Article

Materials, selection, execution – foundation waterproofing (part 3)

Bituminous felt waterproofing – principles of proper execution

Technical documentation: requirements and details

Carefully prepared technical documentation is very important. In this case, the word ‘carefully’ is crucial. The idea is to create detailed drawings of specific elements (e.g. connections and joints between vertical and horizontal insulation, methods of securing the plinth area, sealing expansion joints, pipe penetrations, etc. – this is basically a formal requirement, but it is often overlooked) and an analysis of the costs of installing waterproofing coatings at the design stage. As far as detail drawings are concerned, they should preferably include specific technological and material solutions; they should define the type of material used (e.g. KMB compound, weldable bituminous felt, flexible slurry, self-adhesive membrane, etc.), specifying the parameters of the waterproofing material used that are key to its service life.

Modification of details is only possible due to the specific nature of a given material from a specific manufacturer (e.g. priming or no priming, method of tape application, use of reinforcing inserts or protective interlining, etc.), but it cannot significantly change the essential requirements, such as the most important parameters of the waterproofing material or the number or thickness of layers. In turn, cost analysis is necessary to ensure that there is no ‘need’ to replace the material with a cheaper one at the execution stage.

Supervision and quality of workmanship

Correct documentation is the first step to success, the second is correct execution. Firstly, the contractor must not be allowed to arbitrarily modify the technology used in the works without permission. Secondly, it is essential to monitor each stage of the works. Thirdly, particular attention must be paid to details, which are one of the most common causes of leaks (see photographs opposite).

Technical conditions and acceptance practice

The fact that the technical conditions for the execution and acceptance of works ([1] - [5]) are optional does not mean that they are irrelevant (this is also confirmed by valid administrative court rulings). Compliance with these conditions is essential for the trouble-free operation of facilities.

Acceptance of works: stages and scope

Acceptance of works is an assessment of the correctness and quality of the works performed, in some cases combined with determining their size. The point of reference is the technical documentation of the works performed, i.e. the design documentation (if changes were made, then the as-built documentation), detailed technical specifications, construction log and measurement book, test and control measurement results, as well as documents approving the materials used for marketing (if they are construction products within the meaning of the Acts of construction products).

Types of acceptance

The procedure for accepting waterproofing works consists of the following stages:

acceptance of concealed works,
partial acceptance,
final acceptance,
post-warranty acceptance.

Scope of inspection during acceptance

It is logical that the above-mentioned types of acceptance apply to all types of waterproofing. The procedures for checking the correctness of the works, which are part of the individual stages of acceptance, should include ([1] - [4]):

material (before installation),
repair and/or preparation of the substrate,
preparation of the material for application,
application of the material (during application – testing during works),
waterproofing coating ready for use (after completion of works, but before application of subsequent layers).

This constitutes a kind of checklist, i.e. an algorithm: requirement met – continue work, requirement not met – remove non-conformity.

1 - konstrukcyjny beton podkładowy, 2 - płyta denna, 3 - ściana fundamentowa, 4 - izolacja pozioma płyty dennej, 5 - izolacja pionowa — Fig. 1: (1) structural base concrete; (2) base slab; (3) foundation wall; (4) horizontal insulation of the base slab; (5) vertical insulation.

Slab foundation: assumptions and waterproofing details

A slab foundation is one of several foundation options for buildings. Originally, this option was used for buildings with basements, subject to water pressure (I disregard structural issues that necessitate this type of foundation). Fig. 1 (above) shows a diagram of the waterproofing of the foundations of this type of structure. In the case of underground garages, this type of construction is becoming standard.

Insulation under the base slab: requirements

This variant is characterised by the presence of insulation under the base slab. This means that one of the most critical details of strip footing foundations is missing – the connection between the floor insulation (located on the floor slab on the ground) and the insulation on the strip footings. This solution requires the construction of a structural base concrete slab on which the waterproofing coating will be applied. This should not be a lean concrete screed, but a structural slab with a thickness adequate for the loads.

Material requirements for the base slab

The exposures acting on the waterproofing coating (the structural slab is laid on the waterproofing and must not damage it under any circumstances) require particularly careful selection of waterproofing materials in terms of parameters, as well as the need to protect the coating both during installation and operation (Fig. 2, Fig. 3).

Only polymer-bituminous felt modified with SBS should be used here, in at least a two-layer system, with at least one layer of the bituminous felt having polyester reinforcement in both cases. Bituminous felt with non-woven polyester has very good tensile strength. In addition, it has exceptional elasticity – its elongation is often 50%. No other reinforcement combines these two features so well.

For this purpose, we recommend using bituminous felts from the NEXLER range:

NEXLER PREMIUM PYE G200 S40 (thickness 3.8–4.2 mm, flexibility -20°C, water resistance 200 kPa, 2xSBS modification: linear and radial, reinforcement weight 200 g/m²),
NEXLER PREMIUM PYE PV200 S40 (thickness 3.8–4.2 mm, flexibility -25°C, water resistance 250 kPa, 2xSBS modification: linear and radial, reinforcement weight 200 g/m²),
NEXLER PREMIUM PYE PV250 S48 (thickness 4.6–5.0 mm, flexibility -25°C, water resistance 400 kPa, 2xSBS modification: linear and radial, reinforcement weight 250 g/m²).

Fig. 2 (left): Example detail of the connection between horizontal insulation under the base slab and vertical insulation of foundation walls

Fig. 3 (top): Example detail of the connection between horizontal insulation on the base slab and vertical insulation of foundation walls

Care and preparation of concrete for waterproofing

After pouring, structural base concrete should be cured to prevent it from drying out too quickly and causing shrinkage cracks. Curing begins immediately after pouring the element and consists primarily of protecting the concrete from the effects of sunlight and/or wind (and, as a result, from drying out too quickly). For this purpose, the surfaces of the slabs are covered with waterproof mats, damp mats are laid on the surface or the concrete surface is sprinkled with water. Curing should be continued until the concrete reaches at least 40% of the strength required after 28 days, but for no less than 7 days (for concrete made with Portland cement) or 14 days (for concrete made with blast furnace cement).

Substrate requirements: cleanliness, moisture content, levelling

The concrete substrate (base slab) should be clean, stable, load-bearing, rough (with open pores) and free of scratches and cracks. Greasy stains, dirt, efflorescence or other substances that may impair adhesion are unacceptable. During insulation, the maximum mass moisture content of the substrate (in cross-section) should not exceed 5%. Although the aforementioned NEXLER PREMIUM series bituminous felts are more tolerant to substrate moisture, their use on substrates with a moisture content above 5% requires individual assessment, taking into account the specific nature of the application site.

It is permissible to lay a waterproofing coating on a substrate with higher moisture content, provided that a case study shows that the increased moisture content has no negative impact on further works and the durability of the entire structure. It should also be remembered that sealing concrete base slabs at elevated moisture levels and high temperatures can lead to the formation of osmotic blisters under the waterproofing coating.

The substrate must also be even – sharp edges, cracks, cavities and unevenness are unacceptable. Particular attention should be paid to sharp, protruding fragments that could damage the bituminous felt layers. This is why characteristics such as adequate resistance to tearing (nail shank) and high SBS modification of the bituminous felt are so important – they allow for better handling of unevenness in the substrate, ensuring very high resistance to static loads.

The substrate temperature at the time of works should be between 0°C and +30°C. The substrate must not be frozen or exposed to moisture condensation.

Cleaning and repairing the substrate

The concrete substrate should be cleaned using mechanical or manual methods appropriate for the type of substrate and the identified contaminants, e.g. by grinding, milling, chipping, washing with water, etc. Cement laitance and formwork oil residues should be removed from the concrete. Protruding fragments should be chipped/milled off, and any defects, cracks, edges, expansion joint edges (if any) in the concrete substrate should be repaired with NEXLER Renobud R 103 or Renobud R 105 repair mortars.

Pre-application checks: strength, moisture content, temperature, cleanliness

Substrate condition checks should include verification of:

strength parameters, if necessary (e.g. using Schmidt hammers or pull-off testers),
substrate moisture content using moisture meters (meters for indirect testing should be calibrated for the type of substrate – concrete; direct, destructive, weighing-dryer methods and CM devices, i.e. the carbide method, are used in justified cases),
air and substrate temperature using pyrometers and thermohygrometers,
substrate cleanliness, usually by visual inspection (the presence of anti-adhesive agents – particularly important on reinforced concrete walls – greasy contaminants, etc., can also be detected, for example, by moistening with water. Visual assessment consists in inspecting the substrate in diffused light from a distance of 1–1.5 m),
correctness of expansion joints (if any), their placement is checked by comparison with the documentation, and their width – by measurement.

The correctness of substrate repairs should be checked primarily by visual inspection and tapping the repaired areas with a wooden hammer. A dull sound indicates that the repair layer is coming loose from the substrate.

Corner details and facets

External corners (both vertical and horizontal ones) should be chamfered at an angle of 45° over a distance of at least 4 cm from the edge. In the case of internal (concave) corners, the bituminous felt cannot bend perfectly at a right angle, so the bend is always rounded. Therefore, in such places (where there are internal right angles), a facet should be made. For this purpose, you can use, for example, PCC mortars (NEXLER Renobud R 103, triangular or rounded shape, rounding radius 4 - 6 cm) or KMB-type bituminous compounds (rounding radius of 2 cm). The single- or two-component products of the NEXLER Bitflex line, which are based on an ecological water dispersion, are ideal for this purpose. If there are no other technical contraindications, hard mineral wool wedges can also be used.

Preparing the bituminous felt and priming

Before application, unroll the bituminous felt, lay it on a hard surface, and after it has straightened out, cut it to the desired dimensions and roll it up again. Cut the fittings to seal the corners and edges. The concrete substrate on which the roofing bituminous felt will be welded must be primed with, for example, NEXLER BITFLEX Primer.

Welding bituminous felt: overlaps and technique

Bituminous felt strips should be joined with 12-15 cm wide cross overlaps in the direction of water flow. The longitudinal overlap should not be less than 8-10 cm ([5] - [7]). With two layers, the overlap should be half the width of the strip, and with three layers, 1/3 of the width.

When welding bituminous felt, the torch flame should heat both the substrate and the bituminous felt across the entire width of the roll. The mass on the underside of the bituminous felt should melt. Then move the torch and unroll the roll evenly (the bituminous felt should melt into the melted asphalt), while squeezing the excess bitumen to the sides. A layer of liquid asphalt several centimetres wide should form in front of the roll and should flow out beyond the sides of the roll to a width of about 1.5 cm. Care should be taken to ensure that the bitumen compound melts across the entire width of the bituminous felt.

When performing waterproofing with weldable bituminous felts, it is very important to meet the requirement for complete welding of the underside of the bituminous felt to the primed surface of the structure. Asphalt flow is required at the overlaps of individual strips of insulation material for weldable bituminous felt. When installing two-layer insulation, the second layer of roll materials should be completely bonded to the previous layer of bituminous felt by welding.

Laying horizontal and vertical insulation

Horizontal insulation should extend at least 10 cm beyond the vertical edge of the foundation slab.

Vertical waterproofing of foundation walls with roll materials can be done in two ways: from the bottom or from the top, but it is strongly recommended to plan it so that it starts from the lowest point of the structure – bituminous felt plates are laid in such a way that water flows from the plate above to the plate below (in accordance with the rules of overlapping).

Work on walls: team organisation and fastening

When gluing bituminous felt to vertical surfaces, two or sometimes even three workers are required. This requires a high level of technical skill on the part of the contractor – the bituminous felt must not be overheated, the asphalt must not be melted too much, and it must not be melted too little. It is recommended that the height of the bituminous felt strips welded vertically to the foundation wall does not exceed 2 m. For higher walls (or in the case of other technical conditions), the bituminous felt strips should be laid in sections and mechanically fastened under the cross overlap. The aforementioned mechanical fastening is generally performed only on the underlay bituminous felt, but in justified cases it is also possible to fasten the second layer of bituminous felt (use half the number of fasteners used for the underlay bituminous felt). The fastener must be covered with another strip of bituminous felt. In order to ensure the tightness of the uniform waterproofing coating, the overlaps should be welded to obtain a 2-4 cm wide asphalt melt.

Corners and detail work elements

Corners should be made of appropriately cut bituminous felt elements that facilitate full-surface welding. For this purpose, it is worth choosing NEXLER detail work bituminous felt, factory-cut to a width of 50 cm, which will speed up the installation works. In these areas, ensure that the bituminous felt overlaps by 8 cm and that the underside of the bituminous felt is properly melted, pressing it against the substrate for a moment after removing the torch flame.

Extension above ground level and pressure strips

Vertical waterproofing must be led above ground level and finished in such a way as to prevent rainwater penetration. In the case of a gravel splash strip surrounding the building, the waterproofing should be led at least 30 cm above ground level, and in the case of a concrete strip – at least 50 cm.

The upper edge of the wall insulation bituminous felt should be additionally secured mechanically with a pressure strip and sealed with, for example, NEXLER Full Fix Pure hybrid sealant.

Technological breaks: how to secure joints

Breaks in the application of the coating require particular care. Do not leave the end of the bituminous felt strip unstuck in order to stick another plate underneath it later. In such cases, a so-called reverse joint should be made: the plate must be stuck to the substrate right to the end. When resuming works, carefully clean the joint strip (approx. 20 cm wide). If there is significant contamination, melt the bitumen compound on the contaminated surface with a torch and remove the contamination together with the melted compound using a metal spatula. Stick a new plate onto the prepared surface. During application, do not allow the bituminous felt to be damaged (burned).

Layering system for foundation slabs

Let us now analyse the possible ways of founding buildings on foundation slabs in terms of the waterproofing-thermal insulation system. This is important because, in the case of underground garages, thermal insulation of the base slab is not usually provided, whereas in the case of public or residential buildings it is. The arrangement of waterproofing coatings is shown in Fig. 1. This option is universal in that it does not matter whether the wall is made of monolithic concrete or small-size elements. Horizontal insulation is laid on structural base concrete (it should not be laid on lean concrete). The critical point is the connection between the vertical and horizontal insulation. The connection detail is shown in Fig. 2.

Please note two things. The first is the reinforcement of structural base concrete screed in the edge zone. The second is a layer of protective mortar directly on the horizontal bituminous felt insulation. The function of this layer is to directly protect the waterproofing against damage during reinforcement and concreting works. Its execution in the form of a layer of mortar or concrete is quite troublesome, but a substitute solution may be to use ordinary, cheapest bituminous felt on cardboard reinforcement. This bituminous felt will most likely rot, but it does not perform any waterproofing function, only a protective and temporary one (during the concreting of the base slab). The second method of connection is shown in Fig. 3.

Sequence of works and protection during works

Different types of insulation are installed at different times. The earliest stage involves installing horizontal insulation under the base slab, followed by vertical insulation of the walls. This sequence of waterproofing works has certain consequences. The operational stage requires comment. It does not begin when the waterproofing coatings are fully applied and the excavations are backfilled, but much earlier – when the waterproofing material is physically applied to the substrate. For horizontal insulation, the period of operation begins when the base slab is constructed. The fact that the connection between vertical and horizontal insulation takes place much later is irrelevant. Nevertheless, this time shift necessitates adequate protection of the horizontal insulation strip protruding beyond the face of the slab or wall. Contamination and/or damage to this strip results in subsequent, very characteristic leaks. For this reason, a distinction must be made between protective layers used for temporary protection of the waterproofing coating during further works and layers (materials) for protection during backfilling of foundation excavations or during normal operation of the facility. Therefore, two more steps must be added to the above-mentioned stages of waterproofing coating inspection:

Inspection of the protection of exposed waterproofing sections intended for later connection. This must be a continuous inspection.
Inspection of the condition of the waterproofing surface immediately before connection to another section.

Installation penetrations: flanges and sealed connections

When performing waterproofing, special attention should be paid to the installation of penetrations in walls and foundation slabs. These areas should be sealed in such a way as to prevent water leakage and seepage into the building. Pipe penetrations through walls should not only be sealed, but also flexible. A rigid connection may be damaged by the movement of the pipeline or, for example, building settlement, and may contribute to moisture penetration into the structure. In such cases, only pipe penetrations with sealing flanges should be used. The bituminous felt insulation is attached to the movable flange, while the pipe penetration itself is sealed with a clamp. This is the only acceptable way to seal pipe penetrations with pressurised water. Details are shown in Fig. 4.

Fig. 4: Sealing of an installation penetration – diagram.

More practical details on sealing and alternatives to bituminous felt waterproofing – topics often encountered on construction sites – will be discussed in the next part of the series.

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Bibliography

[1] Specyfikacja techniczna wykonania i odbioru robót budowlanych. Roboty hydroizolacyjne. Izolacje przeciwwilgociowe i wodochronne części podziemnych i przyziemi budynków”, wyd. II, OWEOB Promocja, 2017.

[2] Warunki techniczne wykonania i odbioru robót budowlanych. Poradnik projektanta, kierownika budowy i inspektora nadzoru”, praca zbiorowa, Verlag Dashofer, Warszawa 2018.

[3] M. Rokiel, „Hydroizolacje w budownictwie. Projektowanie. Wykonawstwo”, wyd. III, Grupa MEDIUM, Warszawa 2019.

[4] M. Rokiel, „Hydroizolacje podziemnych części budynków i budowli. Projektowanie i warunki techniczne wykonania i odbioru robót”, wyd. IV, Grupa MEDIUM 2019.

[5] Warunki techniczne wykonania i i odbioru robót budowlanych. Część C: Zabezpieczenia i izolacje, zeszyt 5 - Izolacje przeciwwilgociowe i wodochronne części podziemnych budynków, ITB, 2016

[6] DIN 18195, „Bauwerksabdichtung”:

Teil 1: Grundsätze, Definitionen, Zuordnung der Abdichtungsarten
Teil 2: Stoffe,
Teil 3: Anforderungen an den Untergrund und Verarbeitung der Stoffe,
Teil 4: Abdichtungen gegen Bodenfeuchte (Kapillarwasser, Haftwasser) und nichtstauendes Sickerwasser an Bodenplatten und Wänden, Bemessung und Ausführung,
Teil 5: Abdichtungen gegen nichtdrückendes Wasser auf Deckenflächen und in Nassräumen, Bemessung und Ausführung.
Teil 6: Abdichtungen gegen von außen drückendes Wasser und aufstauendes Sickerwasser, Bemessung und Ausführung.
Teil 8: Abdichtungen über Bewegungsfugen.
Teil 9: Durchdringungen, Übergänge, An- und Abschlüsse.

[7] „Zalecenia wykonywania izolacji z pap zgrzewalnych i nawierzchni asfaltowych na drogowych obiektach mostowych”, IBDiM, 2005.

[8] Technische Regeln für die Planung und Ausführung von Abdichtungen mit Polymerbitumen- und Bitumenbahnen, Industrieverband Bitumen-Dach- und Dichtungsbahnen e.V., 2008

mgr inż. Maciej Rokiel [MSc, Eng.] is a renowned mycological and construction expert and engineer with over 20 years of experience in protecting buildings against water and biological corrosion. He is the author of acclaimed publications, such as ‘Hydroizolacje w budownictwie’ (‘Waterproofing in Construction’) and ‘Renowacje obiektów budowlanych’ (‘Renovation of Buildings’), which are compendiums of knowledge for designers, contractors, and experts. His achievements include numerous expert opinions, articles and e-books published in prestigious industry titles such as ‘Izolacje’ (Insulation) and ‘Inżynier Budownictwa’ (Construction Engineer). He specialises in the diagnosis of dampness and salinity in walls, the design of waterproofing and the renovation of problematic buildings, and his works are widely cited in technical literature.