masonry horizontal joint reinforcement 5

[boffintech]

And now for some specification legalistic hairsplitting...

ACI-530 requires:

6. Place joint reinforcement so that longitudinal
wires are embedded in mortar with a minimum
cover of 1/2 in. (12.7 mm) when not exposed to
weather or earth and 5/8 in. (15.9 mm) when
exposed to weather or earth.

However, when I watch masons at work I see them simply place the 10' lengths of HJR on top of the dry block, use the trowel to spread mortar on top of the face shell, and then place the next block. The result is that the HJR is at the bottom of the bed joint in contact with the block.

Question: does this method of placement fulfill the requirement "longitudinal wires are embedded in mortar" as shown above?

 

[JAE]

I would think that the cover requirement is intended for the distance from the wire out to the exposed face of the mortar...not the cover to the adjoining block. But I could be wrong.

 

[concretemasonry]

boffintech -

I think what your are concerned with is the bond between the mortar and the joint reinforcement wires.

The cover for protection from the weather/earth is similar in concept to that required for coverage for steel in poured concrete.

Joint reinforcement is not structural, except in cases where codes allow it to be used to make a stack bond wall act like a running bond wall.

Frankly, crack control recommendations and requirements are not really scientifically established, but are based on historic performance of masonry where joint reinforcement was used. In some areas/applications it is not needed or required. The placement and mortaring process you described is entirely accurate. It is obvious that there is not 100% uniformity of the mortar/steel bond when masonry units are initially laid. Due to the block laying process (unit movement, vibrations, and the vertical loads of other units above), the bond is improved to the level reflected in he historical performance, which has dictated the current requirements.

Crack control is the subject that can be easy to get lost in the variables, theories and possible moisture and temperature conditions. The bottom line is that since few engineers can afford to take the time to to analyze the specific combination of conditions, it is more practical to set a standard for the worst conditions since it is convenient and not cost prohibitive.

Originally, the crack control criterial was set to match the shrinkage results from the old ASTM 426. More recently, a more practical approach was taken to simply the design and requirements. This encompasses ALL the factors that can affect the shrinkage and expansion of a wall.

Crack control/shrinkage of a wall is a function of many factors:

1. The potential curing shrinkage of the individual masonry units.

2. The temperature differential of the wall between as-laid conditions and in-service conditions.

3. The moisture conditions of the wall between as-laid conditions and in-service conditions.

4. The curing shrinkage of the mortar.

5. The carbonation shrinkage of the concrete units and the mortar.

6. The long term expansion of clay units'

7. The differential temperature/moisture reaction between the wyths of a wall composed of different materials (brick & block).

8. If you really want to confuse matters, add in the effect of grouting, bond beam filling and wall exposure to moisture and temperature during construction.

As you see, the factors provide a wide range of results depending on the conditions that a portion/wythe of the wall is subjected too.

On one extreme, you have a wall built with very dry low shrinkage units at a temperature of 80 to 100 degrees and never exposed to a temperature below 70 degrees (interior cavity wall where joint reinforcement is not really needed). The other extreme is the high shrinkage units laid when at the upper permitted moisture limts at a temperature of 80 to 100 degrees and exposed when fully dried to temperatures of -20 degrees (single wythe wall in a cold climate). Because of the crack problems observed, the most critical conditions set the standard.

In the end, the masonry engineering, code and standards organizations decided to look at the performance of conventionally built walls when setting the requirements for joint reinforcement, since an excessive of reinforcement amount posed no downside performance and guanteed performance at least equal to well performing structures.

P.S. - it took many, many years to get a proper standard since it also required modifying material standards also.

Dick

 

[jike]

What you are describing, in the field, is the bond of the mortar over a portion of the perimeter of the wire (say 75% or so). The mortar then in turn is bonded to the lower and upper block.

If you take a 3/8" mortar joint and subtract the typical 9 ga. wire diameter, that cannot leave much mortar above and below the wire. The code would seem to imply "full" embedment into the mortar not "partial" embedment.

How much a difference that makes, I don't know but I would discuss this with the masons to see if their procedure could be modified by spreading the mortar first and then push the wire into it.

 

[golf39]

Concur with jike on previous post. Featherlite, NCMA TEK 12-4D Steel Reinforcement for Concrete Masonry under Joint Reinforcement states, the same requirement as you originally stated, "In addition, joint reinforcement must be placed SO THAT THE LONGITUDINAL WIRES ARE EMBEDDED IN MORTAR with a minimum cover of 1/2" (13mm) when not exposed to weather or earth, and 5/8" (16mm) when exposed to weather or earth." Therefore, laying the horizontal joint reinforcement on a unbedded joint does not comply and should be questioned. I don't see how the Mason could successfully argue with the National Cocrete Masonry Association.

 

[civilperson]

Joints are at most 3/8" thick, therefore the location of the wire vertically can never satisfy the criteria of 1/2" embedment if interpreted as measured from the masonry unit. The common method of wire, mortar and then next masonry course is totally correct. Embedment distance is to exterior face.

 

[henri2]

Boffintech, refer to MIA inspector's handbook Section 5.8.2 and Figure 5.12 4th edition or Figure 5.11 5th edition. The diagram should settle the issue.

 

[UcfSE]

Joint are at most 5/8-inch thick, with tolerance. Placing the wire after the mortar is spread results in loss of production with no real benefit to anyone. You're lucky if you get it in at 16 inches o.c. If you hassle them about it, especially directly, you'll be lucky to get it at all.

I interpret the cover requirements as perpindicular to the block face, as JAE posted.

 

[henri2]

Technically speaking, if effective embedment in mortar joint is desired, then the mortar would have to be spread first,joint reinforcement laid and so on. Who knows, perhaps that's the way it was done by folks in academia, whose work was used as basis for the code provision.

The reality in the field is different, as what Boffintech described, is what happens. For situations where joint reinforcement is meant to control shrinkage cracking, it's not a big deal in my opinion. However, when joint reinforcement is to be used as part or all of shear reinforcement, will the method of placement of joint reinforcement matter?

Check the opinion expressed under sub-heading "Joint Steel versus Bond Beams"

http://www.masonryinstitute.com/hollow/design28-33.html

Have there been any studies done on the influence of method of placement of joint reinforcement in bed joints, on crack control, shear strength resistance...?

 

[boffintech]

http://www.dur-o-wal.com/prod/pdf/prodinfo/Single_Wythe_System.pdf

Per Duro-wall:

Centering and Placement
Place joint reinforcement directly on masonry and place mortar over wire to form bed joint.

Fully embedded...?