When any concrete structure is designed, one of the critical decisions is about what the design life of the structure is. Thus, the importance of car park waterproofing. This then relates to other design elements designed to prevent spalling, corrosion of the rebar, and possible subsequent failure of the structure.  The effect of this might be just that the life span is shortened, but in some cases, it has resulted in catastrophic results with significant loss of human life when the structure collapses.   

Any exposed concrete that is not protected by some form of waterproofing (let’s call it “concrete protection”) runs the risk that water will seep through to the rebar and cause corrosion. Ultimately this is going to shorten the lifespan of the structure. 

Why waterproof parking decks

In the case of car parks where the upper deck is exposed to the elements, some car parks are protected by the designer, whereas others elect to save money and not do so. However, regardless of how high the MPA of the concrete is, NOBODY can guarantee that concrete will not crack, and if it does,  water and other contaminants will soak through to the rebar, potentially causing rusting—which can expand to 6 times its normal size. 

With structures such as rooftop parking decks, the cost of car park waterproofing is minimal compared to the overall cost of the structure. This is particularly the case as the parking garage may be a multilevel structure, and in most cases it is really only the top level that requires concrete protection. The lower levels already get protection from the levels above them, although wind driven rain can come in from a side angle and affect the outer meter or so of the structure.  There are also cases of  basement car parks in colder climates  where cars are driven into the building covered in snow, and as the snow melts, water is deposited onto the concrete.  The writer has personally seen such structures where the concrete had deteriorated due to the effect of snow being brought into the building on the cars that then melted, causing leakage into the concrete and ultimately degradation of the structure. 

It is probable that virtually every designer in the world would ultimately support protecting the exposed concrete of a car park if costs were not a consideration.  Of course the reality is that cost is always a consideration, and for this reason some designers choose not to protect rooftop parking. While this is understandable, it is also fraught with risk, as we will see later in this article.  

Factors Causing Deterioration of the concrete

A number of factors come into play which cause degradation of roof top decks. These decks are exposed to some extreme conditions such as UV, continual exposure to rain and possible ponding water, thermal movement of the structure, and constant load variation due to the cars, possibly trucks, and other vehicles using the building. There is constant heavy wear, and car tires will cause abrasion of the structure. In cold countries other factors come into play, such as the spikes on the tires to give traction on snowy roads as well as deicing salts, which, of course, will cause potentially catastrophic problems to concrete that has not been waterproofed. 

Parking garages are constantly exposed to extreme conditions: heavy wear and abrasion from rolling loads and tires, temperature variations, moisture, oil, and chemical spills. Traditional solutions often rely on membranes that degrade, peel, or require reapplication over time, leading to cracks, corrosion, and costly repairs. Of course there will be constant oil spills and sometimes other chemicals as well.

This is further exacerbated in cold countries, where freeze-thaw damage can permanently break up the concrete and where de-icing salts are used on the roads. The tires of the incoming vehicles carry these deicing salts into the parking garage, and if they soak down to the rebar, they will almost certainly cause severe corrosion to the steel, and the degradation of the structure begins.  In an ideal world this will be detected long before the structure collapses and remediation can take place or the building can be shut altogether even if it results in a shorter lifespan than what the structure was originally designated for.  But this is a lot better than doing nothing and waiting until the structure collapses – and there have been a significant number of these such incidents around the world.  

Costs of protecting the concrete – and the building

From every possible angle apart from short-term financial gain, there can be no argument that protecting these structures is the correct thing to do.  Even from a financial point of view, it is the correct thing to do if a long-term view is adopted. Protecting a rooftop parking area originally with a sound methodology and long-life waterproofing product will actually result in cost savings in the long term once the cost of ongoing maintenance and design life has been factored in. This argument is irrefutable. There is only one possible reason not to waterproof rooftop parking areas, and that is short-term cost savings.  Note we say short-term. In the long -term, if a wise choice of protective product is made, and the savings in maintenance and increase in design life are factored in, this short-term outlay, which is seen as a cost, actually becomes a massive cost saving, not a cost incurred.  

Concrete protection for parking structures

Protection of these structures can be treated similarly to protecting bridge decks. Here again, these same factors of short-term cost savings versus maintenance savings and increase in lifespan have the same argument. Ultimately these factors come down to concrete protection. Does the owner – whether a developer or a government – want to save a few dollars now or spend a few more dollars now and come out with a cheaper long-term result (and at the same time minimize the chances of any catastrophic, life threatening failure)?