A Framework for Durability Design with Strain-Hardening Cement-Based Composites (SHCC)

Inherent crack control to fine widths in strain-hardening cement-based composites (SHCC) suggests that structural elements produced from SHCC or steel-reinforced SHCC (R/SHCC) may be rendered durable by limiting the ingress rates of potentially deleterious substances. Recently, it has been reported that, while the average crack width in SHCC is maintained up to large tensile strains in excess of 3%, the maximum crack width may equal or exceed those are considered to be limiting in terms of durability. Also, the typical range in SHCC average crack width, from 50 to 100 μm, has been shown to be a threshold in water permeability, at which width permeability is restricted to several orders lower than that expected for crack widths ranging from 0.2 to 0.3 mm — a typical reinforced concrete crack width limit in durability standards. However, it has recently been shown that capillary absorption in dry, pre-cracked SHCC is a quick process, with water penetrating into fine cracks within minutes of exposure. In addition to describing these findings, this chapter sets the scene for later chapters on improved ingress rate characterisation and the actual deterioration of cracked SHCC or R/SHCC. Guidelines for the pre-cracking of SHCC towards durability testing are derived, based on the results of recent comparative testing. These include the specimen shape, size, test set-up, crack measurement to sufficient resolution, and crack width distribution presentation. Finally, the field performance of repairs, structures and structural elements produced from SHCC and R/SHCC in the past decade is reported.

The durability of strain-hardening cement-based materials (SHCC) is strongly influenced by the transport of different substances through the material. Since numerous fine cracks are formed in SHCC, the relationship between the crack pattern and different transport properties has been the subject of experimental investigations, including tests of water and gas permeability, chloride ingress, and capillary absorption. It appears to be insufficient to consider only the average or maximum crack width when transport through SHCC is to be modelled. Therefore, the determination of certain crack pattern parameters has been proposed that take into account both crack widths and distances. These parameters may be linked to certain transport properties of the respective material.

The service life of strain-hardening cement-based composite materials (SHCC) is based on the service life of all the system components: fiber, matrix, and fiber–matrix interface. This chapter describes SHCC durability from the fiber perspective, distinguishing between different fiber types commonly used in SHCC: polyvinyl alcohol (PVA), polyethylene (PE), polypropylene (PP), and natural, steel and glass. Their relative strengths should be considered during any design process, especially when determining the long-term durability of the composite material. PVA is considered a chemically stable fiber, resistant to acid solutions, organic solvents, and alkaline environments. It has proven long-term durability, based on its high strength retention after accelerated aging cycles. The fibers are also hydrophilic as the cement matrix, leading to strong bonding between the two. PE and PP, both olefin type fibers, are also known for their high durability performance in the cement matrix. However, these fibers are hydrophobic and therefore do not have any chemical affinity to the hydrophilic cement matrix, resulting in low bonding between these fibers and the cement matrix. Glass fibers are relatively sensitive to the alkaline environment of the cement matrix. Consequently, special glass fibers (known as AR or alkali-resistant fibers) have been developed with improved alkali resistance. Steel fibers have a good affinity with the cement matrix but, depending on the environmental conditions, can undergo corrosion. The new trend of natural fibers in SHCC design offers a major advantage: combining low cost with local availability. However, they are susceptible to weathering, alkaline environments, biological attack, and mineralization.

In this chapter, the influence of various chemical processes on the performance of strain-hardening cement-based materials (SHCC) is discussed. SHCC was found to increase in strength when subjected to long-term elevated temperature and humidity, but to experience a decrease in strain capacity under the same conditions. Also, chlorides tend to decrease the ductility of SHCC when the material is exposed to a chlorine environment for a long time. SHCC, especially when high amounts of fly ash are used in the mix, generally do not experience problems related to alkali–silica reaction (ASR). However, in cases when reactive aggregates are used and the alkalinity in the pore water is high, ASR can occur. The fibres have been found to suppress ASR expansion and, because of the local restraining in the material, the ASR reaction can even be slowed down. The last part of the chapter deals with the self-healing properties of SHCC. Because SHCC is characterised by small cracks it has an intrinsic self-healing capacity. Parts of these cracks close by themselves when the material is stored in a humid environment or under water. Several methods of improving the self-healing capacity of SHCC by adding different types of self-healing agents to the mix are also reported.

When strain-hardening cement-based composite (SHCC) with cracks is repeatedly exposed to freezing and thawing, accelerated deterioration of the SHCC, due to the expansion of water during freezing in the cracks, may be of major concern. This chapter summarises the results of studies on the frost damage of SHCC, in particular the frost damage of SHCC with cracks. In most cases described in the literature, the ASTM method C666A — Procedure A (2008) was applied. It has been found that even when cracking has occurred, SHCC has a high resistance with respect to frost damage. When the water-cement ratio is high enough, only a very slight decrease of the relative dynamic modulus of elasticity occurred, although a small amount of scaling was observed on the surface, regardless of the type of fibre or of the composition of the mortar matrix. In tests that simulated the case in which the concrete surface layer damaged by freeze-thaw cycles had been removed and the cross-section had then been repaired with SHCC, it was found that no deterioration of the repaired surface sections occurred due to freezing and thawing. The greater the depth to which the deteriorated concrete was removed, the less susceptible the repaired surfaces were to further freeze-thaw damage.

In this chapter, the effect of elevated temperatures on strain-hardening cement-based composites (SHCC) is reported. Key features of SHCC such as tensile strength and strain capacity, compressive strength, failure modes, the fiber–matrix interface, and spalling behavior are discussed. Different testing conditions are covered, including not only residual but also high temperature tests. Experimental results addressing various temperature levels (ranging from ambient temperature to 1000 °C) were used to investigate basic knowledge of the thermomechanical response of SHCC under conditions of both uniaxial tension and compression.

Strain-hardening cement-based composite (SHCC) has proved to be a suitable material for repair layers on concrete substrate. Bonded SHCC overlays may bridge cracks in the substrate, and normally exhibit an enhanced resistance to drying shrinkage. In addition to the SHCC material properties, the interface behaviour has a significant influence on the failure process. By varying the interface roughness and bond strength, it is possible to attain a balance between the debonding of the interface and SHCC cracking, and to ensure both monolithic mechanical behaviour of the structure and sufficiently small crack widths in the SHCC overlay. The smaller these crack widths are, the lower the permeability and the greater the self-healing potential of the cracks. In the case of mechanical loading, a rather weak bond may be beneficial, as the crack widths in the SHCC tend to be smaller under such conditions. In the case of drying shrinkage of the SHCC overlay, however, weak bonding may lead to large debonded interface areas, making the use of SHCC inadvisable in such instances.

Currently, a common use for strain-hardening cement-based composites (SHCC) is as a repair material, or for retrofitting reinforced concrete (RC) structures. This is due to SHCC being expected to have high resistance to substance penetration as the cracks that are produced in SHCC are fine. Equally, in retrofitting applications such as those described in Sects. 1.​5.​3–1.​5.​5, or in applications of steel-reinforced SHCC (R/SHCC) in structures in coastal regions, the corrosion of the steel reinforcement is likely to determine both durability and structural service life. Accordingly, this chapter discusses the chloride-induced corrosion of R/SHCC. From reported experimental results, chloride profiles in cracked R/SHCC have been reported in Chap. 2. Here, the chloride contents are evaluated for correlations with observed corrosion damage in the steel bars for different cover depths in cracked R/SHCC. Steel bar damage is expressed in terms of steel mass loss, corrosion depth, and reduction in yield resistance. Finally, a corrosion model is proposed for R/SHCC, incorporating crack width, crack spacing, free chloride content, and cover depth.

Due to their unique properties, strain-hardening cement-based composites (SHCC) are well-suited for structural and non-structural members, as well as for the repair and rehabilitation of ordinary concrete structures that are exposed to severe mechanical or environmental loading. To fully utilise their advantageous durability properties, a comprehensive durability and service life design framework is required. This framework must encompass design concepts for all relevant load cases and load case combinations. For an efficient service life design of members and structures, the design concepts must be performance-based. In this chapter, SHCC applications and relevant degradation processes are classified. Following the classification, possible service life design approaches are described, categorised and assessed regarding their suitability for the service life design of SHCC members and structures. Following an outline of a possible development strategy for a service life design framework, available design concepts for chloride-induced rebar corrosion are presented. It is shown that in order to develop available concepts into a comprehensive durability design framework, the uncertainty of input variables must be reduced, and deterioration models must be further validated. Finally, other limit states, load cases, and load case combinations must be considered. Moreover, it is postulated that for most load cases and load case combinations, suitable deterioration models for SHCC with reasonably well-quantified input variables will require significant additional development. Hence, deterioration model-free durability assessments with fuzzy uncertainty quantifications will likely be the most suitable option for most load cases and load case combinations for the foreseeable future.