CRACKING OF CEMENT AS A RESULT OF THE LOCALLY DELAYED HYDRATION OF CEMENT

UNREINFORCED CONCRETE SURFACINGS ARE SOMETIMES DAMAGED BY IMPORTANT TRANSVERSE CRACKING SHORTLY AFTER THEIR MANUFACTURE WHEN HYDRATION OF CEMENT OCCURS AND THE MECHANICAL STRENGTH OF THE CONCRETE INCREASES RAPIDLY. DURING THIS SAME PERIOD (FIRST AGE) LABORATORY OBSERVATIONS SHOW THE SUCCESSIVE DEFORMATIONS OF CEMENT PASTES: FIRST SHRINKAGE, SWELLING, SECOND SHRINKAGE. IN SPITE OF CAREFULLY DEFINED EXPERIMENTAL CONDITIONS, THE VALUES OF THE PARAMETERS WHICH CHARACTERIZE THE DEFORMATION OF NORMAL CEMENT PASTE REMAIN DISPERSED, ESPECIALLY BECAUSE ANHYDROUS CEMENT IS A MATERIAL THE PROPERTIES OF WHICH EVOLVE UNDER NORMAL CURING CONDITIONS. THE ONLY FACTORS THE EFFECT OF WHICH CAN BE NOTED IN SPITE OF THE ABOVE DISPERSION ARE: (1) THE HYGROMETRY AND WIND SPEED WHICH CAN INCREASE THE MAXIMUM VALUE OF THE FIRST SHRINKAGE TENFOLD BUT DO NOT MODIFY THE SECOND SHRINKAGE NOTICEABLY, AND (2) THE MINERALOGICAL NATURE OF CEMENT ON WHICH DEPENDS THE VALUE OF THE MAXIMUM RATE OF THE SECOND SHRINKAGE; ACCORDING TO THE CEMENT USED, THIS RATE CAN VARY IN THE RATIO OF 1 TO 10. OBSERVATIONS MADE AT THE TIME OF CONSTRUCTION OF SURFACINGS SHOWED THAT CRACKING DEPENDED MAINLY ON THE INGREDIENTS USED AND EVENNESS OF MANUFACTURE; THE WEATHER HAS ONLY A SECONDARY INFLUENCE. SINCE THESE OBSERVATIONS, CEMENTS SELECTED FOR SURFACINGS ARE THOSE THE NORMAL PASTE OF WHICH HAS A LOW MAXIMUM RATE OF SECOND SHRINKAGE; ON THESE SITES, CRACKING IS CONSIDERABLY REDUCED, AND OFTEN ELIMINATED. THE SECOND SHRINKAGE OF CEMENT PASTE CAN BE INTERPRETED AS BEING THE RESULT OF TWO OPPOSITE MECHANISMS: BULKING CAUSED BY TOPOCHEMICAL REACTIONS, AND SHRINKAGE OF THE HYDRATE NETWORK. THIS SHRINKAGE IS PROPORTIONAL TO THE QUANTITY OF HEAT PRODUCED BY CEMENT HYDRATION. A COMPARISON OF IN-SITU AND LABORATORY OBSERVATIONS SHOW THAT: (1) BECAUSE OF IRREGULARITIES IN THE MANUFACTURE OF SURFACINGS, IT IS POSSIBLE THAT TWO VOLUMES OF CONCRETE BE JUXTAPOSED WITHOUT THE HYDRATION OF THEIR CEMENT BEING SIMULTANEOUS, AND (2) IN THE CONTACT ZONE OF THESE TWO VOLUMES, SHEAR STRESSES OCCUR, THE GREATER AS THE MAXIMUM RATE OF THE SECOND SHRINKAGE IS HIGHER; THESE STRESSES, WHICH ARE ADDED LOCALLY TO THE TENSILE STRESSES EXISTING IN THE WHOLE SECTION OF THE SURFACING, WOULD CAUSE THE TYPE OF CRACKING CALLED CRACKING BY LOCALLY DELAYED HYDRATION. TO AVOID THIS TYPE OF CRACKING, THE SELECTION OF CEMENT CAN BE BASED ON THE VALUE OF THE MAXIMUM RATE OF HYDRATION HEAT EMISSION, MEASURED UNDER WELL DEFINED CONDITIONS. /IRRD-IRF/

  • Corporate Authors:

    Laboratoire Central des Ponts et Chausees (LCPC)

    Boulevard Lefebvre 58
    Paris Cedex 15,   France  F-75732

    Equipment & Housing Ministry /France

    ,    
  • Authors:
    • Baron, J
  • Publication Date: 1971-5

Media Info

  • Features: Figures; Photos; References; Tables;
  • Pagination: 106 p.
  • Serial:
    • Issue Number: 15

Subject/Index Terms

Filing Info

  • Accession Number: 00213165
  • Record Type: Publication
  • Report/Paper Numbers: Res Rept
  • Files: TRIS
  • Created Date: Mar 23 1974 12:00AM