Prasada Rao Rangaraju

Investigation into Potential of Alkali-Acetate-Based Deicers to Cause Alkali-Silica Reaction in Concrete

Recent investigations by the FAA into the condition of airfield pavements have indicated increased occurrence of premature deterioration in concrete pavements, particularly caused by alkali-silica reaction (ASR). On the basis of the evidence, it was suspected that certain deicing chemicals used on the pavements may have been responsible for the distress. This paper presents the results from a study conducted to investigate the effect of potassium acetate- and sodium acetate-based deicers on causing ASR in test specimens. A modified ASTM C 1260 test procedure was used in this study, in which mortar bars prepared with reactive aggregates were soaked in deicer solutions, instead of 1 N sodium hydroxide solution as in the standard ASTM C 1260 test. Tests were conducted on four reactive aggregate sources and one nonreactive aggregate, using a low-alkali and a high-alkali cement. In addition, standard ASTM C 1260 tests were conducted on all aggregates considered in this study. The effect of concentration and te...

EVALUATION OF THE POTENTIAL OF DENSIFIED SILICA FUME TO CAUSE ALKALI-SILICA REACTION IN CEMENTITIOUS MATRICES USING A MODIFIED ASTM C 1260 TEST PROCEDURE

Numerous research and field studies in the past have shown the ability of silica fume to improve the mechanical and durability properties of concrete. Despite these beneficial effects, some recent studies have raised concerns about the dispersibility of a few commercial dry densified silica fumes and the potential for development of alkali-silica reaction associated with the presence of undispersed agglomerates of silica fume in cement matrices. In light of these developments, the present investigation focuses on determining the potential of undispersed silica fume agglomerates to participate in the alkali-silica reaction and the influence of such factors as size and morphology of the agglomerates on the extent of this reaction. An experimental program consisting of a modified ASTM C 1260 test procedure was used to identify and study distress associated with the presence of undispersed silica fume in cementitious pastes and mortars. Results of this investigation indicate that undispersed silica fume agglomerates in cementitious pastes actively participate in alkali-silica reaction. The nature and the magnitude of the observed distress were found to be distinctly dependent on factors such as the size of the agglomerates, their morphology, and the water-binder ratio of the cementitious matrix. In addition, this paper presents suggested modifications to the ASTM C 1260 procedure to test for the reactivity of silica fume agglomerates.

Modified ASTM C 1293 Test Method to Investigate Potential of Potassium Acetate Deicer Solution to Cause Alkali-Silica Reaction:

Following the recent investigations by FAA into premature deterioration of some airfield concrete pavements that were exposed to deicing chemicals, a comprehensive research study was undertaken to examine the role of deicing solutions in causing alkali-silica reaction (ASR) in mortar and concrete test specimens. It was conducted to evaluate the influence of a potassium acetate deicer and anti-icer in causing ASR distress in concrete specimens. A modified ASTM C 1293 test method was employed in this investigation, in which concrete prisms prepared with aggregates of known reactivity were exposed to 50% solution of potassium acetate (KAc) or 1 normal solution of sodium hydroxide (1N NaOH) during the course of testing. Expansion of the concrete test prisms was monitored periodically, along with changes in their dynamic modulus of elasticity. In addition, visual and scanning electron microscopic examinations were conducted on polished specimens at the conclusion of tests. The pH of the deicer soak solution was monitored to detect any changes caused by its interaction with the specimens. Findings from the study indicate that potassium acetate deicer solution is capable of causing significant ASR distress in concrete specimens containing reactive aggregates. A secondary reaction product, composed primarily of a potassium sulfate phase, was observed in prisms containing both reactive and non-reactive aggregates. This product was predominantly found as an infilling in cracks and voids. Additional research needs are identified to help to decipher the precise mechanisms involved in this attack.

Mitigating alkali-silica reaction in concrete

This study investigated the effectiveness of using recycled waste glass in portland cement concrete, both as a finely ground powder and as a crushed granular material. For the potential of glass to undergo alkali–silica reactivity (ASR) distress to be assessed, mortar bar and miniature concrete prism tests were conducted with glass as both a powder and a crushed material. Parallel studies were conducted with a crushed natural aggregate. Simultaneously, strength activity index and thermogravimetric analysis tests were conducted on cementitious mixtures to evaluate pozzolanic reactivity of glass powder when used as cement replacement material. Results showed that when glass powder (70 μm average size) was used as cement replacement material, its pozzolanic behavior (measured by thermogravimetric analysis and strength activity index) was minimal. When glass powder was used as aggregate replacement material, the combination of glass powder and ASR-prone coarse aggregates showed significantly lower expansion than did control specimens in accelerated mortar bar and miniature concrete prism tests. This result indicates the beneficial effect of using glass powder in mitigating expansion induced by ASR. The mechanism by which the fine glass powder appeared to alleviate ASR in coarse aggregates, and therefore any significant distress in test specimens, was by rapidly undergoing its own ASR, which depleted alkalinity in the vicinity of reactive coarse aggregates. ASR associated with the fine glass particles was localized, and the reaction product did not appear to generate sufficient expansion to cause global distress. Additional field studies are required to validate study findings before waste glass can be used in concrete on a large scale.

Material Characterization Studies on Low- and High-Carbon Rice Husk Ash and Their Performance in Portland Cement Mixtures

In this study, low- and high-carbon rice husk ash (RHA) in their as-received and ground forms were characterized by means of different methods in order to evaluate their performance in Portland cement mixtures. RHA-cement pastes and mortars, at three different RHA replacement levels of 0 %, 10 %, and 20 %, were prepared at a constant water/cement ratio of 0.485. Results from this investigation indicate that the material characteristics of low- and high-carbon RHA were significantly different in most of the tests conducted. In the as-received condition, the high-carbon RHA had a greater bulk density than low-carbon RHA, but their bulk densities were comparable after grinding. The low-carbon RHA was more effective in its pozzolanic reaction than high-carbon RHA in both ground and as-received conditions. The microstructures of both low-carbon and high-carbon RHA cement pastes were denser than those of control pastes. At a given dosage of RHA and superplasticizer, the ground and the as-received low-carbon RHA mixtures performed significantly better than the high-carbon RHA mixtures. Similarly, mixtures with low-carbon RHA showed significantly higher strength activity indices than those with high-carbon RHA at both dosage levels investigated. Grinding RHA was found to be beneficial in all the tests conducted, as the ground RHA mixtures depleted more calcium hydroxide, registered higher flow values, and possessed greater strength than unground RHA mixtures at all replacement levels. Thus, the grinding process significantly helps in utilizing both high- and low-carbon RHA in concrete.

Effect of Blended Fly Ashes in Mitigating Alkali–Silica Reaction

The role of chemical composition of fly ash in mitigating alkali–silica reaction (ASR) was examined, and findings were used to evaluate blends of high-lime and low-lime fly ashes in their ability to mitigate ASR. In addition, the influence of particle size (fineness) of fly ashes on ASR mitigation was evaluated, so that the relative significance of fineness and chemical composition of fly ash in mitigating ASR could be established. Findings from these studies confirm results from studies on the influence of lime content of ash on ASR mitigation. Blended fly ashes containing content of no more than 16.5% equivalent calcium oxide and no less than 66% equivalent silicon dioxide were found to be effective in mitigating ASR. The performance of blended fly ashes was comparable with that of virgin fly ashes of equivalent chemical composition. Finer fly ashes showed better ASR mitigation in the case of low- and intermediate-lime fly ashes. However, in the case of high-lime fly ashes, the effect of fineness could not be clearly resolved. Findings from this study indicate that both the physical and the chemical properties of fly ash are important in selecting ashes for developing blends that are effective in ASR mitigation.

Role of Potassium Acetate Deicer in Accelerating Alkali–Silica Reaction in Concrete Pavements Relationship Between Laboratory and Field Studies

About 15 years after the introduction of alkali–acetate and alkali–formate deicers, premature deterioration was observed on some airfield pavements that had been exposed to the deicers. A characteristic map cracking pattern was observed on pavement surfaces that had experienced repeated applications of these deicers, and the suspected cause of this cracking pattern was accelerated alkali–silica reaction (ASR). Laboratory-based research indicated that alkali–silica reactive aggregates may undergo active deterioration when intimately exposed to such deicers under conditions promoting accelerated reaction. Investigations were conducted on cores collected from an airport whose deicing operations involved repeated applications of potassium acetate deicer. Detailed microscopic investigation indicated that uniform distress existed throughout the depth of the pavement, although in one, the distress resulted from alkali-carbonate reaction rather than from ASR. However, investigations on the depth of penetration of deicer into these pavement cores showed only limited incursion. A companion laboratory study estimated the extent of deicer penetration under different laboratory exposure conditions. Even in a relatively aggressive wetting and drying exposure regime, ingress of the deicer was limited. Thus, it was concluded that although the potassium acetate deicer can induce severe ASR under aggressive laboratory conditions, penetration into field airport pavements may be so limited in some cases that the potassium acetate deicer does not seem to aggravate the ASR distress should one already exist.

INVESTIGATING PREMATURE DETERIORATION OF A CONCRETE HIGHWAY

Premature deterioration of concrete on Trunk Highway 169 near Hibbing, Minnesota, triggered an investigation to determine the potential cause of the distress. The deterioration manifested in the form of severe cracking and spalling in the top 75 mm (3 in.) of the pavement near the transverse joints. Investigation of cores obtained from locations near joints and midpanel indicated that the air void system in the top portion of the pavement was compromised initially (during construction) by nonuniform vibration across the depth of the concrete pavement and later by in-filling of the marginal air void system with secondary products. Absence of any evidence indicating the structural failure of the pavement or of the loadtransfer devices suggested that inferior concrete and poor construction practices were primarily responsible for the pavement failure.

Role of Ground Glass Fiber as a Pozzolan in Portland Cement Concrete

Millions of tons of fiberglass are produced annually for a variety of applications. Because of stringent quality requirements and operational characteristics of the manufacturing plants, a significant quantity of fiberglass that does not meet required specifications of the industry ends up as waste in landfills. This study investigated the use of ground glass fiber (GGF) that had been discarded by plants because it did not meet prescribed standards, as a supplementary cementitious material (SCM) for portland cement. Three replacement levels (10%, 20%, and 30% by mass) for portland cement in paste, mortar, and concrete mixtures were studied. Mechanical and durability properties of the mixtures were compared with two control mixtures: a mixture made up of 100% portland cement and a mixture with 25% Class F fly ash as a cement replacement material. It was observed in these studies that even though replacement of portland cement with GGF did not lead to any significant changes in the mechanical behavior of hardened concrete, there were significant improvements in durability properties at replacement levels up to as high as 20%. The use of GGF was found to improve significantly the resistance of mortar mixtures to alkali–silica reaction and sulfate attack. In addition, the use of GGF as an SCM significantly reduced the chloride ion permeability of concrete. Results of this study show that using GGF as an SCM can result in a better durability performance compared with a mixture with a similar level of Class F fly ash.

Effect of Sand Content on Properties of Self-Consolidating, High-Performance Cementitious Mortar

The workability and compressive strength of a high-performance cementitious mortar (HPCM) produced by using a natural siliceous sand were studied as a function of sand content [expressed as sand-to-cementitious materials ratio (s/cm)], silica fume (SF) content, and high-range water reducing admixture (HRWRA) dosage. The purpose of this study was to maximize the proportion of sand content without negatively affecting workability and mechanical and durability properties when these characteristics were achieved at a low cost. Test results indicated that the workability of HPCM became less sensitive to sand content when the SF content increased. Statistical analysis showed that the compressive strength of self-consolidating HPCM was not significantly affected by sand content up to a certain maximum level, which was dependent on the HRWRA and SF dosage. On the basis of combined consideration of both workability and compressive strength, the maximum s/cm ratio to produce a self-consolidating HPCM with SF content at 0%, 10%, and 20% was 1.6, 1.6, and 2.0, respectively. Also, increasing the sand content was helpful in improving the durability of HPCM, because chloride ion permeability and drying shrinkage decreased.