Concrete & Masonry Fundamentals

Concrete and masonry are the backbone of construction. Every foundation, floor slab, retaining wall, and block structure depends on workers who understand these materials inside and out. This guide goes well beyond the basics - it covers the material science, mix design, placement procedures, finishing techniques, curing protocols, and masonry methods that separate competent tradespeople from beginners. Whether you are pouring a residential footing or laying CMU walls on a commercial project, the information here will prepare you for day-1 productivity on a real job site.

Understanding Concrete as a Material

Concrete is a composite material made from four ingredients that undergo a chemical reaction called hydration. Understanding what each ingredient does helps you troubleshoot problems in the field.

Portland Cement

Portland cement is the binding agent. When mixed with water, it forms calcium silicate hydrate (C-S-H) crystite - the "glue" that holds everything together. Common types include:

• Type I - General purpose, used for most residential and commercial work
• Type II - Moderate sulfate resistance, common in areas with sulfate-rich soils
• Type III - High early strength, reaches design strength faster, used when forms need to be stripped quickly or in cold weather
• Type V - High sulfate resistance for severe conditions like wastewater treatment plants

Aggregates

Aggregates make up 60-75% of concrete by volume:

• Coarse aggregate (gravel or crushed stone) - Typically 3/4-inch to 1-inch nominal maximum size for residential work. Larger aggregate reduces water demand and shrinkage.
• Fine aggregate (sand) - Fills voids between coarse aggregate particles. Must be clean and well-graded.
• Quality matters - Dirty, organic, or poorly graded aggregates cause weak concrete. Aggregates should be hard, durable, and free of clay, silt, and organic matter.

Water

The water-to-cement ratio (w/c) is the single most important factor in concrete strength:

• 0.40 w/c - Produces approximately 5,000 PSI concrete. Stiff, hard to work, but very strong and durable.
• 0.45 w/c - Produces approximately 4,500 PSI. Good balance of workability and strength.
• 0.50 w/c - Produces approximately 4,000 PSI. The typical residential mix.
• 0.55 w/c - Produces approximately 3,500 PSI. More workable but weaker.
• 0.60+ w/c - Produces weak concrete. Never add extra water to make the mix easier to work with.

Every extra gallon of water per cubic yard reduces compressive strength by approximately 200-400 PSI and increases shrinkage cracking.

Admixtures

Chemical admixtures modify concrete properties:

• Water reducers (plasticizers) - Increase workability without adding water. Mid-range water reducers are standard in most ready-mix plants.
• Superplasticizers (high-range water reducers) - Create highly flowable concrete for congested reinforcement or complex forms.
• Air-entraining agents - Create microscopic air bubbles that protect concrete from freeze-thaw damage. Required for any exterior concrete in freeze-thaw climates. Target 5-7% air content.
• Accelerators - Speed up set time in cold weather. Calcium chloride is common but should not be used in reinforced concrete (promotes corrosion). Non-chloride accelerators are available.
• Retarders - Slow set time in hot weather or for long hauls.
• Fiber reinforcement - Synthetic fibers (polypropylene) reduce plastic shrinkage cracking. Steel fibers add post-crack strength.

Concrete Mix Design and Ordering

Specifying Ready-Mix Concrete

When ordering from a ready-mix plant, specify:

• Compressive strength - 3,000 PSI for non-structural, 3,500-4,000 PSI for residential footings and slabs, 4,500+ PSI for structural elements
• Slump - A measure of workability. 4-inch slump is standard for most flatwork. 5-6 inches for walls and footings. Never exceed the specified slump by adding water on site.
• Maximum aggregate size - 3/4-inch for most residential, 1-inch for mass pours
• Air entrainment - Specify 5-7% for exterior concrete in freeze-thaw climates
• Cement content - Typically 5-6 sacks (470-564 lbs) per cubic yard for residential work

Calculating Volume

Concrete is ordered by the cubic yard (27 cubic feet). To calculate:

• Slabs - Length (ft) x Width (ft) x Thickness (ft) / 27 = cubic yards
• Example - A 20 x 20-foot slab, 4 inches thick: 20 x 20 x 0.333 / 27 = 4.94 cubic yards
• Always order 5-10% extra to account for waste, subgrade irregularities, and spillage
• Footings - Calculate the cross-sectional area times the total linear footage
• Walls - Height x length x thickness / 27

Mixing on Site

For small pours under 1 cubic yard, mixing on site with bags or a portable mixer is practical:

• 80-lb bag of premix yields approximately 0.6 cubic feet. You need 45 bags per cubic yard.
• 60-lb bag yields approximately 0.45 cubic feet. You need 60 bags per cubic yard.
• Add water gradually. The bag will specify the water quantity (typically 3-4 quarts per 80-lb bag).
• Mix for at least 3-5 minutes until uniformly colored with no dry pockets.
• Mix only what you can place within 30 minutes. Concrete that sits too long in the mixer loses workability and begins to set.

Formwork

Forms contain wet concrete and define the finished shape. Good formwork is straight, plumb, tight, and strong enough to resist the pressure of wet concrete.

Form Materials

• Dimensional lumber (2x4, 2x6, 2x8, 2x10, 2x12) - Standard for residential footings, slabs, and low walls
• Plywood (3/4-inch) - Smooth-faced plywood (form-ply or BB-OES) produces smooth concrete surfaces. Used for wall forms.
• Manufactured metal forms - Reusable aluminum or steel panels for repetitive work and tall walls
• Flexible form boards - Thin plywood or composite strips for curves

Form Construction - Slabs

1. Set form boards to the correct elevation using a builder's level or laser
2. Drive wooden stakes every 4 feet along the outside of the forms
3. Nail or screw the form boards to the stakes. The top of the form should be at the finished concrete elevation.
4. Check for straightness with a string line
5. Check for level or the correct slope (1/4 inch per foot minimum for drainage on exterior slabs)
6. Brace any areas that bow or bulge
7. Apply form release oil to the form faces for easy stripping

Form Construction - Footings

• Footing forms must match the dimensions on the structural plans. Typical residential footings are 12 inches wide by 8 inches deep for one-story, 16 inches wide by 8 inches deep for two-story.
• Use duplex (double-headed) nails for easy stripping
• Brace forms at the top with spreaders and at the base with kicker stakes to resist blowout
• Place footing rebar before closing the top of the forms. Typical residential: 2 pieces of #4 rebar continuous, 3 inches from the bottom on chairs

Calculating Form Pressure

Wet concrete exerts lateral pressure on wall forms. At a standard pour rate and temperature:

• Pressure at the bottom of the form = approximately 150 lbs per cubic foot x height in feet
• A 4-foot wall pour exerts approximately 600 PSF at the base
• Snap ties, walers, and strongbacks resist this pressure in wall forms
• Forms that blow out waste concrete, delay the project, and create safety hazards. When in doubt, over-brace.

Reinforcement

Rebar

Steel reinforcement gives concrete tensile strength (concrete is strong in compression but weak in tension):

• #3 rebar - 3/8-inch diameter, used for temperature/shrinkage steel in slabs
• #4 rebar - 1/2-inch diameter, standard for residential footings and walls
• #5 rebar - 5/8-inch diameter, common in structural beams and columns
• Rebar size convention - The number indicates eighths of an inch (#4 = 4/8 = 1/2 inch)

Key rebar rules:

• Cover - Minimum 3 inches for concrete cast against earth (footings), 1.5 inches for formed surfaces (walls), 3/4 inch for slabs not exposed to weather
• Lap splices - Where two bars overlap, the minimum lap is typically 40 bar diameters (#4 rebar = 40 x 0.5 = 20 inches)
• Placement - Support rebar on chairs, bolsters, or dobies to maintain the specified cover. Rebar lying on the ground does nothing.
• Tying - Tie rebar intersections with tie wire using a rebar tying tool. Every intersection does not need a tie, but enough to hold position during the pour.

Welded Wire Reinforcement (WWR)

• 6x6 W1.4/W1.4 (6x6 10/10) is standard for residential slabs
• Must be positioned in the upper third of the slab, not lying on the subgrade
• Use chairs or pull the mesh up during placement to keep it in position
• Some contractors prefer synthetic fiber reinforcement (1.5 lbs per cubic yard) as a replacement for WWR in non-structural slabs

Placing Concrete

Pre-Pour Checklist

Before the truck arrives:

• Forms are secure, clean, level, and oiled
• Rebar is in place with correct cover and tied
• Subgrade is compacted, moistened (not muddy), and free of standing water
• Expansion joints and isolation joints are installed where the slab meets walls, columns, or existing concrete
• All embedded items (anchor bolts, sleeves, conduit) are secured
• Enough crew and tools are on hand for the pour volume
• Access route for the truck is clear and can support the weight (a loaded mixer truck weighs 60,000-80,000 lbs)

Placement Procedures

• Place concrete as close to its final position as possible. Do not drag it horizontally through the forms - this segregates the aggregate.
• Deposit concrete in layers of 12-18 inches for walls, consolidating each layer before placing the next
• Do not dump concrete from heights greater than 5 feet - use a chute, pump, or tremie to prevent segregation
• Work concrete into corners and around reinforcement with shovels and vibrators
• Fill forms from one end to the other systematically, not from random points

Consolidation

Consolidation removes trapped air voids and ensures the concrete fully surrounds the reinforcement:

• Internal vibrators (pencil vibrators) - Insert vertically into the concrete at 18-inch intervals. Let the vibrator sink under its own weight, hold for 5-15 seconds until air bubbles stop rising, then withdraw slowly. Overlap each insertion point.
• Do not over-vibrate - This segregates aggregate and brings too much paste to the surface, causing a weak top layer.
• For slabs, use a screed vibrator or strike the forms with a rubber mallet. Internal vibrators are used for walls, footings, and deep sections.

Finishing Concrete Flatwork

Finishing is a time-sensitive process that depends on reading the concrete's condition correctly.

Step 1 - Screeding

Immediately after placing, level the concrete with the tops of the forms:

• Use a straight 2x4 or a magnesium screed board
• Two workers pull the screed across the forms in a sawing motion
• Fill low spots and re-screed until the surface is flat and slightly above the form tops

Step 2 - Bull Floating

After screeding, smooth the surface with a bull float or darby:

• Push the float away with the leading edge slightly raised to avoid digging in
• Pull it back with the trailing edge slightly raised
• This embeds aggregate below the surface and fills small voids
• Use a magnesium float for air-entrained concrete (wood floats can tear the surface)

Step 3 - Waiting for Bleed Water

After bull floating, stop and wait. Concrete bleeds water to the surface as it settles. Finishing while bleed water is present traps it below the surface, causing delamination, scaling, and a weak wear surface.

• Wait until the bleed water sheen disappears
• The concrete should support your weight with only a 1/4-inch footprint
• In hot, dry, or windy conditions, bleed water evaporates quickly and you must work faster. Apply evaporation retarder spray if the evaporation rate exceeds 0.25 lb/ft2/hour.

Step 4 - Edging and Jointing

• Edging - Run an edging tool along the form edges to create a rounded edge that resists chipping
• Control joints (contraction joints) - Cut with a groover tool or saw-cut within 4-12 hours after finishing. Joints should be 1/4 the slab depth. Space joints at intervals no greater than 2-3 times the slab thickness in feet (4-inch slab = 8-12 foot joint spacing). Control joints guide where cracks form - without them, cracks appear randomly.

Step 5 - Final Finishing

• Broom finish - Drag a broom perpendicular to the direction of traffic for slip resistance. Standard for driveways, sidewalks, and exterior slabs.
• Steel trowel finish - For smooth interior floors. Trowel in arcing motions after the concrete has firmed enough to support the trowel without leaving deep marks. Multiple passes produce progressively smoother surfaces. Use a fresno trowel (large flat trowel on a handle) for larger areas.
• Exposed aggregate - Apply a retarder to the surface, then wash away the top paste layer after partial curing to expose the stone aggregate.

Common Finishing Mistakes

• Adding water to the surface - Never sprinkle water to make finishing easier. This weakens the surface layer and causes crazing and scaling.
• Finishing too early - Working bleed water back into the surface traps it and creates a weak layer.
• Finishing too late - The surface becomes too hard to work. In hot weather, you may have less than an hour after the bleed water disappears.
• Over-troweling - Creates a dense, slick surface that can delaminate from the layer below.

Curing Concrete

Curing is the single most neglected step in concrete work. Proper curing dramatically increases strength, durability, and crack resistance.

Why Curing Matters

Concrete needs moisture and moderate temperature to hydrate fully. If the surface dries out, hydration stops and the concrete never reaches full strength. Concrete that is not cured can lose 40-50% of its potential strength.

Curing Methods

• Curing compounds - Spray-on liquid membranes that seal moisture in. Apply immediately after finishing. White-pigmented compounds reflect sunlight and reduce heat. Most economical for large pours.
• Plastic sheeting - Lay polyethylene sheet directly on the surface after finishing. Tape the seams. Discoloration may occur where the plastic touches the concrete (not a structural issue, but may be cosmetic concern).
• Wet burlap - Lay burlap on the surface and keep it wet for the curing period. More labor-intensive but excellent results.
• Ponding or continuous sprinkling - Flood the surface with water. Effective for large slabs but wastes water.

Curing Duration and Strength Gain

• Cure for a minimum of 7 days at temperatures above 50 degrees F
• Concrete reaches approximately 70% of design strength at 7 days and 99% at 28 days under ideal conditions
• In cold weather (below 50 degrees F), use insulating blankets and extend the curing period. Concrete that freezes before reaching 500 PSI is permanently damaged.
• In hot weather (above 90 degrees F), begin curing immediately. Hot concrete sets faster but can crack from rapid moisture loss and thermal stress.

Masonry - Block and Brick Construction

Concrete Masonry Units (CMU)

Standard CMU dimensions:

• 8x8x16 (nominal) - Actual dimensions 7-5/8 x 7-5/8 x 15-5/8 inches. The 3/8-inch mortar joint brings it to nominal 8x8x16.
• 8x4x16 - Half-height block for coursing adjustments
• Lintel blocks (U-blocks) - Open-top blocks for placing rebar and grout over door and window openings
• Corner blocks - One flat end for exposed corners
• Split-face and ground-face - Decorative units with textured or polished faces

Mortar Types

Mortar is classified by compressive strength:

• Type M (2,500 PSI) - Highest strength. Used for below-grade, retaining walls, and foundations.
• Type S (1,800 PSI) - Standard structural mortar. Used for most above-grade and below-grade applications. The most commonly specified type.
• Type N (750 PSI) - General purpose above-grade mortar. Good for non-load-bearing walls and veneer.
• Type O (350 PSI) - Low-strength mortar for interior non-load-bearing partitions.

Mix proportions for Type S mortar (by volume): 1 part Portland cement, 1/2 part hydrated lime, 4-1/2 parts sand.

Laying Block

Step-by-step procedure:

1. Snap a chalk line on the footing for the wall location
2. Dry-lay the first course without mortar to check the layout and minimize cuts
3. Build corner leads - Lay the corner blocks first, building up 3-5 courses at each corner. Rack (step back) each course. Use a story pole to maintain consistent course height (8 inches nominal per course including mortar joint).
4. String line between corners - Stretch a mason's line between the corner leads at each course height
5. Butter and lay - Apply a full mortar bed on the footing (1 inch thick). Set the first block into the mortar and level it. Butter the end (head joint) of the next block and push it snug against the first.
6. Check constantly - Level across the top, plumb the face, and check alignment with the string line after every few blocks
7. Tool the joints - After the mortar reaches "thumbprint" hardness (you can press your thumb in and leave a clean print without mortar sticking), compress the joints with a jointing tool. This compresses the mortar and creates a weather-resistant joint. Concave (rounded) joints shed water best.

Grouting and Reinforcement

For structural masonry walls:

• Place vertical rebar in the cells as specified on the plans (typically #4 or #5 at 48 inches on center for residential)
• Fill the cells with grout (a fluid concrete mix). Pour in lifts of no more than 4 feet and consolidate with a pencil vibrator.
• Horizontal reinforcement: Use prefabricated ladder-type joint reinforcement every other course (16 inches on center) for crack control
• Bond beams: Use lintel blocks filled with rebar and grout at the top of the wall and over openings

Laying Brick

Brick masonry follows similar principles to block but with some differences:

• Standard modular brick: 3-5/8 x 2-1/4 x 7-5/8 inches. Three courses of brick with mortar equal 8 inches in height.
• Brick patterns (bonds) - Running bond (most common), stack bond, Flemish bond, English bond
• Bed joints should be 3/8 inch thick. Head joints should also be 3/8 inch.
• Weep holes - Leave weep holes every 24-33 inches at the base of brick veneer walls to drain moisture from behind the brick
• Wall ties - Connect brick veneer to the structural backup wall with corrugated metal ties or adjustable ties every 16 inches vertically and 32 inches horizontally
• Flashing - Install through-wall flashing at the base of veneer, over windows and doors, and at shelf angles to direct water out through weep holes

Safety in Concrete and Masonry Work

• Wet concrete is caustic - Portland cement is highly alkaline (pH 12-13). It causes chemical burns on prolonged skin contact. Wear rubber boots, waterproof gloves, and long sleeves. Wash skin immediately if cement contacts it.
• Silica dust - Cutting, grinding, or drilling concrete and masonry generates respirable crystalline silica dust. Prolonged exposure causes silicosis, a serious lung disease. Use wet cutting methods and wear an N95 respirator (minimum) or a P100 half-face respirator.
• Manual material handling - An 80-lb bag of concrete mix, a standard CMU block (35 lbs), or a hod of mortar require proper lifting technique. Lift with your legs, keep loads close to your body, and use mechanical aids for repetitive lifting.
• Formwork collapse - Never walk on fresh concrete in wall forms. Ensure all form hardware is secure before the pour. Post a spotter to watch for form movement during the pour.
• Saw safety - Concrete saws and masonry saws require eye protection, hearing protection, and dust control. Check blade guards and water supply before cutting.

Troubleshooting Common Problems

Cracking

• Plastic shrinkage cracks - Appear within hours of finishing when the surface dries too fast. Prevent with windbreaks, evaporation retarder, and immediate curing.
• Drying shrinkage cracks - Appear over days to weeks. Reduced by proper control joint spacing, low water-cement ratio, and adequate curing.
• Structural cracks - Wider than 1/4 inch, growing, or offset (one side higher than the other). These indicate settlement, overloading, or inadequate reinforcement. Consult a structural engineer.

Surface Defects

• Scaling - Surface flakes off. Usually caused by finishing with bleed water present, inadequate curing, or lack of air entrainment in freeze-thaw environments.
• Crazing - Fine pattern cracks in the surface. Caused by over-troweling, too-wet a mix, or drying too fast.
• Honeycombing - Voids in formed concrete where aggregate is visible. Caused by inadequate consolidation, too-stiff a mix, or congested reinforcement.
• Efflorescence - White powder on masonry surfaces. Caused by water dissolving salts in the masonry and depositing them on the surface as it evaporates. Clean with a stiff brush and diluted muriatic acid (wear PPE).

Tips from Experienced Tradespeople

• "Order the right slump and don't let anyone add water to the truck. I've seen more slabs fail from added water than anything else."
• "Your subgrade is your foundation. Spend the time compacting it right. Soft spots under a slab always show up as cracks."
• "Cut your control joints early. I've seen guys wait until the next day and the slab had already cracked randomly. Within 4-12 hours, no later."
• "When laying block, build your corners first and take the time to get them perfect. If your corners are right, the wall between them will be right."
• "Never finish over bleed water. I don't care if the homeowner is watching and getting impatient. Wait until it's ready."
• "On hot days, have everything set up before the truck arrives. Wet the subgrade, oil the forms, stage your tools. Once that concrete starts flowing, you don't stop."
• "Keep your saw blade wet when cutting masonry. Dry cutting kills blades fast and fills your lungs with silica. It's not worth it."