bituminous material is further heated to a higher temperature, the material itself catches fire and continues to burn; the lowest temperature causing this condition is the fire point. The fire point is always higher than the flash point of a material. ISI vide specifications I.S: 1209-1258 gives the following definitions.

Flash point-“The flash point of a material is the lowest temperature at which the vapor of substance momentarily takes fire in the form of a flash under specified condition of test.” 

Fire point –“The fire point is the lowest temperature at which the material gets ignited and burns under specified condition of test.”

PENETRATION TEST OF BITUMINOUS MATERIAL

The consistency of bituminous material depends upon several factors such as constituents, temperature, etc. at temperature ranges between 25⁰ and 50⁰ C most of the paving bitumen grades remain in semi solid or in plastic states and their viscosity is so high that they do not flow as liquid. The consistency of the bitumen is determined by penetration test, which is very simple test and indirect method also.

BITUMEN-RUBBER

An experimental pavement bound with a bitumen-rubber mixture was laid in Holland in 1929. The first use of this binder in the United States was made in 1947 when a section was laid Akren, Ohio. The pavement was conventional, except that finely divided rubber amounting to 5 – 7.5 % of the bitumen by weight was included since that time experimental roads have been laid by (among others) the state highway departments of Virginia, Ohio, Texas, Massachusetts, California, Colorado, and Utah the cities of New York and Baltimore and the great Britain. In addition, bituminous binders modified with rubber have used for seal coats.

Certain advantages are attributed to rubber additives. For example, skid tests in Virginia, reported in 1950, showed very little improvement in coefficients of friction on newly laid pavements but considerable advantage after six months early test by the Bureau of public roads indicated both favorable and unfavorable results. Rubber added in powdered form brought unfavorable consequence, when preblended with the asphalt, it improved the stability of some but not all laboratory specimens. Recently, greater elasticity reduces temperature susceptibility and brittleness and longer life in the pavement have been claimed. In sum however the conclusions of a 1954 analysis by the Bureau of public roads still appears to be valid. It stands than an appraisal of the real economic value of the addition of rubber to asphalt must wait on further observation of the behavior of experimental pavements under the influence of age, weather and traffic.