Testing for grouts for post-tensioning ducts
Tests shall be done for grout to verify the strength and properties of the grout. laboratory tests shall be done on mix trials sufficiently prior to the beginning of grouting process to confirm the properties of the grout mix, tests such as setting time, compressive strength, permeability, volume change, fluidity, bleed and others. During the grouting process, grouts shall be tested to verify some important features such as fluidity, the temperature of grout and pressure test(to ensure no leakage).
- setting time: grout setting time shall be tested according to ASTM C953“Standard Test Method for Setting Time of Grouts.” grout setting time shall be more than 3 hours and less than 12 hours.
- Grout strength: samples shall be taken for compressive strength test according to ASTM C942 “Standard Test Method for Compressive Strength of Grouts.” cubes dimensions shall be 50 mm(2in). compressive strength of grout at 28 days should be equal or more than surrounding concrete.
- permeability: grout permeability shall be tested according to ASTM C1202 “Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.” result shall be less than 2500 coulombs after 6 hours when grout subjected to a potential of 30volts.chloride will chemically interact with prestressing steel and forms rust. rust will adversely affect the structure by causing a crack and spalling.
- Volume change: grout should be tested for volume change according to ASTM C1090 “Standard Test Method for Measuring Changes in Height of Cylindrical Specimen from Hydraulic-Cement Grout.” volume change shall be from 0 to 0.1% at 24 hours and maximum of 0.2 % at 28 days.
- Pumpability and Fluidity (Flow Cone): flow cone used to test pumbalility or fluidity of grout. the test should be done according to ASTM C939 “Standard Test Method for Flow of Grout. standard cone will be filled with one litter of grout and then efflux is measured. efflux shall be 5 to 30 seconds immediately after mixing. after 30 minutes efflux shall be less than 30 seconds.
- The temperature of grout mix: temperature of the grout mix should not increase to more than 32 C immediately after mixing and at the outlet of the grouted tendon.
- Bleed: Bleeding can be tested using wick induced test method. in wick induced a cylinder of 80 mm(3in) diameter and 1000mm(40in) hight is filled with grout up to 900 mm hight, then a clean seven wire cable (0.5' or 0.6') is immersed in the center of the cylinder. the bleed will be measured after 3hours and at a temperature of 20 C(68F).
- Pressure test: Immediately after completion of grouting for a tendon, grout inside the duct shall be subjected to a pressure of 5 bar for 30 seconds to 1 minute to ensure no leakage and the ducts fill the ducts completely.