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Bulkhead Design for AMD Paae 14 October 27-29, 1998 <br />Concrete Mix Considerations <br />Chemical attack on the bulkhead concrete exposed to sulfate <br />concentrations in the impounded mine water in contact with a <br />bulkhead can be resisted by using Type V, sulfate resistant cement, <br />as required by the ACI (ACI 318-95, Section 9.3). Table 1 presents <br />the ACZ requirements for concrete exposed to various sulfate <br />concentrations. Brown (1992b, p 1) indicated that the 1250 <br />Bulkhead in the Reynolds Adit at the Summitville Mine would be <br />subjected to a 4693 mgJl (ppm) sulfate ion concentration. Zn <br />addition, pozzolan (fly ash) can be added to the concrete mix to <br />decrease concrete permeability and improve sulfate resistance, as <br />recommended by ACI (ACI 318-89, Table 4.3.1) and Troxell et al <br />(1968, p 109) for concrete in contact with "Very Severe", greater <br />than 10000 ppm, sulfate concentrations. <br />A typical 3000 psi bulkhead concrete mix is 1 sack of Type V <br />cement (99 lbs), to 235 lbs of fine aggregate (sand) to 330 lbs of <br />well graded coarse aggregate and 15 lbs of fly ash (pozzolan). The <br />mix proportions are 1:2.5:3.5 (cement, sand, gravel). One yard of <br />concrete would contain 5.7 bags of cement (536 lbs), 1390 lbs of <br />sand, 1881 lbs of well graded 1/2-inch maximum coarse aggregate and <br />86 lbs of fly ash, pozzolan. One yard of the specified concrete <br />would have a dry weight of 3893 lbs/yard and a mixed weight of 4085 <br />lbs/yard when the required 29 gallons of water is added. See ACI <br />211.9R-93 for additional details. The approximate in-place density <br />of the concrete will be 151 lb/cu ft. <br />The typical mix would normally be considered "oversanded". <br />However, the higher than normal sand content is designed to <br />increase pumpability, i.e. slump, at the low water/cement ratio of <br />0.45 required to resist "Severe" or "Very Severe" concentrations of <br />sulfate in acid mine water. High slump concrete can be pumped as a <br />wet mix through a slick-line or pneumatically blown as a dry mix <br />with the water added as placed in the bulkhead as shotcrete. <br />Pneumatic transport is possible over greater distances but with a <br />more variable field controlled water/cement ratio. <br />The Standard Handbook for Civil Engineers (Merritt, 1963, <br />Table 8-4) indicates that a well-graded aggregate with a maximum <br />size of 2 inches can be used with the mix proportions specified. <br />However, it is recommended that 1/2-inch maximum aggregate size be <br />used to minimize voids, segregation and "honey combing". This is a <br />potential problem between the rebar mat and the face of the <br />bulkhead forms. The 1/2-inch maximum aggregate size also enhances <br />pneumatic transport. The fly ash is sufficiently fine grained that <br />it does not occupy space in the mix, but fills voids that could <br />otherwise be present in the concrete. Fly ash also decreases <br />concrete permeability. <br />- 14 - <br />