Geoffrey Hichborn Sr., PE
"Concrete Insights and Experience When You Need It"
BFI, Building Forensics International, Concrete Guy, Concrete Dude, Concrete Petrographer, Distressed Concrete Warehouse Slab, Concrete Consultant, Concrete Cracks, Concrete delaminations, Concrete and Portland Cement, Concrete Failure Analysis, Concrete Forensic Analysis, Concrete Expert Witness, Concrete Expert
The following is an excerpt from a Concrete Expert Witness Report of a Six Story Post-Tensioned Concrete Structure:
The structure is a six story post-tensioned concrete structure with multiple architectural features and accents, the primary ones in this investigation being the two spiral automotive traffic ramps, one downward and one upward , specifically including their perimeter walls. For a parking structure, the feeling of being in the structure is generally one of openness and airiness. The exterior surfaces of the spiral ramps are exposed to the weather in varying degrees, which (depending on the orientation of the sunlight) will also have a seasonal effect.
The basic cause of action for this report is that the perimeter walls of the spiral ramps have cracked and permitted efflorescence to leave white deposits on the light brown painted surfaces. The efflorescence gives the visual effect of a vertical crack that has had a water flow from and then dry up, leaving a chemical residue where the water has flowed. The cracks (and efflorescence) generally start from half to three quarters of the height of the wall and extend downwards. Detailed crack width measurements were not taken, but the cracks were narrow and it was estimated to be unlikely that any of them were over 0.010 in. wide.
At the time of the inspection painted portions of the structure do not appear to have been cleaned, there was no evidence of paint touch up treatments. Any cleaning performed would have been required primarily due to the accumulation of efflorescent materials on painted surfaces.
Observations on the other painted concrete surfaces in other locations around the structure reveal that efflorescence is neither prevalent nor problematic in these areas. Predictable, minor, and acceptable amounts of efflorescence are present on relatively few concrete surfaces which are exposed to moisture and evaporative moisture loss. However, the only painted areas which are generally exposed to moisture and have fairly regular and predictable white efflorescence deposits are the spiral parking ramps. Generally, the patches of efflorescence are nearly all vertical or trend vertical, are associated with narrow cracks, and appear in repeating patterns and spacing, with similar distances between patches. One observation of interest is that not all painted surfaces exhibit efflorescence. It appears that the areas of efflorescence on the painted areas originate from vertical cracks in the concrete surface, travel due to moisture gradients through the paint, travel down and outward from the painted surface, later to dry on exterior faces due to sunlight and heat, leaving behind tracks of once dissolved salts now dried.
Detailed observations of the spiral ramps were made at both the exterior and the interior sides of the walls. In this area, all of the wall faces were painted. All wall sections were cast using forms with through-ties: the voids created by the tie ends were filled using patching materials. Other surface blemishes were also patched and/or concealed by the use of the material Pavecrete, where appropriate. This material is troweled on as essentially a parge, in a thin coat that will fill small voids and provide an architecturally pleasing surface.
As the observations progressed it became apparent that the color and texture of the finished product was not noticeably varied from location to location. Concealed blemishes and filled through-tie holes were concealed very nicely.
Analysis of the Problem
Efflorescence is caused by the progressive leaching, dissolution, transport of soluble mineral compounds by liquid water and their subsequent precipitation and deposition on surfaces by drying. It is necessarily a demonstration of the action of water and heat on materials so exposed. There is a slight amount of such soluble materials in concrete and cement based patching materials; the amount, pattern and distribution of efflorescence in this case would ordinarily be described as trivial, and would not be very noticeable if merely deposited on concrete that had not received any architectural treatment. In this case, however, the problem is magnified by the architectural light brown paint under the efflorescence deposits. This color contrast magnifies the effect of a relatively small amount of efflorescence, and makes it a real cosmetic problem.
The efflorescence is present because small cracks in the concrete surface can take up moisture, which then acquires dissolved salts and is subsequently expelled and evaporated, leaving the salts then known as efflorescence. The cracks have originally formed due to drying shrinkage (occurring from loss of moisture in the original concrete). As the concrete loses moisture, it tries to shrink. In the case of the walls surrounding the spiral ramp, the upper portion of the walls are not restrained and can tolerate some shrinkage without cracking. For the lower portion of the walls, they are restrained by a very solid spiral ramp, generally with a thickened curb to keep cars from hitting the wall as they travel on the ramp. With this stiff section restraining the wall, shrinkage in its lower portion is manifested by the small cracks which in turn provide a vehicle for the efflorescence.