Using the Right Tool for the Job
It’s one of the first things we learn when we’re old enough to pick up tools: use the right tool for the job. You don’t use a knife as a screwdriver, a screwdriver as a hammer or a sawblade as a lever. If you do, the tool’s owner will not take it kindly. As important, one quickly discovers, they just don’t work very well.
The same discovery soon followed with measurements. Sure, that first treehouse out in the backyard was great fun, but there wasn’t a plumb line or a level surface to be found in it. That was all right when we were ten. It’s not so great now. If it needs to be a right angle, out comes the square. When length matters, a tape measure is produced.
The same habit should apply to concrete and water. The amount of water, or the water-to-cement ratio, is one of the most important factors in concrete performance. An extra ten gallons of water in a nine yard load can typically decrease the final strength of the concrete by 5%. Even worse, it increases the susceptibility to cracks and premature wear.
Unfortunately, there’s no way to directly measure the water-to-cement ratio. It’s not possible to slap a gauge on a wheelbarrow of concrete and read the water content. What is possible is to measure the slump, and ensure that the added water doesn’t go over the allowable water-to-cement ratio for the mix design.
A quick overview of how to perform a slump test is available online from the University of New Mexico Civil Engineering department online1. The complete specification is ASTM (American Society for Testing and Materials) C143, Standard Test Method for Slump of Hydraulic Cement Concrete, which can be downloaded at numerous commercial sites on the Internet for a fee. The tools to make the measurement only consist of a cone, a flat place to put it, a tamping rod, a tape measure and a shovel. Most of these are already available at any job site, and the others are quite inexpensive. A well made cone for the measurement goes for $20–$40.
You wouldn’t use your eye’s judgement to determine the level of a floor or the length of a beam. You shouldn’t trust them to tell you whether your concrete is “wet enough.”
Concrete in Practice 26, Jobsite Addition of Water, National Ready Mixed Concrete Association