Concrete Durability

Concrete structures exposed to aggressive aqueous media (waste water, soft water, fresh water, ground water, sea water, agricultural or agro-industrial environments), due to their porous nature, are susceptible to a variety of degradation processes resulting from the ingress and/or presence of water. In addition to chemical and physical degradation processes, the presence of water contributes to undesirable changes in the material properties resulting from the activities of living organisms, i.e., biodeterioration. Since microorganisms are ubiquitous in almost every habitat and possess an amazingly diversified metabolic versatility, their presence on building materials is quite normal often, they can infer deterioration that can be detrimental (loss of alkalinity, erosion, spalling of the concrete skin, corrosion of rebars, loss of water- or air tightness, etc.). The deleterious effect of microorganisms, mainly bacteria and fungi, on the cementitious matrix has been found to be linked, on the one hand, with the production of aggressive metabolites (acids, CO₂, sulfur compounds, etc.) but also, on the other hand, with some specific, physical and chemical effects of the microorganisms themselves through the formation of biofilm on the surface. Moreover, the intrinsic properties of the cementitious matrix (porosity, roughness, mineralogical and/or chemical composition) can also influence the biofilm characteristics, but these phenomena have not been understood thoroughly as of yet.

These deteriorations lead to a significant increase in the cost of repairing structures and to loss of production income, but may also lead to pollution issues resulting, for example, from waste water leakage to the environment. Also, building facades, and notably concrete external walls, can be affected by biological stains, which alter aesthetical quality of the construction, sometimes very quickly, and lead to significant cleaning costs. Microorganisms, mainly algae, responsible for these alterations have been quite well identified. Research is now rather focused on determining colonization mechanisms, and notably influencing material-related factors, and on development of preventive or curative, and preferentially environmentally friendly, solutions to protect external walls. However, up to now, no clear results about the efficiency of these various protection solutions are available.

Concrete biodeterioration is defined as the damage that the products of microorganism metabolism, in particular sulfuric acid, do to hardened concrete. The combination of sulfur compounds and sulfur-dependent microorganisms is the origin of the process, because sulfates are found in certain groundwater, sewer and in sea water, additionally, some sulfur compounds are natural constituents of Portland cement. Along with this, the common presence and activity of microorganisms plays a very important function in the whole spectrum of degradation processes such as biodeterioration of metals and concrete. We report here the development of a possible biodeterioration resistant concrete. We assume that the elimination of sulfur compounds and acid reactive materials in the Portland cement and aggregates will prevent the formation and action of the biogenic acids that cause dissolution of calcium-containing minerals [for a narrative in Spanish see (Rendon, ¿Que es el biodeterioro del concreto? Revista Ciencia de la Academia Mexicana de Ciencias, Vol. 66 Num. 1. http://​www.​revistaciencia.​amc.​edu.​mx/​images/​revista/​66_​1/​PDF/​Biodeterioro.​pdf, 2014)]. This study was carried out on site inside a wastewater sewer drainage in Mexico City. Concrete samples whose main characteristic was being formulated without any sulfur or sulfates in its composition as well as reference concrete samples made with Ordinary Portland Cement (OPC) were used for the experiment. The weight changes and surface changes of both concrete samples were valuated after 7-month exposition to the biodeterioration process. The results obtained on site suggest that both the composition of concrete and duration of aggressive environment are very important. This possible biodeterioration-resistant concrete could give a viable solution to the long known problem of microbiologically induced concrete corrosion (MICC) a typical case of biodeterioration. Furthermore, we recommend the Portland type cement Mexican norm (ONNCCE, Organismo Nacional de Normalización y Certificación de la Construcción y Edificación, S.C. (2004) Norma Mexicana (NMX – C – 414 – ONNCCE - 2004) Diario Oficial de la Federación 27 de julio de 2004, 2004) which does not take into consideration the concrete biodeterioration variable and its mechanism, to be reviewed in this aspect, or at least that a warning be issued as a key measure to mitigate biodeterioration in sewer concrete infrastructure.

The need for an accurate determination of the chloride threshold value for corrosion initiation in reinforced concrete has long been recognized. Numerous investigations and reports on this subject are available. However, the obtained chloride threshold values have always been, and still are, debatable. The main concern is linked to the methods for corrosion detection and chloride content determination in view of the critical chloride content itself. In order to measure the chloride content, relevant to the corrosion initiation on steel, destructive methods are used. These traditional methods are inaccurate, expensive, time consuming and noncontinuous. Therefore, the application of a cost-effective Ag/AgCl ion selective electrode (chloride sensor) to measure the chloride content directly and continuously is desirable. The advantage would be an in situ measurement, in depth of the concrete bulk, as well as at the steel/concrete interface.

The aim of this work was to evaluate the importance of the sensor’s properties for a reliable chloride content measurement. The main point of interest with this regard was the contribution of the AgCl layer and Ag/AgCl interface within the process of chloride content determination in cementitious materials. The electrochemical behavior of sensors and steel, both embedded in cement paste in a close proximity, hence in identical environment, were recorded and outcomes correlated towards clarifying the objectives of this work. The main point of interest was to simultaneously detect and correlate the time to corrosion initiation and the critical chloride content.

The electrochemical response of steel was monitored to determine the onset of corrosion activity, whereas the sensors’ electrochemical response accounted for the chloride content. For evaluating the electrochemical state of both sensors and steel, electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurements were employed. The results confirm that determination of the time to corrosion initiation is not always possible and straightforward through the application of OCP tests only. In contrast, EIS is a nondestructive and reliable method for determination of corrosion activity over time. The obtained results for corrosion current densities for the embedded steel, determined by EIS, were in a good agreement with the sensors’ half-cell potential readings. In other words, the sensors are able to accurately determine the chloride ions activity at the steel/cement paste interface, which in turn brings about detectable by EIS changes in the active/passive state of steel.

The electrochemical response was supported by studies on the morphology and surface chemistry of the sensors, derived from electron microscopy (ESEM) and X-ray photoelectron spectroscopy (XPS). It can be concluded that the accuracy of the sensors, within detection of the time to corrosion initiation and critical chloride content, is determined by the sensors’ properties in terms of thickness and morphology of the AgCl layer, being an integral part of the Ag/AgCl sensors.

This work reports on the influence of stray current on the development of mechanical and electrical properties of mortar specimens in sealed and water-submerged conditions. In the absence of concentration gradients with external environment (sealed conditions) or in their presence (submerged conditions), compressive strength and electrical resistivity change due to: cement hydration alone; cement hydration, affected by diffusion (including leaching-out); or cement hydration, simultaneously influenced by diffusion and migration. The results are compared to equally conditioned control specimens, where stray current was not involved.

In view of material properties development over time, the ageing factor in relevant exposure conditions is addressed, considering reported approaches for its determination. Through implementing existing methodology and based on experimentally derived electrical resistivity values, the ageing factor for sealed conditions was determined. The apparent diffusion coefficients were calculated based on ageing factors and reported relationships, reflecting the effect of stray current on matrix diffusivity.

Two levels of electrical current density, 100 mA/m² and 1 A/m², were employed as a simulation of stray current to 28 days-cured mortar specimens with water-to-cement ratio of 0.5 and 0.35. For the time interval of these tests of ca. 110 days, the experimental results show the positive effect of stray current on mortar specimens in sealed conditions and the negative effect for water-submerged conditions.

For sealed specimens, increase of compressive strength and electrical resistivity were recorded, more pronounced for the higher current density level of 1 A/m². This effect was irrespective of w/c ratio. Increased electrical resistivity and superior performance overall, would determine improved material properties in terms of reduced permeability and diffusivity of the matrix. The results show that for sealed specimens in stray current conditions, the apparent diffusion coefficient was reduced, the effect being more pronounced for the higher current density level of 1A/m²and (logically) for the lower w/c ratio of 0.35.

In contrast, for water-submerged mortar, a reversed trend of material behavior was observed i.e. reduced mechanical and electrical properties were recorded to be resulting from stray current flow.

Defining a threshold for a positive or negative stray current effect was not possible to be determined from this work. Higher current density levels in varying external environment are necessary to be studied, in order to potentially define such threshold. However, the results clearly show that stray current affects the development of material properties of cement-based materials, an aspect that is rarely considered in the current practice.

Stray current, arising from direct current electrified traction systems, further circulating in nearby reinforced concrete structures may initiate corrosion or accelerate existing corrosion processes on the steel reinforcement. In some extreme conditions, corrosion of the embedded steel will occur at very early stage. One of the significant consequences is loss of bond strength and premature failure of the steel-matrix interface. This plays an important role for the integrity of a structure during its designed service life.

In this work, the level of stray current was set at 0.3 mA/cm², applied as an external DC electrical field. This level of stray current was chosen based on preliminary calculations on expected corrosion damage, i.e., in view of material loss at the level of 10% weight loss of the steel rebar (analytically calculated via Faraday’s law). The investigated reinforced mortar specimens were cured for 24 h only and then conditioned in chloride-free and chloride-containing environment. The evolution of steel electrochemical response in rest (no stray current) and under current conditions was monitored for approx. 240 days via OCP (Open Circuit Potential), LPR (Linear Polarization Resistance), EIS (Electrochemical Impedance Spectroscopy) and PDP (Potentio-dynamic Polarization).

The results show that the effect of stray current on concrete bulk matrix properties, together with steel corrosion response, is significantly determined by the external environment, as well as by the level of maturity of the cement-based bulk matrix.

For chloride-free environment the effect of the chosen stray current level was not significant, although lower corrosion resistance of the steel rebars was recorded after longer exposure of ~240 days, compared to control conditions. In fact, even positive effects of the stray current were observed in the early stages, i.e., until 28 days of age: stray current flow through a fresh (non-mature) cement matrix led to enhanced water and ion transport due to migration. The result was enhanced cement hydration, consequently environment, assisting a more rapid stabilization of pore solution and steel/cement paste interface. In chloride-containing external medium, steel corrosion was a synergetic effect of both de-passivation due to chloride ions in the medium and stray current effects. Corrosion acceleration solely due to the stray current flow in chloride-containing medium cannot be claimed for the chosen current density levels and the duration and conditions of the experiment.

What can be concluded is that the effect of stray current for both chloride-free and chloride-containing conditions is predominantly positive in the initial stages of this test. The expected negative influence towards corrosion acceleration was observed after a prolonged treatment, when a stable maturity level of the cement-based matrix was at hand. This also means that the properties of the cementitious material in reinforced cement-based system are of significant importance and largely determine the electrochemical state of the steel reinforcement.

The influence of highly nitrogen-doped mesoporous carbon spheres (NMCSs) (internal pore size of 5.4–16 nm) on the electrochemical response of low carbon steel (St37) in model alkaline solutions of pH 13.9 and 12.8 was studied, using Open Circuit Potential (OCP) monitoring, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). Prior to adding the NMCSs in the relevant solutions, they were characterized in the same model medium by measuring their Zeta-potential, hydrodynamic radius and particle size distribution, using dynamic light scattering (DLS) and transmission electron microscopy (TEM). In alkaline environment of pH 13.9 and 12.8, which simulates the concrete pore water of fresh and mature concrete, the DLS measurements indicated that the hydrodynamic radius of NMCSs particle varied from 296 nm to 183, respectively. According to the Zeta-potential measurements in the same solutions, the NMCSs were slightly positively charged.

The addition of 0.016 wt.% of NMCSs to the model medium induced certain variation in the electrochemical response of the tested steel. In alkaline solutions of pH 12.8, the presence of NMCSs in the passive film/solution interface induced a delay in the formation of a stable passive film. On the other hand, in solutions of pH 13.9, the higher corrosion activity on the steel surface, enhanced by high pH, was limited by adsorption of NMCSs on the film/substrate interface. In addition competing mechanisms of active state, i.e., enhanced oxidation on the one hand, and particles adsorption on anodic sites and oxidation limitation, on the other hand, was relevant in solution of pH 13.9 inducing larger fluctuations in impedance response and stabilization only towards the end of the testing period.

Except steel electrochemical response, the properties of the cement-based bulk matrix were investigated in the presence of the aforementioned additives. The mortar bulk matrix properties were highly affected by NMCSs. The lowest electrical resistivity values were recorded in mortar specimens with mixed-in 0.025 wt.% NMCSs (with respect to dry cement weight). Furthermore, the addition of 0.025 wt.% NMCSs increased the compressive strength when compared to control specimens. The presence of F127 as a dispersing agent for NMCSs was found to be not suitable for reinforced concrete applications, which is in view of the reduced mechanical strength and electrical resistivity of the cement-based bulk matrix. This is in addition to the adverse effect on the formation of electrochemically stable passive layer on steel surface in alkaline medium in its presence.

A number of alternatives have been explored by the cement industry in recent years to reduce CO₂ emissions. One of the alternatives, the subject of this chapter, is an intermediate system between a high volume fly ash concrete and a geopolymer concrete. This concrete includes a hybrid system of 50% OPC–50% fly ash, and an activator. In this study, the long-term durability was studied for laboratory and outdoor cured concretes. It was found that chloride diffusion coefficient was reduced significantly at 90 days and beyond for the activated system compared to control samples (100% OPC and 80% OPC–20% fly ash) of the same water to cementitious content ratio (W/CM). This behavior was exhibited by samples cured under laboratory controlled curing conditions (100% RH and 23 °C). On the other hand, outdoor curing increased concrete permeability for all concretes. Long-term carbonation was also explored and samples under outdoor curing had a significant carbonation depth. Alkali silica reaction problems were mitigated with this activated hybrid system. In order to improve the carbonation resistance of this concrete, a reduction in W/CM seems necessary. Based on these results, activated high volume fly ash systems provide a low CO₂ concrete alternative; however, more studies are needed for establishing specifications and service life prediction models.

Environmental issues related to CO₂ emissions have become a key focus for many different industries, including the cement and concrete industry. An environmentally optimized ‘green’ concrete can provide a much needed alternative to conventional concrete to reduce the carbon footprint of the construction industry. This can be achieved through high Portland cement replacement by fly ash and with the inclusion of activators to enhance the rate of development of strength and other properties. This study evaluates different fly ashes and different activators (Na₂SO₄, lime and quicklime) that are added to enhance the reaction of the fly ash to achieve a comparable performance to that of standard Portland cement in mixes of much lower CO₂ emissions. TGA, XRD and SEM are used to determine the development of hydration products and the consumption of portlandite by the fly ash. It is found that the amorphous content of the fly ash is an important parameter influencing compressive strength evolution. Based on the results, Na₂SO₄ as an activator, and a fly ash with high reactive SiO₂ and Al₂O₃ contents and low Fe₂O₃ are found to provide the best options for producing a high volume fly ash matrix with the potential to show comparable behavior to a Portland cement control mix.