{"id":66345,"date":"2009-01-26T15:05:55","date_gmt":"2009-01-26T15:05:55","guid":{"rendered":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/article\/prepare-before-repair_o"},"modified":"2009-01-26T15:05:55","modified_gmt":"2009-01-26T15:05:55","slug":"prepare-before-repair_o","status":"publish","type":"post","link":"https:\/\/zonda-develop.go-vip.net\/concrete-construction\/projects\/infrastructure\/prepare-before-repair_o\/","title":{"rendered":"Prepare Before Repair"},"content":{"rendered":"
\r

Concrete clarifiers and storage tanks may lose up to 1\/8-inch of material each year when they’re exposed to treatment chemicals, which cause biogenic sulfide corrosion. Many plant managers apply protective coatings to ward off degradation, not knowing that \u2014 when improperly applied \u2014 the coatings speed the destruction of those valuable assets.<\/p>\r

Studies have shown that pinholes that create a discontinuous protective film are the major cause of concrete breakdown at water treatment plants. Cast-in-place and precast concrete sometimes contain imperfections that were formed during placement. Those small voids, caused when entrapped air within the substrate is released into the coating, allow gasses or corrosives to penetrate the otherwise impervious coating and attack the underlying concrete substrate.<\/p>\r

Although resurfacing the substrate improves film quality by eliminating imperfections, applying a coating directly to an unprepared surface causes pinholing, outgassing, and bughole-induced outgassing of the barrier film.<\/p>\r

Fortunately, resurfacing materials and repair methods have been developed exclusively for use under high-performance protective coatings. The best choices for repair materials are cementitious mortars using ingredients with properties that closely match those of the host concrete.<\/p>\r

Four composite cementitious mortars are available for water and waste-water structures:<\/p>\r

  • Epoxy-modified<\/li>\r
  • Acrylic-modified<\/li>\r
  • Portland-based<\/li>\r
  • Calcium aluminate-based.<\/li>

    READ BEFORE ASSEMBLING<\/b>\r <\/p>\r

    Regardless of repair material, though, the first step to a long-lasting, effective coating is to create a proper surface profile.<\/p>\r

    Manufacturer recommendations for surface preparation range from high-pressure power washing to very specific surface profile requirements. This inconsistency causes confusion \u2014 and often failures \u2014 for contractors who must sort through the manufacturer’s instructions and compare them to the requirements of the protective coating system and specification documents.<\/p>\r

    Enlightened manufacturers recommend industry-accepted surface preparation standards, such as the ones created by the Society for Protective Coatings (SSPC), National Association of Corrosion Engineers (NACE), American Concrete Institute (ACI), and International Con crete Restoration Institute (ICRI), to create consistency and clarity.<\/p>\r

    The goal is to meet the prescribed adhesive specification. For example, most requirements in typical water or waste-water repair applications call for the adhesion of the protective coating system, including repair material, to exceed the concrete’s tensile strength (generally around 350 to 450 psi). Using a portable adhesion tester according to ASTM D7234, for example, will confirm it.<\/p>

    MAKE SURE IT’S THICK ENOUGH<\/b>\r <\/p>\r

    Cementitious resurfacing mortars applied at less than minimum recommended thickness may be too weak and friable to support high-build, chemical-resistant protective coating systems specified for water and wastewater environments. Therefore, double-check the \u201cprepared\u201d surface before installing the protective coating.<\/p>

    \r <\/p>

    Typically, epoxy-modified cementitious materials can be applied at a minimum thickness of 1\/16-inch, compared to the \u00bc-inch to 1\/8-inch minimum for acrylic-modified resurfacers, \u00bc-inch to 3\/8-inch minimum for portland-based mortars, and \u00bd-inch minimum thickness for calcium aluminate-based materials.<\/p>\r

    A 6-mgd wastewater treatment plant in Louisiana recently averted what would have been an expensive mistake. A contractor who was about to apply the protective coating doubted the quality of the surface repair, suspecting that the portland-based cementitious mortar used to fill voids and bugholes was inadequate to support the epoxy lining system specified for the plant’s severe wastewater headspace environments.<\/p>\r

    A test was performed using a Type II fixed-alignment adhesion tester, or elcometer, on a 5×5-foot section of concrete wall coated with the portland-based cementitious mortar, a common testing protocol for field-applied liquid coatings.<\/p>\r

    Results revealed 100% cohesive failure (or splitting) of the mortar at less than 80 psi. Adhesion was well below the anticipated 350- to 450-psi minimum generally obtained when applied over a cementitious substrate. The portland-based resurfacer applied at 1\/16-inch thickness to the new concrete did not develop the proper calcium silicate hydrate matrix, ultimately yielding poor cohesive properties.<\/p>\r

    Given those findings, the decision was made to remove and replace the portland-based mortar with an epoxy-modified cementitious resurfacer.<\/p>

    CURING IS CRITICAL<\/b>\r <\/p>\r

    Research indicates that bond strength develops more slowly than compressive strength. Sometimes crews mistakenly abandon curing procedures prematurely when the traditional cementitious mortar seems strong. But once the mortar dries, strength development stops and the mortar patch may fail.<\/p>\r \r \r<\/div> \r","protected":false},"excerpt":{"rendered":"

    Concrete clarifiers and storage tanks may lose up to 1\/8-inch of material each year when they’re exposed to treatment chemicals, which cause biogenic sulfide corrosion. Many plant managers apply protective coatings to ward off degradation, not knowing that \u2014 when improperly applied \u2014 the coatings speed the destruction of those valuable assets. Studies have shown […]<\/p>\n","protected":false},"author":0,"featured_media":66348,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"scheduledNewsletterDates":[],"_scheduled_newsletter_id":0,"footnotes":"","jetpack_post_was_ever_published":false,"zonda_content_type_url_append":"_o","zonda_dfp_zone":"","zonda_exclude_recommended_contents":false,"zonda_exclude_most_popular":false,"zonda_exclude_tagged_results":false,"zonda_eyebrow":"Building & Facility Management","zonda_last_newsletter_date":"2099-12-31T23:59:59.999-05:00","zonda_original_url":"","zonda_pdf_version":0,"zonda_print_headline":"Prepare Before Repair","zonda_promo_abstract":"Eliminating substrate imperfections protects your investment in protective 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