David Stark

The Moisture Condition of Field Concrete Exhibiting Alkali-Silica Reactivity

The moisture condition of field concretes exhibiting evidence of alkali-silica reactivity was investigated utilizing relative humidity (RH) measurements. Prior determinations were made on laboratory mortar specimens to determine the threshold level required to sustain expansive reactivity. Results indicated that RH values greater than 80 percent, referenced to 21 degrees to 24 degrees Centigrade, are required to support expansive alkali-silica reactivity. Field measurements revealed that most of the concrete in highways and dams in desert areas are sufficiently damp to sustain expansive ASR.

Alkali-Silica Reactivity: Some Reconsiderations

Numerous instances have been recorded where the use of low alkali cement with certain volcanic aggregates has failed to prevent deleterious alkali-silica reactivity. Present testing using the ASTM Test for Potential Alkali Reactivity of Cement-Aggregate Combinations (Mortar-Bar Method) (C 227) may fail to identify this reactivity because testing is normally done with only high alkali cements. It is recommended that concurrent testing be done, using a range of both high and low alkali cements, to determine that maximum safe cement alkali level for each aggregate in question. Although the traditional limit of 0.60% alkali has prevented alkali aggregate reaction with many reactive aggregates, there are certain volcanic aggregates that require even lower alkali or the use of suitable pozzolanic materials to avoid alkali aggregate reactivity or both.

CHARACTERISTICS AND UTILIZATION OF COARSE AGGREGATES ASSOCIATED WITH D-CRACKING

Primary considerations in the selection of materials for coarse aggregate are those pertaining to freeze-thaw durability and the development of D-cracking in highway and airfield pavements. Two aspects of the problem are of particular importance: moisture movements and critical saturation of the aggregate, and the response of the aggregate to cyclic freezing and thawing in concrete. Laboratory studies of coarse aggregates have indicated that nondurable materials are generally of sedimentary origin and may reach critical saturation when the concrete is in direct contact with either free water or capillary-held water. Absorption-adsorption and mercury intrusion studies have revealed differences in the pore structure of durable and nondurable materials, while laboratory tests of aggregates in concrete differentiate performance during freezing and thawing in line with field service records. The most feasible method of utilizing potentially nondurable coarse aggregates is to reduce maximum particle sizes. The needed reduction can be determined from laboratory freeze-thaw tests and is found to vary with source. Gravel sources have the option of crushing oversize material for using naturally finer material to improve durability, while crushed stone sources may use alternatively selective quarrying to produce higher quality aggregates. Proper evaluation of aggregate materials is contingent on establishing laboratory procedures directed expressly to the problem of D-cracking.

ALKALI-SILICA REACTIVITY: EFFECT OF ALKALI IN AGGREGATE ON EXPANSION

Tests were performed on samples of alkali-bearing aggregates to determine amounts of alkali that could be removed by leaching in Ca(OH)2 solution and water at 38 deg. C (100 deg. F) and 80 deg. C (176 deg. F). Additional tests were made on one reactive aggregate which was first subjected to leaching and then used to make mortar bars for testing in accordance with ASTM C 227. Leach periods of 7, 28, 90, and 180 days were used. Results show that alkali present within concrete aggregate particles may participate in alkali-silica reactivity. Alkali levels much greater than these present in high-alkali cements can be leached from non-reactive as well as reactive aggregate materials. It is suggested that ion-exchange processes possibly augmented by partial dissolution of the aggregate, facilitate leaching of alkali. Removal of alkali from aggregates, prior to their use in mortar bars, was shown to reduce expansions. Expansions were progressively reduced as greater amounts of alkali were leached from aggregates prior to their use with low- as well as high-alkali cements. Applicability of these findings to concrete in service is discussed.