Civil MDC

Guide for the Design and Construction of Fixed Offshore Concrete Structures 2

Guide for the Design and Construction of Fixed Offshore Concrete Structures


Guide for the Design and Construction of Fixed Offshore Concrete Structures.2.8.1- Recommended water-cement ratios and minimum28-day compressive strengths of concrete for the three ex-posure zones are given in Table 2.1.

2.8.2- Measures to minimize cracking in thin sectionsand to prevent excessive thermal stresses in mass concrete arenecessary if more than 700 pounds of cement per cubic yardof concrete are used (415 kg per cubic meter). A minimumcement content of 600 pounds per cubic yard (356 kg percubic meter) is recommended to obtain high quality paste ad-jacent to the reinforcement for corrosion protection.

2.8.3- The rise of temperature in concrete because of ce-ment heat of hydration requires strict control to prevent steeptemperature stress gradients and possible thermal cracking ofthe concrete on subsequent cooling. Reducing the tem-perature rise may be difficult in the rich mixes and thick sec-tions required in concrete sea structures.The control of concrete temperatures includes selection ofcements which have low heat of hydration, reduced rates ofplacement, precooling of aggregates, the use of ice to replacesome or all of the mixing water and liquid nitrogen cooling,as described in ACI 207.4R. Pozzolans may be used to re-place a portion of the cement to lower the heat of hydration.

2.8.4- When freeze-thaw durability is required, the con-crete should contain entrained air as recommended by Table1.4.3 of ACI 201.2R. Air entrainment is the most effectivemeans of providing freeze-thaw resistance to the cementpaste. Conventional guidelines, such as those contained inTable 1.4.3 generally apply to unsaturated concrete. Whereconcrete is exposed to frost action in a marine environment,care must be taken to insure that critical water absorptiondoes not occur. Using a rich, air-entrained mix of low water-cement ratio, a pozzolan and an extended curing period arethe most effective means of producing a concrete of low per-meability, which is essential in such an environment. Light-weight aggregates behave differently from normal weight ag-gregates. The pores in lightweight aggregate particles arelarge and less likely to fill by capillary action than normalweight aggregates. However, care must be taken to preventexcessive moisture absorption in lightweight aggregates priorto mixing. Such absorption can result in critical saturationlevels if sufficient curing and drying do not take place beforethe structure is subjected to severe exposures. High strengthlightweight aggregates with sealed surfaces are effective inlimiting water absorption.

2.8.5- Where severe surface degradation of the concreteis expected to occur, the minimum specified concretestrength should be 6000 psi (42 MPa). Additional protectioncan be achieved by using concrete aggregates having equal orhigher hardness than the abrading material or by the provi-sion of suitable coatings or surface treatments.

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