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Grout for post-tensioning ducts

After completion of the prestressing of post-tensioning tendons. ducts will be grouted, grout materials will fill post-tensioning ducts completely, the purpose of using grout is:
  • protecting post-tensioning cables or steel from rust
  • providing and enhancing bonds between tendons and surrounding structure
  • improve the ultimate capacity of the tendons
  • in the event of failure, anchorage will not be subjected to all of the strain energy
used grout shall be of high quality, also it should possess a low viscosity to enable proper and continuous pumping of grouts until filling ducts completely, also viscosity should not be very low to avoid bleeding and segregation of grout, grout should possessive a compressive strength equal or more than surrounding concrete.

Figure 1

The main component of grout is cement, ordinary portland cement Type (I and II) is used usually. the properties of grout can be enhanced by adding other pozzolans. fly ash and slag can be added to improve corrosion resistance in an aggressive environment. adding silica fume will decrease concrete permeability and reduce the ingress of chloride, which helps in protecting post-tensioning cables or steel from corrosion. The W/C should be limited to a maximum of 0.45. increasing W/C will adversely affect grout mix strength and increase the bleeding. therefore, adding water at the site shall be strictly prohibited.

admixture for grout

the admixture should be used to improve grout properties. cement and water grout only will exhibits bleeding, segregation, and low workability. 
calcium nitrite can be added to increase corrosion resistance. calcium nitrite will bond to reinforcing steel and form a passive layer that will resist chloride attacks, the side impacts of calcium nitrite on strength and setting time shall be studied and evaluated. 
HRWR or high range water reducer can be used to reduce the amount of water used. this will increase grout strength and reduce segregation and bleeding, also it will assist in producing a workable mix with low water content. other admixture may add such set controlling, air-entraining, expansion creating and anti-bleeding. adding of admixture shall follow the manufacturer's specification strictly.  

Mixing grout at the site is undesirable, grout mixed at the site will not have uniform properties, this arises from the variation in material, the difference from day to day in mixing, crew changes, workmanship,  weather condition, and other factors. therefore, pre-bagged grout considered as a good solution to eliminate the error that arises from the mixing of grout at fields. pre-bagged grout will be delivered in a pre-measured bag, the crew will add the water and mix it thoroughly.


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