Instructions for Using SEAM in "Semi-Rigid" Mixes

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1.) ADVANTAGES OF USING SEAM

SEAM addition to asphalt mixes improves pavement resistance to rutting and its strength and durability. SEAM can be used with all conventional asphalt mixes. As SEAM improves mix stability to a high degree, it is also particularly suitable for use in mixes with rounded sands and sand-asphalt mixes, which normally may not be considered for use because of their low stability.

2.) DESCRIPTION OF "SEMI-RIGID" MIX

SEAM may be used in mixes made with conventional asphalt cement in mixes in binder weight ratios from 30%SEAM/70%Asphalt Cement to 50%SEAM/50%Asphalt Cement. SEAM melts readily into hot asphalt cement and becomes a part of the binder in asphalt mixes.

The SEAM/Asphalt Cement ratio recommended for most mixes is 40%SEAM/60%Asphalt Cement. This is suitable for use in strong road structures for which the static deflection does not exceed 1.25 mm.

The advantage of the 40%SEAM/60%Asphalt Cement binder mixes is that they can be handled and compacted similar to conventional asphalt mixes.

The amount of asphalt cement saved by this type of mix is 30% and it is recommended that this is replaced by 1.7 times this weight by SEAM, based on experience (i.e. replacement value, R = 1.7).

(NOTE: The 50%SEAM/50%Asphalt Cement "rigid" mix is stronger and stiffer and provides more rutting resistance to a pavement, although it is more difficult to use. It has a shorter TAC (time available for compaction) than the semi-rigid SEAM mixture and should not be paved in a thickness less than 5 cm. The rigid SEAM mix should only be considered when the delivery time from the mixing plant to the paving site is short and all roller compaction can be completed before the time the mix temperature cools to 110 C. This mix can only be used in very strong road structures with static deflection less than 0.5 mm. The rigid SEAM mix is recommended only for experienced SEAM users.)

3.) DESIGN OF SEAM MIXES

SEAM mixes may be designed in three ways.

3.1) The Marshall design of a conventional asphalt mix may be used to determine the SEAM content and asphalt cement content required for a SEAM mix using the following formula:

SEAM binder % = 10 000 AR .
10 000R-100Ps(R-1)+APs(R-1)
where:
A = Weight % asphalt in conventional mix design (total mix weight basis)
R = SEAM substitution ratio (R= 1.5 to1.7 recommended based on experience)
Ps = Weight % SEAM in total "binder"

The mix recommended is the "semi-rigid" mix (40%SEAM/60%Asphalt) and the R recommended is 1.7 based on experience.

Example (AC 20 mix used in Tianjin):

Asphalt cement content in conventional mix = 4.6% (total mix weight basis)

Total binder in SEAM mix = 10 000(4.6)1.7 .
10 000(1.7)-(100)(40)(1.7-1) + 4.6(40)(1.7-1)
= 5.5% (total mix)
SEAM % = 5.5 x 0.4 = 2.2%
Asphalt cement % = 5.5 x 0.6 = 3.3%

The reason why a formula can be used without risk is that the Marshall Stability of the "semi-rigid" SEAM mix will always be higher than that of the conventional asphalt mix and also the air void content of the SEAM mix will never be lower than that of the conventional asphalt mix.

Example (Tianjin AC 20 mix):
Marshall Stability of above conventional asphalt mix = 7.5 kn.
Marshall Stability of above SEAM mix = 17.2 kn.; air voids = 5.1%

3.2) SEAM mix design based on a single mix test.

For this approach, use the mix design determined using conventional mix information and the formula in 3.1 above and test this mix in the laboratory.

3.3) Full Marshall SEAM mix design.

Using the binder content determined with the procedure in 3.1 above, prepare 5 different mix formulations using the SEAM and asphalt cement determined using the above formula and at 2 binder contents below this value and at 2 binder contents above this value. Use the 40%SEAM/60%Asphalt Cement ratio.

Procedure 3.1 is the most expedient, procedure 3.2 provides verification of 3.1 and procedure 3.3 will yield the most accurate binder content for the SEAM mix. The procedure outlined in 3.3 is recommended for large SEAM projects.

4.) MANUFACTURE OF SEAM MIXES

SEAM mixes should be prepared at a temperature which is 10 C below the usual mixing temperature. The temperature of a SEAM mix should never exceed 150 C. The safety guidelines in Appendix 1 should be followed.

The amount of asphalt cement used in a "semi-rigid" SEAM mix is approximately 70% of the asphalt cement in the usual asphalt mix. It is recommended that the amount of asphalt cement saved is replaced by 1.7 times this weight by SEAM.

This means that the amount of SEAM added to a mix is usually between 3 and 4%.

An automated system for SEAM addition is recommended such as, for example, using a filler addition system from a storage silo or using a RAP (recovered asphalt pavement) feeder system consisting of a bin and a belt feeder into the asphalt mixing plant.

The SEAM can be added to the aggregate in the mixer at the same time that the asphalt is added to the aggregate. The SEAM pellets are easily melted by the hot asphalt mix and the SEAM is dispersed by mixing during the normal mixing period. It is not necessary to increase the mixing time.

The strength of the SEAM mix odor as well as fume emissions increase with mix temperature. Therefore, it is recommended that all SEAM mixes are produced at a target mix temperature of 140 C.

SEAM pellets contain additives to reduce mix odor and fumes but these are most active for a period of approximately one hour. It is recommended therefore that SEAM mix storage be minimized and that SEAM mixes should be used as soon as possible after production.

5.) COMPACTION OF SEAM MIXES

SEAM mixes produced at 140 C will arrive at the paving site under normal circumstances at a temperature near 135 C. As with all asphalt mixes, SEAM mixes should be compacted immediately at as high a temperature as possible to achieve maximum compacted density. The initial and most of the intermediate compaction should be completed by the time that the mix temperature has reached 110 C, as with conventional mix. The final rolling temperature for SEAM mixes should be carried out at approximately 10 C above that of conventional asphalt mix, i.e. usually above 70 C. As with conventional asphalt mixes, compaction of mix when the pavement surface temperature has cooled, but while the interior of the pavement is still hot, can result in transverse "hairline" cracking of the pavement surface. Also, continued vibratory compaction when the mix has cooled can also result in this type of cracking of the road surface.

Special considerations may be required for "tender mixes". These are mixes which, after having been densified by compaction, will still "move" when roller compaction is applied. This may be observed when mixes with a high content of natural, round sand are used and sometimes also with mixes with a high content of coarse aggregate and a low content of fine aggregate. Such mixes should be compacted as much as possible while hot in the 140 to 120 C temperature range, but not disturbed in the 120 to approximately 105 C temperature range, and compaction should be completed when the mix can again support the roller at a temperature near 100 C.

SEAM mixes may be compacted using steel rollers for initial, intermediate and final compaction in the same way as conventional asphalt mixes. However, many paving contractors prefer using a steel roller for initial compaction, a rubber-tired roller for intermediate compaction and a heavy steel roller for final rolling. These contractors believe that the rubber-tired roller provides a "kneading" action which is more effective in densifying "tender" mixes, however the use of rubber-tired rollers is discouraged with SEAM mixes due to a picking up of the fines and the ravaling of the mix due to this fine redistribution action that can occur prior to the finish rolling.

September 1, 2002