How to Mix Concrete

What you are actually controlling: the water to cement ratio

Everything else about mixing is technique. The water to cement ratio is the part that decides how strong the concrete ends up, and it is set the moment you stop adding water.

The relationship is published. PCA's Design and Control of Concrete Mixtures, Table 9-3, which is the same table as ACI 211.1 Table 6.3.4(a):

Read it the other way round and it is a warning. Going from 0.57 to 0.68, which is one part water in ten more than you meant, is the difference between 4,000 psi concrete and 3,000 psi concrete. Both PCA and ACI caveat that these are approximate and depend on the aggregate, the cement and the curing, so treat them as the shape of the relationship rather than a guarantee.

How little water cement actually needs. ACI puts it at about 0.40 by mass for complete hydration, of which "only 0.2 unit of water by mass combines chemically with cement during hydration." The rest fills the gel pore space in the hydration products. You will see 0.42 quoted everywhere as the minimum; that is not an ACI figure. And below about 0.40 some cement simply never hydrates, which is not a defect. It acts as filler, and those are the strongest mixes.

One caveat that matters if you get freezing weather. ACI 318's durability rules cap the water to cement ratio by exposure class and also require entrained air. Per NRMCA's guide to selecting exposure classes, exposure class F1 caps it at 0.55 with a 3,500 psi minimum, F2 at 0.45 and 4,500 psi, and F3 at 0.40 and 5,000 psi, with a 5 to 6 percent air target depending on class. Bagged concrete mix is not air-entrained unless the bag says so, and you cannot entrain air by mixing harder. If you are pouring exterior flatwork that will freeze while wet, site-mixed concrete is at a real disadvantage against air-entrained ready-mix, and no competitor page seems willing to say that.

Pick a method: how much each one holds

The choice is mostly about volume, and every option holds less than it looks like it does. For reference, the bag yields both major brands publish are 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb, which works out to 45 eighty-pound bags per cubic yard.

Those are our calculations from published capacities and yields, not published batch figures, and the reason is worth knowing: US mixer manufacturers publish drum volume, not batch volume. The clearest statement of the distinction comes from Navy construction training material, which notes that mixer capacity is properly "expressed in terms of the volume of mixed concrete, not of dry ingredients the machine can mix in a single batch." The one manufacturer working figure we could find is Kushlan's own guidance for its 6 cubic foot mixer, which recommends four 80 lb bags per batch. Four bags is 2.4 cubic feet in a 6 cubic foot drum, so the working batch is about 40 percent of the drum rating. We extrapolated the 9 cubic foot row at the same ratio.

Two practical limits follow. Above about half a cubic yard, stop mixing and order ready-mix. Half a yard is 13.5 cubic feet, which is 23 eighty-pound bags, or four to five wheelbarrow loads of continuous mixing. The bag calculator works out how many bags your pour needs, and how much fits in a truck covers the other end.

How to mix concrete in a bucket

A 5 gallon bucket is exactly 0.668 cubic feet, and you cannot use all of it. Collomix, which makes mixing equipment, gives the rule that a bucket should be "about one-third larger than the maximum fill level" so material is not thrown out during mixing. That puts the working fill at about 0.50 cubic feet.

Which means one 60 lb bag fits a 5 gallon bucket and an 80 lb bag does not. A 60 lb bag yields 0.45 cubic feet, comfortably inside the limit. An 80 lb bag yields 0.60, which is over the brim before you allow for mixing action. Split it or use a tub.

There is also a weight argument. At about 145 lb per cubic foot, a bucket filled to the working line holds roughly 73 pounds of concrete, call it 75 with the bucket. Filled to the brim it is about 97 pounds, which is a two-person lift and a genuine way to hurt your back.

1. Measure the water for the bag you are using and pour about two thirds of it into the bucket first.
2. Add the dry mix. Tip the bag rather than shaking it, to keep the dust down.
3. Mix with a paddle on a proper mixing drill, or with a margin trowel and some patience, for three to five minutes.
4. Add the remaining water a splash at a time until it holds its shape when squeezed. Stop there.

Bucket mixing suits post bases, small repairs and anything under about two cubic feet total. Fifty-four bucket loads to a cubic yard is the number that tells you when to stop.

How to mix concrete by hand, in a wheelbarrow or tub

This is the default for one to six bags. A mortar tub is better than a wheelbarrow for mixing because the flat bottom lets you drag material rather than lift it, but a contractor wheelbarrow doubles as the transport.

Use a mixing hoe if you have one. It has a flat blade with two holes in it, and it drags and chops rather than scooping, which is what you want. A shovel works. Army FM 5-428 specifies platform mixing "by turning the pile over and over with shovels until the color is uniform and the mixture is homogeneous," so both tools are sanctioned.

1. Measure the water. Every source agrees on this and it is the step people skip. Collomix puts it bluntly: "Water should be measured precisely. Do not eyeball it."
2. Empty the bag and make a well. Pull the dry mix into a ring with a hollow in the middle.
3. Pour about two thirds of the water into the hollow. Not over the top, or it runs off the pile and out of the barrow.
4. Drag the dry material in from the edges. Chop and fold rather than stir. Keep going until nothing dry is left in the corners, which is where unmixed pockets hide.
5. Add the last third in stages. A cup or two, mix, look, repeat. Stop while the batch is still stiff.
6. Turn the whole batch at least three or four times. See the turning count below, which is better evidence than it sounds.

How many turns is enough? There is no standards answer, but two independent sources land on the same number. Navy training material says "four complete turnings are usually required." And a peer-reviewed study that tested 140 concrete cubes hand-mixed with one to seven turnings found strength "appreciably increased with increase in number of turnings from one to four times but remained almost constant beyond four times." One is US military training doctrine and the other is a laboratory study from a different continent, and they agree: three or four turnings, then it plateaus.

A note on the honest comparison: no US standard publishes a strength comparison between hand mixing and machine mixing. PCA says machine mixing is recommended and leaves it there. The real risk of hand mixing is not the method, it is that shoveling stiff concrete is hard work and the temptation is to make it easier with water.

How to mix concrete in a drum mixer

A drum mixer earns its rental once you are past about four bags, mostly because it keeps mixing while you place the last batch.

1. Start it empty and level. Run it before you load it.
2. Put about half the measured water in first. This is the manufacturers' own sequence, not a shortcut: it stops dry material caking on the drum wall and in the fins.
3. Add the dry mix with the drum turning. If you are batching from scratch rather than from bags, the loading order matters. Navy training material specifies coarse aggregate first, then cement, then sand on top, "to prevent excessive loss of cement as the batch enters the mixer."
4. Add the rest of the water after about a minute, holding back the last of it.
5. Mix three to five minutes. Quikrete's machine instruction is "about 3-5 minutes, until a uniform, workable consistency is achieved."
6. Discharge into a barrow and place it. Do not let a batch ride in the drum while you finish the last one.

The standards floor for mixing time is lower than you would guess. Army FM 5-428: "the mixing time should be at least 1 minute for the first cubic yard of concrete and at least 15 seconds for each additional cubic yard or fraction thereof." Three to five minutes on a one or two bag batch is longer than the minimum because small mixers are less efficient than the stationary plant the standard was written for, so use the vendor figure.

For comparison, a ready-mix truck mixes 70 to 100 revolutions at mixing speed per FHWA construction review guidance, with 90 minimum for front discharge trucks.

How to tell when it is mixed right

The field test is your hand. Squeeze a gloved handful. If it holds the shape of your fingers without slumping, it is right. If it crumbles it needs more water. If it runs, you have gone too far and cannot undo it.

The trade description is Quikrete's: properly mixed concrete "should look like thick oatmeal and should hold its shape when it is squeezed in a gloved hand." Worth being straight about something, though. No standard maps "thick oatmeal" to a number, and anyone who tells you it equals a specific slump is inventing it.

What is published is the slump range. The slump test drops a 12 inch cone filled in three rodded layers and measures how far the concrete settles. ACI 211.1 and PCA Table 9-6 recommend these:

One to three inches is a stiff mix by DIY standards. A batch that holds a squeezed shape is qualitatively in that neighborhood, which is the honest version of the oatmeal test.

What extra water actually costs

There is a clean conversion for this. NRMCA CIP 26 gives the dose as "1 gallon, or roughly 10 lb., of water per yd3 for 1 inch increase in slump."

Put that together with the strength table above and you can price an inch of slump. Take a 4,000 psi mix at 0.57 water to cement with 520 lb of cement per cubic yard, so 296 lb of water. Add a gallon, which is 8.34 lb, and the ratio goes to 0.586. The table's slope between 0.57 and 0.68 is about 9,100 psi per unit of ratio, so 0.016 costs about 146 psi. Run it at leaner and richer cement contents and it lands between 124 and 161 psi.

So one inch of extra slump costs roughly 150 psi. That figure is our arithmetic from PCA Table 9-3 and NRMCA CIP 26, not a published number. We looked specifically for a standards body that publishes a psi-per-gallon figure and there is not one, so the 150 and 200 psi numbers you will find quoted online are not traceable to anybody.

Quikrete states that "adding one extra quart of water per 80 lb bag can reduce the strength of the concrete by up to 40%." That is one vendor's claim and it sounds implausible until you do the arithmetic: a quart per bag across 45 bags is more than 11 gallons per cubic yard. Working it through PCA's table gives a 39 percent loss, so the vendor's alarming number is arithmetically sound. A quart is just a much bigger dose than it feels like.

Mixing from scratch with cement, sand and gravel

Buying portland cement and aggregate separately is cheaper per yard than bagged mix and worth it past roughly a cubic yard. The recipe both major brands publish is 1 part portland cement, 2 parts sand, 3 parts gravel.

Sakrete's portland cement data sheet gives it in weights, which is more useful: 1 part cement, which is one 94 lb bag or one cubic foot; 2 parts concrete or general purpose sand, 160 to 180 lb; 3 parts gravel or stone, 240 to 300 lb, with 3/4 inch recommended; and "approximately 6 gallons of clean water." One 94 lb bag of portland makes 4.5 cubic feet of concrete, and the mix stays workable at least an hour. Quikrete's equivalent sheet gives the same 1:2:3 and says five to six 94 lb bags make a cubic yard, so 470 to 564 lb of cement per yard.

One thing to know before you trust a ratio chart. No US standards body assigns a compressive strength to a nominal volume ratio, so every "1:2:4 equals 3,000 psi" chart in circulation is unsourced. Working back from the vendor's own water dosage instead puts a careful 1:2:3 site mix somewhere near 4,000 to 4,500 psi. The concrete mix calculator has the full argument, a chart of the common ratios and what each is for, and the proportions for whatever volume you are pouring. For what the strength numbers mean, see concrete psi.

Why dry ingredients measure more than the concrete they make

Loose dry material is mostly air. A 94 lb bag of portland cement occupies a cubic foot, but the solid volume of that cement is only 0.478 cubic feet, so more than half the volume of a bag of cement is void space. Loose aggregate is about 40 percent voids. When you mix, the paste and sand move into the gaps between the stones, and the result takes up less room than the ingredients did.

Work the Sakrete recipe by absolute volume and 6 cubic feet of loose ingredients yields about 4.37 cubic feet of concrete, a factor of 1.37. Sakrete's own published yield of 4.5 cubic feet gives 1.33. Both agree on roughly 1.33 to 1.40.

You will also see 1.5 to 1.6 quoted. That comes from an old military rule of thumb that 41 to 42 cubic feet of dry material makes a cubic yard, and that rule has a handling and waste allowance baked into it, because the same manual adds 10 percent for small jobs. Both numbers are useful and they answer different questions: about 1.35 is the physics, and 1.5 to 1.6 is what to buy.

Letting it rest, and what false set is

Experienced hands mix a batch, let it sit two or three minutes, then turn it once more before placing. Being straight about this: no standard prescribes it. No ACI, ASTM, PCA, NRMCA or manufacturer instruction includes a rest and remix step, and we checked.

But there is a real phenomenon behind the habit. ACI on false set: it "is a form of premature stiffening of the paste or concrete, which occurs within 1 to 5 min after mixing," caused by partially dehydrated gypsum in the cement, and it "can be eliminated by continuous mixing or by reworking." That is the legitimate basis. A batch that stiffens in the first few minutes is most likely false set, and remixing fixes it without adding water.

The rest also lets dry aggregate finish absorbing water, so the batch genuinely looks wetter after the second turn than it did at the first. Which is the practical value of the habit: it stops you adding water to a mix that was about to come good on its own.

Mixing in cold and hot weather

Cold. ACI 306R-16 defines cold weather as when air temperature "has fallen to, or is expected to fall below, 40°F (4°C) during the protection period." Note what that is: a trigger that starts the protection requirements, not a ban on placing. The widely repeated "never pour below 40 degrees" is a misreading. ACI 306 sets no absolute minimum air temperature for placement. What it does set is the concrete temperature to hit and hold, by section thickness:

So for a normal slab the question is not the air temperature at pour time, it is whether you can hold the concrete above 55 degrees for the protection period and keep it from freezing before it reaches about 500 psi. The subgrade, forms and reinforcement all have to be above freezing too.

Heating the mixing water is the efficient way to warm a batch, because water has roughly five times the specific heat of cement and aggregate. Mix the hot water with the aggregate first so the cement never meets very hot water directly, which can flash set it. We are not going to give you a maximum water temperature: the 140 and 180 degree figures quoted online are not verifiable from any free ACI, NRMCA or DOT source, so target the concrete temperature in the table instead.

A counter-intuitive point for cold weather. Per NRMCA CIP 12, water demand for the same slump is about 270 lb per cubic yard at 30 degrees against 310 lb at 110 degrees. A cold batch needs less water, not more. It looks stiff because it is cold, and people over-water it for exactly that reason.

Hot. ACI 305.1 limits maximum concrete temperature to 95 degrees at discharge. That limit is in the specification, ACI 305.1, not in the guide ACI 305R, which is a distinction most pages get wrong. Heat costs you set time: NRMCA CIP 12 puts initial set at 12 to 14 hours at 40 degrees against 2 to 4 hours at 120 degrees. Chilled water buys you up to about 10 degrees, and NYC's building department puts it nearer 8. Ice substituted for part of the mixing water buys up to about 20 degrees, because of the latent heat of fusion, which makes ice the practical move for a hot-weather DIY batch. Cure for at least three days. Cure time covers what happens after the pour.

Two hazards worth taking seriously

Most mixing guides skip this. They should not.

Wet cement burns, and it does it without hurting first. OSHA's publication on preventing skin problems from portland cement says wet cement "can damage the skin because it is caustic, abrasive, and absorbs moisture," and then the sentence that matters: workers "cannot rely on pain or discomfort to alert them to cement burns because cement burns may not cause immediate pain or discomfort." Quikrete's own safety data sheet puts the pH at 13 and says exposure "can cause caustic burns as severe as third degree." People have knelt in wet concrete in jeans for an afternoon and needed skin grafts.

OSHA names the protective equipment specifically: "butyl or nitrile gloves (rather than cotton or leather gloves)," boots high enough to keep cement out, waterproof kneepads or dry kneeboards, and eye protection. Cotton and leather soak it up and hold it against you. There is also hexavalent chromium in portland cement, which can cause an allergic dermatitis that OSHA notes "is long-lasting and employees can remain sensitized to Cr(VI) years after their exposure to portland cement has ended."

Dry mix is mostly crystalline silica. Quikrete's safety data sheet lists its concrete products as 70 to 90 percent crystalline silica sand, which is why tipping a bag makes a cloud worth avoiding. OSHA's construction silica standard, 29 CFR 1926.1153, sets a permissible exposure limit of 50 micrograms per cubic meter as an 8-hour average, with an action level of 25.

A correction while we are here: OSHA's Table 1, the list of tasks with prescribed control methods, contains no entry for mixing concrete, dumping bags or using a paddle mixer. It covers 18 cutting, drilling, grinding and crushing tasks. Mixing is not exempt from the standard, it just is not inside that safe-harbor table, so an employer has to assess exposure under paragraph (d) instead. If you see a page citing a Table 1 row for mixing, it is wrong. And OSHA regulates employers rather than homeowners, so none of this is a legal requirement in your own yard. It is still the reason to cut the bag low and tip it rather than shaking it, stand upwind, and wear an N95 while you are dumping dry material.

Where these figures come from

The strength and slump tables are PCA's Design and Control of Concrete Mixtures, chapter 9, which reproduces ACI 211.1. Durability limits are ACI 318 as tabulated in NRMCA's exposure class guide. The hydration ratio and false set are ACI. Water and slump dosing is NRMCA CIP 26, weather effects CIP 12. Mixing times are Army FM 5-428 and FHWA. Safety is OSHA 3351 and 29 CFR 1926.1153. Bag yields, water per bag and the 1:2:3 recipe are the manufacturers' own data sheets.

The batch capacities, the per-gallon strength penalty and the dry to wet volume factor are our calculations from those sourced inputs, shown step by step above so you can check them. Where we could not find a real source for a commonly quoted number, we said so rather than repeating it.

Next: how many bags you need, the mix ratio calculator, mixing Quikrete specifically, mixing mortar rather than concrete, and why dry pouring does not work.