ACI Flatwork Technician key terms

A measure of the consistency and workability of fresh concrete, found by filling a standard cone, removing it, and measuring how far the settled concrete drops in inches or millimeters.

How easily fresh concrete can be mixed, transported, placed, consolidated, and finished without segregating or losing uniformity. It depends on water content, aggregate grading, and admixtures rather than on slump alone.

The weight of water divided by the weight of cementitious material in a mix. Lowering it generally increases strength and durability but reduces workability, so it is balanced with admixtures rather than added water.

Deliberately introducing tiny, evenly spaced air bubbles into concrete with an admixture to improve resistance to freezing and thawing and to deicing salts. Entrained air also improves workability but lowers strength slightly.

A material other than water, aggregate, or cementitious binder added to a concrete batch before or during mixing to change a property such as set time, workability, air content, or strength development.

An admixture that slows the chemical setting reaction so fresh concrete stays workable longer, useful in hot weather or long hauls. Overdosing can delay finishing and formwork removal substantially.

An admixture that speeds up setting and early strength gain, often used in cold weather or when formwork must be stripped early. Calcium chloride accelerators are avoided in reinforced concrete because they promote corrosion.

The separation of coarse aggregate from mortar or paste during handling and placement, often caused by dropping concrete from too great a height or by adding excess water. It produces nonuniform, weak, and hard-to-finish surfaces.

The upward migration of mixing water to the surface of freshly placed concrete as solids settle. Normal bleeding is harmless, but excessive bleeding raises the water-cement ratio near the surface and weakens the finished slab.

The period during which fresh concrete changes from a fluid to a rigid state. Initial set marks when finishing should begin to be planned around; final set roughly marks when the surface can no longer be worked.

The span of time between when bleed water has disappeared and when the concrete becomes too stiff to work, during which floating and troweling must occur. Missing it causes weak surfaces, delamination, or trapped bleed water.

Compacting fresh concrete to remove entrapped air voids and ensure it fully surrounds reinforcement and fills form corners, usually by vibration. Proper consolidation improves density, strength, and surface quality.

A tool with a vibrating head inserted into fresh concrete to consolidate it. It should be inserted vertically at regular spacing and withdrawn slowly so the hole closes, avoiding over-vibration that causes segregation.

Voids or cavities in concrete where mortar failed to fill the space around coarse aggregate, usually from poor consolidation, segregation, or leakage from forms. It reduces strength and must be repaired before finishing is judged complete.

A straightedge or mechanical tool used to strike off excess concrete and bring the surface to the required elevation and plane after placement. It establishes the rough grade that floating and troweling later refine.

The operation of removing surplus concrete above the forms or screed guides to produce a surface at the correct elevation. It is done with a straightedge or screed before any floating begins.

A hand or power tool with a flat plate used to embed aggregate, remove high and low spots, and open or close the surface before troweling. Floating also works paste to the surface and helps level the slab.

A steel-bladed hand or power tool used after floating to densify, smooth, and harden the concrete surface. Successive trowel passes at increasing blade pressure produce progressively tighter finishes.

A very smooth, dense, glossy surface produced by multiple power-trowel passes with blades tilted at a steep angle. It is highly reflective but can become slippery and is prone to blistering if done too early.

A textured surface created by dragging a stiff-bristled broom across the floated concrete, providing slip resistance for walks, driveways, and exterior slabs. Timing and bristle stiffness control the depth of texture.

The ability of a finished concrete surface to resist slipping, especially when wet. It is increased by brooming, grooving, or other texturing and reduced by over-troweling or burnishing.

A horizontal separation of a thin top layer of concrete from the slab beneath it, usually from troweling while bleed water is trapped or from finishing too early. It shows as hollow-sounding, blistered, or peeling areas.

The peeling or flaking of the finished concrete surface, typically caused by freeze-thaw cycles, deicing salts, low air content, or overworking the surface. Proper air entrainment and curing reduce the risk.

Maintaining adequate moisture and temperature in concrete after placement so cement hydration continues and the concrete reaches its intended strength and durability. Methods include wet coverings, curing compounds, and plastic sheeting.

The speed at which moisture leaves the concrete surface, driven by air temperature, humidity, wind, and sun. When it exceeds bleeding, plastic shrinkage cracking can occur before the concrete has set.

Short, often parallel cracks that form on a fresh concrete surface when water evaporates faster than bleed water rises. Windbreaks, fogging, and starting curing early help prevent them.

A planned sawcut or formed groove that creates a weak plane so shrinkage cracks occur there instead of randomly across the slab. Joints must be cut to proper depth and timing to work.

A full-depth separation between a slab and an adjacent structure, column, or wall that allows independent movement and prevents cracking from restraint. It is not intended to control shrinkage within the slab itself.

A narrow groove cut into hardened or setting concrete to create a control joint. Timing matters: cutting too early causes raveling, and cutting too late lets random shrinkage cracks form first.

Volume reduction in concrete as it dries and cures, which creates tensile stress and leads to cracking if the slab is restrained. Control joints and proper curing help manage it.

The planned elevation or slope of a finished concrete surface, set by forms, screeds, or laser tools. Correct grade ensures drainage and matches the design intent for the slab.

A temporary edge or mold that holds fresh concrete to the desired shape and elevation until it hardens. Forms must be sturdy, aligned, and treated so they strip cleanly.

A coating applied to forms so hardened concrete separates easily without damaging the surface or the form. Proper application prevents sticking, bug holes, and surface tearing.

A motorized machine with rotating blades used to float and trowel large slabs faster and with more uniform results than hand tools. Blade pitch and pass timing control the final surface tightness.

A long-handled float with a wide, flat blade used to level and smooth fresh concrete beyond arm's reach before it stiffens. It opens the surface and pushes paste down around aggregate.

A tool with a curved lip used along slab edges to compact the concrete and form a rounded corner that resists chipping. Edging also helps shed water away from the edge.

A tool with a V-shaped or square runner that cuts a groove into fresh concrete to form a control joint or decorative line. Grooving must be deep enough to create a weak plane.

Equipment that moves fresh concrete through hoses or pipes to the placement point, useful where trucks cannot reach. Pumping requires a workable mix and careful control to avoid segregation.

The ability of hardened concrete to resist being squeezed, usually measured on cured cylinders or cubes and expressed in psi or MPa. It is the primary basis for structural acceptance and mix design.