The time span (which usually takes 3 to 28 days) between concrete placement and the assessment of cube strength is a primary shortage of the current quality control technique used in concrete construction.
The growth of accelerated curing techniques has shortened the time span from 24 to 48 hours. But even the 24 hours time is enough to allow the concrete to set and harden in the form previous to the testing of the accelerated cubes.
This very article give details the technique for determination of water-cement ratio of the mix releases from the mixer and conjunction with air-content tests can forecast 28-days strength of concrete in 15 minutes, so that any mix found sub-standard should not be permissible for placement.
Water Content Test (Kelly and Vailís method)
The method for the determination of water content is based on the principle that the free water is obtainable to act as diluents when an aqueous (water-like) solution of sodium chloride of known force is added to a sample of fresh concrete.
1. Weigh out two separate one Kg samples of concrete and place each sample in a wide-mouth bottle. Add 500-ml to 0.5 N sodium chloride solutions to one bottle (sample) and 500-ml of distilled water to the other bottle (blank).
2. Close and seal the bottles and place then in an end-over mixer; operate 3 minutes.
3. Remove the bottles from the mixer and allow the contents to settle for 3 minutes.
4. Pipette 50-ml samples of clear supernatant liquid from the sample and blank bottles and add to separate Erlenmeyer flasks. To each flask (sample and blank) add 10-ml of 50 percent nitric acid, 2-ml of nitrobenzene, and 5-ml of ferric alum. Shake it well.
5. Determine the chloride content of the sample and blank flasks by adding excess silver nitrate (50-ml of 0.5 N AGNO3 for sample and 10-ml of 0.5 N AGNO3 for blank) and back-titrating with 0.05 N potassium thiocyanate (Volhard back-titration).
6. Record the quantity of potassium thiocyanate required to reach the white to reddish-brown end point in both the sample and the blank. Use figure 1 to determine the water content of the mix. Quantity of KCNS (ml) required for sample titration plus the back-titration of blank (100 minus the KCNS required for blank titration) equals the abscissa of figure 1.
Cement Content Test
This technique has been developed in Japan. In this method the cement content is get hold of the virtue of the temperature difference which lifted on account of the heat evolved by the reaction between cement and hydrochloric acid added to the diluted mortar solution. An identified weight of mortar sample is first gained from the example of fresh concrete by sieving. The mortar sample is then diluted by adding a fixed quantity of water. With addition of HCL, the dilute solution of mortar experienced an abrupt exothermic reaction, which decay of the cement contained in the sample. The aggregate (except calcareous) do not react which HCL. The heat of reaction reaches a stable temperature very rapidly and there lives a relationship between the temperature difference and the cement content of the sample of concrete.
The experimental set-up (Fig. 2) consisted of a wooden box in which a 5-litre capacity polyethylene bottle surrounded by glass wool was placed. About one Kg concrete sample was first diluted with 800 cc water to which 500 gm of concentrated HCL was added. The mixture was stirred with a rod and the temperature of the mixture was measured with a thermometer. The experimental investigation resulted in the following regression equation:
Y = 12.28 + 3X
Where: Y = cement content (in gm) of the sample of concrete, and
X = temperature difference of °C
The above technique is appropriate to only for concrete made with OPC, if the pozzolana content in PPC is known, then this technique can be used after making due alteration and correction for the amount of pozzolana in the cement content so determined.
Air content of the sample can be determined by conservative methods. Thus knowing the water-cement ratio and air-content concrete 28-days strength can be forecasted. For a direct approach in predicting the concrete strength, it is desirable that water-cement ratio v/s 28-days concrete cube strength curves be drawn from the actual site materials.
Information and Image Reference : www.engineeringcivil.com and KAUSHAL KISHORE