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Historic Preservation - Technical Procedures

Spectitle:

Monitoring And Evaluating Cracks In Masonry

Procedure code:

0420002S

Source:

Hspg Prepared For Nps - Sero

Division:

Masonry

Section:

Unit Masonry

Last Modified:

02/24/2012

Details:

Monitoring And Evaluating Cracks In Masonry



MONITORING AND EVALUATING CRACKS IN MASONRY


PART 1---GENERAL

1.01 SUMMARY

    A.   This standard includes guidance on monitoring and
         evaluating cracks in masonry.  Three different methods
         are described and include the following:

         1.   Using tape and a pencil,

         2.   Using glass and epoxy, and

         3.   Using the Avongard Crack Monitor.

    B.   Cracks in masonry are evidence that the building material
         has moved or is still moving, (active cracking).

    C.   Some causes of cracking include:  settlement or
         foundation erosion, decay of materials, "vandalism" by
         renovators, structural failure, change in materials or
         geometry, and moisture and temperature changes.

         1.   In foundation piers and piles, general cracking is
              often due to settlement or rotation of the pier
              footing.

         2.   Vertical cracking or bulging of a masonry
              foundation wall is often due to physical
              deterioration of the pier from exposure, poor
              construction or overstressing.

         3.   Horizontal cracking or bowing of a masonry
              foundation wall may be caused by improper
              backfilling, or by swelling or freezing and heaving
              of water saturated soils adjacent to the wall.

         4.   Differential settlement of a masonry foundation
              wall may be caused by many different things
              including soil consolidation, soil shrinkage, soil
              swelling, soil heaving, soil erosion or soil
              compaction.

         5.   Differential settlement of a chimney is often
              caused by inadequate foundations which may cause
              the chimney to lean and crack.

    D.   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).


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.   Crack Monitor:  Avongard, 2836 Osage, Waukegan, Il.,
         60087

2.02 EQUIPMENT

    A.   A pencil, tape, ruler

    B.   Small piece of window glass (single thickness) or glass
         slide

    C.   Epoxy adhesive

    D.   Crack monitor


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Examine the nature and severity of the crack:  

         1.   What direction are the cracks going and where are
              they the widest?
             
         2.   Note sloped floors, bulging walls and doors that do
              not fit.

    B.   Determine the probable cause:

         1.   Foundation erosion.

         2.   Decay and/or improper use of materials.

         3.   Structural failure.

         4.   Change in materials or geometry.

         5.   Changes in moisture content.

         6.   Thermal changes:

              a.   Horizontal or diagonal cracks near the ground
                   at piers in long walls:  due to horizontal
                   shearing stresses between the upper wall and
                   the wall where it enters the ground,

              b.   Vertical cracks near the ends of walls,

              c.   Vertical cracks near the top and ends of the
                   facade,

              d.   Cracks around stone sills or lintels:  due to
                   expansion of the masonry against both ends of
                   the tight fitting stone piece that cannot be
                   compressed.

3.02 ERECTION, INSTALLATION, APPLICATION

    A.   Monitoring Cracks Using Tape and Pencil:

         1.   Place a piece of tape on each side of the crack.

         2.   Draw one short line on each piece of tape at a
              convenient distance apart (2 inches) and parallel
              to the crack.

         3.   If there is movement in the crack, the distance
              between the line on the tape will vary; If the
              crack is long, several monitors will be needed.

         4.   Make a record chart of the distance between the
              marks of the tape at weekly intervals.

         5.   Keep accurate records of these measurements and
              place them along with photographs in file.

         6.   If significant widening occurs, report this with
              back-up data and copies of photographs to the RHPO
              for consideration.

    B.   Monitoring Cracks Using Glass and Epoxy:

         1.   Take a small piece of single strength window glass
              (a microscope slide is good) to bridge over the
              crack.  Tiny glass rods are also made for this
              purpose.

         2.   Epoxy the ends of the glass to the masonry on
              either side of the crack; locate it in an
              inconspicuous place.

         3.   If the glass breaks, it is an indication that the
              walls are still moving and that the crack is
              widening.

    C.   Monitoring Cracks Using the Avongard Crack Monitor:

         1.   Position the monitor over the crack with the
              vertical "0" line on scale parallel with the crack
              to be measured.

         2.   Fix the monitor with screws or adhesive.

         3.   Cut the transparent tape holding the two plates of
              the scale on the monitor in a fixed position with a
              sharp knife; over time, the degree of movement on
              either side of the crack will be measured as the
              two plates slide independently of one another.

                         END OF SECTION