The performance criterion for below-grade waterproofing materials differs from other exterior building component materials. Waterproofing materials are unique because they are exposed too much harsher conditions than any of the other building exterior components. Waterproofing materials, particularly in below-grade assemblies, are subjected to more rigorous conditions than other exterior component materials. Waterproofing materials are exposed to the continual presence of water, environmental conditions and exposure to various soil chemicals, fungi, and alkaline. Most of the exposure elements are continually present at the waterproofing surface and do not dissipate as they do at the other exterior components. For example, water can be present in below grade surfaces for weeks, whereas water on roof systems is to be removed within 48 hours.
In addition, waterproofing systems should have a service life that matches the life of the
building. This is an important characteristic, because unlike other exterior building components, the waterproofing materials are inaccessible and the expenses associated with repairs and replacement are substantial, primarily due to excavation costs.
The best way for a designer to determine the materials best suited for a waterproofing application is through the examination of the materials’ physical properties based on performance requirements. In addition to successful performance in high ground water conditions, the essential waterproofing requirements of the material should include the following criteria:
The determining factor in deciding if waterproofing is required for below-grade structures and what type of application (waterproofing or damp proofing) is required is based on the existence of hydrostatic pressure. Hydrostatic pressure is pressure exerted by stationary liquid water in all directions, against adjacent surfaces. Below grade structures must be able to resist hydrostatic pressures that generally range from 30 to 62.4 psf per foot of depth. The rate of hydrostatic pressure depends on the surrounding soils. Pressures are typically lower in dry, granular soils where water flow remains at the vertical subsurface and higher at wet soils where water flow is continual.
Hydrostatic pressure should be determined by a civil engineer prior to waterproofing design. A general rule in determining hydrostatic pressure is that it increases linearly with depth, which produces a triangular horizontal loading pattern where pressure is exerted from below the structure, above the structure and at the wet face of the structure. The formula to determine hydrostatic pressure at a depth of 10 feet is 62.4 x 10 = 624 psf in all directions.
Hydrostatic pressure occurs at a building component anytime the water table raises above the component and it is directly related to ground water levels. Water rises in most soils by capillary action. The rise can be as extreme as 11.5 feet in soils made of small particles (clay and silt) to zero in granular (gravel) soils that have large spaces between the particles. When ground water levels are at the highest points – in the spring, after flooding from rains, heavy run-off from walls or from clogged drains – the hydrostatic pressure rises. These changes in pressure can occur on an hourly basis. The designer should specify waterproofing to meet the highest pressures under extreme conditions. This can be calculated by one of three methods:
P = wd on top slab
P = w(dth) lateral pressure at the base of the wall
P = w(dth) upward resistance on the slab-on-ground
Proper designs of slabs-on-ground that are subjected to hydrostatic pressure must be
completed to resist uplift from hydrostatic pressure. This can be accomplished in one of the
1. Increased slab weight (to counterbalance upward hydrostatic pressure)
2. Reinforcing the slab for flexural resistance and anchoring it to foundations and
3. Tying the slab to rock anchors
Installing under slab drains and footing drains that are directed to the storm water system can also accomplish prevention of hydrostatic pressure.
The testing requirements to determine a materials resistance to hydrostatic pressure are provided in ASTM 5385. The designer should ensure that the manufacturers materials meet the requirements of these tests for each specific project.
One element of consideration is soil characteristics. Chemicals in the soil can have an adverseeffect on some materials and knowledge of potential chemicals present is required for design.
Chemical properties in soils can adversely affect waterproofing in various ways. Acids and
alkaline in ground water can accelerate the deterioration of concrete and steel reinforcing bars.
Salt in water corrodes reinforcing bars in concrete. Sulfates can have a negative reaction with
Portland Cement resulting in internal sheering stress that causes spalling. Other chemicals that affect waterproofing are calcium hydroxides, oils, and chemicals from fertilizer.
The manufacturer should provide a list of chemicals that the material is resistant too and ensure compliance for each project as required.
The intensity of the hydrostatic pressure is also an important consideration in the selection of
waterproofing materials. When subjected to continuous hydrostatic pressures, membranes with low moisture absorbance rates tend to perform well. High moisture absorbance membranes can be subjected to swelling, disbanding and wrinkling in these conditions. Wrinkled membranes are subject to an increased risk of puncture. Intense hydrostatic pressure can also force membranes in concrete voids exposing the membrane to cracks from flexural stress allowing openings for moisture infiltration.
A key element of waterproofing materials is the water absorption rate. A materials water
absorption rate can be determined by ASTM E96 “Water Vapor Permeability”. The ASTM standard states that a materials water absorption rate for waterproofing applications should be below 5%. Best practices state that a satisfactory rate should be below 4%, preferably in the 1% to 2% range. If materials with absorption rates above 5% were exposed to the perpetually wet conditions found in waterproofing environments, there is a probability of water wicking, which would cause the membrane to swell and disband.
Waterproofing is applied on substrate surfaces to protect the substrate from structural
deterioration caused by water, chemicals, and soil characteristics. The applied waterproofing
material must also be capable of performance if the substrate becomes unstable or if minor
imperfections occur. Some substrates are inherently prone to imperfection and this should be considered prior to waterproofing design.
Over substrates that are vulnerable to cracking from any source, the waterproofing membrane must be elastic and capable of resealing. Cracks can occur in masonry or other waterproofed components that have multiple construction joints. Damp proofing should not be considered in these conditions.
The types of soils at the site can also have an impact on substrate stability. Expansive soils and peaty soils can produce rising and settling footings that induce cracks in footings and foundation walls. All substrate openings can become potential points of moisture infiltration.
Due to suspectable occurrence of substrate cracks the waterproofing material must have the
ability to successfully bridge any cracks. The ASTM C1305 “Crack Bridging” test determines
the materials ability to bridge cracks at different temperatures.
The most important criteria for a waterproofing material is its ability to successfully adhere to
the existing substrate for the service life of the system. All waterproofing applications,
particularly below-grade applications, require full adhesion to the substrate. Partially adhered or un-adhered materials provide an opportunity for moisture to infiltrate into the substrate and/or interior of the facility. Most waterproofing material failures occur due to disbanding of the membrane and the substrate. In addition to moisture infiltration, loss of adhesion of the waterproofing materials leads to splits and openings.
ASTM C794 tests the waterproofing materials adhesion rates to the substrate. Adhesion tests can be completed in both the laboratory and at the site. Due to the importance of adhesion to the success of the project, it is a best practice for designers to require on-site adhesion tests to be completed during application.