Find Precasters or Suppliers that provide products, equipment and services to make precast concrete.
By Tim Frazier
Editor’s Note: This is part two of a three-part series. In the previous issue of Precast Inc., we examined the differences in damp proofing and waterproofing. In this article, we examine published industry guidelines for concrete surface preparation, guides for effective sub-grade waterproofing and related waterproofing performance specifications.
Various methods of sub-grade waterproofing exist, including: hot melt-applied waterproofing membranes and traditional sheet membranes. Today, cold liquid-applied membranes are growing in popularity, offering advantages over previously traditional systems. The waterproofing specifications reviewed here will focus on these cold liquid-applied membranes.
The unique physical and chemical nature of concrete requires special surface preparation techniques prior to coating not associated with other substrates. There are three basic steps for preparing concrete surfaces that result in a uniform, continuous coating with good adhesion:
A bond is only as strong as the weakest link in the chain. Applying a coating to contaminated or dusty concrete will result in poor coating adhesion. Laitance, a layer of weak and nondurable material containing cement and fines from aggregate, occurs when bleed water rises to the surface of formed concrete. Laitance is usually eliminated mechanically by blast abrasion. Contaminates that may need to be removed include form release agents, curing agents and oil. Several good resources related to general surface cleaning methods of concrete are listed in Table 1.
Concrete surface profile is important in that it provides the “grip” that the coating requires to achieve a strong mechanical bond or anchor to the concrete surface. Typically, the thicker the specified coating application the greater the surface profile required. ICRI Guideline 03732 identifies nine distinct concrete profile levels and describes how to achieve each by different profiling techniques. Recommended surface profiles(i) include:
0-3 mils thick
|0-75 micrometer surface profile|
4-10 mils thick
|High Build Films||
10-40 mils thick
Voids and depressions in concrete surfaces (e.g., bug holes, honeycombs and tie-rod holes) should be filled prior to coating so that a continuous coating film can be applied. Successful use of liquid coatings requires that there be no voids in the protective coating film to fully protect the concrete substrate. Similarly, concrete surface protrusions, often called fins, should be removed prior to coating to ensure complete coverage over the concrete surface. Typically, surface protrusions are removed by grinding.
There are several important waterproofing principles worth consideration. The 90%/1% principle says that 90% of all water intrusion problems occur within 1% of the structure surface
area(ii). Further, the 99% principle states that approximately 99 percent of waterproofing leaks are attributable to causes other than material or system failures(iii). These principles reinforce the need for acceptable concrete surface preparation and subsequent error-free coatings application for successful waterproofing projects.
ASTM C 898-09, “Standard Guide for Use of High Solids Content, Cold Liquid-Applied Elastomeric Waterproofing Membrane with Separate Wearing Course,” describes the use of high solids content, cold liquid-applied elastomeric waterproofing membranes in a waterproofing system subject to hydrostatic pressure. Included in this guide are terminology definitions, design considerations and waterproofing membrane requirements among other content. Additional ASTM waterproofing guides are listed in Table 2.
Waterproofing membranes specified in ASTM C898 must meet the requirements of ASTM C836, “Standard Specification for High Solids Content, Cold Liquid-Applied Elastomeric Waterproofing Membrane for Use with Separate Wearing Course.” According to this standard, waterproofing membrane materials shall cure, after application by spreading or spraying, to form an elastomeric film capable of maintaining a seal against liquid water in addition to meeting the physical property requirements as listed in Table 3.
It is interesting to note that ASTM C836 does not directly specify the waterproofing membrane’s resistance to hydrostatic pressure. Recall that the three main differences between waterproofing and damproofing are elimination of water movement under hydrostatic pressure, thickness of the applied membrane and membrane crack bridging performance. ASTM D7088, “Standard Practice for Resistance to Hydrostatic Pressure for Coatings Used in Below Grade Applications Applied to Masonry,” might be a test value that could differentiate or compare similar expected product performance with regard to evaluating stopping water intrusion into treated concrete under hydrostatic pressure. Reviewing Technical Data Sheets (TDSs) from potential membrane suppliers is an important step in selecting the most appropriate waterproofing materials for use in a given construction project.
Besides selecting the waterproofing membrane product based on product performance and physical values, additional criteria will include:
Additional performance features often touted with waterproofing membranes include the following:
Tim Frazier is technical director of Concrete Sealants Inc. He has been involved with coating-related products for 27 years and with concrete coatings for the past 20 years. Frazier holds a bachelor’s degree in chemistry from Wilmington College, and a master’s degree in chemistry from Wright State University.
i Randy Nixon. “The Fundamentals of Cleaning and Coating Concrete.” SSPC, The Society for Protective Coatings, 2001, 101.
ii Michael Kubal. “Construction Waterproofing Handbook.” McGraw Hill, 2nd Edition, 2008, 1.12.
iii Ibid., 1.13.