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Historic Preservation - Technical Procedures

Spectitle:

Patching Scaling Concrete Masonry

Procedure code:

0373202R

Source:

Hspg Prepared For Nps - Sero

Division:

Concrete

Section:

Concrete Repair

Last Modified:

02/24/2012

Details:

Patching Scaling Concrete Masonry



PATCHING SCALING CONCRETE MASONRY


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on patching scaling
         concrete with a mortar patch.

    B.   Scaling of concrete masonry is the local flaking or
         peeling away of the near surface portion of concrete or
         mortar.  It is often caused by:

         1.   Overtroweling:  Weakens the bond of the surface
              concrete to the concrete below by bringing an
              excess of fines to the surface.

         2.   Insufficient concrete strength:  Often a result of
              cool temperatures that do not allow the concrete to
              cure properly and gain sufficient strength to
              withstand freeze/thaw cycles and the application of
              de-icing chemicals.

         3.   The use of concrete with low air contents in areas
              exposed to severe winter weather.

         4.   Heavy application of salts and/or de-icing
              chemicals.

         5.   The use of high slump concrete.

         6.   Improper curing methods or lack of curing.

         7.   Insufficient protection of fresh concrete during
              cold temperatures.

    C.   See 01100-07-S for general project guidelines to be
         reviewed along with this procedure.  These guidelines
         cover the following sections:

         1.   Safety Precautions

         2.   Historic Structures Precautions

         3.   Submittals

         4.   Quality Assurance

         5.   Delivery, Storage and Handling

         6.   Project/Site Conditions

         7.   Sequencing and Scheduling

         8.   General Protection (Surface and Surrounding)

         These guidelines should be reviewed prior to performing
         this procedure and should be followed, when applicable,
         along with recommendations from the Regional Historic
         Preservation Officer (RHPO).

1.02 DEFINITIONS

    A.   Scaling is the local flaking or peeling away of the near
         surface portion of concrete or mortar.

         1.   Peeling:  Thin flakes of mortar are broken away
              from the concrete surface.

         2.   Light scaling:  Loss of surface mortar without
              exposure of coarse aggregate.

         3.   Medium scaling:  Loss of surface mortar without
              exposure of coarse aggregate.

         4.   Severe scaling:  Loss of surface mortar 5 to 10 mm
              in depth with some loss of mortar surrounding
              aggregate particles 10 to 20 mm in depth so that
              aggregate is clearly exposed and stands out from
              the concrete.

         5.   Very severe scaling:  Loss of coarse aggregate
              particles as well as surface mortar surrounding
              aggregate, generally greater than 20 mm in depth.

    B.   Spalling is a more extreme form of scaling.

1.03 MAINTENANCE

    A.   Check to see that all gutters, downspouts and other water
         run-off systems are in good repair and clear of debris.

    B.   Correct conditions of "rising damp", splashback, and
         foundation wetness.

    C.   Remove water-entrapping vegetation from on or near the
         concrete walls.

    D.   Do not use salt to melt snow anywhere near concrete
         walls.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.   Thoro System Products
         7800 NW 38th Street
         Miami, FL  33166
         305/597-8100

2.02 MATERIALS

    A.   Mortar Patching Material:  Composition to be determined
         by Regional Historical Preservation Officer (RHPO)

    B.   Bonding Agent or Cement paste such as "Acryl" (Thoro
         System Products), or as specified by RHPO

    C.   Rebars and/or stainless steel pins as required

    D.   Epoxy as required

    E.   Clean, potable water

2.03 EQUIPMENT

    A.   Hammer and chisel

    B.   Trowel

    C.   Mortar board

    D.   Masonry drill


PART 3---EXECUTION

3.01 ERECTION,INSTALLATION, APPLICATION

    A.   Cut back damaged area to stable material.  Roughen the
         surface with a hammer and chisel.

         CAUTION:  AVOID GRINDING THE MATERIAL IN THIS PROCESS,
         BECAUSE IT LEAVES THE SURFACE TOO SMOOTH TO ACHIEVE A
         PROPER BOND.  

    B.   Remove all rust from any exposed rebar.

    C.   Cut out any rebar which is severely corroded and splice
         in new rebar- consult RHPO.

    D.   Paint freshly cleaned rebar with an epoxy coating to
         prevent further rusting.

    E.   If the patch is unusually large, drill holes into sound
         substrate and insert stainless steel pins anchored with
         epoxy.

    F.   Remove all dust and debris by water blasting, air
         blasting, or with a broom or vacuum.

    G.   Square off the perimeter of the area to be patched so
         that a feathered edge will not be required.

    H.   About 1 hour before making repair, moisten surface of
         area to be patched.

    I.   To insure a good bond between patch and substrate, brush
         substrate surface with either a cement wash or a bonding
         agent such as Acryl.

    J.   Apply patching material, with a trowel, in 3/4" layers,
         compacting thoroughly after each layer.

    K.   Work the finished surface carefully making sure to match
         texture and appearance of surrounding surfaces.

         NOTE:  PATCHING MIX SHOULD MATCH AS CLOSELY AS POSSIBLE,
         THE TEXTURE AND COLOR OF THE ORIGINAL.

                         END OF SECTION