Too stiff, and what it costs you
A low slump load barely moves when the cone comes off. It holds its shape, which is exactly what you want in the bank of a footing trench or on a sloped driveway where nobody wants concrete running downhill before it sets. The tradeoff shows up in labor. Stiff concrete is harder to screed, harder to consolidate around steel, and harder to finish, and a crew that works it too long can do more damage than the stiffness itself ever would.
- Holds its shape on a slope or in a form
- Harder to screed, place around steel and finish
- Less bleed water and less shrinkage risk
Too loose, and what it costs you
A high slump load spreads out fast and finishes easy, which is why a loose load can feel like the better choice on a hot day with a crew short on hands. The tradeoff is strength and stability. Past a certain point the mix carries more water than the cement needs, and that is really a water to cement ratio problem wearing a slump number, since the extra water raises the ratio right along with the pile spread. That extra water either bleeds to the surface or weakens the paste once it leaves.
A load that is loose enough to pour itself is usually a load that gave something up to get there.
Who actually sets the number
There is no slump figure printed on a chart that fits every job. The right one comes off the ready-mix plant's own mix design for the strength and the exposure being ordered, air entrainment for outside flatwork included. Tell your supplier the strength you need and what the concrete is going into, and their mix design people land the slump in the workable middle instead of anybody guessing at it on site.
On our own pours that phone call is ours to make before the truck ever loads. We tell the plant the strength, the job, and that it needs to travel through 2.5 or 3 inch line as a pump mix, and they hit the number from there.
What happens to slump once the truck is loaded
Slump is not fixed for the whole trip from plant to job site. Concrete stiffens on the way, faster in summer heat and slower in cold weather, which is one reason a load that tested right at the plant can show up feeling different an hour later on a hot afternoon. That drift is normal within limits, and it is a separate question from whether the mix was designed correctly in the first place.
Adding water at the truck to chase a number back up is a different move entirely, and it trades away strength the load was already paid for. If a load looks wrong when it arrives, that is a call to the plant, not a hose.