SURFACE POPOUTS CAUSED BY ALKALI-AGGREGATE REACTION

Small popouts often appear in the surfaces of hard-troweled concrete flatwork in some parts of the Midwest. This study seeks to determine the mechanism of popout formation, evaluate the factors believed to influence the formation of popouts, and develop remedial techniques. These popouts are caused by the shale particles in the local sand and are the result of alkali-silica reactions involving opaline materials in shale and alkali contributed by the cement. The popouts are a surface phenomenon and their severity appears to be a function of the alkali content at the slab surface. Alkali content at the surface depends on the portion of the cement alkali that is readily soluble, and the amount of alkali that is brought to the surface during drying of the concrete. Migration of alkali during drying can significantly increase the alkali concentration of the surface layer. High temperature, high wind velocity and low humidity can increase alkali migration towards the surface of a concrete slab during placing and curing. Alkali migration is also be increased by long periods of drying during finishing and the first stages of curing. Inappropriate curing procedures aggravate popouts. If high-alkali cement is used in hot weather, wet curing is essential and should be initiated as early as possible. Fresh concrete should be protected from drying prior to the final finishing operations. Findings show ponding of slabs or the use of continually moist burlap, soil, or sand can eliminate popout formation. It is also recommended that hard troweling be avoided whenever possible.

Language

  • English

Media Info

  • Features: Figures; References; Tables;
  • Pagination: 13 p.

Subject/Index Terms

Filing Info

  • Accession Number: 00960732
  • Record Type: Publication
  • ISBN: 0893122238
  • Report/Paper Numbers: R&D Bulletin RD121,, R&D Serial No. 1300
  • Files: TRIS
  • Created Date: Jul 1 2003 12:00AM