Concrete Strength & Testing Methods 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 Concrete Strength & Testing Methods flashcards as text
The unit weight (density) of fresh concrete is measured per ASTM C138. What is this test primarily used to calculate?
Answer: Yield and air content of the concrete batch
ASTM C138 unit weight is used to calculate batch yield and, in combination with theoretical density, to estimate air content.
Which admixture is used to slow down the strength gain of concrete in hot weather?
Answer: Retarder
Retarding admixtures slow cement hydration, counteracting the accelerated set caused by high ambient temperatures.
What does a consistently low slump reading on delivered concrete most likely indicate?
Answer: Low cement content or insufficient water relative to mix design
Low slump typically indicates insufficient water or that cement content is too high, making the mix stiffer than specified.
How does the addition of fly ash as a supplementary cementitious material (SCM) affect concrete strength development?
Answer: Fly ash slows early strength gain but can increase long-term strength
Fly ash reacts pozzalanically with calcium hydroxide, slowing early strength but contributing to long-term strength beyond 28 days.
What is the primary cause of plastic shrinkage cracking in concrete slabs, which can reduce surface strength?
Answer: Rapid evaporation of bleed water exceeding the rate of bleeding
Plastic shrinkage cracks form when surface evaporation exceeds the bleeding rate, causing tensile stress in the still-plastic concrete.
In the context of quality control, what does 'coefficient of variation' indicate about a concrete production facility's strength data?
Answer: The consistency and control of the concrete production process
Coefficient of variation (standard deviation divided by mean) measures the relative variability of strength results, indicating production consistency.
Which test method measures the resistance of hardened concrete to chloride ion penetration, relevant to durability in coastal or de-iced environments?
Answer: ASTM C1202 rapid chloride permeability test (RCPT)
ASTM C1202 (RCPT) measures electrical charge passed through a concrete disc to assess its resistance to chloride ion penetration.