1、 1 ACI 364.16T-18TechNotePhysical ProPerties and characteristics affecting the sensitivity to cracking of cementitious rePair materialsKeywords: coefficient of thermal expansion; cracking; creep; dimensional compatibility; drying shrinkage; durability; elastic modulus; phys-ical properties; repair m
2、aterial; tensile strength.Question:What properties and characteristics of cement-based repair materials influence cracking in repairs?DiscussionOne of the main factors assuring the durability and long-term performance of concrete repairs is its resistance to cracking. Cracks are open pathways provid
3、ing aggressive agents easy access into the repair, promoting the development of reinforcement corrosion and deterioration of the repaired structure (Vaysburd 1995; ACI 224.1R). Repair materials typically crack as a result of restrained volume changes. To achieve dimensional compatibility and minimiz
4、e cracking in repairs, the relevant physical properties and characteristics of the repair material should be appraised during the selection process, notably in the case of cement-based repair materials. The cracking sensitivity of concrete repairs is also influenced by factors such as surface prepar
5、ation; patch geom-etry; and presence of reinforcing steel, placement conditions (notably the temperature and relative humidity of both the existing concrete and ambient air, which may result in significant gradients within the repair), curing, and expansive forces in the existing concrete, which are
6、 addressed elsewhere and not discussed herein.Structural and nonstructural repairs are considered in this discussion. Structural repairs are intended to increase the load-carrying capacity of a structural component beyond its current capacity or to restore a damaged structural component to its origi
7、nal design load-carrying capacity. For example, repairs to structural elements such as columns subjected to applied loads should accommodate these loads. Conversely, protective repairs performed to reestablish the original configuration without altering the structural capacity of a member are genera
8、lly defined as nonstructural. Nevertheless, in ACI 562, the definition of a structural repair has been widened to include any repair that creates an unsafe condition in the event it fails, irrespective of the structural capacity consideration.Cracking caused by restrained contraction occurs when the
9、 induced tensile stress exceeds the tensile strength of the repair material. External and internal restraint conditions that induce tensile stresses that can lead to cracking are discussed in ACI 207.2R and ACI 224.1R. Restrained contractions that can induce cracking are primarily caused by shrinkag
10、e and thermal volume changes. In the case of cementitious repair materials, these volume changes may occur individually or in combination, while the material is in either a plastic or hardened state.The magnitude of shrinkage is generally the dominant factor, but not the only factor affecting the cr
11、acking sensitivity of a repair material. The other important factors and characteristics interplaying in the overall cracking behavior of the material are (Vaysburd et al. 1999, 2000, 2014; McDonald et al. 2002; Bissonnette et al. 2015; Courard et al. 2015):a) Degree of restraintb) Tensile strengthc
12、) Modulus of elasticityd) Creepe) Coefficient of thermal expansionThe combination of properties that are most desirable to reduce the advent of cracking in cement-based materials can be lumped into the single notion of extensibility (Mehta and Monteiro 1993), which corresponds American Concrete Inst
13、itute Copyrighted Material www.concrete.org2 CHARACTERISTICS AFFECTING THE SENSITIVITY TO CRACKING OF CEMENTITIOUS REPAIR MATERIALS (ACI 364.16T-18)the sum of their short-term (elastic) and long-term (creep) tensile strain capacity. Cement-based materials with a high degree of extensibility can resi
14、st volume changes (deformations) without cracking. Therefore, to limit the potential for cracking, cement-based repair materials should have low shrinkage, and should also exhibit as much extensibility as possible, through a combination of low elastic modulus, high creep, high tensile strength, or a
15、ll these.Overall, the requirement for long-lasting monolithic behavior is that the repair materials have properties and dimensional behavior that will make them compatible with the existing concrete substrate for the application considered. Dimensional compatibility is defined as a balance of strain
16、s between a repair material and the existing substrate, such that the composite repair system withstands all stresses induced by the various volume changes without distress and deterioration over a designed period of time.A comprehensive investigation conducted by the U.S. Army Corps of Engineers (V
17、aysburd et al. 1999; McDonald et al. 2002) evaluated the relationship between these properties, characteristics, and the repair materials field performance with respect to crack resistance. While the results did not reveal conclusive trends when consid-ering the properties and characteristics indivi
18、dually, a correlation with the cracking sensitivity was found when considering them together. The in-depth analysis of the data generated in both laboratory and field experiments led to the identification of preliminary performance criteria for the most influential repair material properties and cha
19、racteristics. The relative importance of the latter varies depending on application and service condi-tions; therefore, the requirements should be modified as necessary to achieve compatibility with the existing substrate.Tensile strengthIntuitively, the easiest way to improve the resistance of ceme
20、ntitious materials to cracking would be to achieve substantially high tensile strength. In reality, because the tensile capacity of cement-based materials cannot be increased substantially, the primary goal should be to reduce tensile stresses to minimize cracking. Nonetheless, it was concluded in t
21、he study by the U.S. Army Corps of Engineers (Vaysburd et al. 1999; McDonald et al. 2002) that the tensile strength of the repair material should be a minimum of 400 psi (2.8 MPa). Such a value would be expected for conventional concrete with a compressive strength of approximately 4000 psi (28 MPa)
22、.The term “tensile strength” has no absolute meaning for cement-based materials and needs to be expressed in terms of the specific test procedure used. Three types of tests are primarily used for cementitious materials tension testing: the direct tension test, the flexural test, and the splitting te
23、nsile test. The direct tension test is challenging to perform because of the inherent difficulty of ensuring that the load is truly axial. In a ductile material, some eccentricity will not have much effect on tensile strength, but in brittle cementitious mate-rials, there is relatively little stress
24、 redistribution and, consequently, the test gives an underestimate of true tensile strength. Still, when properly conducted, a direct tension test such as the one described in the USACoE CRD-C 164 procedure yields a more representative measure than indirect methods. The results from the flexural str
25、ength or beam test (modulus of rupture) and the splitting-strength test are both known to overestimate the tensile strength of the material. Data by Price (1951) demonstrate that flexural strength testing tends to over-estimate the tensile strength of concrete by 50 to 100 percent. Despite their sim
26、plicity, the indirect tests fail to reproduce the state of stress developed within a centrically tensioned specimen and, consequently, the values of the corresponding strengths differ from the pure tensile strength values.The compressive strength of a repair material intended for structural repair s
27、hould ideally be similar to that of the existing concrete. Generally, it is not desirable to have repair materials with compressive strengths in excess of 4000 psi (28 MPa) for nonstructural repairs. Rather, it is generally agreed that high-strength repair materials are more prone to cracking becaus
28、e of high stresses developed from restrained shrinkage, as a result of typi-cally high elastic modulus and lower creep. In addition, the use of rapid-setting materials to achieve high early strength often leads to increased cracking because of higher early-age volume changes, increased stiffness, an
29、d less creep.Modulus of elasticityIn a repair, the modulus of elasticity controls, to some extent, the load sharing between the repaired area and the rest of the element, as well as the level of tensile stress generated due to volume changes such as shrinkage and thermal deformations. For structural
30、 repair, the modulus of elasticity of the repair material should be compatible with that of the existing concrete. Conversely, it was concluded in the USACoE CRD-C 164 investigation that the modulus of elasticity of materials for nonstructural repairs, deter-mined in accordance with ASTM C469/C469M,
31、 should be specified not to exceed 3.5 106psi (24 GPa).American Concrete Institute Copyrighted Material www.concrete.orgCHARACTERISTICS AFFECTING THE SENSITIVITY TO CRACKING OF CEMENTITIOUS REPAIR MATERIALS (ACI 364.16T-18) 3Such a low limit might appear somewhat discriminant in view of the range of
32、 values typically found for cementitious materials, especially considering the recommended minimum tensile strength of 400 psi (2.8 MPa). What should primarily be seen in that recommendation is the need for the lowest effective material stiffness to minimize the restrained shrinkage stresses and the
33、 highest extensibility to prevent shrinkage-induced cracking. The effective stiffness (ACI 209R), takes into account both the short-term (elastic modulus) and the long-term (creep) deformational behavior of the material. The rationale of this approach is that shrinkage is a time-depen-dent phenomeno
34、n and restrained shrinkage stresses are induced progressively within the repair material, thus leaving time for creep and stress relaxation to occur.Nevertheless, lowering the repair material stiffness is very desirable in any nonstructural repairs. The factors affecting the modulus of elasticity of
35、 cement-based materials are related to compressive strength and density. Hence, factors that affect strength such as cement content, w/cm, aggregate size, type and grading, curing condi-tions, and age at the time of testing should similarly influence modulus. The modulus of elasticity of cement-base
36、d materials can be reduced with a higher w/cm and aggregate with a lower modulus of elasticity.Cracks exist in a composite repair system for reasons other than service loads. Some of these are caused not only by the drying shrinkage of the repair material and the difference in the coefficients of th
37、ermal expansion from the substrate material, but also to the differences in the elastic moduli of the repair material and substrate concrete.The compatibility in elastic moduli, therefore, becomes, in cases of structural repairs, an important factor because incompatibility may lead to considerable s
38、tress concentration when widely differential volume changes of the repair material in relation to the concrete substrate occur. Because, in such situations, the interfacial bond region (transition zone) is the weak link in the repair system, cracks will tend to form in this region. In certain cases
39、where bond strength is high, cracks will occur in the matrix of the material, which has the highest modulus of elasticity. When external load is perpendicular to the bond line, as in the case of repaired pave-ment, differences in modulus of elasticity regarding the varying stiffness between repair m
40、aterials and concrete substrate is normally not problematic (Vaysburd et al. 2014).Conversely, in repairs where the service load is parallel to the bond line, differences in modulus of elasticity may cause load transfer to the high-modulus material if the other materials yield under the stress. If t
41、he load transfer is beyond the load-bearing capacity of the higher-modulus material, it will fracture and damage the structure.CreepRelaxation through tensile creep reduces the stresses induced in a repair system by restrained drying shrinkage, thereby enhancing the crack resistance of repairs. Test
42、 results show tensile creep capacity is depen-dent on material composition, often more so than shrinkage (Pigeon and Bissonnette 1999). Because a reduction in paste content reduces shrinkage and appears to increase tensile creep, a proper repair mixture should have the lowest practical cement conten
43、t. Also, in general, creep of cement-based materials is inversely proportional to the modulus of elasticity and compressive strength; therefore, high-strength, high-modulus materials are generally not desirable for nonstructural concrete repairs.The phenomenon of gradual increase in strain with time
44、 under a given level of sustained stress is called “creep”. In the same fashion, the phenomenon of gradual decrease in stress with time under a given level of sustained strain is called “stress relaxation”. Both manifestations are typical of viscoelastic materials. When the volume changes of a cemen
45、t-based repair material are restrained, their viscoelastic behavior will translate into a lower stress-increase rate. Thus, under the restraining conditions present in the repair material, the interplay between elastic tensile stresses induced by shrinkage strains and stress relief due to creep is a
46、t the heart of the risk of cracking.Compressive creep properties are easier to obtain and evaluate, but their use in place of tensile creep to analyze the extensibility and cracking resistance of cement-based repair materials was found to produce signifi-cant errors (Vaysburd et al. 1997). The works
47、 by Bissonnette and Pigeon (1995) and Bissonnette et al. (2005) have highlighted the criticality of tensile creep among the properties of cement-based materials influencing the behavior of a concrete repair.Coefficient of thermal expansion (CTE)The CTE gives a measure of dimensional contraction or e
48、xpansion with changes in temperature, and therefore is an essential property of the repair materials used in composite repair systems. When significant changes in temperature occur, a marked difference in the CTE between repair and substrate will produce different volume changes, resulting in excess
49、ive stresses at the interface, which may cause bond failure or failure within the lower strength material of the composite repaired structure.American Concrete Institute Copyrighted Material www.concrete.org4 CHARACTERISTICS AFFECTING THE SENSITIVITY TO CRACKING OF CEMENTITIOUS REPAIR MATERIALS (ACI 364.16T-18)Except under extreme temperature conditions, surface concrete repairs generally suffer very little or no distress from changes in temperature, because the CTE of most cement-based repair materials is very close to that of the existing concrete. The risk for therm
