Making “good” concrete, mortar, cement paste or Hypertufa is a lot like baking a cake. You not only need the right ingredients…you have to put them together properly. Here are a few suggestions for improving the mixing process.
The “right” way to measure out ingredients for any cementitious mix is by weight. Keep an eye out for a small scale. It will give you much greater control & consistency, particularly if you are prone to experimenting. If you can’t find a scale, use a consistent form of measurement for all your ingredients & projects. Whether it is a coffee can, yogurt container or kitchen measure, stick with it.
Keep Good Notes
Unless you have a total photographic recall, this is the only way to tell over time what works, what doesn’t work, and more importantly…why. Start with the working environment. Hot? Cold? Humid? Dry? Windy? Then record all the ingredients, dry & wet, by whatever “standard” measuring system you are using. Make note of how long you dry mixed. Record when you added the first wet ingredients and how long you “wet-mixed” the batch. Also, record when you began applying the mix & when you finished. If you run into any problems while working (sets too quickly, too much slump, etc.) make note of that as well. I keep a big batch of 3 x 5 cards handy and put my formulations on one side & working notes on the other. That way, whether a problem shows up minutes or months later, I can go back and re-examine the mix & processes to help determine what went wrong. It also helps prevent those “Duh” moments, like the…”Oh yeah, I still need to add some cement” one. By the same token, there is absolutely nothing more frustrating than having something turn out really great…and not know why. Good notes are the difference between wild & wooly experimenters…and consistently creative craftsmen & women. And if you include all of your finishing processes as well, you’ll be just that much further ahead of the game. It may seem like a hassle at first, but if you make it a habit, I can positively guarantee that you will wind up spending more time creating and less time scratching your head.
Before you begin worrying about how much water to add, make sure all of your dry ingredients are thoroughly mixed. You absolutely CANNOT overdo it. Whether you are mixing by hand or have a ninety horse motor doing it for you, make certain that all the dry ingredients are fully and evenly distributed throughout the mix. I typically run my small mixer for at least fifteen minutes before I ever introduce any of the wet media. When hand mixing smaller batches, here’s a handy trick that makes for a well-combined mix. Put all of the dry components into a plastic, 5-gallon bucket that has a tight fitting lid and the handle removed…then roll it all around the yard for several minutes. The tumbling action does a surprisingly good job. And if you want to go one step further, there is a little manual mixer available called the “Oddjob” that is designed for tumbling mixes. While I personally found it to be leaky and pretty close to worthless for actually mixing anything but very wet concrete, it seemed to be quite adept at dry mixing the ingredients. It has “fins’ cast into its poly drum body that helps kick the mix around better than a standard 5-gallon bucket.
And speaking of “dry”…try to make sure and use ingredients that really are. Wet sand & gravel carry a surprising volume of water on their surface and will definitely affect your water/cement ratio. Commercial ready-mix plants routinely test their aggregates with a hydrometer and adjust the “added water” volume accordingly in order to properly match the mix design. I don’t own a hydrometer, so I allow all of my sand to dry for a week or more in my barn before I put it into a sealed container for later use. Other good arguments for keeping your sand (and other aggregates) in sealed containers involves Mother Nature. Leaves, twigs & such invariably wind up in outdoor exposed materials and some contain chemicals and things you don’t necessarily want in your mix. Oak trees, for example, are very high in Tannic Acid. Pine needles are heavy with resin. And don’t forget the alternative name for a “sand pile”. It is also often referred to by some as…a “litter box”.
And, if you are using reinforcing fibers (alkali resistant, of course), be aware that most are designed to be dry-mixed. That is based on the commercial assumption that a power mixer will be handling the task. And, since I employ a small mixer for just about all of my work, they perform as advertised and I have never had a “clumping” problem. That said, there are today a wide and growing number of fiber reinforcement products in a variety of lengths & materials and a goodly number of craft folks swear by pre-wetting some of the types they are using first. Especially if they are hand-mixing small batches. As for myself…I’m going to stick with what works for me. I suggest you experiment a bit and do likewise. The only caution I can offer here is to take into account that, just like using wet sand, you will also be adding a volume of water and need to adjust your mix accordingly.
Adding Wet Ingredients…Like Water…
Start by calculating just how much water the mix should require. That’s the “Water/Cement Ratio” that seems to confound so many people. “How much is enough/too much?” Like so many things in life, the answer is…”Well now,…that depends…”. Sorry, but that’s just how it is. There are many variables and a few hard, fast rules. But, here are a few…
- Water content should always be calculated by weight and expressed as a relative percentage of the mixes total cementitious ingredients (See the “Glossary” on this site if you have any questions about “Cementitious” or “Water/Cement Ratios”).
- Pre-calculating and measuring out the desired amount of water or wet mix in advance will give you the best control and the most consistent results.
- The higher the water content, the lower the strength.
- Conversely, the lower the water content, the higher the strength.
- Most “common” mix designs contain around 24 to 33% water…but some go below 10% while others can be as high as 150%.
- Be aware that most discussions regarding minimum water to hydrate apply to commercially placed mixes that tend to be on the “wet” side for contractor convenience. “Dry Pack” (or “Dry Tamp”) Concrete, Roller-Compacted Concrete and the original Roman Concrete all have water/cement ratios that are far below what is considered average. And they are all exceedingly strong. Most “sculpting” formulations are also significantly dryer than the average mix to attain the proper workability.
- Never use anything but clean, “potable” (fit to drink) water in your mix.
- Hot mixing water will greatly accelerate the set and sometimes even the rate of cure. It is a very common cause of early-onset cracking and mix failure due to its rapid evaporation. Always check the temperature of your water supply before you add it. A hose lying in the sun can heat water to a point that it can cause the hose to burst and the failure of just about any mix. However…warm water can be successfully employed when working out in cold weather to help the mix achieve a proper set. Just don’t overdo it. Water temperatures of around 125 degrees F are about as high as you probably ever want to go.
- Cold mixing water will retard the set and can affect the cure similar to low ambient temperatures. If it is too cold, hydration may not take place properly or at all. Groundwater supplies in some areas can be surprisingly cold (just above freezing), so, once again it is important that you check your supply’s temperature before adding it to the mix. And…coolwater (like warm) can be employed to offset ambient temperature. When the thermometer rises, using cool water will help offset the heats’ tendency to accelerate setting. Just remember that you never want to go below the 50-degree mark. From about 60 degrees F, up to anything that is below the ambient temp, and some degree of retarding the set will take place. Since it soon rises to match ambient, it will have little or no effect on the actual cure rate.
- Also, be advised that not all water is created equal. Mineral & salt content in water can vary greatly from area to area, even from “approved” public supplies. If you experience consistent problems with an otherwise proven mix, you may be encountering water problems. Sometimes a simple “taste test” may give you a clue as to an unusual water situation, but short of lab testing, this is a difficult issue to nail down. About the only way I know of isolating & identifying this problem is to test a carefully prepared mix formulation with the water supply source as the only variable. Distilled water can be used to test this possibility but nix on the Perrier or “Mineral” water.
- SUMMARY: There is no one, single “right” consistency. The only way to make sure the water content and consistency are right…is to first decide what results you want to achieve, then design the mix accordingly.
EXAMPLE ONE – Molds & Casting: If you are casting into a mold or a form, the water content will be partly driven by the nature of the mold material involved. If it is a soft, stretchy or flexible material, like latex with no solid backing to support it (like a rigid Mother Mold) the mix will need to have a lot of “flow”, i.e. a higher water content…or, have an admix such as a superpalsticizer introduced that will increase its flowability. A rigid or well-backed mold or form, on the other hand, can tolerate having the mix tamped or packed in vigorously without deforming, and can, therefore, accommodate a dryer mix. Much, much dryer. “Dry Packing” is a commonly employed commercial technique for two main reasons. The product is much stronger, and…the castings can be de-molded almost immediately. This allows the same mold to produce many items per day, versus having to wait several days for a wet mix to set before de-molding and casting it again.
EXAMPLE TWO – Hand Forming or Sculpting: Hand forming or sculpting onto an armature or framework requires a whole different approach and mix. Instead of “flowing into”, you want it to “cling onto”. And that equates as dryer and possibly “stickier”. Latex admixtures and old fashioned Hydrated Lime both make for a stickier mix. But be advised on each. Adding plastic will densify your mix and high volumes will yield a surface that “skins over” and is often difficult to work. I have some “experimental” pieces made with very high proportions of latex that even resist acid etching. Not what you want if you are planning on using acid etch color. And Hydrated Lime is extremely caustic and dangerous to work with. Most experienced masons have both a very high respect for its use…and the scars that taught them that respect. It can blind and burn in very small amounts and is not advisable for use by amateurs. The addition of reinforcing fibers will also add a significant amount of “cling” to just about any formulation and is particularly helpful when applying the mix onto stucco lath or other such materials. But bear in mind that fibers dramatically limit the finishing techniques that can be employed on any surface they are used on and often must be “torched” off. I personally never use them in any of my finish layers…only in the sub-layers.
Admixture Note: If you are incorporating any wet admixes such as a latex modifier, bonding agent, liquid retarder or anything else…do not just dump it into the mix. First estimate approximately how much total water the full mix will require, measure out about half that amount, then combine all of the wet ingredients into that and mix them up well. Once they are well mixed, you can combine it with the dry ingredients and then very slowly add the balance of your water.
Superplasticizer Note: If you employ a superplasticizer (High-Range Water Reducer)…be advised that the effect of adding water to your mix will be greatly exaggerated. It can go from way too dry to soup in an instant with the addition of seemingly minute amounts of water. That’s just the plasticizer doing its’ job. Just go slow and be extra conservative when adding water.
And One final piece of advice…just when you think you need to add “just a little more water”…don’t. Try mixing it a bit longer instead.
“Shaken, not stirred.” Hand Mixing VS Power Mixing
Sean Connery’s inimitable & enduring character (actually, author Ian Flemmings’) had a distinct preference for how his Martini was mixed for a reason. Shaking vigorously with the ice results in a more thoroughly combined mixture than just stirring. Admittedly, running it through a blender would combine it even better, but for whatever reason, Flemming apparently didn’t think sipping a Martini Slush through a straw seemed to fit James Bonds’ personality, so he settled for just shaking it. You, on the other hand, need to do whatever is necessary to ensure that your cement-based cocktail, whether concrete, mortar, cement paste, ‘Tufa or whatever, is as thoroughly combined as possible. All cement-based mixtures require a very thorough blending of all their ingredients if they are ever to achieve their full potential. We’ve already stressed the importance of “dry-mixing” prior to the addition of any liquid components, so now let’s look at how we can optimize the process of combining dry with wet.
Hand Mixing Hints
“Tempering” is when wet meets dry and the water + cement magic begins to happen. And once it does, there’s no turning back, so have everything lined up & ready to go. Once you have measured out all of your dry ingredients and measured and prepared your wet ingredients, you are ready to combine them in your chosen mixing container. Preferably one that is made for the task and has a smooth bottom & sides. Inexpensive plastic “mixing tubs” are available in the masonry departments of most Big Box stores and meet the above requirements perfectly. They are wide, shallow and have fully rounded corners that make mixing a lot easier than trying to work in a deep container. Easy to clean, too.
Form all of your dry ingredients into a short “volcano” in your mixing container. A low mound that is hollowed out in the center. Into that center, pour about half of your calculated water or wet mix. A hoe, or better yet, a mortar mixing hoe (with a short handle and a couple of holes in the blade) is handy for folding and combining the wet & dry materials. Work back and forth, pulling & pushing, bringing only small amounts of dry into the wet until the wet mix is as distributed as possible, then slowly add more of your water/wet mix into the center and continue the process. Just don’t try to combine too much wet & dry at once or you wind up fighting “clumps”. What you want is a very fluid working mix that slowly increases in viscosity. And don’t rush it. It takes about ten to fifteen minutes of steady mixing to thoroughly combine the ingredients for most batches up to thirty pounds or so. Once you are satisfied that everything is adequately mixed and the right consistency for the job, it’s a good idea to transfer it all into a more vertical container, like a bucket, rather than working from the tub. This will reduce the surface area and slow the mixes moisture loss to evaporation. A lid or damp towel over the top will aid in retaining moisture as well, especially if you are working in the sun or a breeze. Another suggestion is to let the mix “rest”, that is, sit covered for maybe five minutes before you begin working from it. This will allow the moisture to further distribute & equalize itself. It’s also a good time to hose out your mixing tub before it goes solid.
Machine Mixing…first you have to have one…
If you are one of the lazy people (like myself) who prefer to let mechanical devices handle the tedious tasks, a powered mixer is definitely the way to go. Especially if you mix in any quantity. And, like many modern contrivances, they generally do a much better job than most humans. If you already own a mixer, congratulations and feel free to jump ahead to the next section. If not, here are some thoughts on acquiring one.
Realistically asses what you will use it for and establish a budget that you can live with. To accomplish this, do some homework. Google both “Cement Mixers” as well as “Concrete Mixers” because in spite of the fact that there is technically no such thing as a “cement mixer”…both terms are used freely & interchangeably. There are also dedicated “Mortar Mixers” that are of a totally different design. A mortar mixer has stationary set of paddles around which the drum turns…where a cement or concrete mixer has a rotating drum & paddles. Mortar mixers are great for what they are intended for..mixing mortar…but cannot handle any aggregate much larger than basic coarse sand without something breaking. The tolerance between the drum and paddles is simply too tight. Talk to some pros in your area. What do they look for in a mixer and why? What do they like or dislike about what they own? You get the picture…inquire before you invest. Features & prices are all over the place, and only you can decide which ones are worth how much to you. Here are a few “feature” considerations to take into account:
1. Size There are a wide range of homeowner & light commercial mixer sizes available as measured in cubic feet or cubic yards of load capacity. Typically from about one and a quarter cubic feet to around a cubic yard. Unless you are going into the foundation contracting business, something around two to five cubic feet in capacity should be more than adequate for most art projects. You can only work so much at a time before it sets up, so take that into account as well. Remember…a single “Yard” is 27 cubic feet. That’s a lot of concrete, hypertufa, jelly beans or just about anything else.
2. Power Source Gas or electric? (Sorry, no solar just yet) Once again consider your use & situation. Gas powered units are more expensive, but they can be used just about anywhere that the noise & exhaust can be tolerated. Electrics are fairly inexpensive and quiet but have to be within reach of an outlet.
3. Portability Some units are stationary, some highly portable with self-contained wheels, and some are just too darn heavy for most people to move about when fully loaded…even with wheels…so keep the fully loaded weight and your personal hauling capacity in mind.
4. Drum Construction Metal or plastic? The new generation Poly drums are durable, rust-free and easy to clean. The only caution I can offer to plastic drum owners is to never whack it hard in freezing weather as it might break under those conditions. So why then do I own a steel-drum unit? Probably just old-fashioned I guess and relate “real” durability to iron. That coupled with the outstanding reputation for reliability and durability of the manufacturer involved and the price being “right”.
5. User Features How is the drum emptied? Does it tilt up & down easily or do you have to go through a lot of gyrations to get your mix out?. How about swiveling side to side? A few even have mixing angle adjustments for the drum, a handy feature that is discussed in “Tricks of the (Cement) Trade”.
6. Brand Reputation/Durability This where the biggest differences probably lie. There are a few really outstanding makers out there…and a whole lot of junk. Take a serious look at the warranty & return policies for any unit you are considering and gauge it against the price (Will the store take it back or do you have to ship it to Beijing?). I buy some tools that I classify as disposable. Those are the really cheap ones that I know are gonna’ get destroyed anyway. Others I view as an investment. Some of which I’ve had for over twenty years and still do what they were designed to do quite well. My mixer is not that old, but I definitely acquired it as an investment grade tool. That it was half-price was simply good fortune combined with my dear wife’s negotiating skills. This is where talking to owners & operators can really pay off. If you don’t plan on working it all that hard, you can probably get by with a cheap-o and put the savings to good use elsewhere. If breaking down in the middle of a paying job is of greater concern, go with the best you can afford.
7. Shop, Shop, Shop Once you have your criteria worked out and a unit selected it’s time to get down to some serious price shopping. You’ll soon discover that “the price” ain’t necessarily set in concrete (sorry..I just couldn’t resist that). Look at the many online discounters and you’ll probably find variations of as much as 20 to 30% for any given make & model. Just don’t forget to factor in shipping. Locally, there are some real deals to be had as well. “Scratch & dent” items are often up to half off. And a scratch or a dent on a concrete mixer is hardly the same as on a shiny new Lexus. And don’t be afraid to make an offer on scratch & dents, demos, and floor samples. That’s exactly how I got mine at a bargain basement price…or rather my darling savvy wife did. The Depot had lost the box, instructions and one cotter key to exactly the unit I had been looking for so she cornered the department manager and told him she would give him half the retail price in cash “right now, so decide, will you take it or not?”. He took it. Done. So negotiate. Make an offer. You have absolutely nothing to lose…they will always be happy to take the full price. Oh, and by the way…these days you can download most manufacturers product manuals online.
8. Price Ranges I just saw a little 1.25 cubic foot unit on sale at Harbor Freight for $99 dollars. No, it’s not a top-of-the-line unit, but the price is certainly right for an “entry model” (hey, what do you expect for 99 bucks?). And, “Name Brands” in the 3 to 5 cubic foot size typically go for about $300 to $500 with other models falling somewhere inbetween…so you have a pretty good spread of price options to pick from. These are all electrically driven. If you need the gas or diesel power option, be prepared to pay well over a grand for the low-end units and 2 to 5 thousand for the “good stuff”. My mixer is a 5 cubic foot, name brand electric from Home Depot that sold four years ago for about $360.00. My wife talked them down to $180.00. Happy hunting.
Machine Mixing Hints
For those of you who are using a power mixer, be advised that the piece of equipment I am referring to is called a “Concrete Mixer”. Not a “Cement Mixer”. No matter what you’ve heard or what you may be dumping into it. Or, for that matter, what I may occasionally slip and call it. Personally, I really do use mine to mix neat cement paste, but since just about every professional on the planet will argue that “Cee-ment don’t need no mixin’, son”… I go with the flow on this one if only to avoid having to explain a process that they will also tell me will never work.
Likewise, for this session, we’ll stick to concrete, mortar & hypertufa. The ins & outs of mixing “neat” cement paste for sculpting is a different animal that will be dealt with in a forthcoming segment of the “How-To” tutorial on constructing the Faux Bois table that’s on this site.
OK. By now most of you probably know the differences between cement, concrete, mortar & hypertufa, but here are the short read definitions for the folks who stumbled onto this site by accident, taken right from our very own “Big & Ever Growing Glossary of Cement, Concrete & Hypertufa Terms”…
Cement A building material manufactured by grinding calcined limestone and clay to a fine powder, which can be mixed with water and poured to set as a solid mass or used as an ingredient in making mortar or concrete.
Concrete A mixture of cement (usually Portland), various aggregate fillers (sand, gravel, rock) and water which chemically hardens into an insoluble monolithic composite. The most common formulation is often referred to as a “standard 1-2-3 mix” and consists of One part Cement + Two parts Sand + Three parts Gravel (or Stone).
Mortar A mixture of cement, sand, and water typically used in masonry construction. The mixture may contain masonry cement, or standard portland cement with lime or other ad-mixtures added to produce greater degrees of plasticity and/or durability.
Hypertufa A cement-based formulation designed to reproduce the porous look of the naturally occurring lightweight rock known as Tufa and the resulting products manufactured from this formulation. Mix ratios vary wildly, but the most common “basic recipe” consists of 1 part Portland cement + 1.5 parts Peat + 1.5 parts Perlite. The term literally translates as “Fast” or “Beyond”(hyper)…and… “Tufa” (the name for an unusual porous geological formation).
The common denominator in these concoctions is cement and most standard-issue home concrete mixers will handle all of the above very nicely. But each does have its’ own set of quirks. Most of which are determined by the nature of the “other” ingredients.
Mixing Basic Concrete
Before we even get started, let me state this as emphatically as I know how…
NEVER…EVER…put your hands, arms or any other body part that you want to stay attached into a moving mixer! Even if it is only a ¼ to ½ horsepower motor…the average small mixer is more than capable of completely removing any given limb from your body. And…NEVER…EVER…insert any kind of tool, stick or other objects into a moving mixer. The rotating mixing blades are designed to hook and grab wet concrete…they are certainly up to the task of snagging you, your clothing or anything in your hands and really ruining an otherwise good day. And finally…NEVER…EVER…NEVER…allow a child anywhere near a mixer that has power going to it…let alone while it is running! Concrete mixers are great tools, and they are powerful tools. And like any power tool, they will hurt you badly if you use them foolishly. If you want it to stay a tool for enjoyment, read the manual and observe every safety precaution religiously and consistently.
OK…now…where were we? Oh yeah, Mixing Concrete…
Regardless of the formulation, start by gathering up everything you will need for a given project. Not just what is going in the mixer, but everything. Because once the water hits the cement the game is on, and you don’t want to be chasing around trying to find some special doo-dad that you like to work with or something you really need while the cement is setting. Hydrating cement waits on no one. I generally try to do a complete “walk-through” of all the processes, from beginning to end and think about what I’ll need for each step and then try to pull them all together. Make a shopping list if you have to. Here’s how some of mine go, but they can obviously vary depending on what tasks are involved.
A Typical Walk-Through Scenario…
1. Start by preparing an ingredient list with everything pre-calculated (by weight, of course), including proper water content and all admixtures & additives. I put mine on a 3×5 card and use it to check-off each component as I add them. I also put environmental conditions, working notes & observations on the back. A cell-phone call in the middle of formulating can make it all to easy to lose track of what has & hasn’t been added. (…gee, great story, well gotta’ run now Mom, click…Hmmm…did I put the danged cement in yet or not?) A checklist will tell you. And even before you go tossing things together, use the list to make sure that you have an adequate supply of everything required to complete the task of the moment.
2. Gather up any & all special mixing tools & accessories and make sure they are where they need to be. (…now, where did I put those extra rubber gloves…) I’ve found it handy to keep a small trowel for making a trench in the dry mix to pour my wet mix into and a mason’s hammer for knocking loose any dried build-up from the mixing blades when necessary. And I make sure I have a heavy duty scrub brush within reach of the mixer because cleaning it should always be part of the mixing routine. Do it now it’s easy. Do it later its dynamite.
3. Make sure that all of the sculpting or working tools I anticipate using are where they should be and ready to go. Likewise for any supplies like Bonding Agent or reinforcing material. Fill a 5-gallon bucket with clean “rinse water” and locate it close to where I’ll be working so that I can quickly clean up tools & gloves as needed. And I check to make sure that whatever I will be transferring the mix into for working is clean, dry and ready to host the mix as soon as it comes out of the machine.
4. Insure that everything necessary to begin & maintain proper curing is in place. I often work 8 to 12 hours straight and have to start monitoring & maintaining hydration moisture levels long before I am done working the mix. That means my misting hose, plastic bags or sheeting, tape, spring clips, old towels or burlap, stand-offs to keep anything from touching the surface…anything associated with curing a particular piece of work must be on standby and ready for immediate use.
Bottom line. Everyone works differently and uses what is appropriate for them and the job they are doing. But we all need to prepare as best we can. And a “dry run” walk-through will save a lot of time, materials, money & grief for just about anyone.