WATER _and CEMENT

"... concrete shall be placed with no more than a 3" slump!"

This statement has been synonymous with good quality, durable concrete for decades. As a general rule of thumb it can still be an acceptable guideline, taking into consideration the use of good quality materials, adequate cement content and use of good concreting practices.

There is a danger, however, in simply specifying a 3" maximum slump without considering water-cement ratio, one of the key elements in producing quality concrete. Concrete can be placed at a 3" slump and still have higher water-cement ratio than may be desirable, resulting in a concrete with lower quality.

To better understand this, let's get into a basic discussion of the water used in the manufacture of concrete.

Water is a necessary ingredient in the chemical reaction, hydration, that turns cement into a "paste" matrix that holds aggregate particles together to form concrete. The amount of water added to concrete is expressed as water-cement ratio. For example, if you add 40#'s of water for every 100#'s of cement in a mix, the water-cement ratio is 40#'s Water - 100#'s Cement or 0.40.

Published minimum water cement ratio's necessary for hydration of cement range from the high teens (i.e. 0.19) to the mid-twenties (i.e. 0.26). Rather than target a specific number, let it be sufficient to say that these values are substantially below a typical 0.45 water-cement ratio necessary to produce 3" slump conventional concrete.

The additional water added to cement, sand and rock beyond that needed for hydration of the cement is water added for convenience. This means we need a certain amount of extra water in the mix to get the workability needed for placement and finishing. This extra water in the mix that is not used in hydration remains throughout the cement paste to evaporate at some later time. Higher water-

cement ratios cause the cement paste to become more porous. The more porous the cement paste, the weaker and less durable the concrete.

Getting back to our 3" slump concrete, "what is slump?" Slump is a standard method that has been developed to verify the workability or consistency of concrete mixes. Assuming batches are correctly proportioned and properly mixed with uniform materials at constatnt temperature, a slump measurement taken at the same elapsed time after mixing is a fairly good indicator that the amount of water added to the mix or water-cement ratio of the mix will be similar from batch to batch. For the precaster and readymix supplier, efforts in designing concrete mixes involve measuring water-cement ratio during the development and testing stages. The resulting consistency is measured as slump which is then used as a routine quality control tool.

If the slump test is taken at varying times after batching, slump becomes a much less reliable indicator of the relative amount of water that is in the batch. The reason for this is that "slump is always on the move". Slump decreases with time at varying rates that are dependent on many factors including cement content and characteristics, temperature, and admixtures that are used. Slump loss is particularly a problem when concrete is being transported long distances or if placement is delayed.

Loss of an inch of slump can occur in as little as a few seconds or as long as 20 minutes or more, depending on circumstances. In the past, concrete producers have had two basic ways to deal with slump loss over time. The first way is to add water prior to placing to bring the concrete back up to the slump necessary for placement, or, if water cannot be added prior to placement, often a higher slump will be mixed in anticipation of loss of slump prior to placement.

This 3" maximum slump concrete at the jobsite can have a significantly different water-cement ratio than the 3" slump concrete we think we are getting. Along with this higher water-cement ratio will come reduced concrete strengths and potential durability problems.

Given that the main ingredient concrete producers have under their control with regard to concrete quality is water, the acceptance of chemical admixtures, particularly a High Range Water Reducer, commonly referred to as HRWR or Superplasticizers, has allowed them to maintain desired water-cement ratios or reduce them while maintaining workability for placement. Use of Superplasticizers will also allow placement of concrete at slumps higher than the typical 3" with a lower water-cement ratio.

A comparison of concrete with and without Superplasticizer is shown below.