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What is W/Cm Ratio
There is absolutely no doubt that, except for mechanical damage (such as being hit or dropped), all durability problems with concrete products can be related to the transport of fluids through the concrete. This ties in directly to the w/c ratio of concrete mixes, and permeability. The tighter the concrete, the more difficult it is for fluids to penetrate, and damage the concrete. The lower the W/Cm ratio, the more critical the curing. But how does all of this fit together?
W/Cm ratio is simply the weight of water in the mix divided by the weight of the cementitious materials that go into the mix. The cementitious ingredients include the weight of cement and pozzolans such as fly ash, slag and silica fume. Whenever you see the ratio expressed as W/Cm, it is evident that pozzolans are present in the mix design. For simplicity in this article, however, we will use the expression w/c to include all cementitious materials.
The most important purpose of the water is to hydrate the cementitious powder to form a gel that binds all of the mix ingredients together. Think of this gel as being a type of glue that surrounds all of the aggregate particles, and glues them all tightly together. A good, low w/c mix design utilizes almost all of the water in the hydration process, so there is little excess water left to evaporate, and create voids in the finished concrete product.
With high w/c mix designs, the water also provides flowability, so that the concrete can move around the reinforcing steel, and into the corners of the form. The problem with these mixes, however, is that there is much more water in the mix than is required to hydrate the cement. The excess water evaporates, which leaves voids in the product, increasing permeability. Because the concrete is porous, fluids can penetrate the product and damage it.
There are significant advantages to using as little water as possible, in your mix designs. The less water used, the better the quality of the concrete. Reducing water content:
- Increases compressive and flexural strength
- Reduces permeability, resulting in improved water tightness
- Improves durability
The National Precast Concrete Association recommends the following w/c ratios:
- 0.48 maximum
- 0.45 or less for concrete exposed to freeze/thaw
- 0.40 or less for concrete exposed to deicer salts, brackish water, sea water
Manufacturers of dry cast products use zero slump concrete, with w/c ratios in the order of 0.30 to 0.36. This w/c range provides all of the quality advantages referred to above, but because the mix is so stiff, heavy vibration is necessary to consolidate the concrete. The vibratory forces liquefy the stiff mix, which improves its flowability.
Many manufacturers of wet cast products use w/c ratios in the range of 0.50 to 0.60, because they require flowability. With such high water content, however, it is extremely difficult to make a good quality product. There is an alternative to using high water content to make concrete flow.
Fortunately, water-reducing admixtures (superplasticizers) are available. They allow the producer to reduce the quantity of mixing water (which reduces the w/c ratio), while achieving flowability. Because these superplasticizers (frequently referred to as supers) reduce the w/c ratio, producers can enjoy the benefits of increased strengths, lower permeability and improved durability. Many wet cast manufacturers are now using w/c ratios as low as 0.35, and using supers to make the concrete flow.
A word of caution - most supers increase flowability for only a short period of time (30 to 60 minutes), then there is a rapid loss of workability. Once the effect of the super has worn off, the concrete will stiffen, because of the lower w/c ratio.
Curing is critical when utilizing low w/c ratios. Keep in mind that, because there is not an excessive amount of water in the mix, care must be taken to ensure that the water remains in the concrete in order to properly hydrate the cementitious materials. The freshly poured product must be covered, or sealed with a curing compound, to prevent the escape of moisture from the concrete.
If the product is exposed to wind and sun, moisture will escape, leaving insufficient moisture to hydrate all of the cement. Mixes are designed to provide sufficient gel to cover the surfaces of all the aggregates, in order to achieve a specified strength. If not all of the cement is hydrated, it will not be possible to achieve the mix design strength, because there will not be enough gel to glue all of the ingredients together.
Once you have settled on a mix design that is suitable for the respective product that you are manufacturing, do everything you can to ensure that the actual proportioning of the ingredients is what you specified. If you are purchasing ready mix concrete, ensure that you are getting what you asked for. And, please, don’t ever let your employees put in additional water while they are pouring the concrete, as this will increase the w/c ratio, and weaken strengths.
The advantages of using a low w/c ratio make it worth your while to review your mix designs and see how you can remove some of the water. Using water-reducing admixtures can be a quick and easy method of improving product quality.