ALKALI-AGGREGATE REACTION IN THE CONCRETE OF QINGHAI-TIBET RAILWAY

摘要

It was proved by ASTM C295 that most aggregates used in the concrete of Qinghai-Tibet Railway contained alkali reactive minerals ,such as strong wavelike extinction quartz, microcrystalline quartz, chalcedony, and the aggregates, of which the expansion of mortar bar made from those aggregates tested by ASTM C1260 were more than 0.20%, were above 50%. At the same time, about 21% expansion of mortar bar was between in 0.10% and 0.20%,and 51% expansion of mortar bar was bigger than 0.20%. In order to prevent the alkali-silica reaction in the concrete of Qinghai-Tibet Railway and insure the long-term durability of the bridges and tunnel, measures had been taken in the course of construction, such as restricting alkali content of raw materials used in the concretes, controlling total alkali content of the concretes, adding high quality fly ash and special composite admixture of DZ in the concretes. In order to research the effectiveness of preventing alkali-silica reaction when mixing mineral admixture and admixture in concrete, according the situation of the laboratories in China and ASTM C1260, author raised a test method as below: A certain amount of mineral admixture or additives were adopted instead of Portland cement that was used in the method of ASTM C1260 to make mortar bar. And the mortar bars were maintained in the condition of 90% RH and 20℃for 24 hours, and then curing in 1.0mol/L NaOH under 80℃. The expansion of the samples was measured in different ages. If the expansion after 28-day age was no greater than 0.10%, it could be considered that the mineral admixture or additives were effective in suppressing the alkali-silica reaction. When the above method was Used to evaluate the effectiveness of adding high quality fly ash and special composite admixture of DZ in the concretes of Qinghai-Tibet Railway to resist alkali-silica reaction, it was found the expansion of mortar bar which contained both of them, was under 0.10% at 28-day age generally. Further study shown, the macroscopical conclusion was confirmed by microcosmic test. It is obvious that mixing mineral admixture and composite additive in concrete could prevent the alkali-silica reaction efficiently.