FIELD STUDIES OF FLY-ASH CONCRETE STRUCTURES CONTAINING REACTIVE AGGREGATES

The paper presents the findings from field studies of fly ash concrete structures containing reactive (alkali-silica) aggregates. Data is pr esented from a number of hydraulic structures in Wales and Ontario con structed using geologically similar greywacke-argillite aggregates. Al l the structures with ash showed evidence of damage due to ASR; indeed reaction with this aggregate has lead to replacement of structures in both areas. Reaction may occur at alkali levels significantly below 3 kg/m(3) Na2Oe and has been found in a structure with a measured 'avai lable' alkali content of less than 2 kg/m(3) (NaOe)-O-2. However, the fly ash concrete structures are in excellent condition after more than 25 years service despite having higher alkali contents than many of t he damaged structures; the Lower Notch Dam was actually constructed us ing a high alkali cement (> 1.0% Na2Oe). Microstructural and pore solu tion analysis showed that much of the alkali in the fly ash concrete w as 'bound' in the CSH and not available to the pore solution for ASR. Published findings related to the McPherson Test Road, the cracking ob served in fly ash concrete being attributed to drying shrinkage. Detai ls regarding the use of ash in the Hanshin Expressway are not adequate ly reported, however, it would appear that 18% fly ash was not effecti ve in completely suppressing expansion in this structure. This is not surprising in view of the moderate level of ash and excessive alkali c ontent of the concrete (up to 0.33% Na2Oe). The balance of field perfo rmance data sustains the concept of using Class F fly ash to control e xpansion due to ASR in concrete, provided the material is used at suff icient levels of replacement. Further control of the alkalis from othe r sources would appear to be a prudent precaution even when the concre te contains fly ash. It is surprising that many specifications have ad opted the advice from accelerated laboratory tests and penalize fly as h by assigning an 'alkali contribution' (e.g. one-sixth the total alka li or all of the ASTM C311 'available' alkali) to the material which m ay often preclude its use. Such advice appears to contradict the more cogent argument from the field where Class F fly ash has been successf ully used for many decades with no reported incidences of ASR in struc tures containing sufficient levels of ash.