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Difficulty: MediumConcrete and Masonry Materials

During a concrete pour for a structural foundation, a crew considers adding extra water to a stiff batch to make it easier to place and finish. If excess water is added beyond the engineered mix design ratio, what is the primary effect on the cured concrete?

  1. A
    It accelerates the chemical hydration process, allowing the concrete to reach its maximum design strength much faster.
  2. It lowers the compressive strength and increases the porosity and cracking potential of the hardened slab.Answer
  3. C
    It increases the tensile bond strength between the cured cement paste and embedded steel rebar.
  4. D
    It eliminates volume shrinkage during curing, removing the requirement for sawed control joints.

Answer

It lowers the compressive strength and increases the porosity and cracking potential of the hardened slab.
Concrete strength depends directly on the water-cement (w/cw/c) ratio. Adding excess water beyond what is required for chemical hydration dilutes the cement paste. When the unreacted water evaporates, it leaves behind capillary voids and porous channels that substantially decrease compressive strength and increase susceptibility to cracking.

Step-by-Step Solution

1
Analyze the role of water in concrete mix design.
Only a specific amount of water is chemically required to hydrate the Portland cement; additional water exists solely as a workability lubricant.
Understanding the water-cement (w/cw/c) ratio is critical for predicting concrete durability and load capacity.
2
Determine the physical consequence of excess water evaporating during and after the curing phase.
Unreacted water evaporates, creating microscopic capillary pores and voids throughout the hardened cement matrix.
Porosity directly reduces structural density, leading to lower compressive strength and higher permeability.
3
Identify the primary structural impact on the finished concrete product.
The cured concrete suffers from reduced compressive strength, higher susceptibility to freeze-thaw damage, and increased drying shrinkage cracking.
Maintaining the lowest practical water-cement ratio ensures maximum strength and longevity.

Key Concept

Water-Cement Ratio and Concrete Compressive Strength
Estimated Time:1m 0s
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Topics

Concrete and Masonry Materials

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