- Created: Tuesday, 01 April 2008 08:00
- Published: Tuesday, 01 April 2008 08:00
High-strength concrete will perform well when recycled concrete aggregate takes the place of virgin aggregate, but an admixture is required, according to new peer-reviewed research from China and Alaska, presented at the 87th annual meeting of the Transportation Research Board in Washington, D.C., in January.
More than 10,000 researchers, engineers, technicians and stakeholders attended the meeting this year. Following are summaries of some of the papers of interest to the aggregates community. For more information about TRB, visit http://trb.org.
WITH HELP, WASTE CONCRETE IS FINE FOR HIGH-STRENGTH CONCRETE MIXES
So-called waste concrete, when crushed, has the potential to replace virgin aggregate in critical high- strength concrete (HSC) mixes, according to Juanyu Liu, Ph.D., University of Alaska-Fairbanks, and Bing Chen, Ph.D., Shanghai Jiaotong University, in their paper, ìProperty of High Strength Concrete Made with Field-Demolished Concrete Aggregates.î But it should incorporate silica fume, natural sand, and undergo a modified mixing method.
ìEnvironmental and economic factors are increasingly encouraging higher-value utilization of demolition debris,î they write. ìWhile the use of waste concrete aggregate (WCA) is mainly confined to low-grade application[s], it would be more beneficial if good-quality concrete can be produced with demolished concrete for new concrete construction.î
Urbanization, population growth and reconstruction has led to an abundance of construction waste, the authors write. ìThe debris from these demolished buildings is thrown away, causing environmental pollution, or is simply used as filling material,î they add. ìIn the meantime, quality aggregates are in short supply in many places where concrete technology is needed, making it necessary to import quality aggregates from distant locations, which can significantly increase the overall budget of a project. This together with environmental, economic and energy considerations are encouraging the recycling of demolished concrete structures and pavements as aggregates.î
These waste concrete aggregates [the most common industry term is recycled concrete aggregate, RCA] are crushed and ground by means of different methods so that they could be used as concrete aggregates, the researchers say. The use of crushed waste concrete as concrete aggregates began in Europe at the end of World War II, and it has been the subject of investigation for a long time, the authors write, adding the use of WCA is mainly confined to low-grade applications, such as unbound road base and fill. ìIt would be more beneficial if good-quality concrete can be produced by taking advantage of the debris of the buildings damaged in destructive earthquakes or demolished due to special needs, provided that the debris is used properly,î they say.
It's well accepted that WCAs make proper aggregates, they say, but add that the original demolished concretes are often composed of concretes with different design strengths from different structural parts, and contaminated with other building materials such as pigment, gypsum.
ìWCAs are always attached with substantial amount of relatively soft cement mortar paste, making these aggregates more porous and less [resistant] to mechanical attacks,î Liu and Chen write. ìTherefore, there are many unsolved problems encountered in controlling the quality of concrete made with WCAs including low compressive strength, wide variability of quality, high drying shrinkage, large creep and low-elastic modulus, which hampers the application of WCA for higher grade applications.î
In previous studies, WCAs were obtained from crushed concrete specimens made in the laboratory, to control the properties of the original concrete, but this may not well represent the behavior of those obtained from field-demolished concrete. Also, they say, most studies recommend only partial replacement of natural aggregates.
Therefore, the researchers undertook an experimental study to explore use of field-demolished concrete as full replacement of natural aggregates to produce high-strength concrete. The workability of fresh concrete and mechanical properties of hardened concrete made with WCAs also was investigated.
The cement used in this study was ordinary portland cement produced by the Shanghai cement factory, with a 28-day compressive strength of 53.5 MPa. To enhance the bonding, silica fume was used, provided by the Elken company, with particle size of about 0.01 ó 0.1 um. A sulphonated naphthalene formaldehyde superplasticizer also was used. The waste concrete came from one demolished building in Shanghai, China. Before crushing treatment, concrete cores were retrieved from different locations of the building for strength measurement.
ìThe preliminary results indicated that HSC with up to 80 MPa of 28-day compressive strength made with field-demolished concrete as coarse aggregates, and natural sand as fine aggregates, can be obtained with the facilitation of a modified mixing method and addition of silica fume,î they write. ìBy providing a general basis for evaluating demolished concrete as aggregates for good-quality concrete, the investigation should contribute significantly to the understanding of recycled aggregate concrete as a viable option in the HSC construction industry.î
LASER, X-RAY SCANNING PROVIDE RELIABLE AGGREGATE CHARACTERIZATION
Both laser and x-ray scanning provide reliable methods of quickly determining aggregate characteristics, say Linbing Wang, Ph.D., P.E., You Zhou and Christopher Harris, Virginia Polytechnic Institute and State University; and Cristian Druta, Ph.D., Virginia Tech Transportation Institute, Center for Sustainable Transportation Infrastructure, in their paper, ìThree-Dimensional Aggregate Evaluation Using Laser and X-ray Scanning.î
Accurate and quick characterization of aggregates is an important part of aggregate production quality control for pavements, the researchers say. ìAggregates constitute the major part of a pavement structure; thus, properties such as shape, angularity and surface texture substantially affect the overall performance of pavements, the bond with cementing materials, and the resistance to sliding of one particle over another,î they write. ìFlat or thin aggregates, as well as long, needle-shaped aggregates tend to break more easily than cubical aggregates. Those with rough and fractured faces allow a better bond with asphalt binder than rounded aggregates and also allow higher friction strength to develop when aggregates slide against each other.î
Automated, accurate and easy to use scanning systems may offer the ultimate capabilities for evaluating the shape, angularity and texture of aggregates, they say. To better identify an optimal system, the writers studied two commercially available scanning systems, one x-ray-based and the other laser-based, and evaluated them in terms of their capabilities in accurately acquiring three-dimensional surface data.
Imaging systems that employ laser and x-ray are frequently used in mapping three-dimensional surfaces and volumes. Their primary functions reside in acquiring images of the objects of interest and subsequently processing them with the help of analysis tools, which are computer programs used to analyze the acquired images in order to obtain the desired information, such as dimensions, shape, texture, or angularity.
The Surveyor 3D laser scanner selected for this study utilizes a line-range laser probe (RPS 120) for profile measurement of objects ranging from 1mm to over 10 in. in size. The probe is capable of acquiring inputs for surface point reconstruction, as the position of a profile can be calculated by triangulation method, since the position relative to the laser is known.
The x-ray technique used the Skyscan 1174 x-ray micro-CT scanner, employing a sensitive 1.3 megapixel camera that allows scanning of a 4cm ◊ 4cm ◊ 4cm sample in a few minutes. Also, a variable magnification (6 to 30 um pixel size) is combined with object positioning for easy selection of the part of an object to be scanned. The full range of the Skyscan software includes fast volumetric reconstruction, 2D/3D quantitative analysis and 3D visualization.
ìTheir potential in characterizing fine and coarse aggregate's shape properties was assessed as part of the [National Cooperative Highway Research Program] NCHRP 4-34 research [on developing and evaluating 3D aggregate characterization systems and analysis methods], with very good correlation,î they write.
The aggregates used were crushed limestone and granite and uncrushed gravel. Two sets of aggregates, each containing nine particles randomly selected, were chosen for results comparisons, ranging from 1 in. (25 mm) to No. 16 (1.18 mm), and were measured using three means: electronic calipers with a precision of four decimals, Skyscan 1174 Analyzer software and RPS-120 Geomagic software.
ìThe dimension measurements, surface area and volume quantification capabilities of the two systems were evaluated by comparing these measurements with caliper measurements, theoretical calculations and other proved methods,î the researchers say. ìThe evaluations indicate that both the x-ray scanning method and laser scanning method can yield very high resolution, from 6 to 60 um, in determining the dimensions of aggregates, allowing for accurate characterization of the derived characteristics of aggregate shape, angularity and texture in true 3D.î
The two scanning systems can successfully be used in evaluating mineral aggregates, they conclude. Both dimensioning methods, Skyscan 1174 CT Analyzer and RPS-120 Geomagic, agreed with the caliper results. The CT Analyzer software is more accurate in dimensioning samples than Geomagic software. Also, scanning with Skyscan takes less time than scanning with the Surveyor laser scanner.
But there is a very good correlation between both methods, with dimension values featuring R2 values being around 0.95 or higher. There are very good results regarding the ability of the two systems in calculating the surface areas and volumes of aggregates. The evaluations also indicate that both scanning methods can be used to assess the derived aggregate characteristics such as shape, angularity and texture.
QUANTIFY GRADATION AND STRENGTH FOR LARGE-STONE HMA MIXES
Accurate rating of aggregate gradation and strength ó including the effect of stress concentration at contact points of coarse aggregates ó is paramount for large stone premium hot-mix asphalt mixes, but methods are available to quantify these variables, say Cesar Alvarado, Fugro Consultants Inc.; Imad N. Abdallah and Soheil Nazarian, Ph.D., P.E., Center for Transportation Infrastructure Systems, University of Texas-El Paso; and Eyad Masad, Ph.D., P.E., Texas Transportation Institute, Texas A&M University, College Station, in their paper, ìFeasibility of Characterizing the Role of Coarse Aggregate Strength on Mechanical Properties of Hot Mix Asphalt.î
Today's new large stone mixes include both polymer-modified stone matrix asphalt (SMA) and open-graded friction courses (OGFCs). ìThe performance of the new generation of hot-mix asphalt mixtures that rely more on stone-on-stone contact is greatly influenced by the properties of the aggregates such as gradation and strength,î the authors write. ìAs a result, it is important to quantify the quality of aggregates to ensure proper performance of roadways. Aggregate specifications should ensure that particles possess the necessary strengths to avoid degradation during handling, construction and trafficking.î
Several well-known methods are available to determine the aggregate characteristics and their relationship to field performance; aggregate structure in HMA and traffic loading needs to be further investigated and defined, they say; so they undertook an experimental investigation to evaluate the effect of stress concentration at contact points of coarse aggregates that could cause aggregate fracture as presented.
A multifaceted study, which focused on the geological, geotechnical and mix design aspects, was carried out. Their main objective was to identify tests that can characterize the aggregate and the performance of the new generation mix design, and their research effort involved testing rock masses from quarries, individual aggregates, combined aggregates for a specific mix, plus HMA mixes, prepared with different gradations to correlate the performance of particles and blends to their respective characterization test methods.
Three aggregate types were selected from three Texas DOT districts: a granite, a hard limestone and a soft limestone. These aggregates are commonly used in Texas and their performance histories are well-known. Soft limestone aggregates are known to perform poorly in coarser mixtures, while the granite and hard limestone aggregates are classified as high quality in Texas but their performances in coarser mixes have been uneven. ìThe field performance of coarse mixes with granite has been mixed whereas the hard limestone performs quite well in coarse mixes.î
Correlation analysis among a variety of familiar tests was performed. ìTraditional aggregate tests, even though quite valuable, do not seem to provide a full picture of the performance of aggregates,î the authors write. ìA series of simple tests is proposed that can be performed on aggregates and rock specimens. Further studies are currently being conducted under the same research project to include more aggregates and mixes so that a larger database can be developed for more conclusive results. The aggregate properties such as stiffness, compressive strength and tensile strength will be used as input to the model to predict performance for mixes under different loading conditions.î
The authors observed:
* The ACV test and its surrogate parameters were found to be suitable for assessing the performance of aggregates.
* The compressive strength obtained using the Schmidt hammer seems to be the most appropriate test for characterizing this parameter in the quarry. This test is not only easier and faster than the compressive strength test, but also eliminates the need for coring the rock and requires minimal training.
* The V-meter seems to be a great tool for estimating the modulus as well as the quality of the rock in tension.
* For the performance characterization of the mixtures, the indirect tensile test and the modulus with the V-meter seem to be the two methods that can be readily implemented and are faster than other performance test used in this study.
IN MINNESOTA, IRON-ORE TAILINGS REPLACE VIRGIN AGGREGATE
In Minnesota, taconite tailings have been used successfully for decades as a replacement for virgin aggregates in road construction, and could be exported elsewhere in the country, report Julie A. Oreskovich, Marsha Meinders Patelke and Lawrence M. Zanko, Natural Resources Research Institute, University of Minnesota-Duluth, in their paper, ìDocumenting the Historical Use of Taconite Byproducts as Construction Aggregates in Minnesota: A GIS-based Compilation of Applications, Locations, Test Data, and Related Construction Information.î
Taconite is a low-grade iron ore, pelletized for blast furnace reduction. Even as virgin aggregate resources are getting more difficult to permit and extract, Minnesota's taconite mining industry generates more than 125 million tons of mining and processing byproducts annually that hold aggregate potential of trap-rock quality, the authors write.
ìMaterials such as blast rock, coarse crushed rock and coarse tailings (collectively known as Mesabi hard rock) have been staples of northeastern Minnesota road construction for over four decades,î they say. ìInfrastructure is already in place to move these materials to markets throughout the country to augment local aggregate resources.î
Because these highway construction applications are not widely known outside of northeastern Minnesota, the authors assembled related test data and documented how and where taconite byproducts have been used. Letters, interviews, site visits and searches of archived records were the primary modes of data collection. The result is a project report with a stand-alone Microsoft Access (or Excel) database and an ArcView GIS product containing mappable Mesabi hard rock usage locations with accompanying data.
The resulting databases can be queried on a variety of topics or attributes. The researchers found more than 400 documented usages in Minnesota from 1960 to 2006, including 1,120 usage miles of roadway pavement and fill. The primary applications are for bituminous pavement (with desired hardness, durability, with 100% fractured faces ideal for Superpave mixes); and fill (with desired free-draining attribute and containing a ready-made fine aggregate equivalent). The Minnesota Department of Transportation is the top user.