By Peter R. Kolf and Terry J. Willems
Numerous material properties affect the short- and long-term performance of concrete repair products. The diversity of such properties as well as the diversity of product applications has led to a proliferation of specialized proprietary products.
Drying Shrinkage and Shrinkage Compensation
Drying shrinkage presents many problems for concrete structures in general and repair projects in particular. Accommodating a shrinking repair product that is adhered to a dimensionally stable substrate presents difficult issues in the restoration market and leads to an obvious desire for a repair product with reduced drying shrinkage.
One repair material chemistry that might appear to offer the potential for shrinkage compensation as well as accelerated set is expansive cement or calcium sulfate based chemistries. While "shrinkage compensation" is a term that in the repair product industry is loosely defined, per ACI 223 the basis for shrinkage compensation lies in the expansion of cement paste within curing concrete. Properly used, this expansion creates a pretension in embedded reinforcing steel and corresponding precompression in the concrete. Later-age drying shrinkage strains relieve the precompression without inducing tensile stresses in the concrete.
Repair Failure Exhibiting Crack Pattern Indicative of Volumetric Expansion at a Building Slab Edge.
Expansive cements utilize sulfates and aluminates to initiate an expansive reaction resulting in ettringite formation. This expansion in curing concrete can be used to provide shrinkage compensation. This same reaction in hardened, cured concrete is called internal sulfate attack and can be destructive. Therefore, an essential factor in use of shrinkage compensation is controlling this reaction. ACI and ASTM present three types of expansive cements: Types K, M,. and S.(1)(2) Of these, only Type K is currently commercially available in the United States. Nevertheless, product manufacturers use chemistries similar to those of expansive cements to achieve desired results.
In expansive cements, the reacting sulfate is calcium sulfate that may be present in the form of anhydrite (CaSO4), plaster (CaSO4 ½H2O), or gypsum (CaSO4 2H2O). Most failures observed by the authors have been related to chemistries similar to those used in Type S cements. In such chemistries, calcium sulfate reacts with tricalcium aluminate (C3A) present in portland cement clinker. Typically, the product formulations involved in observed failures have incorporated relatively small quantities of gypsum added to an otherwise portland cement based product.
As with any of the shrinkage compensation chemistries, the expansive reaction (formation of ettringite, or calcium sulfoaluminate hydrate) requires significant water. Therefore, proper curing is essential and failure to provide sufficient water during cure has the potential to stop the reaction until sufficient water may become available at a later age.
Repair Failure Exhibiting Spalling and Paste Degradation at a Balcony Corner.
Further, the proportions between reacting sulfates and aluminates are important in controlling the timing and extent of the reaction and ensuring that residual unreacted elements do not remain to react detrimentally at a later age.
Detrimental reactions that may be associated with elevated levels of calcium sulfate include internal sulfate attack and dissolution of residual gypsum from cured repair product. As previously indicated, internal sulfate attack may occur due to lack of sufficient water present during early ages and later exposure to external sources of water. It may also be caused by residual unreacted sulfates or aluminates that become exposed to external sources of reacting agents. Formation of ettringite within the cement paste induces expansive pressures within a hardened cement matrix, causing cracking and associated paste degradation.
Since gypsum is soluble in water, dissolution of residual gypsum over time and consequent degradation of cement paste may occur if materials are exposed to external water sources in service. Subsequent to gypsum dissolution or internal sulfate attack, repair materials may become exposed to further degradation, such as from freeze-thaw cycles, as water penetrates cracked or degraded surface materials.
Products containing elevated levels of calcium sulfate generally have accelerated set times. Although rapid/accelerated set is not a requirement for many repairs, there are instances where it is desirable. One is short-downtime applications that typically involve repairs affecting use of a functioning facility. Another is trowel grade mortar used in vertical and overhead repairs for which accelerated set enhances non-sag characteristics and allows heavier application thickness. Hence, calcium sulfate additions are more likely to be used in such products.
Product formulations will be susceptible to differing modes of deterioration based on their chemistries. Some accelerated-set products have been formulated by blending portland cement with relatively large quantities of gypsum. Such a formulation will support ettringite formation only to the extent that aluminates in the cement are present. Consequently, significant quantities of unreacted gypsum will be present in the hydrated material. Due to the solubility of this residual gypsum, while such a material may have a use in the repair market as a fast-set material, it should only be considered a temporary repair material in exterior environments. Previous work by NCHRP supports this.(3)
There is an inherent flaw in the use of expansive cement chemistry for the vast majority of repair applications
As previously discussed, shrinkage compensation, as defined in ACI 223, involves an early age expansion during hardening of concrete that compensates for later age drying shrinkage. ACI 223 discusses at length the design and detailing needed to allow for the early expansion and subsequent shrinkage. However, typical repair applications do not allow for expansion since they are either bonded to or confined by existing concrete elements. ACI 223 indicates that for applications where high restraint is present, little shrinkage compensation will result.
In the authors' experience, signs of faulty repair material chemistry such as that discussed have been observed within approximately one to five years from time of repair installation, although other time periods are possible. Observed conditions have been consistent with the deterioration mechanisms discussed, i.e. volumetric expansion and dissolution of cementitious binder. Thus, potential indicators of elevated levels of calcium sulfate would include:
- cracking patterns consistent with volumetric expansion often exhibiting crack widths disproportional to the repair dimensions and
- degradation of the repair material in a manner somewhat consistent with freeze-thaw deterioration.
State of the Industry
Engineers or others who have spent significant time and effort studying product data sheets have had the experience that test data presented is often not directly comparable between various products, and may or may not have any applicability to real-world installations. Current tests for shrinkage compensating materials generally allow for expansion with only limited resilient restraint since that is the behavior intended for applications as indicated by ACI 223. Further, these tests generally specify a 7-day laboratory saturation cure that cannot even be simulated in many repair applications.(4)(5)
There have been recent advancements in developing criteria that can be used to provide more meaningful and consistent test data on repair product data sheets. The International Concrete Repair Institute (ICRI) has recently developed ICRI Guideline 03740, Guideline for Inorganic Repair Material Data Sheet Protocol, and a commentary to this protocol is currently in development.
It is understood that product manufacturers cannot have limitless R&D budgets for each of their products. Nevertheless, it is reasonable to expect that significant testing into product properties and possible product limitations would be performed by manufacturers aware of product chemistries and intended markets.
How to Specify?
How, then, is an individual to specify a product if data sheets provide only limited information? We offer the following minimum recommendations:
- Service History:
The product should be able to demonstrate good long-term performance in similar applications. Short-term performance provides only limited information.
- Product Data:
Read the product data sheet carefully, paying close attention to intended applications and noted product limitations. Keep in mind that test data presented may not reflect conditions that are applicable to your project. Be wary of concrete repair products with test data indicating product expansion.
- Manufacturer Involvement:
Obtain the involvement of the product manufacturer. Make sure they are aware of the intended application and service conditions. Reputable manufacturers should provide statements indicating that project conditions have been reviewed and that the specified product is appropriate for use as specified.
Develop a basic understanding of repair materials you may specify. Inquire regarding the source of characteristics such as accelerated set, shrinkage compensation, etc.
Simply put, avoid products possibly marketed as shrinkage compensated or rapid setting repair materials that utilize calcium sulfate additions to otherwise portland cement based products, since the potential exists for premature failure. Based on the authors' experience, such chemistries are at best unreliable with current state of knowledge.
The repair industry has made significant advancements in developing reliable, durable, repair products. Reputable manufacturers have been at the forefront of many of these advancements. However, more work is needed. It would greatly benefit the repair industry to encourage research, increase interaction among chemists, manufacturers, specifiers and users, and develop standards that aid in keeping inappropriate repair materials off the market.
- ACI 223, "Standard Practice for the Use of Shrinkage-Compensating Concrete"
- ASTM C 845, "Standard Specification for Expansive Hydraulic Cement"
- NCHRP Synthesis of Highway Practice Report 45, "Rapid-Setting Materials for Patching of Concrete", 1977
- ASTM C 806, "Standard Test Method for Restrained Expansion of Expansive Cement Mortar"
- ASTM C 878, "Standard Test Method for Restrained Expansion of Shrinkage-Compensating Concrete"
ABOUT THE AUTHORS:
Terry Willems, Principal Materials Scientist, CTLGroup
Mr. Willems' responsibilities include consulting and testing associated with construction materials evaluation, troubleshooting material performance problems, durability evaluations, forensic investigations and litigation support.
American Concrete Institute (ACI)
Committee 524 Plastering -Secretary
Committee 515 Protective Systems for Concrete - Secretary
Committee 303 Architectural Cast-in-Place Concrete
Committee 201 Durability of Concrete
The Society for Protective Coatings (SSPC)
Publications and Presentations
Mr. Willems has authored or coauthored 6 publications regarding petrographic examination of concrete, evaluation of buildings, and materials testing.
ACI Wason Medal for Materials Research, 2001
Peter Kolf, Principal Structural Engineer, CTLGroup
Mr. Kolf is active in a wide range of projects involving structural evaluation, strengthening, rehabilitation, and durability enhancement.
Typical projects have involved evaluation of the extent and causation of structural distress or deterioration as well as development and implementation of repairs.
Licensed Structural Engineer - Illinois
Licensed Professional Engineer - Illinois
American Society of Civil Engineers, ASCE - Member
International Concrete Repair Institute, ICRI - Member
Repair Materials and Methods Committee
Mr. Kolf has authored and coauthored eight articles published in industry trade publications and trade conference proceedings.
© 2011 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2011.