Water / Cement Ratio Calculation 2

[40A]

I am currently at issue with an existing equation used to calculate the weight of water in a concrete mix and ultimately the water/cement ratio.

Known:
Sand Wt: 13,270 lb's ---- 4.9% Moisture Content
Coarse Agg. Wt. 6,800 lb's ---- -0.5% Moisture Content

Wt. of H2O added at plant: 1,650.33 lb's
Wt. of H2O added onsite: 83.35 lb's

Cement Wt.: 4,945 lb's
Flyash Wt.: 1,240 lb's

This is the equation used to determine the free water in the sand. (is this correct?)

[(13,270 lb's)*(0.049)] / (1 + MC/100) = 619.86

.....

and the water/cement ratio we come up with is:

0.375

I know this is simple, but any and all input is appreciated.

 

[hokie66]

It's been a long time since I was involved with mix design, but I would think the water in the sand would just be calculated as 13270 x .049 = 650 lb. So I get about 0.39 as the w/c ratio.

 

[Ron]

You have to consider the absorption of the aggregate. Assuming the fine aggregate absorption to be about 0.9 percent, then the free water content is 4.9-0.9 or 4 percent. The calculate as hokie66 stated.

 

[40A]

I had seen on another post, by conceng(materials), that dividing the actual batched quantities by [1+(mc/100)] would yield the saturated surface-dry aggregate weights. From further analysis, I would think that the first part of that equation would give you total moisture in the batched quantity, then dividing it by the saturated surface-dry weights would yield the amount of free moisture contributing to the water/cement ratio. ??? Is there any truth in that? I have been peeling throught ACI 211.1 as well as ACI 318 and can't seem to find anywhere breaking it down.I appreciate all of your responses.

 

[bimr]

ASTM C566, “Standard Test Method for Total Evaporable Moisture Content of Aggregate by Drying,” should be followed when determining the aggregate moisture content. Take a representative sample from the aggregate stockpile, avoiding the first few inches as this material is probably dry and not representative of the entire lot. Take the samples in accordance with the procedures established in ASTM D75, “Standard Practice for Sampling Aggregates,” except for the sample size.

Weigh the collected sample and record it prior to drying. Use a hot plate, microwave oven or some other means of drying. Note that very rapid heating may cause some particles to explode, resulting in the loss of particles, which may render your calculations inaccurate. The sample is considered dry when further heating would cause less than 0.1 percent additional loss in mass. Allow the sample to cool to avoid damaging the scale. Weigh the sample to the nearest 0.1 percent. Calculate the total aggregate moisture content (MC) using the follow equation:

MC = (Wi - Wod)/Wod * 100%

MC = Moisture Content

W initial = Weight of the sample prior to drying

W od = Weight of the sample after drying

By using the moisture content and absorption of the aggregates, you can adjust the batch weights to account for the actual moisture condition. If the moisture content is higher than the aggregate’s absorption value, the aggregates will contribute free water to the mix. If the moisture content is below the absorption value, the aggregates will absorb a portion of the mix water.

Calculate the adjusted fine aggregate (FA bw) batch weights using the following equation for each material:


FAbw = 13270

FAbw = FAdw * (1 + MC/100) = 12650 * 1.049

Therefore, the MC of the sand would be:


FAbw - FAdw = 13270 - 12650 = 620

The water-to-cement ratio is the weight of water provided in a mix divided by the weight of cementitious materials. The total weight of water includes all batch water and free water from the surface of aggregates.


w/c ratio = (619.85 lb + 33.83 Lb + 1650 Lb + 83.35 Lb)/(4,945 lb's + 1,240 lb's)

w/c ratio = 2387.03/6185 = 0.3859

http://www.precast.org/publications/mc/2006_julyaug/wc_ratio.htm

 

[40A]

Excellent! That hits the nail right on the head. I appreciate your input.