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Historic Preservation - Technical Procedures

Spectitle:

Patching Chips And Cracks In Ornamental Concrete Block

Procedure code:

0422003R

Source:

Developed For Hspg (Nps - Sero)

Division:

Masonry

Section:

Concrete Unit Masonry

Last Modified:

11/30/2012

Details:

Patching Chips And Cracks In Ornamental Concrete Block



PATCHING CHIPS AND CRACKS IN ORNAMENTAL CONCRETE BLOCK


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on repairing chips,
         cracks or holes in ornamental concrete block.  Small
         areas may be patched using a mortar mixture; larger areas
         may require pins and mortar for additional reinforcement.

    B.   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 REFERENCES

    A.   American Society for Testing and Materials (ASTM)
         www.astm.org

1.03 PROJECT/SITE CONDITIONS

    A.   Environmental Requirements:

         1.   Check manufacturer's literature for precautions and
              effects of products and procedures on adjacent
              building materials, components, and especially
              vegetation; Take appropriate protective measures as
              necessary.

         2.   Wet Weather:  Do not apply or mix mortar on outside
              surfaces with standing water or outside during
              rain.

         3.   Do not proceed with patching under adverse weather
              conditions, or when temperatures are below or above
              manufacturer's recommended limitations for
              installation;  Proceed with the work only when
              forecasted weather conditions are favorable for
              proper cure.

         4.   Cold Weather, winter construction is not allowed
              without consent of Regional Architect;  Winter
              construction is defined as any time when surface
              temperature of masonry is below 50 degrees F. or air
              temperature is predicted to be below 40 degrees F. within
              48 hours; Heat mortar materials to above 50 degrees F. if
              necessary.

         5.   Work must not be done at temperatures above 80 degrees F.
              unless shading and water-misted burlap over new
              work is provided.  Mortar mixing should be done
              only in the shade; cover mortar in hot weather to
              reduce evaporation.  Pointing work should be done
              in the shade.  Work around the building during the
              day so that the fresh work will be shielded from
              direct sunlight to reduce evaporation rate.


PART 2---PRODUCTS

2.01 MATERIALS

    A.   Clean, potable water

    B.   Portland cement

    C.   Hydrated lime

    D.   Sand

    E.   Epoxy cement

    F.   Teflon or nylon pins with scored and threaded surface.
         (Stainless steel or bronze may be used if teflon or nylon
         are unavailable).

2.02 EQUIPMENT

    A.   Stiff bristle brushes or vacuum

    B.   Trowels

    C.   Hawks

    D.   Carbon-tipped masonry bit


PART 3---EXECUTION

3.01 ERECTION, INSTALLATION, APPLICATION

    A.   Patching small cracks and holes with grout (a wetter
         version of mortar mix):

         1.   Cut out the deteriorated area to a sound surface.
              Under-cut the edge where possible to create a
              "key".

         2.   Brush or vacuum out all dirt or debris.

         3.   Flush the area with clean, clear water.  Be sure no
              standing water remains.


         
4.   Mix 1 part Portland cement, ASTM C150, Type I, 
              part hydrated lime, and 2 to 3 parts sand

              -OR-

              1 part Portland cement Type P (lime pre-mixed), and
              2 to 3 parts sand

              NOTE:  ADJUST MIX TO MATCH COLOR, TEXTURE AND
              PHYSICAL PROPERTIES OF THE ORIGINAL MORTAR.

         5.   Thin mortar to a slushy batter consistency.

         6.   Trowel apply mortar to damaged area in layers no
              more than 1 inch thick.  Several layers may be
              required.  DO NOT APPLY PATCHES OVER JOINTS.

    B.   Patching large chips, holes or broken corners using pins
         and mortar:

         1.   Using a stiff bristle brush, clean surfaces to be
              joined or patched.

         2.   Using a carbon-tipped masonry bit, drill staggered
              rows of holes approximately 2 inches deep (no more
              than 4 times the pin diameter), 1-1/2 inch apart and
              1/8 inch wider than the pin diameter.

         3.   Again, brush debris from the surface.

         4.   Fill the holes with mortar mix or epoxy cement.

         5.   Set the pins in the holes.

         6.   Trowel apply mortar to damaged area in layers no
              more than 1 inch thick.  Several layers may be
              required.  DO NOT APPLY PATCHES OVER JOINTS.

         7.   To patch a chipped corner, fill the cavity with the
              concrete mix or bonding material, forcing it in and
              around the exposed pins, which will act to support
              and reinforce the patch.


    C.   To rejoin two broken parts (such as a concrete baluster),
         coat both broken surfaces with epoxy adhesive and gently
         tap the parts together.

                         END OF SECTION
 


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