1.) ADVANTAGES OF USING SEAM
SEAM (Sulfur Extended Aphalt Modifier)
addition/extender 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.
6.) DESCRIPTION OF "RIGID" MIX made with ARP
ARP (Asphalt Replacement Pellets) are a replacement
binder that allow you to make a Portland cement concrete (PCC) type heavy
duty pavements that is made and placed with conventional asphalt paving
equipment and similar methods. It has been likened to a "hot" PCC pavement
that is paved similarly to mixes made with conventional asphalt cement.
ARP melts readily into hot aggregate during mixing in the hot plant mixing
chamber and becomes the binder of the heavy duty pavement mixes.
The ARP substitution ratio (ratio of regular asphalt
(bitumen) replaced with ARP) recommended for most mixes is 1.8 to 2 times
the regular asphalt percentages. This is suitable for use in strong road
structures for which the static deflection does not exceed 1.25 mm.
The advantage of the ARP binder mixes is that they can
be handled and compacted similar to conventional asphalt mixes but with
performance of an acid resistant PCC. This type of pavement was originally
created and paved with the help of the US Bureau of Mines and Federal
Highways "plasticized Sulfur" projects of the '70's and '80's. ARP was
designed and approved by Dr. McBee, the champion of plasticized Sulfur for
the Bureau. It was designed as a total replacement for the asphalt
(bitumen) portion of the pavement. ARP is the binder for a heavy duty
pavement that is acid resistant and very durable. Several plasticized
Sulfur projects have been in place in for over 20 years and ARP allows for
easier construction of these high performance pavements.
The amount of asphalt cement replaced by this type of
mix is 100% and the recommended replacement is 1.8 - 2 times the weight of
the original binder, based on experience (i.e. replacement value, R =
1.8).
While paving ARP in Las Vegas we noticed the mix appeared dry compared
to regular asphalt mixes and a brownish color to the mix. This is quite
normal and there can be a colorant added to the mix if desired. Minimum
thickness' of ARP mixes should be 6 inches to accomplish a structure
similar to the PCC pavements.
7.) MANUFACTURE OF ARP "rigid" MIXES
ARP mixes should also be prepared at a temperature which is 10 C. below
the usual mixing temperature. The temperature of an ARP mix should never
exceed 150 C. The safety guidelines in Appendix 1 should be followed.
The amount of ARP used in a "rigid" ARP mix is approximately 180 -200%
of the asphalt cement in the usual asphalt mix. It is recommended that the
amount of asphalt cement normally used is replaced by 1.8 -2 times this
weight by the ARP.
This means that the amount of ARP added to a mix is usually between 8
and 14% and the P200 should have a similar percentage to act as a mastic
builder. Proper mix design is critical and the finer mixes will work
better.
An accurate automated system for ARP 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 ARP can be added to the aggregate in the mixer the same as the
regular asphalt is added to the aggregate. The ARP pellets are easily
melted by the hot aggregate and is dispersed by mixing during the normal
mixing period. It is usually not necessary to dramatically increase the
mixing time.
As with SEAM, the strength of the ARP mix odor as well as fume
emissions increase with mix temperature. For every 10 degree increase in
temperature the fume volume doubles. Therefore, it is recommended that all
ARP mixes are produced at a target mix temperature of 145 C +/- 5% (290
+/- 10 F).
ARP 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 ARP mix storage be minimized and that these mixes be used
as soon as possible after production.
8.) COMPACTION OF ARP MIXES
ARP mixes produced at 145 C should arrive at the paving site under
normal circumstances at a temperature near 135 C. and no tarps should be
required unless the ambient air is cool. ARP mixes should be compacted
immediately at if possible to achieve maximum compacted density. After the
temperature of the mat is lower than 95 C (200 F) compaction efforts
should be stopped. Extensive rolling is not necessary due to the fact you
should be paving a thick lift (6-8") similar to PCC. The final rolling
temperature for ARP mixes should be stopped when the pavement surface
temperature has cooled. Also, continued vibratory compaction when the mix
has cooled can also result in cracking of the road surface. Only one
roller should be required and more as a cosmetic concern than for
compactive effort as ARP is treated like it was a "hot" concrete treated
base.
Although many paving contractors use a rubber-tired roller for
intermediate compaction and a heavy steel roller for final rolling. These
rollers are not needed with ARP pavements. Rock Binders has paved ARP
pavement without any compactive effort other than the screed of the paver
with good results.
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