Crystalline Admixture as Healing Promoter in Concrete Exposed to Chloride-Rich Environments: Experimental Study

The requirements on service life of reinforced concrete structures, as prescribed by design codes, may be difficult to be fulfilled in highly aggressive environments such as marine ones, in which premature degradation is most likely to occur. In the aforementioned situations, to avoid expensive repair activities, different protective systems, including, among the others, self-healing concrete, could be adopted. Researchers have found self-healing as a way to face degradation problems in chloride-rich environments. If a significant degree of crack sealing can be achieved, the physical properties of a cracked element can trend back to those of an identical uncracked element, which may also result in a slower penetration rate of aggressive substances. The main problem in exploiting this methodology is related to its reliability. In this context, an experimental program aimed at investigating the effectiveness of crystalline admixtures as healing stimulating agent in chloride-rich environments was carried out. The influence of the exposure conditions on the compressive strength development and on its recovery in predamaged specimens was first analyzed. Afterwards, crack sealing and chloride permeability of sealed cracks were evaluated for specimens continuously immersed or subjected to wet/dry cycles in a 16.5% NaCl aqueous solution. Both an enhanced recovery of compressive strength and an improved crack sealing ability were observed for samples containing the healing agent. A microstructure study of the healing products was conducted by means of scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis as well

Una metodologia sperimentale per valutare la capacità di autoriparazione di calcestruzzi con additivi aerocristallizzanti

The self-healing capacity of cementitious composites employed for either new or repairing applications opens challenging perspectives for the use of materials intrinsically able to recover their pristine durability levels, thus guaranteeing a longer service life of the designed applications and a performance less sensitive to environmental-induced degradation. One possibility of achieving the aforementioned self-healing capacity stands in the use of additives featuring a “delayed crystalline” activity, which, when in contact with water or atmosphere humidity, form chemical compounds which are able to reseal the cracks thus guaranteeing the partial recovery of the pristine mechanical performance. In order to quantify this self-healing capacity and its effects on the recovery of mechanical properties of the material, a methodology has been developed and will be presented in this paper. The method starts with pre-cracking, up to different crack opening levels prismatic beam specimens (a three point bending scheme with COD measurement has been employed in this study), made with concrete, both added or not with the aforementioned additives. Specimens are then submitted, for different exposure times, to accelerated temperature and humidity cycles, which may be chosen as representative of climate conditions relevant to the intended application. Finally, three point bending tests are performed once again on the same “conditioned“, either uncracked or pre-cracked specimens, and results, in terms of load-COD curves, are compared with those obtained from virgin specimens before any “conditioning”. This allows crack “self-closure” to be evaluated and “self-healing” indices to be defined and correlated, e.g., to the load-recovery capacity

An experimental methodology to assess the self-healing capacity of cementitious composites with “aero-crystallizing” additives

The self-healing capacity of cementitious composites employed for either new or repairing applications opens challenging perspectives for the use of materials intrinsically able to recover their pristine durability levels, thus guaranteeing a longer service life of the designed applications and a performance less sensitive to environmental-induced degradation. One possibility of achieving the aforementioned self-healing capacity stands in the use of addi-tives featuring a “delayed crystalline” activity, which, when in contact with water or atmosphere humidity, form chemi-cal compounds which are able to reseal the cracks thus guaranteeing the partial recovery of the pristine mechanical performance. In order to quantify this self-healing ability and its effects on the recovery of mechanical properties of the material, a methodology has been developed and will be presented in this paper. It consisted in pre-cracking up to different crack opening levels (a three point bending scheme with COD measurement was employed) prismatic beam specimens, made with both concrete added or not with the aforementioned additives. Specimens were then submitted to accelerated temperature and humidity cycles, representative of autumn climate conditions in northern Italy, for dif-ferent exposure times. Finally, three point bending tests were performed on either uncracked or pre-cracked speci-mens and results, in terms of load-crack opening curves, were compared with those obtained from virgin specimens before any “conditioning”. This allowed crack “self-closure” to be evaluated and “self-healing” indices to be defined and correlated, e.g., to the load-recovery capacity

AN EXPERIMENTAL METHODOLOGY TO ASSESS EFFECTS OF HEALING ON FREEZE-THAW DAMAGED ULTRA HIGH-PERFORMANCE CONCRETE

This paper presents the experimental investigation of the self-healing capacity and ability to maintain the structural performance of a Ultra High-Performance (Fiber Reinforced) Concrete (UHPC/UHPFRC), with a crystalline admixture to stimulate the healing, after freeze-thaw cycles. To the aforesaid purpose ultrasonic pulse velocity tests, four-point flexural tests (before and after freeze-thaw, and after self-healing), and crack closure quantification have been performed. 20 mm thin beams were pre-cracked up to a cumulative crack width of 0.3 mm by means of four-point flexural test and subjected to freeze and thaw cycles between -20° C to 38° C for 17 days, each cycle lasting for 20 hrs. The flexural tests showed that freeze-thaw did not deteriorate the specimens' flexural strength. However, freeze-thaw caused some damage which was noticeable in the ultrasonic test. After the freeze and thaw cycles specimens were immersed in water for self-healing. The self-healing progress was measured periodically after 1, 2, 3, and 6 months of healing through ultrasonic test and microscopy image processing. The results showed that the freeze-thaw damages were healed throughout the specimens, and that previously undergone damage didn't affect neither the stimulated autogenous healing capacity of the investigated material nor its mechanical performance. This can be likely attributed to both closure of the cracks, which were almost fully healed within 3 months, and likely also to improved bond strength between the fibers and concrete matrix, due to the deposition of the healing products along the interface

Concept of Ultra High Durability Concrete for improved durability in chemical environments: Preliminary results

The aim of this work is to analyze the enhanced durability performance of an Ultra High Durability Concrete (UHDC) exposed to chemical attack (XA exposure conditions), with reference to an intended application into infrastructures serving geothermal plants. This study is based on a reference Ultra High Performance Concrete (UHPC) with steel fibers and crystalline admixtures (reference mix) and other two mixes that modify in some aspect the reference one: addition of alumina nanofibers (ANF) and addition of cellulose nanocrystals (CNC). Accelerated and short-term tests, complemented with mineralogical and microstructural characterization, have been employed to measure critical durability indicators according to an XA environment. Based on the monitoring and damage evolution of the UHPC under the intended exposure conditions, a criterion for durability performance assessment is being defined in order to understand the differences due to the incorporation of nanoadditions

EFFECT OF AUTOGENOUS SELF-HEALING ON HIGH TEMPERATURE EXPOSED ULTRA HIGH-PERFORMANCE CONCRETE

Mechanical properties of Ultra High-Performance Concrete (UHPC) degrade when exposed to elevated temperatures, even more than ordinary concretes due to its dense microstructure. Concerning, in particular, the special application of nuclear power plants, in which UHPC can find a promising use, concrete can be subjected to moderately high temperature (usually lower than 400 °C) along the working life, this making of interest the study on the influence and persistence of UHPC's innate self-healing capabilities over the thermal degradation. In this context, the paper focuses on an experimental study of UHPC recovery ability by autogenous self-healing after being exposed to high temperatures. The UHPC specimens have been made with hybrid fibers, that is, polypropylene and steel fibers, and have been pre-cracked up to a cumulative crack width of 0.3 mm under 4-point flexural test. The pre-cracked specimens have been exposed to a temperature of 200 °C or 400 °C, with a heating rate of 1 °C / minute from room temperature and kept at the target temperature for two hours, with a following slow cooling at a rate of <1 °C / minute. The specimens have been kept in the lab environment for 24 hours after reaching room temperature. Then they have been tested for residual flexural capacity or allowed to self-heal under water immersion for six months. The damage and healing evolution have been monitored periodically using ultra-sonic pulse velocity survey and digital microscope inspection. In spite of the thermal degradation, during the healing period UHPC showed a significant recovery in terms of strength assessed by ultrasonic pulse velocity tests

An Overview on H2020 Project “ReSHEALience”

In the framework of H2020, the European Commission recently funded the project ReSHEALience (www.uhdc.eu). The main idea behind the project is that the long-term behaviour of structures under extremely aggressive exposure conditions can highly benefit from the use of high performance materials, in the framework of durability-based design approaches. The project consortium, coordinated by Politecnico di Milano, features 14 partners from 8 different countries, including 6 academic/research institutions and 8 industrial partners, covering the whole value chain from producers of concrete constituents to construction companies to stake-holders and end-users. The main goals of the project are the development (a) of an Ultra High Durability Concrete (UHDC) and (b) a Durability Assessment-based Design (DAD) methodology to improve structure durability and predict long-term performance under Extremely Aggressive Exposures (EAE). The project will tailor the composition of UHDC, by upgrading the UHPC/UHPFRC concept through the incorporation of tailored nanoscale constituents