Is Water Ponding on Carpark Floors Normal?
By: Dr. Zack Lim
Nowadays water ponding has become a common construction dispute as any water appearing stagnant on the floor surface us generally not acceptable by owners and/ or architects. Hence, is water ponding normal or should it be rectified?
Two issues are commonly encountered when rectification is made to water ponding. Firstly, if the affected area is rectified, there is a tendency that a new puddle of water my be created adjacent to the rectified area. Secondly, it is very difficult to carry out thin topping repairs on low areas successfully. Any mason can repair a floor but what is the assurance that the repaired area will not delaminate or fail months later?
Before understanding the causes and finding a solution, everyone should know the difference between 'flat' and 'level'. Flatness relates to the bumpiness of the floor surface and levelness relates to the tilt or slope of the floor. To describe a floor profile clearly, flat and level are two inseparable entities with different types of floor profile as shown in Fig 1.
Water ponding is common on concrete floor surfaces, especially at today's large carpark areas. This problem is unavoidable as most finished floor levels are specified to meet a 'perfect level', and even on a floor that is constructed to be super-flat and super-level, the water will still stagnate on the floor surface as the water has nowhere to fall. Therefore, the only way to eliminate ponding problem is to design and build the floor surface to fall or slope to facilitate water flow. Thus, the expectation of the property owners and architects by not having any water ponding (unless to fall) is not realistic. There must be an acceptance of reasonable tolerance on the level of ponding.
What causes water ponding?
Large areas of water ponding or small localised ponding known as birdbaths are results of depressions created on the concrete floor surface during concrete strike-off (screeding) and power trowelling. The other reason for water ponding can be resulted by a deflection of a floor slab caused by the weight of the slab itself after the removal of scaffoldings or a possible change of floor profile on an elevated slab after post-tensioning the slab.
Idealistically, to avoid water ponding, concrete floor need to be built with a surface profile that is very flat but not level (designed to fall), to cater for water runoff from a higher to lower elevation.
However, modern carpark floors are so large that it is impractical to build floors with gradients, as concrete at the centre need to be much thicker to enable water to fall to the lower perimeter scupper drains. Alternatively, the formworks to receive the concrete can be installed to tent-shape so that the concrete can be laid with consistent thickness with falls. However, it is extremely difficult for builders to actually control and do it precisely. As floors are generally designed and built without gradients, any high spots will restrict water to flow freely to any low-lying areas which then trap and hold water, resulting in water ponding.
Floor specification and acceptance
The QLASSIC standard mentioned that specification for floor surface evenness allowed is 3mm over 1.2-meter starightedge whereas BS8204-2:2003 (refer Table 1) has three classifications of floor flatness for different usages. For example, SR3 specification, which is a 10mm tolerance under 2-meter straightedge is generally recommended for carpark floors.
Fig 2 shows how a SR floor is measured using a 2-meter straightedge placed anywhere on the entire floor surface and uses a slip-gauge to measure the gap under the straightedge. This only measures the flatness of the floor but does not measure the level tolerance from the control datum or finished floor level (FFL). As mentioned earlier, flat and level are two inseparable entities, therefore floor level tolerance also needs to be specified for the entire finished floor. For example, +/- 15mm tolerance from the finished floor level is a very stringent specification imposed on levelness control. If a level tolerance of +/- 15mm is also specified from the finished floor level, logically water ponding of up to 30mm may be observed over the entire floor area.
Rectification of water ponding
Generally, the method of repair is carried put by grinding the high spots that caused water damming and topping up the low areas with suitable cementitious materials. However, if the high areas are grinded down too much, it will cause new ponding areas around the newly filled up areas. As such, rectification works to eliminate water ponding may backfire as most repair areas will looked worse than original and any repairs not carried out professionally may results in delamination.
It is near impossible to build a concrete floor with no water ponding. As such, if the depth of water ponding is very little (not more than 10mm deep), it is bearable and acceptable as it can easily be removed. As today's carparks are mostly dry, any slight imperfections on the floor profiles are unnoticeable when dry. Low spots are evident only if waters is present.
Understanding of Surface Regularity (SR)
Theoretically, a floor specified to SR2 tolerance accepts 5mm water ponding on every localised measurement using a 2-meter straightedge. However, the common misinterpretation of SR2 is assumed to be a +/- 5mm level tolerance from the control datum (level control), which is near to impossible to achieve. As a result, huge spending for repairs are incurred due to such misunderstandings in complying to the SR standard. Another ambiguity of this standard is that it does not indicate how many bumps are allowed if measurements need to be taken. Generally, at least 5% of the floor area should be measured. For example, for a floor area of 500-square meters, twenty-five equally spaced out points need to be selected and measured randomly.
Without water stagnation on the floor, any low spots may not be noticeable. Any corrective action taken on insignificant ponding areas is unfavourable, as any repair carried out may make the smooth floor appear worse than before. If the repairs is not carried out properly, in addition to patchiness and colour variations, the repaired area may delaminate. The only areas where water ponding is unacceptable are main walk paths, vehicle turning points and areas which may be a safety hazard, causing people to slip and fall or a vehicle to skid.