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Mastic Asphalt

The most well-known use of mastic asphalt is to waterproof flat roofing. This fantastic material commonly referred to as ‘The King of Waterproofing’ can also be used basement tanking, step waterproofing, flooring, loading bays, balcony and car park waterproofing.

Mastic asphalt is composed of suitably graded limestone aggregates bound together with bitumen or modified bitumen. This combination makes a dense material which contains no voids. Mastic asphalt cannot be compacted so it must be spread with an asphalt float or spatula rather than rolled.

The beauty of mastic asphalt is that it is fully waterproof, carbon zero rated and does not degrade when subject to weathering. Even though the cost of mastic asphalt is higher than other waterproofing materials (due to a combination of the raw material and the skilled labour needed to apply it). The price of waterproofing in mastic asphalt when considered on a price per year of service basis is extremely favourable. When you also consider the easy repairs and the ability to recycle the material. You can see how ‘The King of Waterproofing’ has a reputation for being both environmentally friendly and great value for money.

The Original Composition of Mastic Asphalt

The original mastic asphalt used for waterproofing was made from compositions containing natural rock asphalt. This is naturally occurring limestone rock combined with asphalt. The asphalt came from the famous Pitch Lake in La Brea in Trinidad (often referred to as Lake Asphalt)1. The naturally occurring mixture of bitumen and finely divided mineral matter found in this lake is used to improve the waterproofing characteristics of mastic asphalt. When refined this Lake Asphalt contains between 52 and 55% soluble bitumen.

The limestone aggregate it is mixed with are from rocks which can be found in rock formations across Europe. Nowadays this original composition is not manufactured on a large scale and the production has completed ceased in the UK.

In its place has come ‘Modified Mastic Asphalt’.

Modified Mastic Asphalt

Now mastic asphalt works are often carried out using modified mastic asphalt. This is the case for all applications from asphalt roofing to asphalt steps and walkways. This asphalt consists of bitumen which is modified with the addition of polymers. This advanced technology provides a combination of long term durability, increased fatigue resistance. And improved temperature stability which is critical in in climates like the UK where the temperature can fluctuate great throughout the year.

How Is Mastic Asphalt Produced

Mastic Asphalt is produced using a batch production process. The raw materials in this process include asphaltic cement, along with fine and coarse limestone aggregates.

This asphaltic cement consists of modified bitumen, bitumen or a hybrid of bitumen and lake asphalt. The purpose of this asphaltic cement is to bind the aggregates together. In the manufacture of mastic asphalt the tolerance for aggregate is fairly wide. In most roofing grade asphalt mixtures you will see approximately 20% of the content being made up of coarse aggregate. In situations where extra hardness is needed this may go up to 35%.

The aggregates and asphaltic cements are fed into a hot mixer in controlled quantities. The mixture of aggregates and asphaltic cements is not consistently agitated until it is thoroughly mixed. Samples are now taking and tested in a laboratory. Once the samples pass the laboratory test the mixture is discharged into steep moulds to form blocks of mastic asphalt. Once the asphalt is cooled it is ready to delivery to construction sites where the mastic asphalt can be re-melted prior to application.

The hardness of mastic asphalt can be adjusted dramatically by changing the composition of asphaltic cement and aggregate during the manufacturing process.  An increase in coarse aggregate will lead to the mastic asphalt being harder. The process of hardening asphalt can also be achieved on site. This can be done by adding coarse aggregate just like in the manufacturing process.

Another way to increase the hardness of mastic asphalt when on site is during the re-melting process. By heating the asphalt for a longer period at a higher temperature the asphalt will become harder. Prolonged heating of mastic asphalt should be avoided.

During the manufacturing processes allowances for the hardening of the asphalt during the re-melting process are made. However, if mastic asphalt is delivered to a site as a hot charge the mastic asphalt will be manufactured to the required hardness. There will be no significant change in the hardness of the asphalt from manufacturer to the delivery and application of the asphalt.

During the manufacture of mastic asphalt the hardness number is the way in which quality is controlled. The hardness number indicates the depth in tenths of a millimetre to which a flat-ended steel rod 6.35mm in diameter will indent the asphalt under a load of 9.8MN/square metre applied for 1 minute at a specified temperature. This is described in BS 5284:1993, Sampling and testing mastic asphalt for building and civil engineering.

The hardness test can be used for quality control during the manufacturing process as well as during the re-melting or applying of the asphalt. As there is no provision for the application of the hardness test on laid asphalt any result gathered at this stage can only be processed as indicative.

Over time the stability of mastic asphalt increases and numerous investigations have shown that mastic asphalt can show there can be up to a 40% reduction in hardness over the first year in which it is laid.

During the manufacture of mastic asphalt the hardness number is the way in which quality is controlled. The hardness number indicates the depth in tenths of a millimetre to which a flat-ended steel rod 6.35mm in diameter will indent the asphalt under a load of 9.8MN/square metre applied for 1 minute at a specified temperature. This is described in BS 5284:1993, Sampling and testing mastic asphalt for building and civil engineering.

The hardness test can be used for quality control during the manufacturing process as well as during the re-melting or applying of the asphalt. As there is no provision for the application of the hardness test on laid asphalt any result gathered at this stage can only be processed as indicative.

Over time the stability of mastic asphalt increases and numerous investigations have shown that mastic asphalt can show there can be up to a 40% reduction in hardness over the first year in which it is laid.

What Are The Different Types of Mastic Asphalt?

Here we will look at the different types of mastic asphalt used in waterproofing. The types of asphalt we will look at are:

1) Roofing Grade Asphalt

2) Paving Grade Asphalt

3) Modified Asphalt

Roofing Grade Asphalt: Mastic Asphalt Type R988

This type of asphalt consists primarily of limestone powder and aggregate combined with bitumen or a hybrid of bitumen with Lake Asphalt. It is used in all types of asphalt waterproofing apart from asphalt tanking.

The different compositions of roofing grade asphalt are as follows:

Type R988 B – 100% Bitumen, 0% Lake Asphalt

Type R988 T25 – 75% Bitumen, 25% Lake Asphalt

Type R988 T50 – 50% Bitumen, 50% Lake Asphalt

As you can see these compositions vary in the amount of Bitumen and Lake Asphalt present.

By increasing the amount of Lake Asphalt in the mixture it is generally recognised as a means of improving handling and performance. Therefore, Type R988 T50 Asphalt is easier to handle and performs better than Type R988 T25 and Type R988 B. The reason for this improved performance is the addition of Lake Asphalt brings a silky texture to the mastic asphalt making it easier for the asphalter to lay. The very fine particles of clay which are dispersed across the Lake Asphalt provides a thixotropic characteristic (this means the mixture becomes thinner) to the bitumen binder.

The addition of Lake Asphalt also brings an unusual characteristic to finished asphalt surface. This characteristic is the rapid weathering of the bitumen rich skin which can be seen on the finished asphalt. All compositions of roofing grade asphalt will have this bitumen rich skin on the surface of the finished asphalt and it is weathered by rubbing coarse sand into the surface. Out of all the composition Type R988 T50 needs the least amount time to be weathered with and Type R988 B needs the most time.

The surface of roofing grade asphalt consisting of 100% bitumen tends to be a darker colour when finished when compared to roofing grade asphalt which contains Lake Asphalt. The reason for this is the higher bitumen content. Over time this darkness will draw out of the asphalt and it will be hard to tell the difference between roofing grade asphalt of different compositions.

British Standards set out the acceptable range of hardness acceptable for asphalt at the point of manufacture and the time of laying. At the point of manufacture at 25 degrees Celsius roofing grade asphalt must have a hardness number between 45 and 90 (most manufactures will make sure the hardness is above 50 to allow for an increased tolerance once on site).

At the point of laying at 25 degrees Celsius roofing grade asphalt must have a hardness higher than 30.

As you can see there is an allowance for a hardening of at least 15 during the re-melting of the asphalt. When re-melting it is important to make sure the asphalt is not over-heated or heated for too long of a period. This is normally an easy thing to do. However, difficulties can arise if unexpected changes in weather delay the application of the asphalt. If the asphalt must be left in the mixer for an extended period it is important to charge the mixer to maximum capacity and to be put on at the minimum heat.

Paving Grade Asphalt: Mastic Asphalt To BS 1447:1988

Paving Grade Asphalt is used for roads, walkways and as a heavy duty wearing course on top of roofing grade asphalt. An example of paving grade asphalt being used on top of roofing grade asphalt would be a loading bay or a car park deck.

The difference between paving grade asphalt and roofing grade asphalt is the paving grade asphalt contains a harder bitumen, a higher aggregate content which granules are larger in size. The reason for these differences is to provide a harder finish which is more suitable to withstand higher demands.

Paving grade asphalt is not suitable as a waterproofing layer. The reason for this is the increased hardness makes is susceptible to cracking to both bay joint cracking and surface cracking due to thermal contraction2. For this reason, if waterproofing is required paving grade asphalt must be laid upon roofing grade asphalt. The roofing grade asphalt provides the waterproofing. And the paving grade asphalt serves as a heavy duty wearing course.

Following the addition of coarse aggregates to the mastic asphalt there are no hardness requirements. The amount of the coarse aggregate in paving grade asphalt can be anywhere from 20% up to 50%. As more coarse aggregate is added to the paving grade asphalt the harder the surface will be. However, this comes with a trade-off. As the hardness increases so does the asphalts tendency to suffer from problems at the joints due to thermal movement.

The specification will have to decide upon which balance between hardness and ability to endure movement is best for the project. In most cases, it is advisable to endure some indentation from a slightly softer surface rather than suffer from problems along the joints.

Modified Mastic Asphalt

Modified mastic asphalt is a type of mastic asphalt modified with polymer modified bitumen. This polymer modified bitumen replaces the normal bitumen or hybrid of bitumen with lake asphalt. This modified asphalt allows for easier handling and improved performance of the mastic asphalt. These improvements carry over to both roofing and paving asphalt. The polymer can be tailored to the specific application the asphalt is required for.

The Mastic Asphalt Separating Layer

A separating layer is used with mastic asphalt for several reasons. The first is to isolate the asphalt from any joint movement in the substrate. The second is to provide enough friction to restrain the asphalt against contraction in cold weather. It must also allow a free lateral passage for moisture vapour and hot air during the application of the hot asphalt. And it must act as a long-term vapour pressure release layer.

The separating layer is normally provided by a black sheathing felt3. This is a bitumen impregnated batt of loose jute fibre which is partly compressed and retains an open loose texture. It is laid entirely loose with lap joints of 50mm.

Sheathing felt has ideal characteristic to use for normal roofing specifications. However, when having to endure traffic it allows a small amount of compression. Therefore, in circumstances where traffic will be endured a glass tissue separating layer4 if often used. This is laid in the same manner, entirely loose with lap joints of 50mm.

It is important to ensure the separating layer does not adhere to the substrate. Pre-felted decks, pre-felted insulations, or any bituminised surfaces would lead to two problems. The first problem would be the adhesion of the separating layer to the substrate. The second problem is the bitumen can migrate into the asphalt and pollute it.

In surfaces such as these it is important to prevent adhesion and any pollution of the mastic asphalt. This can be done by using one or more layers of building paper under the sheathing felt. This will prevent adhesion to the substrate and prevent any asphalt contamination.

If laying mastic asphalt on a heat sensitive insulation such as expanded polystyrene a heat resistant overlay will be necessary. A heat resistant overlay can be any type of heat resistant board including wood fibreboard, cork or perlite. The overlay boards should be lightly butted to prevent heat strike through open joints. Once this heat resistant overlay board is in place. The sheathing felt is laid on top of it in the exact same way as any other application.

Unlike roofing grade asphalt, paving grade asphalt will not tolerate a small amount of compression in the insulation. As paving grade asphalt is a thermoplastic material, it will soften when temperatures rise and when it is applied over insulation it will not be able to support traffic without the suitable protection in place.

Suitable protection means that if insulating screen or board insulation is overlaid with a lightweight aggregate concrete. The reason for this is this concrete has a high compressive strength is a poor insulator and has a relatively high thermal mass5.

Want More Information On Mastic Asphalt

If you have any question about mastic asphalt or would like a quotation for your next project please get in touch with us by calling us on 01277 375 511 or by clicking here. We have a team of asphalt surveyors including members with an abundance of expertise with a knowledge base accumulated through over 40 years in the asphalt trade. Making it certain you will receive the best possible advice for your own unique situation.

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