Minis and MicrosImprovements in Grouting Methods for Ground Anchors & Post Tensioning Tendons

This brief report outlines recent advances in grouting methods for this type of work. These improvements permit the use of water : cement ratios as low as 0.36 : 1 (by weight), and hence there is very little free water present. Consequently, bleeding and formation of voids is at a minimum a very important consideration in the anchorage zone and also where the grout is relied on to protect bare steel against corrosion.

The problem of void formation was recognised right from the early days of this kind of work. It was quickly found that ordinary cement, without assistance from additives, left numerous unacceptable voids at the water : cement ratios necessary to get grout sufficiently fluid for adequate penetration into the intersticies of cables, spacers, etc. Initial attempts to overcome the problem made use of gas-filled grout. The gas, usually hydrogen or nitrogen, was produced from metallic powders which reacted with the cement. Aluminium powder was one of those commonly used for this. The idea was that the gas was supposed to cause the grout to expand. Although successful in many lab tests and on some jobs, the method failed miserably on other jobs, at times producing froth of milk-shake consistency, and highly variable densities and strengths. In the early 1960s, the H.E.C. in Tasmania sought a better solution to the problem. Vertical post-tensioning of a dam was in hand, where bare steel tendons required grout protection throughout. Dr. H.E. Vivian of the C.S.I.R.O. cement technology group was able to suggest a remedy in the form of methyl cellulose. He knew of the use of this long-chain polymer to stop settlement of particles in other industrial processes, and found it worked equally well with cement. By controlling settlement of the cement, excess water does not bleed upwards but instead remains dispersed within a loose skeleton of cement crystals.

The H.E.C. has subsequently used methyl cellulose, known as nMethocell”, for all of this type of work. Problems with grouting of the segmented roof shells of the Sydney Opera House led to use of Methocell there also. Usage spread overseas, and nowadays many anchors and tendons are grouted with Methocell grout at a w:c ratio of 0.45:1.

This grout has drawbacks, however. It is very viscous and has relatively low strength.

There is very real concern that its viscosity stops it penetrating properly through the many fine passages of modern complex multi-strand cable
assemblies.

Therefore, in order to find a better grout than the Methocell type, informal discussion on the subject took place at the 1982 ASCE Grouting Conference, New Orleans. Dr. G.S. Littlejohn, who is probably the world leader on the subject, put forward some particularly interesting ideas. An outcome was the philosophy of using so little water in the grout that almost none is free to separate out as bleed water. This means using w:c ratios as thick as 0.36: 1. Such grouts are not practical using normal cements, but in the testing reported on here, have been developed into excellent grouts by the addition of a particular superplasticiser, or alternatively by the use of a special cement – Oilwell Cement. Both methods have now been extensively tested by the Water Resources Commisssion, N.S.W. and have been used on large post-tensioning jobs by the Commission.