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The concrete industry has long been adding discrete fibers to cementitious materials to compensate for their (relatively) low tensile strengths and control possible cracks. Extensive past studies have identified effective strategies to mix and utilize the discrete fibers, but as the fiber material properties advance, so do the properties of the cementitious composites made with them. Thus, it is critical to have a state-of-the-art understanding of not only the effects of individual fiber types on various properties of concrete, but also how those properties are influenced by changing the fiber type. For this purpose, the current study provides a detailed review of the relevant literature pertaining to different fiber types considered for fiber-reinforced concrete (FRC) applications with a focus on their capabilities, limitations, common uses, and most recent advances. To achieve this goal, the main fiber properties that are influential on the characteristics of cementitious composites in the fresh and hardened states are first investigated. The study is then extended to the stability of the identified fibers in alkaline environments and how they bond with cementitious matrices. The effects of fiber type on the workability, pre- and post-peak mechanical properties, shrinkage, and extreme temperature resistance of the FRC are explored as well. In offering holistic comparisons, the outcome of this study provides a comprehensive guide to properly choose and utilize the benefits of fibers in concrete, facilitating an informed design of various FRC products.

The current study aims to assess the transport and durability properties of alkali-activated concretes made with hybrid aluminosilicate precursors having different proportions of natural pozzolan as a low-calcium precursor and ground-granulated blastfurnace slag as a high-calcium precursor, which are activated with different concentrations and combinations of sodium hydroxide and sodium silicate. The studied parameters included precursor combination (natural pozzolan'slag combinations of 30/70, 50/50, and 70/30), sodium hydroxide concentration (1, 1.75, and 2.5 M), and activator combination (sodium hydroxide/sodium silicate combinations of 70/30, 75/25, and 80/20). The resulting concrete mixtures were tested for slump flow, setting time, compressive strength, absorption, rapid chloride penetration, rapid chloride migration, resistance to sulfuric acid attack, chloride-induced corrosion, and frost resistance. Mercury intrusion porosimetry and X-ray diffraction were used to justify the observed behaviors. The performance of alkali-activated natural pozzolan/slag concretes was also compared with that of a reference concrete made with solely portland cement binder. In view of overall performance, an equal proportion of natural pozzolan and slag (50/50) and a 30/70 combination of sodium silicate and sodium hydroxide proved to be the optimum precursor and activator combinations. The optimum sodium hydroxide concentration was dependent on the precursor and activator combinations as well as the expected fresh, strength, transport and durability performance. In terms of the measured transport properties (that is, absorption, chloride penetration depth, and passing charges) and resistance to acid attack and chloride-induced corrosion, all the studied alkali-activated concretes performed considerably superior to the reference portland cement concrete. In the case of frost resistance, only alkali-activated concretes with 50 and 70% slag performed superior to the reference portland cement concrete. Keywords: acid attack; activator combination; alkali activation; corrosion; frost resistance; natural pozzolan; precursor combination; slag; sodium hydroxide concentration; transport properties.

This paper examines the freezing and thawing resistance of high early-strength concrete (HESC) developed for rapid repair of pavements and bridge decks. The cement types chosen for this study included ASTM Type III, ASTM Type V, and Calcium Sulfoaluminate (CSA). A cement content of 386 kg/m 3 was maintained for all studied concretes. Specimens were tested after 24 hours and 28 days of curing in order to evaluate compressive and flexural strengths. In addition, the opening time was determined based on the required time to achieve the minimum compressive strength of 20.7 MPa. The freezing and thawing (F–T) resistance of the test samples were evaluated in accordance with the F–T duration of 96 hours per cycle for a total of 25 cycles. Test results revealed that at the opening time and after 24 hours curing, CSA cement concrete displayed the highest compressive and flexural strengths, but lowest resistance to freezing and thawing with de-icing salt. The 28-day cured Type V cement concrete produced the highest strength and de-icing salt resistance, while CSA cement concrete produced the contrary.

This study examines the suitability of ASTM Type V cement concrete for rapid repair applications. To this end, experimental results on transport and durability properties of high early-strength concretes using ASTM Type V cement were compared with those of a more traditional cement used for rapid repair, i.e. Type III cement. A cement content of 445 kg/m 3 (750 lb/yd 3 ) was maintained for all studied concretes. The experimental program included compressive strength, absorption, rapid chloride migration, corrosion resistance, and mass loss due to freezing and thawing regimes. The results of this study revealed that use of Type III and V cements were both effective for concrete rapid repair applications. The opening time to reach the minimum compressive strength of 21 MPa (3000 psi) was found dissimilar. Type III cement concrete showed better strength properties at early ages due to its high fineness. However, as curing age was extended to 24 hours and 28 days, Type V cement concrete produced higher strength results. Moreover, Type III cement concretes failed to display better performance in transport properties, corrosion, and frost resistance when compared to that of the studied Type V cement concretes.

This paper examines the abrasion resistance of high early-strength concrete developed for rapid repair of highways and bridge decks. The cement types chosen for this study included ASTM Type III, ASTM Type V, and Calcium Sulfoaluminate (CSA) cements. A cement content of 386 kg/m 3 (650 lb/yd 3 ) was maintained for all studied concretes. Test samples were tested after 24 hours and 28 days of curing in order to evaluate compressive strength and depth of wear. Test results revealed that the opening time to attain minimum required compressive strength for CSA cement concrete was one hour, whereas the values for Type V and Type III cement concretes were 8.5 and 6 hours, respectively. After 24 hours curing, CSA cement concrete displayed the highest strength, but lowest resistance to wear. The 28-day cured CSA cement concrete produced the highest strength and resistance to abrasion, while Type III cement concrete showed a similar strength, but lower resistance to wear, when compared to those of the Type V cement concrete.

The study presented herein evaluates effects of alkaline activator (sodium hydroxide) concentration, solution (sodium hydroxide solution)-to-binder ratio (S/B), and curing condition on properties of alkali-activated natural Pozzolan mortars (geopolymers). To this end, several mixtures were made having natural Pozzolan as their binder with different concentrations of alkaline activator solution including 2.5, 5, 7.5, 10, and 12.5 molar (M) at various S/B of 0.50, 0.54, and 0.58. The produced mortars were cured at 80 °C under three different conditions of exposed (dry), sealed (wrapped), and moist until testing at ages of 1, 3, and 7 days. Multiple tests were conducted on the alkali-activated natural Pozzolan mortars including flow spread, compressive strength, flexural strength, PH measurement, absorption, and rapid chloride migration. Test results showed the sealed curing condition to be most conducive to strength gain, whereas the exposed curing condition caused dehydration and/or carbonation within the samples and the moist curing condition did not allow for full removal of excess water resulting in reduced bond formations. The moist oven-cured mortars produced higher strength than the exposed cured mortars when alkaline activator with lower molarities was used. The opposite trend was observed for the higher molarities mortars. The compressive and flexural strengths, absorption, and depth of penetrated chloride improved when NaOH concentration increased and S/B decreased.

The objective of this study was to evaluate the effectiveness of colloidal nanosilica (nS) as a nanomaterial and pozzolanic admixture to mitigate the deteriorative effects of sodium sulfate-based physical salt attack (PSA) on portland cement mortars. Mortar mixtures of an ASTM CI 50 Type II (<8% C3A) or a Type V (<5% [C.sub.3]A) portland cement were prepared with 0, 3, and 6% cement replacements with either nS or microsilica (mS). Test samples were subjected to 3 years of exposure under a constant or cyclic PSA-conducive environment. The PSA results were supported with additional water absorption, rapid sulfate ion permeability (RSPT), and porosimetry testing. The Type V cement mortars containing nS exhibited the most observable scaling and flaking under both conditions of PSA exposure. The addition and increase in cement replacement with nS had a clear detrimental effect to PSA resistance for both cement types and both types of PSA exposure. Results indicated nS reduces permeability and diffusion in mixtures of either cement type which, for PSA, the denser and more refined pore network proved conducive to higher damaging tensile stresses and distress. The larger the measured volume of permeable pore space through absorption, the less susceptible the mortars were to PSA, which is counterproductive to conventional good practice of designing high-durability concrete via reducing permeability and sorption, and increasing a mixture s watertightness. Keywords: microsilica; nanosilica; physical salt attack; sodium sulfate; sulfate attack.

Presented is a side-by-side comparison study intended to identify the effects of nanosilica (nS) on chemical sulfate attack resistance of port-land cement (PC) mortars and its effectiveness in comparison to similar replacement levels of the more widely implemented microsilica (mS). Several mortar mixtures were prepared with a 4.1 and 7.2% tricalcium aluminate (CA) PC by progressive cement replacement with nS or mS. The mortars tested were measured for expansion, compressive strength, and mass loss. Results indicated that nS replacement benefited the studied mortars. However, in the dry powderform and method of mixing used in this study, poor dispersion and agglomeration of the nS was suspected to hinder mortar permeability in comparison to mS and lowCA cement mortars. Replacement with nS in aqueous dispersion, however, proved to be significantly more effective than equivalent replacement of dry powder nS and mS. Keywords: durability; microsilica; mineral admixture; nanosilica; sulfate attack.

This study examined the effects of limestone powder as a partial replacement for cementitious materials on transport properties of self-consolidating concrete (SCC). Several SCCs were prepared with a uniform powder content (cement + fly ash + limestone) and water-cementitious materials ratio (w/cm) of 475 kg/[m.sup.3] (800 lb/[yd.sup.3]) and 0.45, respectively. Class F fly ash substituted 20% by weight of cement and up to 30% of total cementitious materials was replaced with limestone powder. A slump flow of 635 [+ or -] 25 mm (25 [+ or -] 1 in.), visual stability index of 1, and passing ability of 25 [+ or -] 12.5 mm (1 [+ or -] 0.5 in.) were used for all studied mixtures. The devised experimental program contained compressive strength, absorption, sorptivity, rapid chloride penetration (RCPT), rapid chloride migration (RCMT), water penetration depth, and chloride diffusion. Inclusion of limestone powder improved absorption, volume of permeable voids, capillary absorption, water penetration depth, and RCPT. Compressive strength and RCMT improved marginally, whereas a slight negative effect was observed for chloride diffusion. Keywords: compressive strength; limestone powder; self-consolidating concrete (SCC); transport properties.

This study investigated the effect of four volume dosages (that is, 0, 0.1, 0.3, and 0.5%) of high-elastic-modulus carbon microfiber, shrinkage-reducing admixture (SRA), and accelerating admixture (ACC) on the 24-hour compressive strength and restrained shrinkage of carbon microfiber-reinforced concrete. Additional 7-and 28-day compressive strength tests, as well as 1-, 7-, and 28-day splitting tensile strength tests, were carried out on the mixtures without and with 0.3% carbon microfiber. Results showed that, overall, increasing the carbon microfiber dosage increased the compressive strength, particularly at early ages. Splitting tensile strength results were used along with the restrained shrinkage ring results to quantify the restrained shrinkage cracking potential of the mixtures. It was found that carbon microfiber and SRA can both significantly reduce the drying shrinkage cracking potential of concrete. The combination of SRA and ACC in concrete provided compatible effects, characterized by increased early-age compressive strength, as well as reduced shrinkage and cracking potential. Keywords: accelerating admixture; carbon microfiber; early-age strength; fiber-reinforced concrete; restrained shrinkage; shrinkage-reducing admixture.