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Historic Preservation - Technical Procedures

Spectitle:

Preparing Lime Mortar For Repointing Masonry

Procedure code:

0410003S

Source:

Hspg Prepared For Nps - Sero

Division:

Masonry

Section:

Mortar & Masonry Grout

Last Modified:

02/24/2012

Details:

Preparing Lime Mortar For Repointing Masonry



PREPARING LIME MORTARS FOR REPOINTING MASONRY


PART 1---GENERAL

1.01 SUMMARY

    A.   This standard includes guidance on preparing lime mortars
         for repointing masonry.

    B.   Lime mortars are preferable to portland cement mortars
         for repointing historic masonry:

         1.   Lime mortars are more permeable by water.  Water
              passing through lime mortar will dissolve a small
              portion of the lime and then will deposit it in
              hairline cracks as the water evaporates.

         2.   Lime mortars expand slightly during setting, and
              resists shrinkage which causes cracking.

         3.   Lime mortars are more durable than generally
              recognized.

    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 REFERENCES

    A.   American Society for Testing and Materials (ASTM), 100
         Barr Drive, West Conshohocken, PA 19428, (610) 832-9585
         or FAX (610) 832-9555.

1.03 DELIVERY, STORAGE AND HANDLING

    A.   Storage and Protection:  Lime and cement must be
         protected from rainwater and ground moisture, as water
         vapor in the air can begin the setting process.  Other
         materials also should be protected from contamination.


PART 2---PRODUCTS

2.01 MATERIALS

    NOTE:  The use of standard specifications for materials, such
    as those developed by the ASTM, provides an easily referenced
    level of quality.  

    A.   Lime:  Should conform to ASTM C207, Type S, high
         plasticity, Hydrated Lime for Masonry Purposes.  

         1.   Lime which meets this standard will "work" well,
              resists drying during curing, and is sufficiently
              strong for the purpose of repointing.  

         2.   Lime expands as it hydrates, making high lime
              mortars more resistant to crack formation.

    B.   Cement:  Should conform to ASTM C150, Type I, White.  It
         should not have more than 0.60% alkali nor more than
         0.15% water soluble alkali.  Use gray portland cement
         ONLY if a dark mortar is to be matched.

         1.   Cement meeting this standard should increase the
              workability of the mortar, accelerate the setting
              time and slightly increase the strength of the
              mortar.

         2.   The low alkali content will prevent efflorescence.

    C.   Sand:  Free of impurities and conforming to ASTM C144.  

         1.   Sand color, size, and texture should match the
              original as closely as possible.  Provide a sample
              of the sand for comparison to the original, and
              have it approved by the RHPO before beginning
              repointing work.

         2.   When possible, use bar sand or beach sand rather
              than crushed sand for the repointing mortar.

              NOTE:  BAR SAND OR BEACH SAND SHOULD BE WASHED TO
              REMOVE THE SALTS BEFORE USING.

              a.   Crushed sand has sharp edges, which makes it
                   more "sticky" and difficult to work into the
                   joints.  

              b.   Bar sand, on the other hand, has rounded edges
                   and flows easily during the mortar
                   application.  

              c.   The working characteristics of mortar made
                   with crushed sand may be improved by adding a
                   slight amount of portland cement.  The amount
                   of cement should be determined by
                   experimentation, but should not exceed 20% of
                   the total lime/cement binder.  20% OR LESS OF
                   CEMENT HAS MINIMAL EFFECT ON THE HARDNESS OF
                   THE MORTAR.  CEMENT CONTENT ABOVE 20% WILL
                   MAKE THE MORTAR TOO HARD.

    D.   Clean, potable water:  If the water must be transported
         or stored in a container, the container must not impart
         any chemicals to the water.

    E.   Stone dust finely ground from the same stone as that to
         be repointed.

    F.   Additives:  NO antifreeze compounds or other admixture
         shall be used.

         NOTE:  DO NOT USE ANTI-FREEZE COMPOUNDS.  THESE COMPOUNDS
         ARE DESIGNED FOR USE WITH CEMENT MORTARS, AND THEIR
         EFFECTIVENESS WITH HIGH LIME MORTARS IS QUESTIONABLE.
         FURTHERMORE, THE COMPOUNDS CONTAIN SALTS WHICH CAN LEAD
         TO SERIOUS PROBLEMS IN THE MASONRY AT A LATER TIME.  

         NOTE:  AIR ENTRAINING AGENTS ARE NOT RECOMMENDED.  THESE
         AGENTS ARE DESIGNED FOR USE WITH CEMENT RATHER THAN LIME,
         AND THEY RESULT IN DECREASED BONDING OF THE MORTAR AND
         THE MASONRY.  AIR ENTRAINING IS NOT NECESSARY WITH HIGH
         LIME MORTARS BECAUSE OF THE NATURAL ABILITY OF THESE
         MORTARS TO FLEX WITH TEMPERATURE CHANGES.

2.02 EQUIPMENT

    A.   Surface temperature thermometer - can be either
         mechanical (less expensive but must be calibrated often)
         or digital electronic

    B.   Wooden mortar boxes

    C.   Hoe

    D.   Mesh screen

    E.   Hawks:  Plywood or steel hawk (mortar board)

2.03 MIXES

    A.   Some factors to consider when mixing lime mortar include
         durability, color and texture, and workability.

         1.   Durability:  Repointing mortar should be softer
              than the masonry units and the original mortar to
              reduce stresses at the edge of the masonry and, in
              the case of lime mortar, to reduce shrinkage which
              can cause cracks in the mortar.

              a.   If the new mortar is harder than the masonry
                   or the original mortar, it can cause serious
                   stresses within the wall during thermal
                   expansion and contraction, which can lead to
                   deterioration of the masonry units rather than
                   the mortar.  

              b.   If the mortar is softer, any deterioration
                   which occurs will take place in the mortar,
                   which is easier to replace than the units
                   themselves.  

         2.   The repointing mortar should allow the passage of
              water, both liquid and vapor.  If the mortar does
              not allow water to pass freely through it, the
              water can become trapped inside the wall, freeze
              and cause serious deterioration to the masonry.

         3.   Color and texture:  The repointing mortar should
              match the original mortar in color, texture and
              physical characteristics.

              a.   Obtaining an accurate color match is best
                   achieved by selecting an appropriate sand.

                   1)   Use sand which is similar to the original
                        in color and gradation.  Sand from more
                        than one source may be required.  

                   2)   For repointing of natural stones, use
                        finely ground stone "dust" in the mortar
                        to match the joints as closely as
                        possible to the stone.


              b.   If the original mortar was tinted, or if it is
                   impossible to obtain a color match through the
                   use of sand, it may be necessary to use a
                   special mortar pigment.  

                   CAUTION:  PIGMENTS MAY REACT WITH OTHER
                   INGREDIENTS IN THE MORTAR TO FORM
                   EFFLORESCENCE.  THEY MAY ALSO WEATHER AT A
                   DIFFERENT RATE THAN NATURAL COLORING AND CAUSE
                   A COLOR VARIATION IN THE MORTAR.  

                   NOTE:  IF PIGMENTS MUST BE USED, PURE MINERAL
                   OXIDES SHOULD BE USED BECAUSE THEY DO NOT FADE
                   OR LEACH OUT OF THE MORTAR.  AMOUNT OF PIGMENT
                   SHOULD NOT EXCEED 2% OF THE MORTAR MIX BY
                   WEIGHT.

              c.   Many mortars used before the twentieth century
                   have small lumps of incompletely burned or
                   ground lime, or other impurities.  To match
                   the original appearance of the masonry, these
                   impurities must be included in the new
                   repointing mortar.  Use identical materials,
                   such as ground oyster shells (obtained at feed
                   stores) or lumps of lime, to duplicate
                   original lumps.

         4.   Workability:  The workability or plasticity of the
              mortar is a direct result of the selection of
              materials.  

    B.   Mortar Mix:  

         1.   Have the existing mortar completely analyzed to
              insure that the repointing mortar will not be less
              permeable/harder than the masonry units or the
              original mortar.  IT IS BETTER TO HAVE MORTAR THAT
              IS MORE PERMEABLE THAN LESS.

         2.   Measure all ingredients by cubic volume using a
              pre-established uniform measure, such as a small
              bucket, rather than a less uniform measure such as
              a shovel.

         3.   For historic masonry set in lime mortar, use the
              following mortar mix:

              1 part portland cement
              3 parts lime
              8-12 parts sand (To match existing mortar as
              closely as possible.)

              NOTE:  The exact mix required will relate to the
              grain size and sharpness of the sand and will vary
              depending on the supply.  

              -OR-

              For historic masonry set in standard mortar, use
              the following mortar mix (ASTM C270 Type "0") as a
              starting point:

              1 part portland cement
              2 parts lime or lime putty
              6 to 9 parts sand and stone dust (To match existing
              mortar as closely as possible.)

              -OR-

              For Limestone (ASTM C270 Type "N"):

              1 part portland cement
              1 parts lime
              4-6 parts aggregate
              Enough water to form a workable consistency

              -OR-

              For Granite (ASTM C270 Type "S"):

              2 parts portland cement
              1 part lime
              7-9 parts aggregate
              Enough water to form a workable consistency

              NOTE:  FOR DETERIORATED GRANITE OR GRANITE WALLS
              INDICATING MOVEMENT, USE ASTM C270 TYPE "N" AS
              LISTED ABOVE FOR LIMESTONE.

         4.   Mix a final "job-size" batch once the correct sand
              color, cement content, etc. have been determined
              through small tests to ensure the on-site mixing
              conditions will result in the same final product.


PART 3---EXECUTION

3.01 ERECTION, INSTALLATION, APPLICATION

    A.   Mix Hydrated Lime:  

         1.   Add dry bagged hydrated lime to water.  Stir and
              hoe the mass to form a thick cream.  

         2.   Allow to stand at least 24 hours before use.

    B.   Prepare Roughage Premix (for later use):

         1.   Accurately proportion the sand and lime using
              measuring boxes constructed to contain the exact
              volume of each ingredient required to make on
              batch.

         2.   Mix sand and lime thoroughly for about ten minutes.
              Store in plastic-lined drums and seal until
              required.

              NOTE:  THIS COMPOUND MAY BE STORED INDEFINITELY IF
              KEPT SEALED FROM AIR AND KEPT FROM FREEZING.

         3.   When required for use, add and mix the correct
              portion of gauging cement as specified and use
              immediately.  ACCURATE PORTIONING IS VERY
              IMPORTANT.

    C.   Add cements to lime and aggregate mixes immediately
         before the use of the mortar.

         1.   Perform all batching with wooden boxes or plastic
              pails of known volume to ensure standardization and
              conformity of measurement; SHOVEL MEASUREMENT OF
              MATERIALS IS NOT PERMITTED.

         2.   Use box sizes that are sufficient for producing a
              batch size equal to one mixer load.

         NOTE:  MIX DRY INGREDIENTS THOROUGHLY BEFORE ADDING ANY
         WATER (APPROXIMATELY FIVE MINUTES).

    D.   Add a small amount of water so that the mortar is just
         wet enough to hang on a trowel.

         NOTE:  EXCESS WATER WILL CAUSE SHRINKAGE AND TOO LITTLE
         WATER WILL RETARD CARBONATION.  RECORD THE AMOUNT OF
         WATER ADDED SO THAT IT MAY BE USED AS A GUIDE FOR FUTURE
         BATCHES.

    E.   Mix mortars at least 10 minutes before using to improve
         workability and ensure thorough mixing.

         NOTE:  AUTOMATIC MIXERS SHOULD HAVE RUBBER BLADES. CLEAN
         MIXING BOARDS AND MIXING MACHINES THOROUGHLY AFTER EACH
         USE TO PREVENT HARDENED LUMPS OF MORTAR FROM
         CONTAMINATING THE NEXT BATCH OF MORTAR.

         1.   Repointing mortars may sit 1-2 hours after initial
              mixing and then may be remixed to a workable
              consistency.  This is done to reduce shrinkage.

         2.   Test the mix by holding a trowel with mortar on it
              upside down and shaking it once.

              a.   If the mortar falls off without shaking, it
                   has too much sand.

              b.   If more than one shake is required, the mortar
                   is too sticky or "plastic" and the lime
                   content must be decreased.

    F.   Coloring Mortars:

         1.   Take samples of freshly-broken mortar from the
              original masonry pointing.  Note color of aggregate
              for color-matching.  DO NOT TRY TO MATCH THE COLOR
              OF THE BINDER.

              NOTE:  USE UNWEATHERED, UNSOILED SAMPLES ONLY.

         2.   Prepare test patties of mortar approximating the
              inner color of the sample and set aside to dry for
              at least 72 hours.  Drying time may be accelerated
              by placing the patty sample in an oven or over a
              hot-plate.

         3.   Break the sample test patties and compare the inner
              portions to the original.

         4.   See Section 2.03 above for additional information
              on coloring mortars.

    G.   Use repointing mortar within approximately 1-2 hours of
         final mixing.  Retemper the mortar as necessary to
         maintain workability.

         NOTE:  RETEMPERING IS PERMITTED TO MAINTAIN WORKABILITY.
         REMIXING IS NOT PERMITTED.  ADD WATER AT THE MORTAR-BOARD
         USING A SPRAY BOTTLE TO REPLACE ONLY WATER LOST THROUGH
         EVAPORATION.

         NOTE:  USE ALL MORTAR WITHIN TWO HOURS OF GAUGING; THROW
         OUT LEFT OVER MORTAR; DO NOT RE-TEMPER OR REMIX MORTARS
         AFTER THIS TIME HAS ELAPSED.

         NOTE:  THIS TIME LIMIT MAY VARY DEPENDING UPON THE
         OUTSIDE TEMPERATURE (LONGER ON COOLER DAYS AND SHORTER ON
         WARMER DAYS).  

    H.   For guidance on repointing, see 04520-02-R.

                         END OF SECTION