1887

oa Civil Engineer in South Africa - Monitoring concrete temperature to determine formwork stripping time

 

Abstract

The Zuikerbosch canal incorporates two cast in-situ concrete inverted siphonpipes, with a tear-drop cross section about 6 m high and lengths of 520 m and 390 m, respectively. Using a travelling form 12,5 m long, the contractor building the siphons needed a three day turn-around time in order to complete the work within a tight construction schedule. The programme called for the inner form to be stripped 12 hours after casting the concrete, and the outer form to be stripped after 36 hours.


Whether or not the concrete could attain the minimum strengths specified by the designers soon enough to permit the forms to be stripped depended on the temperature of the concrete. The night-time winter temperatures on the Highveld were known to be low enough to inhibit the development of concrete strength during the critical first 12 hours after the concrete was cast.
It was known that there is a relationship between the early strength of concrete and its age and curing temperature. To establish the relationship a laboratory test programme was set up which led to the development of a simple control curve for determining formwork stripping time, based on the monitoring of concrete temperature.

Die Zuikerboschkanaal sluit twee omgekeerde duikpype in van in-situ gegote beton met 'n peervormige deursnit, omtrent 6 m hoog en met lengtes van onderskeidelik 520 en 390 m. Met gebruik van 12,5 m lange verskuifbare bekistingstukke het die aannemer wat die duikpype gebou het 'n driedaagse wenteltyd nodig gehad om die werk binne 'n kort konstruksieperiode te voltooi. Die program het vereis dat die binneste bekisting 12 uur nadat die beton gegiet is, gestroop moes word en die buitenste bekisting na 36 uur.


Of die beton die minimum toelaatbare strektes sou bereik wat die ontwerpers gespesifiseer het vir die stroping van bekistings, was afhanklik van die temperatuur van die beton. Dit was bekend dat die hoëveldse nagtelike temperature gedurende die winter so laag is dat dit die aanwas van betonsterkte kan strem gedurende die kritieke eerste 12 uur nadat die beton gegiet is.
Dit was bekend dat daar 'n verhouding is tussen betonsterkte en die temperature en duur van nabehandeling. Om die verhouding vas te stel, is 'n laboratoriumtoetsprogram opgestel. Die resultate het daartoe gelei dat 'n eenvoudige beheerkromme ontwikkel is om die tyd vir stroping van bekisting te bepaal na geland van die temperatuur van die beton.

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/content/civeng1/26/10/EJC24932
1984-10-01
2016-12-06
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