Concrete Expert Witness: Weathering Assessment of Recently Constructed Slip Formed Silo

The excerpt was taken from a BFI concrete expert witness condition survey report assessing concrete defect damage to a recently constructed slip formed silo. The concrete expert witness performed a site investigation and provided the following summary for the client.

"In general, the technical issues attendant to the future performance of the exposed concrete

surfaces of the silo include the quality of the concrete materials delivered, the standard of

workmanship employed in installing them, the quality and nature of the repairs made, where made, the quality, nature and thickness of the cementitious coatings applied immediately following the slip forming operation and the exposure and weather conditions likely to impact the work. Based upon a verbal report, the quality of the underlying concrete satisfied the project requirements for air entrainment and strength, and subsequent to repair activities, for consolidation: therefore, these are outside the scope of this investigation and report. Also reported that the face material of the slip form (that is the form surface in contact with fresh concrete) was plywood and that its grain was oriented horizontally. Items subject to examination in this red material(s); and/or, (4) no additional work or materials added

subsequent to slip-forming activities. [Note that the term “parge” is employed herein to describe the thin cementitious coating used to conceal various surface blemishes. Some in the industry use the label “sacking” or “sack rub” to describe this practice, and are terms generally used and accepted in the concrete construction community.

This report will describe the practice in this case as a “parge” to connote the unusually high extent of its presence and to make the distinction that this coating is both more abundant and frequently much thicker in application than typical sacking.] The specific mechanism

of distress to concrete by freeze-thaw cycles involves the presence of moisture in the concrete matrix, often due to penetration of wind driven rain or sleet, the ease of

moisture penetration into the concrete by means of micro- and macro-scopic

pathways in the concrete, like cracks or crack systems, and the intrinsic

permeability of the materials, and the degree of capillarity from the surface towards the interior of the concrete section.

Effective curing is important to this mechanism to the extent that proper curing tends to decrease cracking, reduce permeability and, to some

extent, closes or makes smaller the pore system, which in turn reduces the rate at which moisture can penetrate and limits the total amount of moisture which can accumulate in the concrete. Freeze-thaw damage occurs as the formation of ice crystals, usually at high pressures, lead to a redistribution of moisture and dissolved salts within the pores of the concrete sufficient to induce progressive distress at each freeze-thaw cycle. Cycles of such redistribution of water under pressure lead to local, and in some cases, larger patches of, surficial distress, thus exposing the large aggregate concealed beneath the exposed surface.

Application of effective water resistant membranes and sealants can reduce freeze-thaw

damage by coating the surfaces and/or impregnating the concrete to some depth, thus limiting the degree of penetration and the amount of moisture, and filling cracks which may be present."


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