IDENTIFYlNG PROBLEMS WlTH WALLS USING HEADER BLOCK

David T. Biggs, P.E. 1

1. ABSTRACT

This paper will illustrate several projects which the author has investigated . Each was built in the last 30 years and has walls which use concrete masonry header block and brick veneer. The cracking and movement pattems for these walls are different in comparison to other masonry wall systems. The problems became progressively worse until restoration was recommended . Restoration techniques which have been used to mitigate further damage or to stabilize cracked walls will be discussed also.

2.INTRODUCTION

Header block were developed over 30 years ago in the United States to provide a less costly alternative to solid brick walls. The header blocks are shaped to allow a veneer to be bonded compositely to the backup (Figure 1) . Generally, the veneer is constructed with c1ay bricks. The course of header brick is placed continuously to tie the two wythes together.

Keywords: Masonry; Header block; Restoration

1 Vice President, Ryan-Biggs Associates, P.C., 291 River St. , Troy, New York, 12180, USA

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Concrete masonry header block .every other course

Continuous header brick

FIG.l - HEADER BLOCK W ALL

3. ADVANTAGES

The header block wall system represents a very efficient use of masonry since no metal ties are required to bond the two wythes together. The concept was that the two wythes could be constructed simultaneously and be bonded just like a solid brick wall .

In comparison to a solid brick wall , this wall system is less costly since the cost to make and install each concrete masonry block is less than the bricks it replaces. The bonding of the wythes avoids the use of metal ties and provides the strength of a composite wall section.

The shape of the header block doesn't require any cutting; the block was created to match the standard system of three 3 bricks per block course. This is consistent with standard, concrete masonry construction.

4. DISADVANTAGES

A basic problem with this system is one that is common to any solid wall - the water tightness of the wall is based upon the principie that the wall acts as a barrier. However, the header bricks trom the brick veneer to the backup provides a path for water transmission. Thus, leaks are difficult to control once they begin.

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The wall generally lacks insulation because the cores of the backup are the only voids available for insulation. The header bricks between the two wythes short circuit the insulation and generally counteract any beneficia.l effects of the core insulation.

As with solid brick walls, the header bricks are rigidly bonded to the backup. Since there is little flexibility between the wythes, ali the stresses due to wall movement are built up in the header bricks. In older brick buildings, it is common to find broken header bricks trom this stress.

Another disadvantage is that two different materiais are tied together. Though they have nearly similar thermal coefficients, they have different shrinkage and expansion properties 1 •

Thermal Coefficient

Concrete masonry: 8.1 x1 0-6 cm/cm/oC (4.5x10-6 in/in/cF)

Clay brick: 7.2x1 0-6 cm/cm/oC (4.0x1 0-6 in/in/oF)

Shrinkage Coefficient

6.5x10-4 em/em (6.5x10-4 in/in)

Expansion Coefficient

3.0x10-4 em/em (3.0x10-4 in/in)

The most significant factors are drying shrinkage for the concrete masonry and expansion due to moisture growth for the clay products. These two factors act opposite to each other.

5. CASE STUDIES

5.1 Corner Cracks

Moisture growth of brick veneer is a common phenomenon causing the corners of the walls to expand outward. Walls with header block exhibit the same movement. Figure 2 is taken from a school building which had cracks at both an inside and an outside corner.

In a cavity wall , the veneer tends to move independently of the backup; and the movement is taken by the flexibility of the ties as well as by expansion joints in the veneer. For the wall with header block, the wythes are bonded rig idly. In this case, the backup at an outside corner often cracks at the corner either stepwise or horizontally. At an inside corner, the brick veneer expansion causes the abutting wall to crack vertically frem the stress of the movement. In each case investigated, the backup was the weak link; the header bricks did not break, but they did translate slightly relative to the backup which weakened the bond.

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);

.. Wall continues 65m(214ft.)

~ U

Masonry cracking ~

I

• ~

z Brick Wall continue s

~ f--O expansion 35m (115ft.) ~r-- at inside - r-- corner - '--- -""

Landing separating

J~~ trom walls

LAJ l · Brick expansion at outside corner

Location of brick cracking

PLAN - ST AIRWELL

FIG. 2 - CORNER CRACKS This problem is partially a result of designing the wall as though it were a solid brick wall. The cracking does not usually exist with brick walls because both the interior and exterior wythes undergo the sarne moisture growth. Using header block, there is a build up of stresses due to shrinkage of the concrete masonry and expansion of the brick veneer.

The correct design procedure would have placed control joints in the wall; a practice not common with solid brick walls. This would have allowed the wall to move in smaller increments without causing a large build up of stresses at any one point and cracking.

The restoration details used for repairs to this cracking include:

a. Introducing a vertical relief joint in just the brick veneer along the wall. This is to prevent a further buildup of stresses trom movement due to thermal effects (Figure 3) because by the time the cracking has appeared, the bricks

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have already undergone their moisture growth and the concrete masonry has already experienced its drying shrinkage. Thus, the only likely further movement will be due thermal effects.

In addition to the relief joints placed as shown on Figure 3, additional joints were placed at approximately 9.1 m (30 ft) along both walls and at the coping .

b. Supplemental anchorage of the brick veneer to the backup. Brick movement has been measured as much as 10 mm (3/8 inch) at comers. Mechanical or adhesive masonry anchors are used to reattach the bricks2

.

This is to compensate for any loss of bond with the backup. The anchors must overcome any shearing stresses caused by subsequent therrnal and moisture movements. See Figure 4.

c. Repointing the cracks and sometimes adding horizontal joint reinforcement to strengthen the brick veneer (Figure 5).

New brick rellef joint ---------, , -

r o FIG.

o 4

o

o

~ o o . . o

o

J:)'

New masonry / anchors (RE>point after installation)

Outside corner of

slt ...... ~~.w b,;ck

t reliaf joint

o · · · o

· o

o

· · ·

~.

~

o

o

o

New masonry anchor aach slde of stair corner at 102m (4ft.)0.c.

PARTIAL ELEVATION

FIG.3 - RESTORATION DETAILS

563

T

Concrete masonry backup ----, r----0.3m(1 ft.)

INTERIOR

~T---;,L-f--DrjIJ & insta" 1"--4....-1.'-"- --"-~.L-'-1t:---'---'--"- masonry anchor.

l Repoint over anchor.

Brick veneer

Sawcut 10mm(3/8inJ wide joint full depth of bricks. Install backer rod & sealant

FIG.4 - PLAN DF RELIEF JDINT

Remove mortar & repoint. Insta" 6mm Helibar crack reinforcement -~~

r-7''---7'\---- - 38 mm (1 1/2 in)

,LJ---- Existing crack in brick (concrete masonry not shown)

FIG.5 - CRACK REPAIR DF BRICK VENEER

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5.2 Vertical Cracks

A second problem which is commonly found in these types of walls is vertical cracking. Since the walls were built like solid brick walls, they generally have no expansion or control joints. As previously stated, the backup wants to shrink while the brick wants to swell. Cracking often results somewhere along the length of the wall. Two results are possible:

a. The backup cracks in one spot (usually at mid-Iength of a wall) and also cracks the brick veneer (Figure 6) or;

b. The backup undergoes a series of small cracks at intervals along the entire wall, and the brick growth accommodates the shrinkage with no resulting cracks in the brick veneer.

The pro per design would have utilized control joints for the backup and expansion joints for the brick veneer. The joints would have been aligned inside and out.

The restoration details vary for each case. Figure 6 also shows the repairs for a. above. The cracked bricks at the one distinct crack were sawcut or removed to create a relief joint; the adjacent bricks were reanchored ; the backup was repointed ; and sealant was installed in the relief joint.

Stone coping

Crack

- - -r-r----'----j

BEFORE

Replacement veneer adjacent to new relief joint -----,

-",' --..,...--,-.,...

Placement of new tles in .~ joint (staggered) ---é--'-'----r,.*:i &-,""""-:r:-~L-..(

Brick veneer

AFTER

FIG.6 - ELEVA TION

The repair for b. is much simpler. Usually there is no exterior cracking in the brick veneer and the wall remains watertight. The interior backup cracks are usually hairline. If they beco me noticeable, they can be repointed; otllerwise, a fresh coat of paint hides the cracks.

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5.3 Interior Cracks

A third condition occurs where an interior wall becomes an exterior wall above a low roof area. In this case, a single wythe of concrete masonry is used inside the building below the low roof. Above the low roof, the wall is changed to the headered wall system to maintain the interior, concrete masonry appearance but provides a brick exterior (Figure 7). In some buildings, a control joint was used in the single wythe masonry; in others, there is no control joint. In ali buildings, however, the single wythe of interior masonry experiences cracking.

HIGH ROOF

Single wythe concrete masonry -----<~

Concrete J­masonry backup Header block wall

Brick veneer LOW ROOF

FIG.? - WALL SECTION

The lack of expansion joints in the header block wall above allowed ali the moisture growth in the brick veneer to accumulate. This growth created a tension in the backup of the upper wall which transferred to the lower concrete masonry (Figure 8) .

The correct design would have utilized control joints and expansion joints for the wall's full height.

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The restoration details are similar to ,those used for the vertical cracks (Figure 9). New relief joints are installed in the brick veneer (aligned with the control joints in the lower concrete masonry wall, if they already exist) . The brick veneer should be reanchored and the cracks repointed.

H Brick veneer growth

LOW ROOF

--J; ..... - Interior step crack in concrete masonry of Iower wall

FIG.8 - ELEVATION OF WALL

I HIGH ROOF

! o o o

o Masonry

I anchors

o ' o I--- ReHef joints

o (see FIG. 3&-0 0 o (see FIGA)

LOW ROOF....,.. o O · o

-'-r- T

I ~ Crack reinforcing 2

::::J & repoint (see FIG.S)

I

FIG.9 - RESTORA TION DET AILS

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6. CONCLUSIONS

Composite walls built of concrete masonry header block .and clay brick veneer function differently than either solid, brick walls or concrete block and brick veneer cavity walls. The effects of moisture growth of the clay brick and shrinkage of the concrete masonry backup have to be controlled through a pattern of expansion and control joints.

For walls which were constructed using this header block wall system, the cracks which appear need to be repaired and relief joints created in the brick veneer. Reanchoring the brick veneer is required once movement of the brick veneer away from the backup is observed.

7. REFERENCES

1. American Concrete Institute, American Society Df Civil Engineers, "Building Code Requirements for Masonry Structures", ACI 530-92/ASCE 5-92, New York, NY

2. Biggs, David T., "Reanchoring Techniques for Masonry Veneers," Proceedings of the Sixth North American Masonry Conference, Philadelphia, PA, 1993, pp. 517-528.

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