Sodium Sulfate Resistance of Fast-Track Portland-Cement Concretes

ACI Materials Journal, Nov/Dec 2007 by Ghafoori, Nader, Diawara, Hamidou, Beasley, Shane

The research study presented herein is intended to examine the sodium sulfate resistance of early opening-to-traffic portland-cement concrete. The trial matrixes of two categories of opening-to-traffic times, four cement factors, and three cement types are exposed to very severe external sodium sulfate attack based on ASTM C1012. The mixtures are examined for fresh properties (slump, bleeding, setting times, and adiabatic temperature), bulk characteristics (unit weight and compressive strength), and resistance to sodium sulfate (expansion and loss in mass and strength). The influence of parameters such as curing age, cement factor, and cement type on sodium sulfate resistance of the selected concretes is discussed. In addition, the expansion of the test specimens is compared with proposed acceptance limits found in the literature based on ASTM C452 or C1012.

Laboratory test results conclude that, irrespective of cement type, the rate of expansion reduced with increases in cement factor throughout the immersion period. Neither mass of concrete residues nor notable strength loss was found in any opening time fast-track concretes used in the investigation. There was a statistically significant correlation between the sulfate expansion (the dependent variable) and immersion age, cement factor, cement type, and compressive strength (the independent variables).

Keywords: curing; ettringite; gypsum; sodium sulfate resistance; strength.

INTRODUCTION

As roadways are becoming increasingly congested and delays for construction and maintenance are increasingly routine, engineers are faced with the task of developing a way to alleviate some of the problems associated with repairs. A step in the right direction was taken with the development of fast-track portland-cement concrete (FTC).1

Pavements constructed using FTC principles provide highquality surfaces with quick opening to the traveling public. With public agencies offering incentives for timely completion of projects, this new method has become increasingly popular with contractors and state departments of transportation. Using fast-track technology, contractors can complete projects more quickly without increasing crew sizes or altering construction schedules.1 Because it requires less curing time, the fast-track technique allows engineers to consider concrete for projects once thought unfeasible, allowing roadway surfaces to meet opening strength requirements in less than 12 hours.2,3,4

The benefit of fast-track concrete is often associated with an increased pavement cost of roughly $1.30 to $2.60 per m^sup 3^ to account for the ASTM Type III portland cement and curing blankets. The cost, however, is somewhat offset by reduced curing time and traffic rerouting.3,5

Typical FTC components consist of ASTM C150^sup 6^ Type I or III portland cements in quantities of 356 to 564 kg/m^sup 3^ (600 to 950 lb/yd^sup 3^).1 The chemical composition of cements used in FTC is of particular interest. The two chemical components of cement related to early strength gain are tricalcium silicate (C^sub 3^S) and tricalcium aluminate (C^sub 3^A). Tricalcium silicate is directly related to early strength gain, and tricalcium aluminate accelerates the cement setting by its strongly exothermic fast hydration. ASTM Type III cements, because they are ground finer, have a much larger surface area and thus hydrate faster than Type I cement.1

The water used in FTC systems is integral in the development of early strength. Hot water facilitates early hydration7,8 and thus, in fast-track applications, the water mixing temperature should be between 27 and 49 �C (81 and 120 �F). To avoid flash-set, the hot water and aggregates should be combined before adding the cement.1 Air entraining and waterreducing admixtures corresponding to ASTM C260 and ASTM C494, respectively, are acceptable as deemed necessary. The curing process can be further shortened by the use of an accelerating admixture, which aids the strength development by increasing the reaction rate of C^sub 3^A and C^sub 3^S. Calcium chloride (CaCl^sub 2^) is the most commonly used accelerating admixture, most often proportioned by 2% of weight of cement.9 Curing blankets are also commonly used in fasttrack concrete applications. It is recommended that a thermal resistance (R) rating of at least 2.45 � 10^sup -5^ hm^sup 2^�C/Joule (0.5 hft^sup 2^�F/Btu) is used and that the blankets remain in place until the required opening-time strength is reached.7,8

RESEARCH SIGNIFICANCE

Much research has been conducted to explore the mechanical properties of fast-track concrete.2,3,7 However, little is known about its long-term durability, particularly when it is exposed to external sodium sulfate attack. The research presented herein is expected to be beneficial to highway departments and transportation officials by providing information related to sodium sulfate resistance of various matrix constituents and proportions used in fast-track concretes.

BACKGROUND ON SULFATE ATTACK