Thesis on blending of Aggregates for Bituminous Mix Design without conducting individual gradation

 Introduction

Bituminous mixes like Bituminous Macadam (BM), Dense Bituminous Macadam (DBM), and Bituminous Concrete (BC) require precise gradation of aggregates to ensure the pavement's durability, strength, and workability. This thesis explains the method used to calculate the proportions of aggregates for each mix type, incorporating theoretical and actual retained weights for 6mm and dust. Additionally, adjustments for 6mm and dust for BM, BC, and DBM are discussed to highlight their role in achieving better gradation control.

Objectives

To explain the methodology for calculating theoretical and actual retained weights for BM, DBM, and BC.
To present the rationale for adjustments in 6mm and dust amounts for BM, BC, and DBM.
To provide a clear understanding of how the final proportions of aggregates are derived.

Data Layout and Structure

There are two tables for BM, DBM, and BC.

Left-Side Tables:

These tables display theoretical retained weights calculated using the passing percentages, conforming to the midpoint of the gradation limits for each sieve size. This theoretical calculation serves as a reference point for the actual retained weights.

Right-Side Tables:

These tables display retained weights when 1 kg each of 6 mm and dust sample is passed through the 4.75 mm and 2.36 mm sieves respectively. Based on the retained weights, Contribution Factors (CF), Adjusted Weight, and proportions of the aggregates are worked out. Both tables are essential for deriving the final gradation of aggregates for BM, DBM, and BC.

Methodology of the Mix Design

1. Sieve Analysis

The sieve analysis identifies the distribution of aggregate particle sizes. Sieve sizes are selected as per the grading requirements for BM, DBM, and BC. Passing and retained weights are theoretically calculated for each sieve to ensure the mix conforms to the desired gradation.

2. Theoretical Retained Weights (Left-Side Table)

Theoretical retained weights are calculated using the percentage passing for each sieve, conforming to the midpoint of the specification limits for each type mix.

Retained Weight (g) = Weight of Sample × (Passing Percentage at Previous Sieve −Passing Percentage at Current Sieve)

This approach allows us to estimate the expected retained weights for each sieve to prepare 1200 grams of sample required for the Marshal Density Test.

3. Actual Retained Weights (Right-Side Table)

In contrast to theoretical retained weights, actual retained weights are determined by physically passing a 1 kg sample of material through the sieves. The retained weights for 6mm and dust are measured directly. This provides a more accurate representation of the particle distribution.

4. Calculation of Contribution Factor

Using the actual retained weights, the contribution factor for each aggregate is computed as shown under:

Contribution Factor (CF) = Retained Weight for Aggregate / Total Weight of Sample (1000 grams)

For stone dust, CF should be within 0.11 to 0.15. The stone dust falling lower or exceeding the range should be discarded. The CF for 6mm grit should preferably be within 0.25 to 0.40.

5. Calculation of Adjusted Weight:

Bituminous Macadam:

For 40 mm aggregate, the retained weight corresponding to 13.2 mm sieve is considered, for it directly contributes a significant portion of coarse aggregate.

For 10 mm & 20 mm aggregates, the retained weight corresponding to a 4.75 mm sieve is considered. 

Then to derive the adjusted weight for 6 mm, the retained weight corresponding to 0.300 mm sieve is divided by the contribution factor and subtracting the retained weight corresponding to 2.36 mm sieve.

For dust, similarly, the retained weight corresponding to the passing below 0.075 mm sieve is divided by the contribution factor and subtracted from the sum of retained weights corresponding to 0.300 mm & 0.075 mm sieves to get the adjusted weight.

Dense Bituminous Macadam:

For 20 mm and 10 mm aggregates, the retained weight corresponding to a 4.75 mm sieve is considered. 

Then to derive the adjusted weight for 6 mm,  the retained weight corresponding to 4.75 mm sieve is subtracted from the weight retained on 0.3 mm sieve divided by the contribution factor for the contribution factor is not zero like in the case of coarse aggregate.

For dust, similarly, the retained weight corresponding to the passing below 0.075 mm sieve is divided by the contribution factor to get the adjusted weight.

Bituminous Concrete:

For 10 mm aggregates, the retained weight corresponding to a 4.75 mm sieve is considered. 

Then to derive the adjusted weight for 6 mm,  the retained weight corresponding to 4.75 mm sieve is subtracted from the weight retained on 0.300 mm sieve divided by the contribution factor for the contribution factor is not zero like in the case of coarse aggregate.

For dust, similarly, the retained weight corresponding to the passing below 0.075 mm sieve is divided by the contribution factor and the sum of retained weights corresponding to 1.18 mm and 0.300 mm sieves are subtracted to get the adjusted weight.

Calculation of Proportion

The proportion of each aggregate in the total mix is calculated as:

Proportion (%) = Adjusted Weight of the Aggregate * 100 / Total Adjusted Weight of all the aggregates.

This final proportion guides the blending of materials to produce BM, DBM, and BC conforming to specified Grading Limits corresponding to the respective sieve.

Replica of Theoretical and Actual Tables

Below are the theoretical and actual retained weight tables for BM, DBM, and BC.

Bituminous Macadam (BM)

(Left side table)

Sieve	Limits		JMF	Retain %	Retained Weight
45	100		100	0	0
37.5	90	100	95	5	60
26.5	75	100	88	7	84
13.2	35	61	48	40	480
4.75	13	22	18	30	360
2.36	4	19	12	6	72
300	2	10	6	6	72
75	0	8	4	2	24
Pan				4	48
Total					1200

(Right Side Table)

Agg Size	40mm	20mm	10mm	6mm	Dust
Sample Weight	1000	1000	1000	1000	1000
Sieve	Retained Weight
37.5	0
26.5		0
13.2			0
4.75				400
2.36					139
Total	0	0	0	400	139
CF	0	0	0	0.4	0.139
Weight	480	360	360	108	249
Proportion	31	23	23	7	16

Dense Bituminous Macadam (DBM)

(left side table)

Sieve	Limits		JMF	Retain %	Retained Weight
37.5	100		100	0	0
26.5	90	100	95	5	60
19	71	95	83	12	144
13.2	56	80	68	15	180
4.75	38	54	46	22	264
2.36	28	42	35	11	132
300	7	21	14	21	252
75	2	8	5	9	108
Pan				5	60
					1200

(right side table)

Agg Size	20mm	10mm	6mm	Dust
Sample Weight	1000	1000	1000	1000
Sieve	Retained Weight
26.5	0
13.2	0	0
4.75		0	400
2.36				139
Total	0	0	400	139
CF	0	0	0.4	0.139
Adjusted Weight	264	264	366	432
Proportion	20	20	28	33

Bituminous Concrete (BC)

(left side table)

Sieve	Limits		JMF	Retain %	Retained Weight
19	100		100	0	0
13.2	90	100	95	5	60
9.5	70	88	79	16	192
4.75	53	71	62	17	204
2.36	42	58	50	12	144
1.18	34	48	41	9	108
600	26	38	32	9	108
300	18	28	23	9	108
150	12	20	16	7	84
75	4	10	7	9	108
Pan				7	84
					1200

(left side table)

Agg Size	10mm	6mm	Dust
Sample Weight	1000	1000	1000
Sieve	Retained Weight
13.2	0
4.75	0	400
2.36			139
Total	0	400	139
Contribution Factor	0	0.4	0.139
Adjusted Weight	204	66	388
Proportion	31	10	59

Verification & Recasting of Gradation

To ensure the derived proportions yield the desired gradation, it is essential to verify the mix by testing a sample prepared using the calculated proportions. If any of the passing limits do not conform to the specified gradation limits, recasting the gradation and making slight adjustments to the proportions would fine-tune the gradation to meet the required specifications without compromising the overall balance of the mix. This straightforward approach eliminates the need to conduct physical gradation, enabling streamlining the process while ensuring optimal performance of the final asphalt mix. 

Table showing Recast Gradation

Conclusion

Using theoretical and actual retained weights ensures the design mix meets the specified gradation limits for BM, DBM, and BC. Theoretical weights provide a reference point, while actual retained weights reflect the true particle distribution. Adjustments made for 6mm and dust in BM, BC, and DBM help to maintain proper gradation, ensuring that the final mix meets specification requirements.

During the execution of work at the HMA plant site, quality control engineers have consistently faced significant challenges in ensuring that the material adheres to the specified grading limits. Adopting this method provides a practical approach to adjusting the proportions of aggregates. However, since the research is still in its early stages, it is advisable to exercise caution by conducting a comprehensive combined gradation analysis of the mix to ensure optimal performance of the asphalt pavement.