Saturday, March 31, 2012

DETERMINATION OF OPTIMUM BITUMEN CONTENT

Five graphs are plotted with values of bitumen content against the value of
                        i)          Density Gb g/cm3
                        ii)         Marshall stability, S kg
                        iii)         Voids in total mix, Vv %
                        iv)        Flow value, F (0.01 mm units)
                        v)         Voids filled with bitumen, VFB%.
Let the bitumen contents corresponding to maximum density be B1, corresponding to maximum stability be B2 and that corresponding to the specified void content Vv (4.0%in the case of dense AC mix) be B3. Then the optimum bitumen content for mix design is given by:
B0 = (B1 + B2 + B3) / 3
The values of flow and VFB are found from the graphs, corresponding to bitumen content B0. All the design values of Marshall Stability, flow, voids and VFB are checked at the optimum bitumen content B0, with the specified design requirements of the mix.

RELATED FIGURES/GRAPHS ARE GIVEN IN NEXT ARTICLE : LABORATORY TEST RESULT OF MODIFIED BITUMEN

Friday, March 30, 2012

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 Gb of the specimen is calculated from the weight and volume. The voids analyses are made as given below:

Where,
            Vv = air voids in the mix, %
                        Vb = volume of bitumen
                        VMA = voids in mineral aggregates, %
                        VFB = voids filled with bitumen %

SPECIFIC GRAVITY OF COMPACTED SPECIMEN OF BITUMINOUS MIX


The specific gravity values of the different aggregates, filler and bitumen used are determined first. The theoretical specific gravity Gt of the mix is given by-
 Where,
W1 = percent by weight of coarse aggregate
                        W2 = percent by weight of fine aggregate
                        W3 = percent by weight of mineral filler
                        W4 = percent by weight of bitumen
                        G1 = apparent specific gravity of coarse aggregate
                        G2 = apparent specific gravity of fine aggregate
                        G3 = apparent specific gravity of mineral filler
            G4 = apparent specific gravity of bitumen

Tuesday, March 27, 2012

DESIGN REQUIREMENTS OF THE BITUMINOUS MIX


As per IRC: 29-1968, when the specimens are compacted with 50 blows on either face, the design AC mix should fulfill the following requirements of the mix.

i)          Marshall stability value kg (minimum) = 340
ii)         Marshall flow value, 0.25 mm units = 8 to 16
iii)         Voids in total mix, Vv% =3 to 5
iv)        Voids in mineral aggregates filled with bitumen, VFB% =75 to 85

The highest possible Marshall Stability values in the mix should be aimed at consistent with the other three requirements mentioned above. In case the mix designed does not fulfill any and 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. IRC: 29-1968

Saturday, March 24, 2012

DESIGN PROCEDURE OF BITUMINOUS MIXES


In the design procedure of a bituminous mix, following steps are involved.

i) Selection of aggregate to be employed in the mix.

ii) Selection of aggregate grading.

iii) Determination of the proportion of each aggregate required to develop the design grading.

iv) Determination of the specific-gravity of the aggregate blend.

v) Determination S.G. of the bitumen.

vi) Make up trial specimens with varying bitumen contents.

vii) Determination of the specific-gravity (S.G) of each compacted specimen.

viii) Make stability test on the specimen.

ix) Determine the percentage of voids in each paving specimen and if the design method in use requires it, calculate the VMA and percent voids filled with bitumen.

x) Selection of the optimum bitumen content from the obtained data.

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