M. T. Bassuoni

Enhancing the Reliability of Evaluating Chloride Ingress in Concrete Using the ASTM C 1202 Rapid Chloride Penetrability Test

The rapid chloride penetrability test (RCPT-ASTM C 1202) is commonly used to evaluate the resistance of concrete to chloride ions ingress owing to its simplicity and rapidity. However, it has been criticized for various shortcomings such as giving favorable results to supplementary cementitious materials (e.g., silica fume), and bias against calcium nitrite corrosion inhibitors (CNI). Based on the ASTM C 1202 induced voltage concept, this study aims at enhancing the reliability of rapidly evaluating concrete resistance to chloride ions ingress. It is proposed that upon test termination, not only the passing charges are recorded, but the depth of chloride front migrating into concrete is also measured. Concrete mixtures herein were prepared incorporating selected materials (silica fume and CNI) that have been known to cause misleading results for the RCPT test. The effects of silica fume and CNI dosages on RCPT results were investigated and correlated to porosity trends evaluated by mercury intrusion porosimetry (MIP). The study reveals that measuring the migrating chloride front in concrete subsequent to the ASTM C 1202 test can eliminate the bias induced by electrolysis conductivity resulting from silica fume and/or CNI. This improves the reliability of assessing concrete resistance to chloride ions penetration using the RCPT procedure.

Nano-Modified Fly Ash Concrete: A Repair Option for Concrete Pavements

Efficient repair of concrete pavements typically requires a rapid-setting material to accelerate opening the road to traffic. While numerous high-early-strength cementitious repair materials are commercially available, many of them are vulnerable to premature deterioration. On the other hand, despite its improved long-term performance, concrete incorporating fly ash is rarely used as a repair material due to the delay in setting time, strength gain, and microstructural development at early ages. Nevertheless, these performance limitations can be mitigated by incorporation of nanoparticles (for example, nanosilica) in fly ash concrete. In this study, an effort was made to develop nano-modified fly ash concrete as a repair material for concrete pavements. The performance of the newly developed mixtures was compared to that of two commercial cementitious products. The results indicate that the nano-modified fly ash concrete has balanced performance in terms of hardening time, strength development, bonding with substrate concrete, and resistance to infiltration of fluids and salt-frost scaling.

A Test Protocol for Evaluating Absorption of Joints in Concrete Pavements

Premature deterioration at joint spresents a critical durability issue of concrete pavements associated with considerable repair costs. Durability of concrete exposed to aggressive environments depends mainly on the penetrability of its pore structure. Because absorption has been used as an important indicator for quantifying the durability of concrete, the aim of this study was to develop a customized test protocol for determining the absorption capacity of joints in concrete pavements. The study involved three phases with thorough statistical analyses of results. In Phase I, different absorption procedures were applied to laboratory specimens prepared with water-to-binder ratios (w/b) of 0.3, 0.5, and 0.6, representing variable qualities of concrete. The most efficient procedure was identified from Phase I and further verified in Phase II on concrete specimens prepared with a close range of w/b (0.35 and 0.40). In Phase III, the performance of the absorption protocol selected from the previous phases was assessed on cores extracted from recently constructed pavement sections in Winnipeg, Manitoba, Canada. To further complement and verify the trends obtained from the absorption protocol, mercury intrusion porosimetry tests were conducted on the field cores to capture the characteristics of the pore structure. The results indicated that the proposed absorption protocol was efficient, robust, and reliable in reflecting the physical features of the microstructure of field pavement sections, including joint locations.

Effect of Freezing-Thawing Cycles on the Resistance of Self-Consolidating Concrete to Sulfate Attack

SCC has been used in various applications, such as pavements, marine structures, shallow foundations, etc. that can be concomitantly exposed to sulfaterich environments and frost action. To evaluate the resistance of SCC mixtures to sulfate attack considering the effect of frost action, the present study introduces accelerated testing procedure combining the exposure to aggressive magnesium and mixed magnesium-sodium sulfate solutions with freezing-thawing cycles. After five months of exposure, the results showed that SCC mixtures incorporating high dosage of fillers (limestone and ultrafine kaolin) had inferior physicomechanical properties relative to the other SCC mixtures. Thermal and microscopy analyses indicated that mutual effects of sulfate attack and freezing-thawing cycles caused severe distress in such cementitious systems.

Performance of concrete with blended binders in ammonium-sulphate solution

Concrete elements in agricultural, wastewater treatment, mining, and industrial applications can be vulnerable to chemical attack by ammonium-based solutions. In particular, ammonium sulfate (commonly used as a fertilizer) is extremely deleterious to concrete. This is due to its dual acid–sulfate action, which may disintegrate the hydrated cement paste to various levels based on the prevailing exposure conditions and key mixture design parameters of concrete. The aim of this study was to investigate the response, in terms of physico-mechanical and microstructural features, of concrete comprising different types of cement (general use [GU] or Portland limestone cement [PLC]) with various combinations of supplementary cementitious materials (SCMs: fly ash, silica fume, and nanosilica) to a severe ammonium sulfate exposure. The study comprised 12 months of immersing test specimens in 5% ammonium sulfate solutions with a pH level of 6.0–8.0. The results revealed that the type of binder along with the dosage and nature of SCMs dictated different modes and levels of deterioration, and consequently the physico-mechanical trends of concrete were characterized by softening with (single binders) or without (blended binders) significant expansion. PLC may slightly improve the resistance of concrete to ammonium sulfate attack, whereas among the blended binders tested, binary binders comprising 5% silica fume, 5% nanosilica, or 30% fly ash improved the resistance of concrete to this type of chemical attack.

Investigation into enhancing and evaluating curing efficiency of joints in concrete pavements

Signs of premature deterioration of concrete pavements are often indicated by shadowing, resulting from a network of micro-cracks in the vicinity of joints, which causes significant loss of material over time. In Canada, the construction sequence of concrete pavements typically involves continuous casting, applying curing compound and subsequently saw-cutting, which may compromise the durability of joints, due to insufficient curing and uncontrolled evaporation. The aim of this study was to assess the effect of overfilling joints with curing compound immediately after saw-cutting (early- and late-cuts) on improving the quality of concrete microstructure at joint regions in laboratory slabs and trial field sections. The study involved an absorption test customised to the joint geometry of pavements, mercury intrusion porosimetry and microscopy tests that were conducted on cores. The results indicated that overfilling the joints with curing compound immediately after late saw-cutting significantly improved the microstructure and durability of joint zones.

Resistivity, Penetrability and Porosity of Concrete: A Tripartite Relationship

Demands for using electrical resistivity techniques (surface and bulk resistivity) as an alternative to the rapid chloride penetrability test (RCPT) have been growing, for example by a number of transportation agencies in North America, to give an indication of the relative penetrability of concrete. While resistivity measurements may reflect the quality of pore structure in the cementitious matrix, their accuracy might be affected by a multitude of parameters including the concentration of ionic species in the pore solution, particularly when supplementary cementitious materials (SCMs) are incorporated in the binder. Hence, a systematic investigation on the resistivity of concrete and its corresponding physical penetrability is warranted. The main objective of this study was to establish a tripartite relationship (nomogram) to correlate surface resistivity with penetrability (migration coefficient) and porosity of concrete using a wide range of concrete mixtures, taking into account the effect of key mixture design parameters (water-to-binder ratio, air-entrainment, SCMs, and type of cement). Relationships between surface and bulk resistivity as well as migration coefficient and porosity of concrete were also established. In addition, a penetrability classification of concrete based on the corresponding ranges of surface resistivity, migration coefficient, and porosity has been proposed. The efficacy/practicality of the proposed classification was evaluated by comparing the predicted migration coefficient and porosity, based on surface resistivity of concrete, to the measured ones for field cores extracted from concrete pavement in Winnipeg, Canada. For newly constructed and aging pavement, the nomogram and penetrability classification provided reasonable assessment for the condition of field concrete.

Physical salt attack on concrete incorporating nano-silica

The damage induced by crystallization of salts (physical salt attack, PSA) has been often misidentified as chemical attack. Under certain environmental conditions, PSA may cause notable surface damage of concrete partially embedded in salt-rich soils. The process is similar to salt weathering of natural rocks. The current study investigated the effect of refining the microstructure of concrete by nano-silica additions on its resistance to PSA in an accelerated procedure. Specimens were partially immersed in a high-concentration sodium sulphate solution and simultaneously exposed to cyclic temperature and relative humidity. Specimens’ deterioration was monitored for more than 100 cycles of exposure. Also, an effort was made to analyse the relationships between specimen damage, characteristics of pore structure and absorption capacity. Finally, the immersed and drying portions of specimens were tested by X-ray diffraction and Rietveld analysis and scanning electron microscopy to identify the root cause of damage.

Properties of Nanomodified Fiber-Reinforced Cementitious Composites

AbstractThis paper presents the development of nanomodified fiber-reinforced cementitious composite (FRCC) mixtures. A total of seven mixtures were prepared using general use cement and a constant ...