Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures. 

Part 10: Pack Rust

Pack rust refers to the expansive corrosion of ferrous metals. This powerful mechanical force can break apart adjacent concrete and masonry. Pack-rusted fasteners and structural elements can cause the deterioration of adjacent brick, stone, terra cotta, and concrete. In structural iron and steel, pack rust leads to overall structural weakening.

Pack rust in a reliving angle causing adjacent brick to spall

Iron oxidizes in the presence of oxygen and water, often accelerating with exposure to salt, and expands rapidly as it corrodes. When exposed surfaces rust, the brittle corrosion product weathers away, but when rust occurs within a confined space – for example, within reinforced concrete or between two plates of a bridge truss – the rapidly expanding corrosion product forces apart surrounding materials. Where iron and steel fasteners were designed to be protected from the elements, this action causes more water to enter, creating a positive-feedback cycle of deterioration.

pack rust between steel plates of a bridge truss

Next in this series: Displacement

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Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures. 

Part 9: Guano

Guano refers to the excrement of birds and bats. Most of the guano we encounter during building investigations is produced by birds, but both bird and bat guano can be present in interior building spaces. Guano can accumulate on any building material that presents a horizontal or near-horizontal surface.

guano accumulation on marble

The occasional bird dropping is not of concern, but large deposits of guano can produce chemical reactions that break down building materials. The presence of bat and bird droppings in significant quantities can be a health hazard for people accessing the affected areas of a building or structure. Hazardous buildups of guano must be removed following appropriate precautions. Anti-roosting installations are often used to prevent birds from landing and perching on skyward-facing surfaces of buildings and structures.

guano accumulation on granite

Next in this series: Pack Rust

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Click here for an index of all posts in this series, or download a pdf of the complete series.

Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures. 

Part 8: Biological Growth

Biological growth refers to the accumulation of living organisms on the surface of building materials. This can include fungi, algae, lichens, mosses, and vascular plants. The term biological colonization may be used to refer to microorganism and plant growth, as well as colonization by animals. The term biofilm is sometimes used to describe areas of microbial growth.

microbiological plant growth on limestone

Biological growth can occur on all building materials where moisture can accumulate and linger. When investigating wood building components, we distinguish between fungi (including molds and mildews), lower plants (non-vascular), and higher plants (vascular).

vascular plant growth in terra cotta

Plants and fungi require water in order to survive. Thus, unwanted biological growth often occurs in sheltered areas that do not dry out quickly. Higher plant forms appear where poor drainage or surface features allow dirt and debris to build up. Some biological growth may be tolerated as simply a cosmetic nuisance, but other forms may cause or contribute to deterioration. For example, lichens secrete oxalic acid, which dissolves some types of stone, and the roots and tendrils of vascular plants have a mechanically-destructive effect on building materials.

Next in this series: Guano

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Click here for an index of all posts in this series, or download a pdf of the complete series.

Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures. 

Part 7: Atmospheric Soiling and Black Crusts

Atmospheric soiling is a discolored build-up of airborne pollutants. It can build up on all materials, but this discussion focuses on its effects on masonry and concrete. Black crusts are accumulations of gypsum deposited at the surface of masonry materials and discolored by atmospheric soiling.

atmospheric soiling on brick

Atmospheric soiling results from airborne pollutants – usually soot, soil particles, and fly ash – accumulating on the surface of a building material. Porous and rough-textured materials, such as brick and stone, are more susceptible to atmospheric soiling than metals and painted surfaces. Some types of atmospheric pollution contribute to the deterioration of masonry materials via acid rain. Black crusts are accumulations of insoluble gypsum formed by the reaction of particulate pollutants within the masonry material, and darkened by atmospheric soiling. Black crusts typically form in sheltered areas, where moist air delivers a high concentration of dissolved pollutants.

black crusts on granite

Atmospheric soiling may be cleaned in order to prevent further deterioration, or for cosmetic purposes.

Next in this series: Biological Growth

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Click here for an index of all posts in this series, or download a pdf of the complete series.

Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures. 

Part 6: Failed Joints in Masonry

Failed masonry joints occur when the mortar or sealant material between masonry units or between a masonry unit and an adjacent material has broken down and is no longer providing a weathertight seal. Failed masonry joints occur in brick, stone, terra cotta, cast stone, and precast concrete. Failed sealant joints can also occur adjacent to areas of cast-in-place concrete.

missing mortar in brick masonry

Failed mortar occurs where mortar has cracked, eroded, or separated from the adjacent masonry units. When this deterioration process has continued for a long enough period of time, the result is missing mortar. Failed sealant occurs at joints between masonry and another material, such as wood or metal window assemblies, or at expansion joints. It is distinguished from caulked mortar joints and failed caulked mortar joints, where caulk has been applied as a repair material to an already-failing masonry joint.

failed mortar in cast stone masonry

Because mortar is sacrificial by design, all masonry joints are expected to fail eventually. A well-made masonry wall that is adequately protected from water infiltration should be expected to last 50 to 100 years before needing repointing, but some factors contribute to mortar joints deteriorating before their time.

caulked mortar joints in brick masonry

Poor design can be a cause, as when a too-narrow joint does not contain enough mortar to resist thermal expansion and freeze stresses. Lack of weatherproofing or failure to maintain copings and roofing elements leads to water infiltration, and joint failure along with other types of deterioration. Repointing with inappropriate mortar can cause failure of mortar joints and can severely damage the surrounding masonry units.

failed sealant at an expansion joint in precast concrete panels

The service life of sealants and caulk is much shorter than that of mortar; these materials deteriorate relatively quickly and must be replaced as part of regular building maintenance.

Next in this series: Atmospheric soiling and black crusts

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Click here for an index of all posts in this series, or download a pdf of the complete series.

Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures. 

Part 5: Surface Loss

Surface loss describes the disappearance of material at the outer faces of masonry units or concrete, and includes erosion, delamination, exfoliation, chipping, glaze loss and face spalling. Surface loss can lead to the material becoming friable. Exfoliation may also be referred to as blistering.

Eroded limestone

Erosion of concrete and masonry materials can occur due to scouring by wind and water, or abrasion by other materials. Delamination applies only to natural stone, and results from the separation of layers along bedding planes.

Delaminated sandstone

Exfoliation is the loss of the outer surface of a material in thin layers. Water infiltration is a major cause of delamination and exfoliation.

Exfoliated granite

Chipping refers to shallow surface loss, typically at the edge of a masonry unit. Glaze loss applies only to terra cotta, resulting from water infiltration or poor glaze-body “fit”. Face spalling generally applies only to brick, and can result from thermal expansion forces resisted by inappropriately-hard mortars.

Face-spalled brick

A material is described as friable when a loss of cohesion within the unit or concrete, caused by water infiltration, has rendered it powdery or crumbly. Inappropriate surface treatments or cleaning techniques can contribute to several types of surface loss.

Next in this series: Failed Joints in Masonry

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