Asphalting is a recognized specialist trade in the construction industry and the type of asphalt applied to building structures is known as Mastic Asphalt.
As defined in British Standards “it is composed of suitably graded mineral matter and asphaltic cement, in such proportions as to form a coherent, voidless, impermeable mass, solid or semi-solid under normal temperatures conditions, but sufficiently fluid when brought to a suitable temperature to be spread by means of a hand float”.
It is used to waterproof basements and roofs and for lining tanks to contain water or weak liquors in industrial processes. It is also extensively used to provide waterproof and hardwearing surfaces for floors, courtyards and balconies. Specialists' applications include its uses in cold stores and abattoirs.
Characteristics of Mastic Asphalt
The principal properties of mastic asphalt may be summarized as follows;
thermoplastic and is capable of withstanding normal movement encountered in well-designed building structures.
impervious to water and dampness.
hygienic, vermin-proof and rot proof, non-toxic and odorless after laying.
unaffected by sulphates in soils and therefore protects concrete when used as external tanking.
unaffected by many dilute industrial liquors , chemicals and acids.
non combustible, due to its high content of inert material, with a AA fire designation in accordance with BS 476 Part 3 External fire exposure roof tests.
British Standards play an important role in providing specifications for the building industry and for its related professions. The Standards are prepared by committees representative of users, of manufacturers and of research and professional bodies.
Definitions Of Mastic Asphalt
Asphalt. A mixture of bitumen with a substantial proportion of inert mineral matter.
This term is normally qualified by the indication of the origin (eg. Natural rock asphalt, Trinidad Lake asphalt ) or type (eg mastic asphalt, rolled asphalt.)
Mastic Asphalt. A type of asphalt composed of suitably graded mineral matter and asphaltic cement, in such proportions as to form a coherent, voidless, impermeable mass, solid or semi-solid under normal temperatures conditions, but sufficiently fluid when brought to a suitable temperature to be spread by means of a hand float”.
The Materials Used In Compounding Mastic Asphalt Are:
The asphaltic cement, which consists of bitumen, lake asphalt, asphaltite or blends of these with one another or with flux oils, having adhesive qualities suitable for the manufacture of mastic asphalt.
Bitumen - a viscous liquid, or a solid, consisting essentially of hydrocarbons and their derivatives, which is soluble in carbon disulphide. It is substantially non-volatile and softens gradually when heated. Brown or black in colour and possesses waterproofing and adhesive properties. Obtained by refinery process from petroleum, and is also found as a natural deposit or as a component of naturally occurring asphalt in which it is associated with mineral matter.
Lake Asphalt - An asphalt which, as found in nature, it is in a condition of flow or fluidity.
Asphaltite - A naturally occurring substance allied to bitumen, substantially soluble in carbon disulphide and characterized by a high softening point.
The fine aggregate, which can be :
Natural rock asphalt - to British Standard requirements - limestone naturally impregnated with bitumen, which is mined or quarried in various parts of the world, notably in France, Switzerland and Sicily. It is crushed before compounding with other materials.Natural rock only is used in compounding “natural rock aggregate mastic asphalt”. (Not available in NZ).
Limestone - to British Standard requirements - crushed to a fine powder. Crushed limestone only is used in compounding “limestone aggregate mastic asphalt”.
The coarse aggregate, which can be:
Crushed angular stone of igneous origin, eg granite.
Crushed angular stone calcareous origin, eg limestone.
Naturally occurring graded siliceous material, eg grit.
The stability of mastic asphalt is dependant upon the type of asphaltic cement and aggregates used in the formulation of the material. This choice will be dependant upon the functional requirements of the finished mastic.
For example, the prime consideration in the formulation of a tanking mastic is impermeability and ductility. The same considerations apply to roofing mastic, but a wide range of temperature performance is also required.
Flooring materials cover an even more extensive range of functional requirements such as those for extremely hard materials suitable for under-floor warming systems, or as underlays for other floor finishes and for very onerous conditions such as factory floors where heavily loaded forklift trucks are used.
British Standard Asphalt Specifications
Most of the materials are covered by BSS (British Standard Specifications), the present range of specifications for asphalt used in building construction in New Zealand are:
BSS 6925 combining:
BSS 1097 - Mastic Asphalt for tanking and damp-proof courses (limestone aggregate)
BSS 988 -Mastic Asphalt for roofing (limestone aggregate).
BSS 1076 -Mastic Asphalt for flooring (limestone aggregate).
BSS 1451 -Mastic Asphalt for flooring (limestone aggregate).
BSS 1447 - Mastic Asphalt for paving - rooftop car-parks, balconies and footways (limestone aggregate).
British Standard Codes of Practice
The British Standard Codes of Practice are documents concerned mainly with craftsmanship and with details of application. Three such codes are available:
CP 102 - Protection of buildings against water from the ground.
CP114 - part 2 - Roof coverings; mastic asphalt.
CP204 - Insitu floor finishes.
Details of Mastic Asphalt to be given at tendering stage
To ensure that tenders are based upon the specific grades of mastic asphalt required, it is essential that the asphalt contractor is given precise details of the BSS number and of the table and column number therein, ie
Natural rock aggregate mastic or limestone aggregate mastic.
The Standard applicable.
The type of asphaltic cement, which is usually defined by reference to the appropriate columns in the tables of the BSS.
In addition, the asphalt contractor must be advised, as appropriate, of:
The type of isolating membrane, eg sheathing felt, fibreglass tissue, kraft building paper underlay etc.
The material to be used fro surface reflective treatment eg , chippings, bituminous aluminium paint, acrylic coloured paint etc.
Recommended Form Of Specification
Architects, engineers and quantity surveyors are recommended to include the following clause in their invitations to tender
The mastic asphalt shall be in accordance with the requirements of BSS * ……………., the asphaltic cement to comply with table §……………… col. …………. Etc.
Table of British Standards for mastic asphalt specifications indicating the asphaltic cement recommended in each case
§ Asphaltic cement
BSS 6925 - 988
Table 3, col. 2
Tanking and Damp-proof coursing
BSS 6925 - 1097
Table 3, col. 2
BSS 1076 - grade A
Table 5, col. 2
BSS 1076 - grade B
Table 6, col 2
Table 1, col. 3
Covering Capacity and Weight of Mastic Asphalt
The weight of mastic asphalt varies because of differing compositions and the precise coverage obtained will depend upon a number of factors such as the nature of the added coarse aggregate. For the purpose of calculating loading and as a general guide to material coverage,
Nominal weight allowance
APPLICATION OF MASTIC ASPHALT
Remelting of mastic asphalt in block form must be performed in a cauldron or mechanical mixer which is suitable for the size of contract involved
The temperature of the mastic asphalt should not exceed 230oC.
Buckets used for transporting mastic asphalt should be lined with fine inert dust (limestone in the case of limestone aggregate mastic asphalt and silica dust in the case of acid resistant mastic asphalt flooring).
Each coat should be laid as evenly as practicable using a hand float and undue delay between coats should be avoided to prevent contamination.
Timber or heavy steel gauges should be used for horizontal work.
Any blows should be pierced and made good whilst the mastic asphalt is still warm.
The junction between continuous bays of mastic asphalt should not be less than 75mm from the corresponding junction in the proceeding coat. The edge of the previously laid bay should be warmed and cleaned by the application of hot mastic asphalt, which is then removed before the joint with the new material is made.
The final coat should normally be rubbed with coarse sharp sand using a wooden float.
It is essential that mastic asphalt is fully protected from:
Mechanical and impact damage, including damage from contractors plant, equipment and materials.
Trafficking by following trades
Contamination by spillage of solvents, diesel fuel and paints.
All repair work to a mastic asphalt surface must be performed by a specialist mastic asphalt contractor. If it is necessary to remove an area of mastic asphalt, the lines of the cuts should be covered with molten mastic asphalt until the underlying material has softened. The asphalt should not be removed until this has taken place.
IN NO CIRCUMSTANCES SHOULD A HAMMER AND CHISEL BE USED TO CUT COLD MASTIC ASPHALT.
The cut edge of the existing mastic asphalt should be softened and removed to half its depth for a width of approx. 75mm. A proper lapped joint with the re laid mastic asphalt can then be formed.
MASTIC ASPHALT TANKING
It is essential that a clear distinction should be drawn at the initial design stage between Tanking and Tank Lining to liquid containing structures, as these two waterproofing systems differ in their fundamental purpose and impose their own design constraints which cannot be ignored without a risk of failure and possibly contravention of British Standard Code of Practice recommendations.
Tanking with mastic asphalt is a method of protecting accommodation areas below ground level against the penetration of ground or sub-soil water. This water will invariably exert a pressure against the basement structure. The amount of water pressure will depend upon two interrelated factors
the maximum water table level and
the depth of the basement below the water table.
In practice it is advisable to assume that the head of water is developed from the gound level, to the full depth of the excavation to allow for seasonal fluctuation of the water table.
Water pressure increases with depth and is determined by the formula
Pressure in kN/m2 =9.8 x depth in metres.
This pressure is exerted equally in all directions vertically and horizontally irrespective of the slant of the surface on which the pressure bears.
Two methods of tanking are available to the designer. They are to apply the mastic asphalt waterproofing either to the outside of the structure or to the inside and are referred to as external and internal tanking respectively. The decision on the method to be employed will depend upon site conditions and other design considerations. The use of external tanking is preferered if site aand design conditions permit, because the ground water pressure forces external asphalt against the structure, whereas in the case of internal tanking the onverse occurs.
Thicknesses and number of coats of mastic asphalt.
In all cases of tanking and tank lining the mastic asphalt must be applied in three coats to a total thickness of 30mm on the horizontal and 20mm on the vertical, with 2 coat vertical and horizontal fillets.
When the mastic asphalt is applied to the external walls of the basement structure, the concrete base must extend for at least 150mm beyond the outer face of the basement wall. The horizontal mastic asphalt is laid on the base concrete in three coats to a total thickness of 30mm and must cover the full slab area. A sand and cement screed of 50mm minimum thickness must be laid immediately to protect the asphalt against damage by following trade operations taking car to ensure that the asphalt is not damaged during screeding. A loading coat of concrete of sufficient strength to resist the maximum anticipated water pressure is laid over the screed as soon as possible and prior to any other work being undertaken. No loading concrete need be applied over the minimum 150mm wide pick-up, but the 50mm protective screed must be laid on building paper over the asphalt pick-up to avoid contamination of the asphalt surface.
The walls of the basement are erected on the loading slab concrete and the vertical tanking asphalt is applied to the external face of the walls in three coats to a total thickness of 20mm after the 50mm horizontal protective screed and building paper have been removed. A 2 coat fillet must be applied at the angular junction between the vertical tanking and the asphalt pick-up. It is essential to ensure that the pick-up is completely free of all cement slurry, mortar droppings or other contamination in order that complete fusion of the fillet to the asphalt is achieved. A protecting wall of brickwork or blockwork should be built against the external vertical tanking membrane leaving a 40mm gap between the protecting wall and the asphalt which must be filled with mortar, course by coarse, as the work proceeds. This procedure is essential to protect the asphalt membrane against the effects of solar heating and subsequent damage during the backfilling operation or in the event of subsequent excavation.
Note: The laying of dry jointed blockwork is to be avoided or any form of blockwork or brickwork built directly against the asphalt without the 40mm mortar filled gap referred to above, as such methods do not provide a sound and secure tanking operation.
An alternative protection is to apply an insitu application of a stable industrial bitumen emulsion and sand plaster mix applied in 2 coats to a thickness of 12mm, directly to the vertical mastic asphalt tanking.
It is essential that the ground water table is kept below the level of the base concrete throughout the entire tanking operation and for this purpose site de-watering or pumping must be provided on a continuous and un interrupted basis until the loading slab and concrete walls have hardened and /or the mortar in the brickwork has set.
It must be emphasized that complete continuity of the pumping operation and no interruption of the pumping may be permitted for whatever reason until the loading slab and walls have fully hardened and set.
When the mastic asphalt tanking is to be applied to the internal surfaces of the basement structure, the concrete base and the structural walls of brickwork or concrete should be completed prior to the commencement of the asphalting.
To ensure a sound waterproofing membrane, continuity in the asphalt application is essential to avoid any contamination by mortar droppings, slurry, foot traffic etc by other trades.
On completion of the horizontal asphalt, the bottom bay of the vertical asphalt, and the angle fillets, a minimum 50mm thickness of protective sand and cement screed must be carefully laid over the asphalt without delay and preferably before the greater part of vertical asphalt is applied. Once the asphalt tanking is completed, the structural concrete slab should immediately be laid and the loading walls erected. Both slab and walls must be designed to resist the maximum anticipated water pressure.
Tank Linings For Liquid Containing Structures
The purpose of a mastic asphalt lining is to prevent the escape of liquids from a containing structure. The lining is always applied in three coats to a total thickness of 30mm on horizontal and 20mm on vertical surfaces. The internal pressure of the contained liquid against the lining should be calculated in a similar way as the external pressure in tanking applications, but taking into account the density of the liquid and where the level of the liquid in the container is generally at a relatively constant high level, this pressure will be sufficient to prevent any loss of adhesion of the mastic asphalt to the structural surfaces
MASTIC ASPHALT ROOFING
Roof Falls (Slope)
Whilst ponding is not detrimental to the life of mastic asphalt it is generally desirable that falls are incorporated in flat roofs to assist in the discharge of rain - water and minimize ponding. BS CP 144 “Roof Coverings” Part 4: 1970 requires falls to be not less than 1:80 (1.25%) and that they be provided in the structure or by plaster screeding, not in the mastic asphalt.
To allow for normal construction tolerances and achieve a finished fall of 1:80, a design fall of at least 1:50 (2%) is required.
A design fall of not less than 1:50 should always be used in areas subject to pedestrian traffic such as balconies or clothes drying areas or play areas where freedom from ponding is essential.
Where pedestrian trafficking other than for maintenance or fire escape purposes is anticipated, the following specifications can be employed:
1. 25mm two-coat mastic asphalt to BSS 6925/988, the second coat being 15mm thick and having 5-10% additional grit incorporated.
2. Concrete tiles or pavers bedded in a sand cement screed, laid on a waterproof building paper or 1000 gauge polythene seperating membrane over the mastic asphalt . The pavers must be kept back 75mm from the perimeters and a 25mm movement gap incorporated for every 9m2 of paving laid.
The first specification is frequently used for access balconies, playing and drying areas, but is not suitable for applications where point loads are anticipated. The second specification is suitable for applications involving point loads, such as private balconies, podia and public walkways.
Planter Boxes and Roof Gardens
The same specification as used for roofs, ie 20mm two-coat work on the horizontal and 13mm two-coat work on the vertical (or three -coat work if over 300mm high) may be used for planter boxes. In the case of roof gardens, however, where the mastic asphalt waterproofing will not subsequently be accessible the specifications for Tanking apply, ie three-coat mastic asphalt on the horizontal of 30mm and on the vertical of 20mm. In-situ concrete or plaster, concrete paving slabs or similar materials should be used to protect the mastic asphalt from mechanical damage.
In the case of roof gardens, falls should be incorporated and drainage provided at the lowest point of the mastic asphalt waterproofing membrane.
Solar Reflective Treatment.
The use of a solar reflective treatment on mastic asphalt roofs is recommended and is essential where the mastic asphalt is laid directly over an insulant. For horizontal surfaces, either light coloured chippings bedded in a bitumen-based adhesive compound or a solar reflective paint may be used. The latter should always be applied to skirtings and vertical work. Generally chippings are to be preferred for horizontal work and the solar reflective treatment should be applied as soon as practicable after the mastic asphalt has been laid.
Care should be taken to ensure that paints used as a solar reflective treatment on mastic asphalt are suitable for the purpose (ie do not degrade due to ponding water) and compatible with the compounds used in the manufacture of the mastic asphalt.
In the case of trafficked roofs, overlaid light coloured tiles or pavings will act as a solar reflective treatment.
Thickness and Number of Coats
Horizontal work - (ie asphalt laid on surfaces up to and including 10o pitch):
20mm average thickness laid in two coats on a separating membrane. Note: in the case of work at an angle exceeding 5o and where the asphalt is laid over an insulant, an additional coat may be required to achieve the specified thickness or a reduction in thickness may have to be accepted.
Sloping - (ie asphalt applied to surfaces over 10o up to and including 45o ), or Vertical work - (ie asphalt applied to surfaces over 45o work), other than to timber and excluding skirtings:
20 mm average thickness in three coats without a separating membrane.
Vertical or sloping timber :
20mm average thickness in three coats on expanded metal lathing over a separating membrane.
Additional coats and thicknesses may be specified for special applications.
Skirtings should be not less than 150mm high. On new brickwork or concrete they should have a total average thickness of 13mm laid in two coats and unless continuous with a horizontal damp-proof course, should be tucked into a chase at the top edge. See Roofing Detail1. click here.
Skirtings over 300mm are regarded as vertical work.
On old or irregular brickwork it is usually necessary for the skirtings to be applied in three coats, the first only as a leveling coat, and in such cases, the total thickness would be between 15 and 20mm.
Chases in brickwork, blockwork and concrete should be provided by the main contractor and should be not less than 25x25mm
After asphalting, the chase should be pointed up by the main contractor using a sand/cement mortar containing a plasticising additive.
Black sheathing felt complying with BS747 TypeA(i) is normally used under horizontal mastic asphalt roofing and under expanded metal lathing on vertical timber or lightweight concrete surfaces. Under horizontal mastic asphalt it is laid loose with 50mm lapped joints.
Keying to timber surfaces
The key for mastic asphalt to vertical or sloping timber surfaces is obtained by fixing bitumen coated plain expanded metal lathing to BS 1369, measuring between 6 and 10mm short way of mesh and not less than 0.55mm thick, over a separating membrane, with the long side of the diamond horizontal and with the pitch of the horizontal metal strands inclined upward and outward from the substrate surface. The expanded metal lathing is fixed by nailing with extra large head galvanized clout nails to BS1202 or with galvanized staples at 150mm centres.
The edges of the sheets should be butted.
The expanded metal lathing and separating membrane are supplied, laid and fixed by the asphalter, immediately prior to laying of the mastic asphalt and should not be allowed to get wet. Chicken wire must not be used in place of expanded metal lathing.
Keying to concrete surfaces
Damage to the mastic asphalt and loss of key (bond) will be caused by excessive use of mould oil (shuttering release agents).
Where vertical or sloping concrete is very smooth eg where steel shuttering has been used, some form of surface treatment is necessary to provide a satisfactory key for the mastic asphalt. One of the following treatments may be adopted and may also be necessary if excessive blowing is experienced:
Removal of surface laitence by wire brushing or grinding.
Application of a proprietary sand cement plastic emulsion in accordance with the manufacturers recommendations.
A light application of a proprietary bitumen/rubber emulsion.
Fixing of expanded metal lathing.
Expanded metal lathing should always be used in the case of lightweight concrete blockwork.