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Laboratory test results - shrinkage of HPC

This result includes the information obtained when measuring shrinkage of laboratory produced mixtures. Three batches of 22 mixtures each were produced in the different European regions and tested in various labs. In the Nordic group, length change of 10 x 10 x 50 cm (length) samples was measured and was reported as a percentage of length change. Drying shrinkage measurements began after 1 month of water curing, followed by drying at 20C degrees and 40% RH. In Germany, the drying shrinkage was measured on beams 10 x 10 x 40 cm (length) at 20C degrees and 65% RH.
In the Nordic group the autogenous shrinkage was measured from day 1, at the time of demoulding and while samples were stored at 100% RH (water soaked). Autogenous shrinkage was only measured on mixtures with the lower w/c ratios (< 0.40), since it has been shown that only in these cases is there a lack of water for hydration and thus autogenous deformation.
In the German region during the first 24 hours up to time of demoulding the early age shrinkage was measured. After measuring the early age shrinkage the same specimens were used for measuring the autogenous shrinkage.

The drying shrinkage results of the HPC mixtures ranged from 0.1 to 0.4 mm/m after 1 year of drying. This range was below the cracking risk level and lower than typically seen on OPC mixtures. By decreasing the w/b ratio or the paste content, the drying shrinkage was reduced while at the same time the autogenous shrinkage increased. Adding more mineral additives, such as fly ash, resulted in a higher paste content and thus greater drying shrinkage.
The same trend was seen with additions of air entrainment or other pozzolans, such as silica fume or blast furnace slag.

The autogenous shrinkage results of the HPC mixtures under sealed conditions ranged from 0.2 to 0.4 mm/m after 1 year. The magnitude of these results was equivalent to the drying shrinkage, indicating that autogenous shrinkage is of greater risk with HPC. The autogenous shrinkage was much higher than typically seen for OPC mixtures, which is as expected since the autogenous shrinkage only exists below w/b of approximately 0.45. In the tests, HPC mixtures with greatest autogenous shrinkage had low w/b ratios and dense microstructures (such as the silica fume mixtures). The autogenous shrinkage results for water-cured specimens ranged from -0.1 (swelling) to 0.1 mm/m, indicating that even though these small specimens were very dense they could still absorb water and lower the autogenous shrinkage cracking risk.

When designing HPC mixtures, it is important to keep the w/b ratio and paste content to relatively low values to prevent excessive drying shrinkage. But these parameters should not be lowered too much (i.e. less than w/b of 0.40) or else the autogenous shrinkage can become significant. Both autogenous and drying shrinkage can be minimized by providing sufficient water curing immediately after placement, which is also critical during the early ages on a construction site to prevent plastic shrinkage cracking.

Reported by

Technical Research Centre of Finland
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