Jobsite addition of water is the adding of water to a ready mixed batch of concrete in a truck mixer after it has arrived on the jobsite. This is done to increase the slump of the concrete, or in some rare cases to decrease the temperature of the concrete when ambient conditions cause an increase in concrete temperature.
While ASTM C94 Specification for Ready Mixed Concrete allows for jobsite water additions, let’s take a quick break to discuss what is happening to the load when water is added to concrete.
Adding one gallon of water to one cubic yard of concrete will:
May cut the compressive strength by about 150-200psi (depending on cement content)
May waste the effect of ¼ sack of cement
Increase slump by about one inch
May increase air content by about 1%
May increase segregation
May increase the shrinkage potential by about 10%
May increase the concrete’s potential to crack.
Thus, water can be the most expensive ingredient in concrete. The obvious takeaway is “don’t add water to loads of concrete”.
I cannot count how many jobsites and projects I have visited, from large scale 1,000yd+ SOGs to 10yd driveways where there has been a disconnect between the order maker and the placing crew on site. Concrete is typically ordered at a 4” slump, but the crew almost always wants to place at 5”, 5.5”, 6”+. This ultimately leads to the crew onsite to add water at the jobsite. Owners/project managers/estimators need to take into account the slump that the crew wishes to pour at, or take into account if a slump of 5”+ is preferred, then 6 sack mixes with water reducers need to be used to achieve the high slump without the negatives of adding additional water on site.
In addition to the above listed problems, this can lead to water being added above the mix design, thus increasing the water/cement ratio and decreasing the expected performance of the concrete. A common, but usually overlooked problem is that mixer drivers, being under pressure to move faster on sites to unload faster to keep up production, will temper the loads on their own, bringing what they think is the slump the crew placing for the customer really wants, and therefore making 7/11 liable. So we can get to a scenario where drivers are showing up with wet loads (slump above what is posted on the truck tag), and a crew, who has been adding water to loads already, could add water to the already wet loads without checking. This happens frequently, and probably accounts for the majority of problems found in placing concrete (cracking, dusting, low compressive strengths, etc.), outside of improper cylinder making/handling/storage/testing.
With this in mind, 7/11 Materials always recommends to have pre-pour meetings to discuss a number of topics, but also to discuss the addition of water. Who is authorized to ask for water? Who is authorized to add water? Does the customer want a 7/11 rep onsite to check and temper loads to maintain rate of placement? If specific site specs do not allow for the addition of water, then the meeting needs to address what steps will be taken at the batch plant, or onsite addition of plasticizers, to address slump needs. Another often time over looked topic is the slump itself. What slump does the crew want to place at? This should be addressed at the meeting, and the order maker needs to double check to make sure the correct slump is ordered. 7/11 drivers are trained to bring the slump on the tag. The practice of “bringing what the customer really wants” is a relic of the past, and has no place in proper concreting practices. We want to deliver the best product possible to our customers, and unnecessary water additions can prevent us from doing so.
There are times when water added at the site is used to reach a preferred slump; for example a load that has been on site for 45mins plus. While 7/11 generally does not advocate adding water to concrete loads, if these water adding guidelines are followed, the preferred slump can be achieved, and the negatives of adding water can be decreased:
Establish the maximum allowable slump and water/cement content ratio permitted by the job specifications. Generally 7/11 can trim water from the mix to allow for jobsite additions. These are great topics to discuss and work out at pre-pour meetings.
7/11 always includes an allowable water to add section on its truck tags. It will show the trim water and what can be added to the load to remain within the ordered water/cement ratio. Do not exceed this water to add if you want to ensure compressive performance.
Always look at the first section of concrete being discharged to estimate or take an actual slump test. Never add water to concrete unseen.
Add an amount of water that does not exceed the maximum slump range (generally the slump spec +/- 1”) or the minimum water/cement ratio.
Always measure and record the amount of water added to the truck tags. Only the concrete purchaser may authorize water added above the allowable add on the truck tag.
A good rule of thumb is that 10 gallons to a 9.5yard load (standard 7/11 mixer load) will increase the slump by 1”. Add the least amount of water as possible to get a workable slump.
As water is being added, and after it is added to the mix, the drum should be spinning at full mixer speed for a minimum of 30 revs, but 45 revs would be preferred.
Do not add water if:
The max water/cement ratio is reached
The max slump is obtained
More than ¼ cubic yard of concrete has already been discharged.
If the slump of concrete at the time of placement is such that the mix is unworkable, following the above general guidelines will allow for a more workable mix during adverse site conditions, while managing the potential negatives and risks of adding water at the jobsite. Remember, the slump can be adjusted at the plant (depending on the mix design, since a mix with less than 517lbs of cement per yard or 5.5sk should not be poured at greater than a 5” slump without using plasticizers) by notifying an onsite 7/11 rep or by calling the dispatch office to make a change. It will produce a more consistent and higher quality mix for your project.