New developments in quality assurance of concrete pavements can provide important information about quality, durability, and compliance with hybrid design codes.
The construction of concrete pavement can see emergencies, and the contractor needs to verify the quality and durability of cast-in-place concrete. These events include exposure to rain during the pouring process, post-application of curing compounds, plastic shrinkage and cracking hours within a few hours after pouring, and concrete texturing and curing issues. Even if the strength requirements and other material tests are met, engineers may require the removal and replacement of pavement parts because they are worried about whether the in-situ materials meet the mix design specifications.
In this case, petrography and other complementary (but professional) test methods can provide important information about the quality and durability of concrete mixtures and whether they meet work specifications.
Figure 1. Examples of fluorescence microscope micrographs of concrete paste at 0.40 w/c (upper left corner) and 0.60 w/c (upper right corner). The lower left figure shows the device for measuring the resistivity of a concrete cylinder. The lower right figure shows the relationship between volume resistivity and w/c. Chunyu Qiao and DRP, a Twining Company
Abram’s Law: “The compressive strength of a concrete mixture is inversely proportional to its water-cement ratio.”
Professor Duff Abrams first described the relationship between water-cement ratio (w/c) and compressive strength in 1918 [1], and formulated what is now called Abram’s law: “The compressive strength of concrete Water/cement ratio.” In addition to controlling the compressive strength, the water cement ratio (w/cm) is now favored because it recognizes the replacement of Portland cement with supplementary cementing materials such as fly ash and slag. It is also a key parameter of concrete durability. Many studies have shown that concrete mixtures with w/cm lower than ~0.45 are durable in aggressive environments, such as areas exposed to freeze-thaw cycles with deicing salts or areas where there is a high concentration of sulfate in the soil.
Capillary pores are an inherent part of cement slurry. They consist of the space between cement hydration products and unhydrated cement particles that were once filled with water. [2] Capillary pores are much finer than entrained or trapped pores and should not be confused with them. When the capillary pores are connected, fluid from the external environment can migrate through the paste. This phenomenon is called penetration and must be minimized to ensure durability. The microstructure of the durable concrete mixture is that the pores are segmented rather than connected. This happens when w/cm is less than ~0.45.
Although it is notoriously difficult to accurately measure the w/cm of hardened concrete, a reliable method can provide an important quality assurance tool for investigating hardened cast-in-place concrete. Fluorescence microscopy provides a solution. This is how it works.
Fluorescence microscopy is a technique that uses epoxy resin and fluorescent dyes to illuminate details of materials. It is most commonly used in medical sciences, and it also has important applications in materials science. The systematic application of this method in concrete started nearly 40 years ago in Denmark [3]; it was standardized in the Nordic countries in 1991 for estimating the w/c of hardened concrete, and was updated in 1999 [4].
To measure the w/cm of cement-based materials (ie concrete, mortar, and grouting), fluorescent epoxy is used to make a thin section or concrete block with a thickness of approximately 25 microns or 1/1000 inch (Figure 2). The process involves The concrete core or cylinder is cut into flat concrete blocks (called blanks) with an area of approximately 25 x 50 mm (1 x 2 inches). The blank is glued to a glass slide, placed in a vacuum chamber, and epoxy resin is introduced under vacuum. As w/cm increases, the connectivity and number of pores will increase, so more epoxy will penetrate into the paste. We examine the flakes under a microscope, using a set of special filters to excite the fluorescent dyes in the epoxy resin and filter out excess signals. In these images, the black areas represent aggregate particles and unhydrated cement particles. The porosity of the two is basically 0%. The bright green circle is the porosity (not the porosity), and the porosity is basically 100%. One of these features The speckled green “substance” is a paste (Figure 2). As the w/cm and capillary porosity of concrete increase, the unique green color of the paste becomes brighter and brighter (see Figure 3).
Figure 2. Fluorescence micrograph of flakes showing aggregated particles, voids (v) and paste. The horizontal field width is ~ 1.5 mm. Chunyu Qiao and DRP, a Twining Company
Figure 3. Fluorescence micrographs of the flakes show that as the w/cm increases, the green paste gradually becomes brighter. These mixtures are aerated and contain fly ash. Chunyu Qiao and DRP, a Twining Company
Image analysis involves extracting quantitative data from images. It is used in many different scientific fields, from remote sensing microscope. Each pixel in a digital image essentially becomes a data point. This method allows us to attach numbers to the different green brightness levels seen in these images. Over the past 20 years or so, with the revolution in desktop computing power and digital image acquisition, image analysis has now become a practical tool that many microscopists (including concrete petrologists) can use. We often use image analysis to measure the capillary porosity of the slurry. Over time, we found that there is a strong systematic statistical correlation between w/cm and the capillary porosity, as shown in the following figure (Figure 4 and Figure 5) ).
Figure 4. Example of data obtained from fluorescence micrographs of thin sections. This graph plots the number of pixels at a given gray level in a single photomicrograph. The three peaks correspond to aggregates (orange curve), paste (gray area), and void (unfilled peak on the far right). The curve of the paste allows one to calculate the average pore size and its standard deviation. Chunyu Qiao and DRP, Twining Company Figure 5. This graph summarizes a series of w/cm average capillary measurements and 95% confidence intervals in the mixture composed of pure cement, fly ash cement, and natural pozzolan binder. Chunyu Qiao and DRP, a Twining Company
In the final analysis, three independent tests are required to prove that the on-site concrete complies with the mix design specification. As far as possible, obtain core samples from placements that meet all acceptance criteria, as well as samples from related placements. The core from the accepted layout can be used as a control sample, and you can use it as a benchmark for evaluating the compliance of the relevant layout.
In our experience, when engineers with records see the data obtained from these tests, they usually accept placement if other key engineering characteristics (such as compressive strength) are met. By providing quantitative measurements of w/cm and formation factor, we can go beyond the tests specified for many jobs to prove that the mixture in question has properties that will translate into good durability.
David Rothstein, Ph.D., PG, FACI is the chief lithographer of DRP, A Twining Company. He has more than 25 years of professional petrologist experience and personally inspected more than 10,000 samples from more than 2,000 projects around the world. Dr. Chunyu Qiao, the chief scientist of DRP, a Twining Company, is a geologist and materials scientist with more than ten years of experience in cementing materials and natural and processed rock products. His expertise includes the use of image analysis and fluorescence microscopy to study the durability of concrete, with special emphasis on the damage caused by deicing salts, alkali-silicon reactions, and chemical attack in wastewater treatment plants.
Post time: Sep-07-2021