Before I talk to you about the actual placement of the ready mix, I would like to explain what the concrete slump test is.
Slump is the method by which how loose the mix is and in everyday ready mix the amount the amount of water that the mix contains.
Once when I was driving I made a concrete delivery to a homeowner that was doing his own driveway. He had plenty of help and all of the right tools to do the job. I got out of my truck and went to the homeowner to discuss how we were going to proceed.
The first thing he said to me was he wanted the mix to be a four inch slump. So I told him ok. To which he replied “I have a friend in the business and if you do not give me a four inch slump, I have the right to send the mix back”. I told he was absolutely right and that I would give him a four inch slump.
I mixed the load and when everybody was ready, I started emptying the truck. The homeowner looked at me with a stern look and said “is that a four inch slump?” I told him yes it is and he smiled and said all right then let’s get going.
The fact of the matter is that he had no idea what a four inch slump was. I could have given him the mix any way I wanted to and he would not have known the difference.
A concrete slump test is measured by using slump cone (a concrete slump cone looks like a traffic cone with the top cut off). The cone is twelve inches tall.
It is filled according to a certain procedure with a sample of the concrete from the truck. The cone is then carefully lifted from the sample and placed upside down beside the mix. As the cone is lifted the sample will start to fall and spread. A bar is then placed across the cone and over the mix. A measurement is then taken from the bar to the top of the sample.
How ever many inches it is, that is the slump. If the measurement is one inch then it is a one inch slump if the measurement is seven and one half inches then is a seven and a half inch slump.
The higher the slump the weaker the concrete mix will be. However a slump from zero to three inches would be almost impossible to work with a slump of four to six inches is optimal and anything higher than that is not acceptable.
As the homeowner had informed, you have the right to send the load back and ask for a new one. So how do you do a slump test without a slump cone? You cannot get it exactly, but if you watch the mix and it starts to build up to a cone by itself it obviously too low of a slump and the mix should be adjusted. If it looks well mixed and spreads as wide as it builds up it is probably ok. If it comes down the chute of the truck very fast splashes or separates or spreads farther than it builds up, send it back.
When it comes to concrete water is a necessary evil. The mix needs water to hydrate the cement and cause the chemical reaction that makes it get hard. But if too much water is added to the mix it weakens the mix. One gallon of water will raise the slump of the mix by one inch and in turn weaken the mix by one hundred and fifty to two hundred psi. The funny thing is you will need water again, but more on that in the curing section.