Concrete Mix Selection, Estimating, and Ordering vs. Bag Mixing

Estimating Concrete Volume (and Why It’s Never Just Length × Width × Thickness)

Accurate estimating prevents two common problems: running short mid-pour (cold joints, rushed finishing) or paying for excess. Start with the basic slab volume, then add allowances for thickened edges, footings/haunches, and waste.

Step-by-step: basic slab volume

1. Convert all dimensions to feet. Thickness is often in inches; convert by dividing by 12.

2. Compute cubic feet: Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft)

3. Convert to cubic yards: Volume (yd³) = Volume (ft³) ÷ 27

Worked example: simple pad

A 10 ft × 12 ft pad, 4 in thick:

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• Thickness: 4 in ÷ 12 = 0.333 ft

• Cubic feet: 10 × 12 × 0.333 = 39.96 ft³ (≈ 40 ft³)

• Cubic yards: 39.96 ÷ 27 = 1.48 yd³

Add thickened edges (common on small slabs)

Thickened edges add volume beyond the “flat slab” calculation. Treat the thickened portion as an additional strip around the perimeter.

Method: calculate the extra thickness (beyond the slab thickness) times the thickened-edge width times the perimeter length.

Extra edge volume (ft³) = Perimeter (ft) × Edge width (ft) × Extra thickness (ft)

Example add-on: same 10 × 12 slab, 4 in thick, with a 12 in wide thickened edge that is 12 in thick total.

• Perimeter: 2 × (10 + 12) = 44 ft

• Edge width: 12 in = 1.0 ft

• Extra thickness: (12 in − 4 in) = 8 in = 0.667 ft

• Extra edge volume: 44 × 1.0 × 0.667 = 29.35 ft³

• Extra edge in yards: 29.35 ÷ 27 = 1.09 yd³

Total estimate before waste: 1.48 + 1.09 = 2.57 yd³

Waste factors (don’t skip this)

Waste comes from uneven subgrade, spillage, over-excavation at edges, and leftover in wheelbarrows/mixers. Typical allowances:

• 5% for tight forms, experienced crew, easy access

• 10% for most small projects

• 15% for irregular shapes, thickened edges, limited access, first-timers

Example: total 2.57 yd³ with 10% waste → 2.57 × 1.10 = 2.83 yd³. For ready-mix, you would typically order 2.75–3.0 yd³ depending on supplier minimums and your confidence in the forms.

Converting Volume to Bags (and Why Bag Counts Surprise People)

Bag yields vary by product and bag size. Always read the bag for “yield” (often listed in cubic feet). Common approximate yields:

Step-by-step: bags needed

1. Compute total volume in cubic feet (including thickened edges and waste).

2. Divide by bag yield: Bags = Total ft³ ÷ Yield per bag (ft³)

3. Round up and add a small buffer (1–3 bags) for touch-ups.

Worked example: the 10 × 12 slab with thickened edges

We had 2.57 yd³ before waste. Convert to ft³: 2.57 × 27 = 69.39 ft³. Add 10% waste: 69.39 × 1.10 = 76.33 ft³.

• Using 80 lb bags at 0.60 ft³: 76.33 ÷ 0.60 = 127.2 → 128 bags

• Using 60 lb bags at 0.45 ft³: 76.33 ÷ 0.45 = 169.6 → 170 bags

This is why many “medium” slabs quickly become impractical as bag-mix projects: the handling and mixing time become the limiting factor, not just the material cost.

Concrete Mix Components and Beginner Terms You’ll Hear

Concrete is a controlled recipe of cement, water, fine aggregate (sand), coarse aggregate (gravel/stone), and often admixtures. The terms below affect strength, workability, finishing, and durability.

Compressive strength (psi)

Strength is usually specified as a 28-day compressive strength (for example, 3000 psi, 3500 psi, 4000 psi). Higher strength generally means more cement and/or lower water-cement ratio, which can change finishing feel and cost.

• Typical small flatwork: 3000–4000 psi depending on exposure and local practice.

• Trade-off: higher strength mixes can be “stickier” and may set faster depending on cement content and admixtures.

Slump (workability)

Slump is a measure of how fluid the concrete is. Higher slump = wetter/more flowable. Beginners often assume “wetter is better,” but excess water reduces strength and increases shrinkage cracking.

• Common target slump for hand-placed flatwork: roughly 3–5 in (varies by mix and placement method).

• Key idea: ask for workability via slump and admixtures, not by adding water on site.

Aggregate size

Coarse aggregate is specified by nominal size (for example, 3/4 in is common). Smaller aggregate can help flow through tighter forms and around reinforcement, but may increase cement demand and cost.

• Rule of thumb: choose an aggregate size that fits your form dimensions and spacing; 3/4 in works for many slabs, while 3/8 in pea gravel mixes are useful for tighter spaces or more detailed work.

Air entrainment

Air-entrained concrete contains many tiny, intentionally created air bubbles that improve freeze-thaw durability and resistance to deicing salts. It’s commonly specified for exterior flatwork in cold climates.

• Typical air content: often around 5–7% for freeze-thaw exposure (local specs vary).

• Finishing note: air-entrained mixes can feel creamier but may be more prone to surface issues if overworked or finished while bleed water is present.

Water-cement ratio (w/c)

The water-cement ratio is the weight of water divided by the weight of cement in the mix. Lower w/c generally increases strength and durability, but reduces workability unless you use water-reducing admixtures.

• Practical takeaway: control water carefully—especially with bag mixes. “Just a splash more” adds up fast.

Admixtures you may encounter

• Water reducer / plasticizer: improves flow at the same water content (better than adding water).

• Mid-range/high-range water reducer: more dramatic workability increase; can help with placement and finishing.

• Accelerator: speeds set (useful in cold weather); may reduce working time.

• Retarder: slows set (useful in hot weather or long placements).

• Fiber reinforcement: helps reduce plastic shrinkage cracking; not a replacement for rebar/mesh where those are required.

Ready-Mix vs. Bag Mixing: How to Decide

Project size thresholds (practical guidance)

• Under ~0.25 yd³ (≈ 6–7 ft³): bag mixing is usually reasonable (small pads, posts, small repairs).

• ~0.25 to 1.0 yd³: depends on access, helpers, and how continuous the pour must be. Bag mixing can work with good staging and at least one helper.

• Over ~1.0 yd³: ready-mix often becomes the better choice for consistency and speed, unless access prevents delivery.

• Over ~2.0 yd³: bag mixing is typically impractical for most DIYers due to the number of bags, mixing time, and risk of cold joints.

Access and placement logistics

• Ready-mix wins when the truck can get close, you can chute directly into forms, or you can use wheelbarrows for short distances.

• Bag mix wins when access is tight (gates, backyards, stairs) and you can’t get a truck near the pour, or when you need very small quantities over time.

Labor and consistency

• Ready-mix: consistent batching, fast placement, less physical strain, fewer chances to vary water content.

• Bag mix: quality depends on your batching discipline. Inconsistent water measurement is the #1 cause of variable strength and finish.

Cost considerations (what actually drives cost)

• Ready-mix costs: per-yard price + delivery + short-load fees (common for small orders) + possible waiting time charges if unloading is slow.

• Bag mix costs: higher material cost per cubic yard + tool wear + your time + helper time + potential rework if the pour is not continuous.

When comparing, convert bags to an equivalent yard price and include the value of time. A “cheap” bag pour can become expensive if it takes twice as long and compromises the finish.

What to Tell a Ready-Mix Supplier (Order Checklist)

When you call, be ready with clear specs. Suppliers can help, but you get better results when you know the key terms.

• Quantity: total cubic yards you want delivered (include waste). Ask about minimum load and short-load fees.

• Compressive strength: e.g., 3500 psi or 4000 psi (or whatever your local requirements are).

• Air entrainment: “air-entrained” if exterior freeze-thaw exposure applies; specify target air % if needed.

• Slump range: request a workable range (for example, “target 4 in slump”). Ask for a water reducer if you need more flow without adding water.

• Aggregate size: typically 3/4 in; request smaller if forms are tight or placement is difficult.

• Admixtures for weather: retarder for heat, accelerator for cold, as appropriate.

• Delivery constraints: driveway weight limits, overhead wires/trees, narrow access, steep grades, gate widths, HOA restrictions, time windows.

• Unload method: chute placement, wheelbarrow distance, or if you plan to use a pump (if applicable).

Planning discharge location and time

• Pick a discharge point that minimizes re-handling. Every extra shovel move costs time and increases the risk of segregation (stone separating from paste).

• Plan the truck path and turning radius. Confirm the truck can enter and exit without backing long distances.

• Stage wheelbarrow routes if needed. Use plywood paths over soft ground and remove trip hazards.

• Schedule for conditions. Early morning is often easier in hot weather; mid-day can help in cold weather.

• Have enough labor at the start. Ready-mix arrives all at once; you need to place and finish at the pace the concrete dictates.

Bag Mixing: Consistent Batching for a Continuous Pour

Bag mixing can produce excellent results if every batch is consistent and the pour doesn’t stall. The goal is to keep the mix uniform from the first wheelbarrow to the last.

Staging so the pour stays continuous

• Pre-stage bags: stack them near the mixer but protected from moisture.

• Pre-measure water: fill marked buckets or jugs so each batch gets the same amount.

• Assign roles: one person mixes, one transports/places, one levels/finishes (as available).

• Keep a steady rhythm: avoid long gaps between batches to reduce the chance of visible transitions.

Step-by-step: consistent mixing procedure

1. Measure water first (don’t guess). Start slightly under the bag’s recommended water range; you can add small amounts if needed.

2. Add about 2/3 of the water to the mixer. This reduces dry pockets sticking to the drum/pan.

3. Add the bagged mix. Let it blend until the dry material is fully wetted.

4. Add the remaining water gradually. Stop as soon as you reach a workable consistency; do not chase “soupy.”

5. Mix for a consistent time each batch. Use a timer. A common target is about 3–5 minutes after all material is in, depending on mixer type and batch size.

6. Dump promptly and repeat. Keep batch size consistent; don’t overload the mixer.

A simple water-control system (highly recommended)

Create a “batch card” and stick to it:

Mix: Brand X 80 lb high-strength concrete  Bag yield: 0.60 ft³  Water target: 3.5 qt per bag  Mixer load: 2 bags per batch  Water per batch: 7.0 qt  Mix time: 4 minutes after full wet-out

If the mix needs slight adjustment due to temperature or moisture, adjust in small measured increments (for example, 0.25 qt at a time) and record the change so the next batch matches.

Avoiding retempering (adding water later)

Retempering is adding water to concrete that has started to stiffen. It can reduce strength and increase shrinkage and surface dusting.

• Best practice: make each batch workable at the start through correct water measurement and proper mixing time.

• If a batch is too stiff: first verify it was mixed long enough. Under-mixed concrete can look dry. If still too stiff, add a very small measured amount of water early in the mixing cycle, not after it has been sitting in a wheelbarrow.

• Do not “revive” concrete that has been sitting and starting to set by adding water.

Temperature Management for Mixing Water (Hot and Cold Conditions)

Concrete temperature affects working time and early strength. With bag mixing, you can influence temperature most easily through the water you use and how you store materials.

Hot weather: keep the mix cooler and slow the set

• Use cool water. Store water containers in shade; use cool tap water.

• Chill the water if needed. Ice can be used to replace part of the mixing water (measure total water so you don’t exceed the target). Let ice melt during mixing.

• Keep bags cool and dry. Store out of direct sun; hot bags speed set.

• Smaller batches, faster cycle. Don’t let mixed concrete sit in the sun in wheelbarrows.

• Consider a retarder admixture for larger bag-mix pours if the product allows it, but follow manufacturer guidance carefully.

Cold weather: keep the mix warm enough to hydrate

• Use warm (not boiling) mixing water. Warm water helps early hydration and workability. Avoid extremely hot water that can cause flash set in some mixes.

• Keep materials above freezing. Store bags indoors if possible. Do not use frozen aggregate or allow ice chunks into the mix.

• Mix and place promptly. Cold conditions reduce workability and slow strength gain; delays between batches are more noticeable.

• Consider an accelerator if appropriate for the product and conditions, especially when temperatures are near freezing.

Practical target: consistent feel batch-to-batch

In both hot and cold conditions, the goal is not a specific water temperature number but a repeatable, consistent batch. If you change water temperature, keep it consistent for the entire pour so slump and set time don’t drift from batch to batch.