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The Importance Of Water Cement In Countertops Design Concrete Mix Ratio

  • Listed: March 5, 2019 7:33 am


The w/c ratio refers to the ratio of the weights of water and cement used in the concrete mix. The simplest way to think about the w/c ratio is to think that the greater the amount of water in a concrete mix, the more dilute the cement paste will be. The strength is reduced mostly because adding more water creates a diluted paste that is weaker. As the crystals grow, they are too far apart to knit together and form strong bonds. Shrinkage leads to micro-cracks, which are zones of weakness.When there is a large excess of water, that water bleeds out onto the surface.

Using a low w/c ratio is the usual way to achieve a high strength and high quality concrete, but it does not guarantee that the resulting concrete is always appropriate for concrete countertops. The water to cement ratio largely determines the strength and durability of the concrete when it is cured properly. This not only affects the compressive strength, it also affects the tensile and flexural strengths, the porosity, the shrinkage and the color. Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself. The micro channels and passages that were created inside the concrete to allow that water to flow become weak zones and micro-cracks.

Concrete Countertops How Cement Particles Are Like Socks. | Duration 6 Minutes 30 Seconds

Unless the aggregate gradation and proportion are balanced with the correct amount of cement paste, excessive shrinkage, cracking and curling can result.

High Performance Mix Design For Concrete Countertops by concretedecor.net

There are many different roads leading to an acceptable mix. I would recommend finding a good source, preferably local, and sticking with it.

You can use aggregate larger than 3/8-inch but your mix would be less homogenous in thin-section countertops.

You would use either a mid-range or a high-range water reducer, but not both. The super makes the concrete more fluid, which can cause the mix to experience segregation. The end result is one component in slab curl and bug-hole prevention.

You can halve the strength of your mix if you overdose with them.

You will move between weight and volume for each ingredient by dividing the batch weight of each ingredient by the weight per cubic foot of each ingredient to arrive at the batch volume for each ingredient. Somewhere around 750 to 800 pounds per cubic yard of cement, the strength curve begins to flat-line, eventually going downward somewhere around 1, 200 pounds.

img concrete mix ratio The Importance of Water Cement in Countertops Design Concrete Mix Ratio

You could go as high as 20 percent, but this would severely affect the setting times (longer) without giving you much additional benefit (depending on the pozzolan).

Your mix ingredients will react differently to chemicals than mine will. Remember you are dosing the superplasticizer high enough to maintain your water-to-cement ratio. If your sand is damp and you guessed at 5 percent moisture you would be within a couple of percentage points, which is close enough. He has worked in the architectural/structural precast concrete field for the last 30 years, completing more than 15 million square feet of projects. Some of the practices, such as leaving the countertop in the mold for days on end, made no sense to me. Everything from ingredients to casting and curing practices works in concert with everything else to produce a quality end product. Portland cement is hydraulic cement, which means that it reacts with water to hydrate. I would start with portland – get good with it, and then experiment.

You do not need to use pozzolans to make concrete countertops, but you can make denser, stronger concrete when you use them. Concrete made with a pozzolan may require an accelerator and heat to equal the early strength of concrete made without a pozzolan. Just about any sound inorganic aggregate can be used to get the look you want. The goal is to get the concrete fluid enough to place without adding water, which weakens the mix.

What happens when you use all of the above chemicals in different combinations?

Determine a dosage that works for you, get a base line, then add other chemicals one at a time and decide if they make your mix better.

You will use simple volume calculations to arrive at batch weights for your yield. I stated already, you don’t need a pozzolan, but you can make denser, stronger concrete when you use one. Use the cement weight plus pozzolan to calculate your water-to-cement ratio. I would start with the manufacturer’s recommended dosages and work from there. Stone can only hold about 2 percent moisture before water freely runs out of it. He is also an international distributor of concrete countertop manufacturing equipment, and he holds monthly training sessions for professionals at his plant.

Concrete Mix Design by concretenetwork.com

That mix could be completely wrong for your application and could even be inferior concrete. Placeability is simply the traits that the wet or plastic concrete has that allow it to be placed and finished. Long-term durability is at least as important as placeability and strength, but it is sometimes sacrificed if there are compromises to be made. Durable concrete must resist freeze-thaw action and prevent chloride from penetrating to the reinforcing steel to contribute to corrosion. Nothing sours a client more than a beautiful floor that is cracked or spalled, or a decorative wall that is honeycombed. With decorative concrete, surface hardeners will provide a higher strength, more wear resistant and less permeable surface layer, the overall concrete strength can be less. The requirements for good performance in each of these three stages can actually conflict.

You can then tell him what your application is, when you plan to place, and what the specification requires and he can develop the ideal mix. There’s nothing wrong with this if you have a very knowledgeable producer and tell them precisely what you need. Placeability includes a mix that doesn’t segregate but that can be consolidated. For flatwork, though, strength is seldom the controlling factor, because higher-strength concrete mixes that produce very stiff concrete can result in more shrinkage which shows up as curling and cracking. Durability is achieved by getting a low-permeability, low-shrinkage concrete that has the proper amount and distribution of entrained air. Appearance issues are obviously more important for decorative concrete than other applications. With slabs, shrinkage is the number one concern and lower strength concrete (with less cement paste) will often shrink and curl less. As we all know, wet, easy-to-place concrete is not likely to be durable concrete. But understanding what’s going on in the mix is still your responsibility. Many decorative contractors will have 4 or 5 mixes that they use for different applications or weather conditions.

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