Properties of Concrete in Contact with Pressured Hydrocarbons and Seawater
Authors: Maxson Orwin G. , Achenbach Gary D. 
Published: October 2008

Hydrocarbons do not react chemically with high strength concrete to alter its physical properties. Concrete test samples showed minor changes in properties due to the effects of pressurized hydrocarbons on the internal moisture in the concrete. Pressurized seawater had no adverse effect on the properties of the concrete test specimens. Properties studied were compressive strength, creep rate, fatigue life, and permeability as they might affect the design of a gravity-founded oil storage/ production structure in the North Sea.

INTRODUCTION

Prestressed concrete gravity-founded structures for oil production storage applications are being designed, built, and installed in the North Sea. The environmental effects of hydrocarbons on the properties of concrete for this relatively new application are not well known. At the initiation of this study, no investigation into the effects of pressurized hydrocarbons on concrete properties had been reported in the available technical literature. Only limited information as subsequently appeared. The large investments and potentially high costs of failure justified our investigation in this area.

Chemical reactions between mature concrete and hydrocarbons were not anticipated; however, hydrocarbons were expected to affect the availability of moisture in concrete. The net results of any such effect on the properties of the concrete was uncertain. The role moisture plays in the curing of concrete has been the subject of extensive investigations. Briefly, the ultimate potential strength of concrete is obtained by a low water-to-cement ratio in the initial mix, maximum available water during the curing stage, and minimum free water at the time of the test. Hydrocarbon flooding during the curing stage could alter the strength and related properties of the concrete.

Crude oils from North Sea production typically contain a significant percentage of pentane and' lower molecular weight hydrocarbons. These lower molecular weight hydrocarbons are likely to have the greatest penetration and affect on concrete. Our initial experiments involved high-strength concrete subjected to pentane at several different pressures and time intervals. The effect of crude oil, pentane, and seawater on high-strength, lower-strength, and air entrained concretes was then investigated. The preliminary results of an ongoing study of the effects of: these same environments on the creep and fatigue behavior are included in this report.

TEST SPECIMENS, APPARATUS, PROCEDURES, AND FACILITIES

Standard 6-inch × l2-inch cylinders were cast in cardboard molds using local materials similar to the materials that might be available in England. Type I Portland cement at seven bags per cubic yard was used with mined river gravel and mined sand. Comparisons were made between cylinders of the same age from one batch or a minimum number of equivalent batches. Mixing, casting and testing techniques conformed to ASTM standards. Special 4-inch × 8-inch cylinders were used for the fatigue tests. All samples received a 28-day fog room cure prior to any testing or exposure.

Pressure vessels were constructed to contain two 6-inch × l2-inch cylinders. The concrete cylinders were flooded with the environmental fluid and pressured with a nitrogen blanket.