Multi-storey car parks

The weatherproofing of early car park structures was somewhat hit and miss. Some had protection simply to the top deck, others to the ground (entry deck) and roof deck and some had no waterproofing at all.

Asphalt was common for roof decks, although roofing grade asphalt is generally insufficient in itself to deal with traffic. For this reason, paving grade asphalt was often applied as a wearing course on top of the actual waterproof membrane. Slip and skid resistance is often provided in the wearing course by sand rubbing or surface crimping - sometimes precoated chippings were used. Surface dressings are often found; usually post-applied epoxy resin coatings incorporating bauxite chippings to provide good frictional resistance.

Where heavy point loads were identified (for example, the car park and loading deck of a covered shopping centre) steel grilles were set into the paving grade asphalt to resist indentation.

Although asphalt is a durable material, particularly when laid direct to a concrete deck that can act as a heat sink, it is still vulnerable to defects arising mainly from:

• poor detailing of upstands;
• lack of provision for movement;
• perforation by the fixings for car park barriers and lighting bollards; and
• general neglect.

Aside from keeping the water out of a structure, the waterproofing membrane also (and possibly more importantly) protects the concrete against the ingress of chlorides or other harmful substances. Neglecting the covering may permit water penetration, which given the utility nature of a car park might be acceptable, but it carries with it the risk of more serious deterioration to reinforcement.

At normal temperatures asphalt is relatively inflexible. Solar radiation results in a build up of heat in the covering and, although this is mitigated by the ability of the roof deck to act as a heat sink, high summer temperatures can cause asphalt to soften and degrade. When heated, asphalt can accommodate imposed movements by plastic deformation but at colder temperatures it is prone to fracture if subjected to shock or impact loading.

Exposure to UV light hardens the asphalt, effectively promoting the stabilisation of the bitumen binder after melting and cooling so that it loses tensile strength. Consequently, the roofing may not be able to accommodate temperature and loading changes that affect the deck, resulting in tensile failure. Temperature-related stresses act in 3 dimensions and, while this should not affect flat surfaces, upstands and vertical work are prone to damage.

Asphalt surfacing can also be damaged by:

• water ponding on the surface, which causes crazing and gradual deterioration; and
• oil spillages from vehicles, particularly in the mid-levels of car parks that are not subject to the degree of regular washing that occurs at roof level.

Defects in traditional asphalt roofs are often dealt with using traditional asphalt repair methods. More recently, the car park refurbishment industry has been influenced by the use of a variety of resin-based roofing systems or generically cold-applied liquid roofing systems. These systems can be a useful method of upgrading waterproofing systems without necessarily going to the expense or disruption of replacing the original asphalt. Being lightweight, they can also be used to upgrade intermediate floors and by introducing contrasting colours, define pedestrian walkways, parking spaces and traffic routes as part of a refurbishment programme.

Cold-applied systems are now generally applied by spray to ensure consistency and fall into 2 broad camps:

• polyurethanes; and
• acrylics.

The systems usually comprise a priming coat that is used to seal the substrate and ensure adequate bond - a one or two coat membrane incorporating a reinforcement fleece, a wearing layer and a finish coat giving a total thickness of around 4.5-5mm.

A key attraction of this type of system is that it can be applied over complex details. Imperfections in an asphalt substrate need to be cut out and made good, although given the inherent flexibility of most materials a certain degree of crack bridging can be provided by the membrane. However, the apparent attractiveness of these systems does not mean that they can be used as a substitute for a lack of attention to detail or indeed a lack of proper preparation.

Failure in membrane - asphalt has insufficient tensile strength to accommodate the level of movement experienced.

Joint in precast planks immediately below the failed joint in the above figure. Water penetration has led to the severe corrosion of a retained lifting lug.

Profound adhesion failure of a cold applied waterproofing system probably caused by its application over a contaminated surface or application in unsuitable conditions.

Plastic deformation in asphalt skirting has resulted in the perforation of the asphalt, water logging and plant growth.