FLATWORK/SLAB
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BY DAVID ROTHSTEIN, CHUNYU QIAO
Are You Confident in Your Concrete Pavement Mix
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A fluorescent thin section of concrete in UV light. All photos and figures courtesy of Chunyu Qiao and DRP, a Twining Company.
New developments in concrete pavement quality assurance can provide important information on the quality, durability, and compliance with mix design specifications.
8 CONCRETE CONTRACTOR
CONC0421_08-15_FlatworkslabMix.indd 8
APRIL/MAY 2021
he construction of concrete pavements can see unexpected events that require contractors to verify the quality and durability of the in-place concrete. Such events include exposure to rain during placement, late applications of curing compounds, plastic shrinkage cracking hours within hours of the placement, and problems texturing and curing the concrete. Even when strength requirements and other material tests are satisfied, engineers may call for the removal and replacement of sections of pavement over concerns regarding the compliance of in-place material with mix design specifications. In such cases, petrography and other complementary (but specialized) test methods can provide important information on the quality and durability of concrete mixtures and their compliance with job specifications. W/CM & CONCRETE QUALITY Professor Duff Abrams first described the relationship between water-cement ratio (w/c) and compressive strength in 1918, formulating what is now known as Abram’s Law: “the compressive strength of a concrete mix is inversely related to its water/cement ratio.” In addition to
controlling compressive strength, the water-cementitious materials ratio (w/ cm), is now favored as it recognizes the replacement of portland cement with supplemental cementitious materials such as fly ash and slag. It is also a critical parameter for concrete durability. Many studies show that concrete mixtures with w/cm below ~0.45 are durable in aggressive environments, such as areas exposed to freeze-thaw cycles with deicing salts or areas where high concentrations of sulfate are present in soils. CAPILLARY POROSITY & WHY IT MATTERS Capillary pores are an intrinsic part of the cement paste. They consist of spaces between cement hydration products and unhydrated cement grains that were once filled with water. Capillary pores are much finer than entrained or entrapped air voids and should not be confused with them. When capillary pores are connected, fluids from the external environment can migrate through the paste. This phenomenon is known as percolation and it must be minimized to ensure durability. Durable concrete mixtures have a microstructure where capillary pores are segmented, rather than connected, which occurs when w/cm is less than ~0.45. Although accurately measuring the w/cm of hardened concrete is notoriously difficult, a reliable method to do so can provide a significant quality assurance tool for investigating hardened in-place concrete. Fluorescence microscopy provides one solution. Here’s how it works.
www.ForConstructionPros.com/Concrete
4/7/21 10:13 AM
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