ACI Surface Preparation & Finishing Techniques 1 — Questions and Answers

Question 1: What is the purpose of surface preparation in concrete finishing?

To polish the surface
To remove reinforcement
To ensure adhesion and cleanliness (Correct answer)
To change color

Correct answer: To ensure adhesion and cleanliness

Surface preparation in concrete finishing is critical to ensure proper adhesion of subsequent coatings, toppings, or repair materials. It involves cleaning the surface, removing contaminants, and sometimes roughening it to create a profile that allows for a strong mechanical bond. This step is essential for the longevity and performance of the finished product.

Question 2: Which tool is commonly used for initial surface leveling?

Float
Trowel
Screed (Correct answer)
Brush

Correct answer: Screed

A screed is a long, straightedge tool used for the initial leveling and striking off of freshly placed concrete. It removes excess concrete and brings the surface to the desired elevation and flatness, preparing it for subsequent finishing operations like floating and troweling.

Question 3: What does a broom finish provide?

Glossy appearance
Color variation
Slip resistance (Correct answer)
Structural reinforcement

Correct answer: Slip resistance

A broom finish is created by dragging a stiff-bristled broom across the surface of fresh concrete after floating. This process creates a textured, non-slip surface, which is particularly important for exterior concrete slabs like sidewalks, driveways, and ramps to enhance safety, especially when wet.

Question 4: When should final troweling be done?

Immediately after pouring
After surface has set slightly (Correct answer)
While bleeding water is present
The next day

Correct answer: After surface has set slightly

Final troweling should be performed after the concrete surface has stiffened sufficiently, typically when the bleed water has evaporated and the concrete can support the weight of a finisher without leaving deep indentations. Troweling too early can bring excess water and fine particles to the surface, weakening it, while troweling too late makes it difficult to achieve a smooth finish.

Question 5: What tool is used to create edges on concrete slabs?

Groover
Bull float
Edger (Correct answer)
Tamping rod

Correct answer: Edger

An edger is a specialized tool used to create a smooth, rounded, or beveled edge on concrete slabs. This process helps prevent chipping and spalling of the slab edges, improving both the durability and aesthetic appearance of the concrete. It's typically used after the initial floating.

Question 6: What causes surface laitance in concrete?

Proper curing
Aggregate segregation
Overworking and excess water (Correct answer)
Using coarse sand

Correct answer: Overworking and excess water

Surface laitance is a weak, chalky layer that forms on the surface of concrete. It is primarily caused by overworking the concrete surface, which brings excess water and fine cement particles to the top, especially when the water-cement ratio is too high. This weak layer can compromise the durability and bonding of subsequent finishes.

Question 7: Why is timing critical in surface finishing?

It increases color uniformity
It prevents early curing
It ensures optimal texture and integrity (Correct answer)
It improves vibration

Correct answer: It ensures optimal texture and integrity

Timing in concrete finishing is crucial because it dictates when the concrete has reached the ideal stage of setting. Working the concrete too early when it's still bleeding water can trap water and weaken the surface, while working it too late after it has begun to set can make it difficult to achieve a smooth, durable finish. Proper timing allows for the creation of the desired surface texture, prevents defects like scaling or dusting, and ensures the overall structural integrity and longevity of the concrete slab.

Question 8: Which finishing technique is used for decorative texture?

Troweling
Edging
Stamping (Correct answer)
Saw cutting

Correct answer: Stamping

Stamping is a specialized concrete finishing technique used to create decorative patterns and textures on freshly placed concrete. Molds or "stamps" are pressed into the surface before the concrete fully sets, replicating the appearance of natural stone, brick, wood, or other materials. This method allows for a wide range of aesthetic designs, transforming plain concrete into an attractive and durable surface for patios, driveways, and walkways.

Question 9: Why should finishing not be performed over bleeding water?

It improves hydration
It increases slump
It traps water and weakens surface (Correct answer)
It extends drying time

Correct answer: It traps water and weakens surface

Bleeding water is excess water that rises to the surface of freshly placed concrete. Performing finishing operations while this water is present can trap it beneath the surface, leading to a higher water-cement ratio in the top layer. This trapped water weakens the concrete's surface, making it more susceptible to dusting, scaling, and reduced durability once it dries and cures. It's essential to wait for the bleed water to evaporate or be reabsorbed before proceeding with finishing.

ACI Surface Preparation & Finishing Techniques 1

What is the purpose of surface preparation in concrete finishing?