Concrete strength 8

1. Increase cement. 2. Lower W/C ratio. 3. Add additives.

Note that with increased strength, there is generally increased shrinkage potential as there is a lower W/C ratio in most cases.

retired13 pretty much nailed it.

jayrod12. I agree with your first point but not your second. Lowering w/c ratio does not increase shrinkage. Increasing the total free water content is what increases shrinkage.

All well and good. However, these days I hear this story from my technician inspector son on the jobs. He goes through the steps of checking slump on the job. Supposedly if there is increased slump beyond the design that weaker strength may result. However, he has given up long ago to trying to do anything about this. Once the slump test says the load passed,the foreman on the job says "XX more gallons " and away goes the water into the drum. Son has given up on trying to prevent this but has given up long ago. So far I hear nothing about failures as a result.

The problem with that is that there is no one on the job with authority to overrule the foreman. That authority should not be with the technician, but rather with the design engineer or his representative.

Although I fully understand the engineering behind these comments, I have some sympathy with the foreman. If the techie advises the foreman that the test cylinders will be the final arbiter , and the foreman will have to completely rip out the pour at his expense if the tests come in below par , he at least is likely to be a bit more judicous on the quantity of added water.

Test cylinders likely are made up before that added XX amount of water. Those foremen are not that crazy to do cylinders after adding the extra water.

Concrete is ordered with two things in mind....complying with the specified requirements and placement capability based on placement conditions. The contractor orders the concrete, hopefully based on approved mix designs. Design engineers are usually concerned about strength, not necessarily durability. They often consider placement conditions; however, that depends on the engineer's familiarity with concrete technology.

I've found over the years that many engineers do not understand concrete technology. They don't understand the impact of aggregate size, cement content (other than its impact on strength), the influence of admixtures on properties and strength gain, water additions at the site, placement techniques and other issues. This is unfortunate but true. The same is true of many contractors....even the superintendents and foremen. A foreman who tells the transit mixer driver to add more water likely has no idea of the impact of that water addition other than it makes the concrete easier and faster to place....his only concerns.

The person who knows or should know the concrete that's delivered better than anyone else is the QC representative of the concrete supplier. He/She should be the only person to dictate water addition at the site, particularly if the concrete is delivered to the site within the specified ranges of slump and air content.

retired13's synopsis is correct and good. I would add that once the mix design is correct for the application and is approved, it should not be changed by anyone without going back through an approval process. This includes adulteration of the mix at the site by water or admixture supplements.

Concrete specimens taken for compressive strength testing at the time of delivery are meant to test one thing....the mix design. They do not represent the concrete in the structure whether conventionally placed or pumped. That is not their intent. The intent is only to determine if you got what the mix design indicated you would get. That's what the structural design is based on...f'c.

One thing I throw out in my concrete lectures.....good concrete is made from water, cement and aggregates. Bad concrete is made from the same.

Pay attention to the details of concrete mix designs, placement conditions and site control. Concrete seems like a simple product, but it's actually a complex polymer with a lot of nuances that can be screwed up easily in the wrong hands.

.....good concrete is made from water, cement and aggregates. Bad concrete is made from the same.

Click to expand...

I don't know who can say it better!

Ron, I was hoping to read something from you addressing a recent thread on "bridge inspection" after a fire event. Any thought/suggestion on that regard? Link