By G S Littlejohn, Colcrete
This paper was first publsihed in GE’s April 1981 issue.
While a degree of proof loading and acceptable limits for load-extension behaviour are generally in close agreement throughout the world, by contrast acceptance criteria related to service behaviour are widely divergent in regard to duration of monitoring, and whether load relaxation or creep displacement should be monitored.
Engineers in countries such as Britain, USA, South Africa and Australia tend to favour relaxation criteria, e.g, a prestress loss of up to 5% in 24 hours (Britain), whereas in South America, Continental Europe and Eastern Block countries, engineers prefer creep criteria, e.g. a creep displacement of up to 4mm in 72 hours (France), or a creep rate of less than 0.135mm/m of free tendon for every tenfold increase in time (Czechoslovakia). All these criteria have been used as upper thresholds of acceptability in practice, but it is widely recognised by the specialists concerned that the figures are arbitrary in nature and often incompatible except for a specific free tendon length, cross-sectional area and elastic modulus.
For economic as well as operational reasons the time involved in stressing and testing anchorages on a construction site should be minimised. Thus many ‘engineers have attempted to classify ground which is susceptible to creep, e.g. fine grained as opposed to coarse grained soils in DIN 4125, in order to reduce the period of monitoring down to 1 hour. Since these particle size distinctions are not always reliable for this purpose, a standard sequence of time intervals is ideally required so that only the behaviour of the anchorage dictates the overall period of monitoring and not a prior judgement of the type of ground.
This paper discusses the interpretation of short-term service behaviour in relation to on-site suitability and routine acceptance tests, with the objective of recommending universally applicable criteria based on load relaxation or an equivalent creep displacement. In addition, it is suggested that short duration acceptance tests of less than 1 hour are possible provided that the accuracy of the monitoring equipment is sufficient to record a trend towards stabilisation.