Because concrete is one of the few building materials that can be impacted after transport, the addition of water to concrete is one of the biggest challenges faced by many construction sites today. Ready-to-mix concrete requires the addition of water, but many people prefer to do this after the product has arrived from the manufacturing facility to the job site. While adding water on-site is certainly possible, there are considerations to be made in order to make sure both quality and performance are maintained. Just like the use of proper concrete and quality Symons forms, the use of water requires skill and knowledge.
When to Add Water at the Job Site
Typically, water will be added on-site when the slump needs to be brought to a level that is either acceptable or the specific level for a project. In order to do this, it is crucial that some of the water from the initial mix design was held back during the initial mixing stage of the project in order to avoid liability on the part of the worker. When water is added, it should be added only in the precise amount necessary as deviation above or below the design mix allowance or specification will often have an adverse effect on the ability to place the concrete properly.
Avoid Adding Too Much Water
The major risk of adding water on-site is exceeding the standards for that particular mix. A ready-mix concrete will follow industry standards related to its intended purpose and regarding how much water to use in each project. When too much water is added, the strength of the concrete can be reduced, leading to a structure more susceptible to future cracking.
It is important that the addition of water does not increase the water to cement ratio above the maximum allowed for in the mix design. Therefore the person adding the concrete should be aware of the ratio and the amount that can be added without exceeding this ratio. Some concrete producers include this information on delivery tickets now in order to avoid confusion and problems.
If the allowed amount of water has already been added and the slump is still below the requirement, more can be added at the discretion of the buyer or owner. Doing so accepts liability for any issues that occur as a result of the additional water. A common alternative to adding more water is to use a water-reducing concrete admixture or superplasticizer to increase concrete slump.
How to Add Water On-Site
If you are planning to add water to your job, the first thing you must do is ensure you know the specified nominal slump plus tolerances in order to determine the maximum allowable slump. In addition to this important step, below are tips to achieve a good outcome.
Determine the Actual Slump Before Discharge
You can measure the slump using a sample taken from the first quarter cubic yard of discharged concrete. The result of this measure can be used as an indicator of consistency rather than a true acceptance test.
Add Water to the Full Batch
By adding water to the entire batch of concrete, you will be able to assess the volume of the retempered concrete. A good rule is to use one gallon of water for each cubic yard in a one-inch slump increase.
Measure and Record Any Added Water
Be sure to document any water added on-site.
Increase Revolutions Following Water
After adding water to concrete, you should add an extra 30 mixer drum revolutions at mixing speed. If the truck can mix at 20 or more revolutions per minute, 10 revolutions will be sufficient.
Control the Amount of Water
In order to avoid surpassing maximum slump and water/cement ratio limits, water volume should be carefully controlled. It is better to start with small amounts and then add more as needed.
Prohibit Extra Water On-Site
Once the appropriate slump or ratio has been reached, many sites will prohibit the addition of any other water for the duration of a project.
Establish Procedures in Advance
Before beginning a pour, it is helpful to establish rules and procedures. For example, you need to establish who has the authorization to request more water and what documentation is required when water is added.