Damp diagnosis case studies

Stage 1 experience

Property details A 1950s multi-storey commercial building with 5 floors above ground and 3 floors below ground, originally used as a manufacturing base. The building was undergoing major change – the intention was to house a major computer system network on a massive scale – a full refurbishment and modification programme was underway Site: situated in Paris on a level site within a high-density building area of similar industrial and residential multi-storey properties Construction: solid walls built on concrete ring beams and columns under a flat roof; retaining walls below ground constructed of poured reinforced concrete with an asphalt core Foundation: poured reinforced concrete Damp course: incorporated asphalt core vertically and horizontally between layers of reinforced concrete External finish: rendered blockwork over-clad with marble panels Internal finish: rendered walls and plastered internal partitions Roof and floors: flat roof; communal areas outside of the building at ground floor level also formed part of the roof of the upper-most basement level which was constructed of concrete with an inner core of asphalt Windows: Steel-framed, single-glazed, opening, side-hung, casement sashes Services: basements below ground accessible through an integral elevator system, internal stairs and by vehicular access via a descending, purpose-built, spiral, concrete roadway Survey dates: January/February 2001 – weather: cool and raining

Aims of the survey

To assist the on-site architects to formulate concepts and methods for the elimination or management of dampness which was manifest at several points over the building. There was to be particular emphasis on the below-ground ‘galleries’. To advise on design proposals for the elimination or prevention of water ingress.

Research before the investigation

Full sets of drawings and background information were provided by the architects for the project. Details were also forwarded of the main areas of concern and locations within and around the building.

The previous use of the building had involved the installation of a well head and pump to provide the building with its own water supply (required in the manufacturing processes that took place there).

The level of the water table was known to be fairly high, at least to the height of the second basement level below ground.

It was also established that the datum point for the 100-year flood level was +150mm above street level. The 100-year flood level is a predicted worse case scenario. Should the river Seine burst its banks, the highest level the flood water would rise to would be approximately 150mm above the floor of the ground level of the building. But with 3 basement levels below the streets of Paris and ramped access it would be impossible to prevent flooding within the basement. An additional leased risk.

The internet was used to locate a mapping site to produce a precise ‘on-the-ground’ location of the building. 

Investigation

The investigation started with an early site meeting and briefing with the on-site team:

• the principle architect,
• the project manager,
• the site engineer, and
• key site personnel, e.g. security officers.

The client was represented on site but not in attendance at the meeting. A full agenda was prepared ahead of my arrival. The 2-hour briefing was followed by an extensive visual inspection, with the site engineer accompanying the surveyor around the building for the remainder of a full day.

Due to the sheer scale of the operation, and the nature of defects and design considerations, no physical testing of the building needed to be undertaken. This was a visual inspection on a grand scale.

The survey (wisely) started at the top of the building on the roof area. Additional huge steel-strengthening beams had been placed on the flat roof, mounted on preformed concrete pad stones from which the load was distributed onto the structural steel work. Large computer control cabins (of a type used by mobile phone companies for aerial transmission from the tops of strategically chosen buildings to complete a signalling grid pattern) were placed on these structures.

The additional plant and equipment on the roof required a network of steel cooling pipes and new drainage pipes, passing through the roof structure and connected to a network of water-carrying pipes going down through the building. One major problem was cutting through the roof slab to facilitate the pipe installations. Pipes in cluster created a risk to seals to the flat roof slab. Pipe housings to accommodate pipe clusters would be required as well as lead-burnt sleeving around individual pipes to keep out rainwater. The flat roof was holding considerable quantities of rainwater (ankle deep), indicating that:

• the roof was reasonably watertight, and
• falls to displace rainwater to the integral drainage outlets were inadequate.

Consideration of reproofing was already part of the design brief. The client was advised to consider using a specialist company to work out a computerised cutting list for the flat roof insulation slabs to be machine cut to provide the correct system of falls onto which asphalt or an elastomeric roofing felt could be overlaid. Creation of a new drainage system outside the building and covering over of the integral drainage and piped system were also recommended.

New and existing pipe housings on the roof, where pipes passed through the structure horizontally in one side and out the other, posed the additional problem of rainwater tracking along the pipes and dripping water onto an area of roof within what may have been an old tank room. This was not sufficiently waterproof to prevent rainwater from passing through the slab into the habitable space on the top floor of the building.

To overcome the water ingress, purpose-formed acrylic panels (using material which would resist fire in accordance with French Building regulations) cut to the profile around the pipes, and a purpose-made rubber apron placed between the pipe and the new panel were proposed. A purpose-formed GRP (glass-reinforced plastics or glass-fibre-reinforced polyester) tanking trough was recommended for the floor area with the housing slightly raised and fitted with an overflow pipe routed out onto the roof.

The option of placing a pitched roof over the entire roof was dismissed for a number of reasons:

• cost;
• physical and practical problems (including ventilation) associated with additional plant on the roof;
• additional weight;
• height and angle of the pitch; and
• consents such as planning, building regulations, etc.

The new network of water-carrying cooling pipes, drainage pipes and internal plumbing had not been water or pressure tested despite the advanced stage of the work. Offices on upper floors were taking shape: plasterboard partitions and false ceiling work had begun; electricians were on site installing the miles of network cables and galvanised cable-carrying trays. The testing of potable water and plumbing pipes, and the completion of the roofing works, were placed as a high priority.

For the remainder of the initial walk-over survey the main focus was the below-ground basements (galleries). Here the considerable problems of water ingress and the threat of flooding had to be overcome. The main problems could be summarised as follows:

• Ground water was found to be seeping from cracks in the vertical concrete earth-retaining walls.
• Rust and water staining marks were noted on the underside of large reinforced concrete lintels at ceiling level in line with the access road ramp.
• Large steel water tanks were located below the floor at the lowest basement level.
• A well head pump and a drainage sump pump were also located in the lower basement.
• Large areas of the lower basement floor were holding water.
• Water penetration was occurring between the abutment of the ground floor concrete slab and the external wall of the building above ground.
• Part of the basement had been affected by a burst water main under an adjoining street.
• There was a high water table.
• A newly-formed additional lift shaft resulted in part of the concrete slab of the mid-basement level being cut into. This caused the base of the lift shaft to fill with water.
• ‘Arko’ type drainage channels set into the concrete ramped access road which led to the basement levels were allowing rainwater to overspill and soak through the concrete slab.

A huge series of mainframe computers and sensitive electronic equipment were to be housed within the basement rooms. The equipment had to be protected from the hostile conditions that existed.

Figure 1: Lower basement well head and pump – provided the previous owners with a ready supply of water pumped from the water table below the streets of Paris

Figure 2: Previous attempts to repair cracks to the earth-retaining walls were unsuccessful and ground water was beginning to break back through. More drastic measures of waterproofing would eventually be required

Figure 3: Constructed inner ‘bund’ wall. The dwarf bund wall followed the contour of the external building line. It formed a waterproof trough to divert any water ingress through the earth-retaining walls into the collecting trough and direct it to falls toward the sump collection tanks where the water would be pumped back up and into the street drainage system

Management or cure?

Measures to tackle the problems identified included the following:

• Overhaul and maintain the use of the lower basement below-floor water tanks as these could serve to take in any flood water. If connected to industrial-sized sump pumps, the pumps could then pump the water back up and into the network of street drainage (although it would be doubtful if such a system would cope with the 100-year flood).
• The creation of a raised or mezzanine-type floor in the lower basement would also give a greater capacity for the subfloor to hold increased quantities of flood water. There was no problem with headroom restrictions due to the generous height of each basement level. The lower basement level contained a large number of support columns which could also be easily coped with when raising the floor level. (Attempting to damp-proof the lower floor would be unwise, given the aims of managing not only the present dampness but also future potential flooding.)
• The perimeter earth-retaining walls should be protected from water ingress in 2 principal ways:
  • By injecting a fast acting thixotropic waterproof grout at intervals vertically either side of the cracks. This would require deep drilling through the wall’s thickness. Using high pressure pumps to apply the thixotropic agent through to the water-logged earth-retaining side of the wall would form a diffuse band to seal the crack and self-seal the holes needed for the injection.
  • By providing and erecting a dwarf ‘bund’ wall parallel to the external earth-retaining perimeter walls within the lower basement. Between the bund wall and the earth-retaining wall, a waterproof channel laid to falls would be formed. This would connect with the water-collection sumps leading to the sump pumps. (The installation of a ‘bund’ wall is a ‘belt and braces’ approach. It provides additional insurance against future cracks occurring elsewhere to the perimeter earth-retaining walls, which would undoubtedly cause groundwater to ingress.)
• Remove and cap off the existing well head.
• Completely drain the base of the newly formed lift shaft. Apply a fast-curing, waterproof, cementitious render to completely tank the lift shaft well area and to cover the crude pneumatic chisel marks within the cut concrete. An asphalt dpm would then be applied and overlaid with a formed concrete screed.
• Remove all the ‘arko’ purpose-formed drainage channels and clear away any blockages to the connecting drain outlets. A pitch epoxy damp-proof coating would then be applied inside the purpose-formed concrete cut-out before replacing the ‘arko’ channels.
• Remove the damaged concrete (where cracking of the concrete has occurred due to corrosion of the metal reinforcing bars) and expose the reinforcing bars and, using a concrete restoration specialist, to use a high-pressure cleaning system to remove all rust corrosion from the metal bars. Apply a modified SBR (see box below) mortar repair system to reform the concrete lintels that had suffered from prolonged water ingress.
• Install industrial-scale heating and ventilation systems to manage the heat and humidity levels. This would need to be referred to a mechanical engineer.

Figure 4: Lift shaft wall, lower basement, affected by severe water penetration. The brown staining is caused by the corrosion of steel work within the structure and lift shaft. The cause was rainwater ingress from the defective flat roof of the lift motor room on the roof. This had continued for a long period during which the property lay empty, prior to the new owners acquiring the building

Figure 5: Excavated section of concrete slab at second basement level to form the base of a new lift shaft. The excavation broke through the existing asphalt and concrete dpm causing the ingress from the natural water table. The level that the inflow of water reached was a good indicator of the height of the water table. This demonstrated that the lowest basement level would be completely below the height of the water table

Styrene butadiene rubber (SBR) Styrene butadiene rubber forms part of the polymer latex group of which there are a number of different types. They create stronger bonds and increase strength, making them ideal for repair purposes. They also resist chemical attack. The group includes: SBR PVDC (polyvinylidene chloride) PVA (polyvinyl acetate) Carbonation of concrete is often exacerbated by chloride attack and deterioration of metallic reinforcement. This is a specialised area of work.

In this study it was necessary to eliminate as much of the dampness as possible to help the mechanical engineers design an effective air management heating and ventilation system that could cope with the sheer volume of the internal envelope (without the additional burden of hydration caused by extensive water ingress).

This could apply equally in a domestic setting. You would not simply install extra heating and ventilation to deal with damp walls without first understanding the origin of the damp and its cause(s). The aim would be to eliminate cause as far as possible and leave only moisture produced by use and occupation of the building.

Lessons learned

• The principles of managing or curing dampness in the built environment remain broadly the same irrespective of the scale of problems that industrial premises can often pose.
• The intended use and occupation as well as the severity of the problems are key factors in how to deal with dampness in any building.
• Product selection and method of application are vital cornerstones.
• A degree of innovation and new design is required to tailor particular solutions for keeping out water.
• Full behaviour of the building under maximum flood conditions could only be estimated. Some engineers on site were of the opinion that the building could actually be raised out of the ground to some degree due to the upward force of such a large volume of water. 
• It was interesting to witness similar problems caused (and the common reactions of anxiety invoked) by dampness in buildings within the commercial property sectors in both London and Paris.