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Artificial Rock Concrete Mix Recipe

Mixing concrete - the most important step in artificial rock making

Mixing concrete is no different than baking a cake. If you do not put the right ingredients into the mix it will not turn out properly. In fact, mixing and curing a batch of concrete is virtually identical to mixing and baking a cake. Except that the concrete bakes at room temperature!

There are many different concrete mixes and various additives and substitutions for ingredients. It is true that lightweight concrete can be created by adding vermiculite, perlite, peat moss or saw dust in place or partially in place of sand. The short answer is - yes you can do this with varying degrees of success and with more than a little trail and error. The concrete mix that is detailed below is specifically designed for two reasons:

1) Maximum strength
2) Maximum sculptability

This mix is engineered to produce a final concrete strength of 50-75 mPa. This is STRONG in fact this is VERY STRONG concrete. This will produce concrete that is far stronger than sidewalks and curbs, even stronger than swimming pool concrete and most concrete found in high rise construction.

It is important to note that you can make concrete batches of all sizes using the simple principal that the ingredients are all proportional to one another. For example the ratio of sand to cement is generally 3:1 or three parts sand to every one part cement. All other measurements in the mix will be based on the 'one part' cement that you are using. For this example the measurements are based on a standard 40kg bag of type ten portland cement being equal to one standard five gallon bucket.

respirator is a MUST as silica, concrete dust and fiberglass are dangerous to breathe!

Dry Ingredients:

-One bag type 10 portland cement
-Three buckets fine, clean washed sand
-One and 1/4 buckets un-densified silica fume
-1/4 bucket Fly Ash
-Small handful of glass fibers (like a pinch of salt - not precise, but a small amount)

Wet ingredients:

-800ml of poly-plex or liquid latex (Liquid acrylic can be substituted where liquid latex is not available)
-300ml liquid water reducer

NOTE: The liquid additives can be omitted and replaced with water with acceptable results. The water reducer and liquid latex are needed only where maximum strength and workability is essential.

**In order to make the concrete strong and workable the amount of water is critical and can change depending on a number of given criteria. For example, if the sand you are using is slightly damp you would likely need HALF the amount of water than if your sand was bone dry. If it is hot outside you may find yourself needing more water than on a cool day. Adding the correct amount of water takes practice.

Mixing the concrete ingredients is the most important stage

You can mix concrete in a mixer, a bucket or on a piece of plywood with a square head shovel. This depends on the amount of concrete you need to mix. With all methods A thoroughly mixed, well proportioned cement mix is the secret to easily workable concrete. If you are a beginner you should probably mix the concrete much longer than you would otherwise think- and it can be hard work. The best concrete mix in an entire day of pouring concrete is the one that spins in the mixer throughout a long lunch break!

Mix all dry ingredients thoroughly before getting ready to add water. You will add water in SMALL quantities mixing thoroughly between adding more. You will find that the small amounts of water that you add will get immediately absorbed by the overly dry mix and you will feel compelled to add a big splash of water. DO NOT DO THIS!

The concrete will readily absorb water to a certain point of saturation, after which seemingly a thimbleful more will reduce your fine sculpting concrete to a watery mess. Concrete that has been over watered is completely compromised in terms of strength. You may be able to add ingredients or wait a while to get some sort of useable substance but this seriously weakens the end result in an undetectable fashion to the naked eye.

Have patience with adding water and you can quickly find yourself becoming more familiar with the total amount needed on any given day. If you take the time to measure how much water you need in a mix, the next time you make it you can immediately use half the anticipated amount of water required, and add the second half very slowly to ensure that you do not over water.

Making perfect concrete mixes takes patience and practice

What you are going for with this mix is a relatively firm (not too watery) end result. The concrete should be able to be formed into a ball, but still squished between your fingers without breaking off into clumps. Instead it should almost ooze through your fingers. If you dropped a fist sized ball of concrete onto the ground from the height of your knees you should end up with an almost flat circle of concrete, but not totally flat. Pictures of the desired consistency are included in the sculpting section.

Try taking your point trowel and smoothing out the concrete like spreading icing on a cake. You should be able to get a smooth glossy-like finish by working it back and forth gently. If the concrete just spreads out to nothingness under the gentle weight of your trowel you have too much water. If the concrete tends to break apart instead of spreading smoothly you likely need just a touch more water. A little goes a LONG way at this point.

Once the concrete is mixed - pay attention to the ticking clock

You will have a few hours in most cases to work the concrete into its desired shape before it cures too much to be worked further. Direct sunlight and hot dry weather can considerably increase the curing rate of concrete so it is always better to err on the side of caution with concrete drying times as you certainly do not want a batch of concrete drying in the mixer before you can get to it. This goes for tools as well. Always keep your tools clean or you will quickly find all of your tools rendered useless by dried concrete.

You can work the concrete for a few hours, misting with water if it looks like it is drying too fast (cracking in direct sunlight). After the initial curing begins and the concrete begins to stiffen you should stop working the concrete as further disturbing will weaken the strength of the final product.

Be prepared in case you can not finish your project in a single session

If you find that you are not able to complete the project in one shot, be sure to leave your concrete work finished in an inconspicuous spot. You will always be able to see the joint between where new concrete was added to cured concrete and in most cases this will cause a weak link in the overall structure called a cold joint.

If you need to add new concrete to old concrete you must complete an additional step to ensure that the two sides bond together. You need to make a watery mix of cement, water and concrete adhesive called slurry.

Making slurry for bonding old concrete to new concrete

Prepare your new (regular) mix of concrete, cover and leave in a cool shaded place. Now you are going to make another very small mix, just enough to cover the joint where new concrete and old concrete will overlap.

Mix only water and cement powder until you have something resembling soft butter. Then add a generous dollop of glue to the mix and mix further. This soft butter looking mixture (slurry) must be applied along the entire joint where you wish to add new concrete while the slurry is still wet.

Right before you apply the slurry mix you must spray down the existing concrete piece with water so that it is thoroughly wet.

NOTE- This slurry mix dries FAST. On a hot day the slurry can cure in a matter of minutes so do not waste any time in applying the slurry to the cold joint and getting your fresh waiting concrete mix spread over top.

There are some products as well that can be purchased that will perform the job of this slurry mix. Generally these mixes are designed to set fast for joining surfaces and fixing active leaks. There are concrete mix additives that can cure in 30 seconds or even underwater to patch a leak. Many times dealing with these products you will find yourself distracted for a moment only to find your trowel firmly embedded in the solidified bucket of mixed concrete moments later. The home made slurry mix described above is much more forgiving than these commercial products and produces reliable results just the same.

Concrete curing times

Concrete takes almost a full month to reach maximum strength. All too often with beginners the anticipation gets the better of you and you end up destroying or damaging your creation by moving or working the concrete too soon after sculpting. I recommend waiting a minimum of 3 full days to begin working the concrete and a full 14 days before attempting to lift or move. Be sure to keep the concrete moist with water at least 2-3 times per day for the initial 3 days.

Now that you have a better understanding of how to make concrete that is great for sculpting artificial rocks, we can take a look at how you are going to sculpt it to look exactly like real stone.