Iano's backfill

Mortar for Masonry part 1: Cement

Cement-lime mortar used for brick and stone masonry is made up of cement, aggregate, lime and water. Aggregate forms the structural body of the mortar, cement is the glue that binds the aggregate, lime improves the workability of the uncured mortar, and water chemically combines with the cement to cause the mortar to harden as well as contributes to the plastic consistency of the wet mortar.

PORTLAND CEMENT
Portland cement, the most common cementitious material in mortar consists primarily of calcium silicates. Common sources of calcium include limestone, marble, and other minerals, and of silica, clay, sand, shale, and marl. Lesser quantities of compounds of iron, aluminum, magnesium, and sulfur are added to improve the hydration properties of the finished cement (hydration is explained below).

The raw materials are ground to a small particle size, blended, chemically combined in a large, high-temperature kiln, finely ground again, and mixed with a controlled amount of gypsum to produce portland cement. In its finished state, the cement particles are very small, ranging in size from 0.0004 to 0.0006 inches in diameter.

Portland cement is an hydraulic cement: It hardens by chemically combining with water, a process called hydration. The small size of the cement particles is important to ensuring the fullest possible hydration of the cement during the curing process.

OTHER CEMENT TYPES
Blended hydraulic cement is a mixture of portland cement with other cementitious materials such as blast furnace slag (itself an hydraulic cement), or any of a number of pozzolans (materials that react with the calcium hydroxide in wet portland cement to form other cementing compounds) such as fly ash, silica fume, or natural pozzolans derived from shales or clays.

The use of blended hydraulic cements reduces the quantity of portland cement in the mortar, reducing the energy required to manufacture the cement. Depending on their formulation, blended hydraulic cements can produce mortars with increased strength, lesser permeability, and other favorable properties in comparison to portland cement-lime mortars.

Masonry cements and mortar cements are proprietary blends of various cementitious materials, lime, and other ingredients. Unlike mortar made from portland cement or blended hydraulic cement (cement-lime mortars), mortar made from either masonry cement or mortar cement does not need the addition of lime as a separate ingredient. Both of these cement types offer the convenience and consistent quality of factory mixing, and can produce mortars with improved workability when wet, less shrinkage while drying, and other benefits. Masonry cements, which rely, in part, on air entraining (the generation of very small air bubbles within the mortar mix) for their workability, have a lower bond strength than cement-lime mortars, and may not be suitable for some applications, such as masonry subject to high wind or seismic forces. Mortar cements are produced to a different standard that produces mortars with bond strength comparable to cement-lime mortars.

Lime mortar is made with just lime, aggregate, and water--in this type of mortar, lime itself acts as the aggregate binder. In the absence of any hydraulic ingredients, lime mortar cures as atmospheric carbon dioxide gradually combines chemically with the mortar, a process called carbonation. Lime mortars are primarily used in the restoration of historic masonry where it is important that new materials used to repair such structures have physical properties close to those of the the original materials. Lime mortars with varying degrees of hydraulic properties and greater compressive strength can also be produced by adding varying amounts of other cementitious materials to the mortar mix.

	Hydraulic cement	Lime and other workability agents	Aggregate
Portland cement-lime mortar	portland cement	lime	sand
Mortar with blended hydraulic cement	blended hydraulic cement	lime	sand
Mortar with masonry cement	masonry cement		sand
Mortar with mortar cement	mortar cement		sand
Lime mortar	(none)	lime	sand

Keeping Masonry Walls Warm and Dry

Recently, this author was reviewing proposed details for the renovation of an historic brick masonry building located in downtown Seattle. During this review, questions were raised regarding how much insulation to add to what has been until now, a massive but uninsulated solid masonry wall system, and whether the addition of a vapor retarder membrane to the wall system would be beneficial. After some research on the topic, our conclusions were not as obvious as one might expect.

Normally, building assemblies are designed with the understanding that insulating buildings is good practice, and that higher levels of insulation are implicitly better than lower levels. Building designers are taught this in architecture and engineering school; and this understanding is reinforced by energy codes that set minimum insulation values for buildings and green building guidelines that provide incentives for even greater reductions in the energy required to condition the spaces within our buildings.

However in the case of an existing historic structure, "more insulation is better" may be too simple an answer. While it is generally true for all buildings that higher insulation values will reduce building energy consumption, in the case of older buildings, this benefit must be traded off with the potentially harmful effects that adding insulation to an existing wall system may produce.

In the case of our building under consideration, the proposal was to add rigid foam insulation on the interior side of the 12-inch thick masonry walls. (Adding insulation to the interior would preserve the historic appearance of the building exterior.) However the concern became that the more insulation added to the interior side of the wall, the lower would be the temperature of the brick during the colder months of the year. And as discussed in a previous article on this site, Air, Moisture, and the Building Envelope, keeping a wall warm can be an effective strategy for keeping a wall dry as well.

So our concern was that, up to this point in the life of the building, the exterior walls had been kept relatively warm--and dry--by the building heating system. By changing this, would we risk creating a colder wall system that could become more vulnerable to the effects of greater temperature extremes and added moisture? After some research on the subject, our decision was to apply just one inch of rigid foam insulation (with an insulation value of R-5) to the interior side of the walls. Essentially this strategy aims to balance the greatest possible reduction in energy use with the least reduction in exterior wall temperatures.

Additionally, we decided to add a polyethylene sheet membrane vapor retarder to the interior side of the wall assembly. The intent of adding this component was to reduce moisture movement into the wall system from the building interior, both by water vapor difusion and by the direct passage of moisture-laden air. While the benefits of vapor retarder membranes in a relatively mild climate such as Seattle can be debated (such membranes can reduce wall drying in the warmer months of year), our judgement was that the overall effect would be a net positive.

More Info
Loadbearing masonry construction is discussed in Chapter 10 Masonry Loadbearing Wall Construction of the textbook.
In researching this topic, this author found Rehabilitation of Solid Masonry Walls (National Research Council of Canada) particularly informative. This Council's Rehabilitation of Masonry Assemblies page has a variety of additional articles and references.

Self-Consolidating Masonry Grout

PCA Masonry Today (Winter 2004/2005) devotes its two main articles to a relatively new material self-consolidating grout (SCG). Like its close relative self-consolidating concrete, SCG is a cementitious grout material formulated with special additives to achieve very high-flow, low-slump properties without the addition of excess water to the mix. The most significant benefit of using SCG in grouted masonry construction is its ability to flow easily through complex, extended void spaces and around densely spaced reinforcing within the masonry assembly. This simplifies the masonry construction, speeds the grouting process, and greatly reduces the chance of voids or partially filled cores in the completely construction.

According to Characteristics of Self-Consolidating Grouts, an SCG mix has a lower water-cement ratio and includes superplasticizing admixtures (polycarboxylates) to increase its workability or flow in comparison to conventionally formulated grout. When the standard test for slump is applied to SCG, the test is more accurately termed a “flow test”. When the test cone is lifted, the SCG typically spreads out in a shallow pool or “pat” roughly 22 to 30 inches in diameter. In addition to verifying adequate flow, the pat is also checked for absence of segregation, bleed water, or aggregate clumps--all signs of deficiencies in the mix that could lead to poor quality in the finished construction.

Other common tests that are used to ensure the quality of both conventional grout and SCG construction include compressive strength testing of cured samples, and the post-placement inspection of strategically located openings in the masonry assembly to confirm the full flow of grout throughout the intended spaces within the masonry.

Grout Goes Straight on Prison Project discusses additional benefits of SCG in grouted masonry construction. While SCG itself may cost 8% to 11% more than conventional grout, it can be placed in roughly half the time. This reduces off-loading time for the ready mix trucks that deliver the grout to the construction site and speeds construction. Additional labor savings accrue from the absence of any need to vibrate SCG to ensure proper consolidation, and potential savings in the simplification of the masonry construction due to the high-flow capabilities of SCG.

More information
Grouting of reinforced brick masonry construction is discussed on page 294, and grouting of concrete masonry unit construction on pages 324 - 328 of the textbook. Additional information is also contained in Chapter 10, Masonry Loadbearing Construction.
The concrete slump test, the compressive strength testing of test cylinders, and the use of self-consolidating concrete are discussed on pages 477 - 478.

Metal Stud Masonry Connector

FERO Corporation is advertising its Side Mounting Rap-Tie masonry anchor for steel stud backup systems. According to the manufacturer, advantages of this anchor design for improved performance with steel stud backup systems include:

• The anchor fastens into the side of the steel stud, rather than through the stud's face, putting the fasteners in shear rather than withdrawal.
• Voids in the anchor strap are reduce thermal bridging.
• Optional integrated insulation supports provide for a continuous insulation layer exterior to the steel stud framing.

Ties are available manufacturerd from hot-dip galvanized steel or stainless steel.

Masonry Performance and the WTC

Building Safety Journal (September 2003), Masonry Performance and the World Trade Center Disaster, discusses a new report by The Masonry Society, Masonry Aspects of the World Trade Center Disaster. The report evaluates the performance of buildings with masonry construction surrounding the twin towers during the WTC event. Conclusions appear to reflect favorably on masonry's greater mass and durability in comparison to other materials. For example:

• Framed buildings with exterior masonry walls generally performed better than newer buildings with lighter, curtainwall construction
• Masonry infill for walls and beams provided significant fire protection
• Masonry infill provided alternate load paths where columns were damaged, in some cases preventing partial structural collapse
• Interior masonry partitions provided redundant lateral force resistance
• Masonry flat arch floors built to early 20th standards provided better fire protection than newer steel framed construction.

08 - Brick Masonry Links

This article contains external links to resources on the Web relevant to Chapter 8 Brick Masonry.

BIA Technical Notes on Brick Construction

Brick Industry Association's extensive technical reference information related to brick masonry construction.

FERO Corporation Masonry Connectors and Accessories

Product information, specifications, details, installation animations, design software...

Glen-Gery Brick

Manufacturer's web site with a good range of product and technical information

MasonrySystems.org

The industry association-sponsered web site provides a useful variety of information regarding wall system selection guidelines, wall system examples, and more.

The Masonry Society Workshops

Provides annual workshops for university instructors on the teaching of masonry design and construction. This site also includes course materials and othe technical resources.