Tips for mixing and casting concrete countertops

Mixing and casting the concrete may seem like the simple part. You’ve made the templates, built the forms and double- and triple-checked them. Now you just dump the ingredients in the mixer, and then dump the concrete in the forms, right? Actually, this is a critical point in creating high-quality concrete countertops.

Here are some tips to make sure your pour goes smoothly and make your life easier in the subsequent steps.

  • Safety first!

All workers should wear dust masks, eye protection and gloves. Designate one person as the Pour Master (or whatever creative name you want to use for him/her). Before turning on the mixer, the Pour Master is responsible for checking around the area to make sure everyone’s hands and limbs are clear of the mixer, then confirming this by yelling, “Clear!”, and waiting for a response from each worker. The Pour Master also checks off on the batch report and the quality checklist.

  • Use a double-checkmark system.

Before you start, print out or write out a batch report that lists every ingredient and exactly how much in each batch. When you measure out the ingredients for each batch, put one checkmark beside that ingredient for each batch. When you add each ingredient to the mixer, put another checkmark beside that ingredient. This applies to bagged mixes as well as from-scratch mixes.

CCI’s Precast Mix Calculator includes a calculator that generates a printable batch report with space for double checkmarks. You just enter how many square feet you’re making at what thickness, the color, and the calculator does the rest.

But regardless of whether you use CCI’s formulas and calculator or your own, be sure to use a batch report. The batch report also serves as a record of exactly how you made the mix. You can keep it in the customer’s file and refer to it if you ever need to duplicate that customer’s color.

  • Make sure everything you need is close at hand.

Pre-measure all of your ingredients in buckets or containers, including your water (especially your water!) and any admixtures you might need (set retarder, superplasticizer, etc.). This applies even for bagged mixes that you “just add water” to. If you have multiple batches, use a set of buckets/containers for each batch. Also make sure you have gloves, scoops, trowels, etc.

  • Pour adjacent slabs from the same batch.

Even if all ingredients are measured to the gram, subtle color differences can result from very slight variations in sand moisture, cement color, aggregate gradation and other factors. If you’re doing a large pour with multiple batches, plan out which slabs will be adjacent in the installed kitchen or project, and use a single batch for adjacent slabs. If adjacent slabs are too big for a single batch, pour the face of all adjacent slabs with one batch, and then fill in the depth with another batch. Stir the edges to prevent casting lines.

  • Add ingredients to the mixer in the correct order.

For a from-scratch mix, here’s the best order to add ingredients to the mixer:

1. Sand, rocks and pigments (dry or liquid)

2. Water and all admixtures

3. Fibers

4. Pozzolans

5. Add cement gradually

Here’s why:

If you start with the cement in the mixer and add water to it, clumping can occur. Also, mixing everything except cement with the water first ensures that all of the ingredients are fully wetted and dispersed. Then when the cement goes in, it can combine properly with all the other ingredients to make high quality concrete.

Remember, water first, then cement.

If you’re using a bagged mix, you won’t be able to do this. That’s okay. Just follow the manufacturer’s instructions, and measure your water precisely.

  • Use a quality checklist.

If you are hand packing a stiff mix, you need to make sure to pack the concrete into every corner and edge. If you’re using a fluid mix and hand vibrating, you need to make sure to apply vibration evenly to all edges/surfaces. You need to make sure that the reinforcing is placed at the correct level in the form. You need to screed the back of a stiff mix to make sure it’s level with the forms.

These are just a few items, and you may think it’s easy to remember, but when you’re hustling to get concrete into the forms, it’s easy to miss details. Write down a quality checklist and have the Pour Master double-check it. Doing something like forgetting to screed the back now can result in a lot more work later.

Concrete Countertop Mixes: Stiff versus Fluid, part 2 of 2

This article concludes the discussion of stiff versus fluid concrete countertop mixes, with the final three differences.

Difference #3: Mix Design

Stiff Mixes

Stiff concrete mixes can be either all-sand mixes (no coarse aggregate) or coarse-aggregate based mixes. Generally they are all-sand mixes due to the ease of spreading and packing a fine-grained clay-like concrete. Coarse aggregate makes the mix “chunky”, often making hand-packing difficult or uncomfortable.

All-sand mixes have far more surface area, so require more cement paste. But the fine grained nature of sand inhibits movement due to the increased particle friction generated from many times more contact points. All-sand mixes are typically stiff, zero slump. It is possible to make anall-sand mix fluid with a combination of a powerful superplasticizer and strong vibration, but this is rarely done.

Fluid Mixes

Fluid concrete mixes are generally based on coarse aggregate.  Aggregate based mixes are more easily made fluid because of the large amount of coarse aggregate in the mix. Pound for pound, coarser aggregate has less surface area than fine sand. A mix that has less surface area requires less cement paste to coat the particles, and any excess paste in the mix acts as lubricant, allowing the particles to move past each other more easily.

Difference #4: Forming Techniques

Stiff Mixes

Because of the nature of stiff mixes, watertight forms are not necessary. As such, careful caulking of the seams isn’t required. If the form edges are caulked, it is because a cast rounded edge is desired instead of grinding a rounded edge. Forms should be tight, but the added step of caulking is not necessary.

Fluid Mixes

It’s a different story with fluid concrete. The forms must be watertight with fluid concrete because any leakage will result in discoloration of the concrete and loss of material.

Fluid concrete also requires that reinforcing steel be tied to the sides of the forms, since the steel will sink if simply placed in the form. However, it is very important not to pour the concrete over the steel. Doing so will result in ghosting. To prevent ghosting, either tie the steel after the forms are mostly filled with concrete, or very carefully pour the concrete through the holes between the steel, taking care not to pour onto the steel. Also, if the concrete is to be vibrated, the steel must be tied in place so that it does not move.

Comparison of Forms

Stiff mixes require less form work than fluid mixes when complex or 3 dimensional pieces are being cast. This is especially true for integral sinks cast into countertops. On the other hand ordinary flat countertop slabs use essentially the same forms for both stiff and fluid mixes.

form for fluid concrete countertop mix

Fluid concrete mixes need more complex forming for an integral sink.

form for stiff concrete countertop mix

Stiff mixes require less forming because the concrete is self supporting.

Difference #5: Casting Effort

Stiff Mixes

Stiff mixes take more effort to cast, and if care is not exercised in placement, air will get trapped between the concrete and the form. This results in large, shallow “craters” that often are on the order of 1/16”deep. The resulting surface is ugly and inconsistent in appearance, and it is difficult to repair by grouting.

craters in stiff cast concrete

Fluid Mixes

Fluid mixes are quick and easy to cast, as long as you take care with the reinforcing steel as noted in Difference #4: Forming Techniques.

Conclusion

As you can see, the choice of a stiff mix versus a fluid mix has many implications for all steps of the concrete countertop production process. Understanding the interplay of these implications is important for producing a high-quality concrete countertop.

Concrete Countertop Mixes: Stiff versus Fluid, part 1 of 2

There are several considerations to make when choosing a concrete countertop mix consistency. Some factors include the appearance of the finished piece, the forming and casting methods, and the complexity of the mold geometry.

Other factors that are not so obvious also drive the selection process. These can include the type of mixer that will be used, how the forms are assembled, the shrinkage tendency of the concrete and subsequent vulnerability or resistance to curling and hairline shrinkage cracks.

This article outlines the basic differences between two mix styles, stiff and fluid, in design, look and technique.

There are two basic types of mix consistencies: stiff and fluid.

A stiff mix is a versatile, all-purpose mix that can beadapted to a wide variety of looks, from a “solid” appearance, to a variegated/veined, or a terrazzo look. A stiff mix is typically made using only sand as an aggregate, and it has zero slump.

zero slump concrete countertop mix

A fluid mix is a high slump mix that is more conventional in its ingredients, since it uses both fine and coarse aggregates. While coarse-aggregate based mixes are easy to make fluid, it’s vital that the fluidity is achieved by using superplasticizers and not by adding water. Concrete countertops require high quality concrete for aesthetic and long term performance, and using water to enhance workability instead of superplasticizers results in weak concrete that is prone to shrinkage, curling and cracking. Of course being disciplined with mix water applies to all types of concrete, but fluid concrete is more susceptible to water abuse.

flowable concrete countertop mix

 

Difference #1: The Look

Stiff Mixes

Stiff concrete mixes will always have voids in the cast surface. This is because the cement paste (and the mix as a whole) is so stiff that the entrapped air bubbles cannot escape. Often larger voids are irregularly shaped, and this tends to lend a more organic, stone-like look to the concrete.

pressed concrete stool

small round sink in concrete countertop

copper embedded in concrete countertop

variegated integral sink concrete countertop

The fact that the concrete is stiff and will always have air pockets can be exploited by purposefully introducing voids and patterned fissures. The resulting look resembles the veins in natural stone. Sometimes the voids are left open, but more often they are filled with contrasting cement grout. This highlights the voids and reinforces the resemblance to natural stone.

Fluid Mixes

Fluid concrete creates castings with very high surface quality. Edges are crisp and precise. Surface color and visual texture is smooth and even.

fluid mix pinholes

ramp sink in concrete countertop

Fluid concrete mixes can sometimes have a few small pinholes, or if the casting is done carefully, no pinholes at all. Concrete can be made fluid by using an appropriate mix design and superplasticizer, or astiff mix can be made fluid by vibration. Either way the cast surface will very closely reflect the surface characteristics of the mold. If the concrete was cast against smooth, glossy plastic then the virgin cast surface will be shiny and almost glass-smooth.

Difference #2: Casting Techniques

Stiff Mixes

Stiff concrete is placed by hand, often in small handfuls.The stiff nature of the mix allows the concrete to be packed onto vertical forms and place in thin layers.

hand packing sink form

hand packing in form

Fissured concrete, sometimes called variegated or hand-pressed concrete, is placed so that air gaps between areas of concrete result in fissures once the concrete is demolded.

variegated round table

Fluid Mixes

Fluid concrete is often poured into forms, but it can also be placed by hand.

describe the image

casting fluid concrete countertop mix

 

The next article will explore three more differences between stiff and fluid concrete countertop mixes.

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