Design Method

The SMA design procedure consists of the following stages:

• Selection of the design aggregate mix using an analysis of the impact of the coarse aggregate content on SMA properties

• Determination of the optimum design content of the binder for the selected gradation

Step by step, the design proceeds as follows (for SMA 0/11).

A. Design an aggregate mix

1. Determine the properties of raw materials.

1.1 Gradation of aggregates

1.2 Penetration at 25°C and softening point (R&B) of the binder

1.3 Establishing the compaction temperature for preparing samples (adjusted to the type of binder)

2. Design an aggregate mix gradation according to the required gradation limits; using this method the aggregate mix No. 3 (referred to later as mix 3) is evolving.*

3. (Based on experience)[35] [36]‘ we arbitrarily accept an optimal binder content for mix 3.

4. At this point, mix 3 has an optimal binder content (temporary); next exam­ine the influence of changes to the aggregate mix on SMA features.

5. Design four new variants of an SMA aggregate mix in the following way.

5.1 The binder and filler contents remain unchanged.

5.2 Design four new aggregate mixes.

5.2.1 Mix 1—decrease the content of HDK by -5.0% to -7.0%, and increase the content of fine aggregate by + 5.0 to + 7.0%

5.2.2 Mix 2—decrease the content of HDK by -2.5% to -3.5%, and increase the content of fine aggregate by + 2.5% to + 3.5%

5.2.3 Mix 4—increase the content of HDK by + 2.5% to + 3.5%, and decrease the content of fine aggregate by -2.5% to -3.5%

5.2.4 Mix 5—increase the content of HDK by + 5.0% to + 7.0%, and decrease the content of fine aggregate by -5.0% to -7.0%

— With changes to the content of HDK fraction (> 4 mm), appro­priately decrease or increase the content of the fine fraction (0.09/4 mm); the filler remains unchanged.

— While changing the quantities of HDK and fine fraction, the inter­nal proportions of fractions (e. g., 4/8 and 8/11) should probably be maintained at a constant level.

5.3 Produce four Marshall samples of each mix (1 through 5), each con­taining the same quantity of binder that was adopted for mix 3 as optimal.

5.4 For each mix, determine the following:

5.4.1 Stability according to Marshall

5.4.2 The binder volume

5.4.3 The content of air voids in compacted 2 x 50 samples (M)

5.4.4 The content of air voids in the aggregate mix (Mk)

5.4.5 The voids filled with binder (Sv)

5.5 Draw graphs of relationships between the elements in Step 5.4 and the content of coarse aggregate HDK.

5.6 Analyze the parameters of the mixes (1 through 5) and select the best one, based on:

5.6.1 The analysis of the inflection point at the relationship between the content of air voids and the content of HDK in SMA samples

5.6.2 The designer’s experience

5.7 Based on the results of the analysis of the previous item, select the best gradation curve or determine a new one (i. e., mix 6).

B. Design an optimum binder content

5.8 Based on the results from Step 5.4, the optimum binder content for the selected gradation of 5.7 may be determined by producing a series of Marshall samples again, with a binder content 0.3% (m/m) higher and lower than the amount initially adopted as optimal for mix 3.

5.9 Select an optimal variant of the binder content based on the following:

5.9.1. Stability according to Marshall greater than or equal to 6 kN

5.9.2. Binder volume

Greater than or equal to 14.5% (v/v) for SMA 0/11 Greater than or equal to 15.0% (v/v) for SMA 0/8

5.9.3. The content of air voids in compacted 2 x 50 (M) SMA sam­ples, which should be from 3.0 up to 4.5% (v/v)

5.10 Conduct additional tests for the optimum content of binder in SMA.

5.10.1. Air void content in SMA samples compacted with an excessive effort of 2 x 100, minimum required greater than or equal to 2.5% (v/v)

5.10.2. Resistance for rutting, 10,000 cycles at a temperature of 50°C, required maximum less than 1.6 mm

5.10.3 Test draindown with Schellenberg’s method, required to be less than 0.3% (m/m)

5.10.4 If requirements are satisfied, design is completed.

7.3.3 Summary

To sum up the Czech method, it is worth noting that, despite leaving the simple basic principles of design, it facilitates examining the influence of the coarse fraction on the properties of an SMA mixture. Generally it takes into consideration all of the essential rules and relationships explained in greater detail in Chapter 6.