Joint Layout & Crack Control Flashcards
7 cards from real ACI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Joint Layout & Crack Control flashcards as text
Which sealant material is generally best suited for sealing contraction joints in an industrial floor subject to forklift traffic?
Answer: Rigid epoxy joint filler that supports joint edges
Rigid epoxy or semi-rigid polyurea fillers support the joint edges under hard wheel traffic, preventing spalling and chipping better than flexible sealants.
Subgrade settlement beneath a slab most commonly causes which crack pattern?
Answer: Random cracks or a crack pattern following voids below the slab
Voids or soft spots in the subgrade cause the slab to lose support and crack in a pattern that mirrors the location of subsurface settlement.
The term 'aggregate interlock' at a cracked concrete joint refers to:
Answer: Load transfer across a crack through the interlocking of coarse aggregate particles
Aggregate interlock transfers shear loads across a crack when coarse aggregate on opposite crack faces mesh together, without any reinforcement.
To minimize crack-inducing restraint at the base of a slab-on-grade, finishers should ensure:
Answer: A bond breaker or polyethylene sheet is placed between the slab and subbase
A bond breaker or slip sheet reduces friction between the slab and subbase, allowing the slab to shrink freely and reducing tensile stresses that cause cracking.
What is the primary cause of 'curling' in a concrete slab on grade?
Answer: Differential moisture and temperature between the top and bottom of the slab
The top surface dries and cools faster than the bottom, causing differential shrinkage that lifts (curls) the slab edges and corners upward.
When laying out control joints for a slab that includes a re-entrant corner (L-shaped slab), the finisher should:
Answer: Place a diagonal control joint from the re-entrant corner into the slab interior
Re-entrant corners concentrate stress and almost always crack diagonally; a diagonal control joint directs that crack to a predetermined location.
When the w/cm (water-to-cementitious materials) ratio of a concrete mix is reduced, the effect on drying shrinkage is:
Answer: Drying shrinkage decreases because less evaporable water is present
Lower w/cm means less excess water to evaporate, which directly reduces the volume change (drying shrinkage) that drives crack formation.