The efficacy of limestone-blended cements ... recycled concrete aggregate (RCA) in pervious concrete ... the durability of pavements containing RCA. They’re just a few of the themes of research papers on ready-mixed concrete and cast-in-place methods presented at the late-January 91st Transportation Research Board meeting in Washington, D.C.
For the first time, registration reached 12,000 at TRB, where delegates heard or studied over 4,000 peer-reviewed technical papers or poster presentations on transportation design, planning, construction, materials and operations.
Concrete Products was there, and follows last month’s review of the best new research in precast/prestressed with a look at research in ready-mixed concrete and cast-in-place concrete. For more information about TRB, visit www.trb.org.
Limestone-Cement Blends Work, Experience Shows
Portland cement blended with limestone—with the latter at up to 15 percent by mass—shows acceptable performance in the field, and improves cement properties through other functions, say Paul D. Tennis and John Melander, Portland Cement Association, in their paper, Environmental Benefits and Performance Equivalence of Portland-Limestone Blended Cements.
The authors define the subject as portland-limestone cements, and stay away from suggesting they are blended cements as little as possible. “Portland-limestone cements (PLCs) are hydraulic cements used for general concrete construction,” Tennis and Melander say. “They are fine powders similar to blended cements in that they are composed of portland cement (or portland cement clinker) and an inorganic material, in this case limestone, rather than pozzolans or slag used in existing blended cement types.”
These cements have similar performance to portland cements for properties such as strength development and setting time, but, since they contain a proportion of uncalcined limestone as an ingredient, they have lower environmental impact, as the pyroprocessing energy and emissions required to produce a ton of cement are reduced, they write.
And it’s the environmental sustainability attribute that is driving codification of PLCs, they write. “The sustainable development focus of the cement industry, proposed implementation of more restrictive environmental regulations on cement manufacturing, and potential global climate change legislation have prompted the U.S. cement industry to propose provisions for a new cement type within specifications ASTM C595 and AASHTO M240: Portland-limestone blended cement, Type IL,” Tennis and Melander say. “In addition, provisions for ternary blended cements with limestone, Type IT are also being proposed. Collectively these blended cements with limestone are referred to as portland-limestone cements (PLCs).” Those proposals are being considered by AASHTO and ASTM.
The writers conclude:
- Ground limestone has been commonly used in Europe and other countries for several decades, Canadian specifications have contained provisions for portland-limestone cements since 2008, and cements meeting ASTM C1157 with around 10 percent limestone have been successfully used.
- The environmental benefits of cements with limestone are appreciable. Although more grinding energy can be required, the energy saved by reducing clinker in the finished cement clearly outweighs the extra grinding energy. As well, by not calcining limestone to produce clinker, CO2 emissions are reduced directly and through lower combustion fuel usage.
- Although relatively inert compared to clinker or supplementary cementitious materials (SCMs), limestone appears to contribute directly to concrete performance properties in three ways: particle packing effects, which can reduce water demand (and therefore water-to cement ratios for equivalent workability) and subsequently increase strengths; nucleation effects, in which hydration products of traditional cement reactions are accelerated slightly; and chemical reactions, which only occur to a minor extent, to produce carboaluminate phases, which can reduce porosity.
- Evidence that fresh and hardened concrete properties are within normal ranges (compared with concretes without limestone) were documented. “Control of limestone particle size distribution and overall fineness of the cement, along with sulfate content optimization, provides for equivalent behavior or even slight benefits when limestone is used in amounts up to 15 percent,” they write.
- Recent case histories of field placements in Canada and the U.S. (up to 10 percent limestone), in addition to decades of experience in European and other countries, demonstrate that cements with up to 15 percent limestone can be effectively used in concretes with or without SCMs and that limestone can be used as an ingredient in ternary blended cements.
Cement Blends with Coarse Limestone Require Attention
Portland-limestone blended cements in which the limestone particles are larger than cement particles are less costly—due to savings in fine grinding—but require special attention, say Narayanan Neithalath, Arizona State University, Tempe, and Hieu T. Cam, New York State Department of Public Service, in their paper, Strength and Transport Properties of Coarse Limestone Powder Modified Concretes Proportioned to Compensate for Dilution Effects.
“Cements incorporating limestone powder are widely used in Europe where as in the United States, ASTM allows the use of a small amount of limestone powder in cement,” the authors write. “The fresh and hardened properties of mortars containing 5 percent or less limestone powder interground with cement are found to be comparable or better than those of plain mixtures.”
In addition to ensuring efficient particle packing when appropriate size ranges are used, the limestone powder addition also helps to improve the workability of concretes, thereby facilitating the production of self-consolidating concretes, Neithalath and Cam state.
“Carbonate additions to cementitious materials and their influence on cement hydration [have] been studied,” they say. “The beneficial effects of limestone powder in providing nucleation sites for cement hydration, and thus accelerating the C3S hydration is well-reported. Nano-size limestone powder has been reported to beneficially influence setting in high volume fly ash mixtures. The hydrate assemblage is influenced by the incorporation of fine limestone powder, with stable carboaluminate structures being formed.”
Most studies deal with limestone powder that is as fine as or finer than the portland cement, wherein the dilution effect caused by the incorporation of the inert filler is compensated (almost) entirely through the increased cement hydration, particle packing effects, and carboaluminate phase formation, Neithalath and Cam say.
“However the use of limestone powder that is coarser than the cement has not received enough attention because of the definite dilution effect that will result in inferior concrete properties,” they observe. “The production of coarse limestone powders requires lesser energy, thereby making its use a sustainable option for concrete production. It is also conceivable that the coarser fillers result in reduced heat generation and consequently reduced cracking propensity. Hence it is desirable to devise methodologies to facilitate the use of coarse limestone powder in concretes by tailoring the material design process.”
To this end Neithalath and Cam studied limestone powder with a median particle size that is five times more than that of the cement, and evaluated the strength and chloride ion transport in concretes proportioned to overcome the dilution effects.
The authors find:
- The incorporation of coarse limestone powder in concretes as a direct replacement for cement results in significant dilution effects, and consequently lower compressive strengths, and higher rapid chloride permeability (RCP) values and non-steady state migration (NSSM) coefficients.
- A methodology that combines a reduction in water-to-powder ration (w/p) and incorporation of small amounts of silica fume has been shown to be capable of negating the dilution effect caused by coarse limestone powder incorporation in concretes. A reduction in w/p from 0.40 to 0.37 for 10 percent limestone powder incorporation, and from 0.40 to 0.34 for 15 percent limestone powder incorporation resulted in comparable strengths as that of the 0.40 w/c benchmark plain concrete.
- The chloride transport parameters of the modified concretes were evaluated along with their pore structure parameter extracted from electrical impedance data. The impact of w/p reduction and silica fume incorporation was quantified through this pore structure parameter.
It was shown that it is possible to proportion concretes containing coarse limestone powder to have similar pore structure parameter as that of control concrete of similar w/p. The reduction in pore solution conductivity realized through the incorporation of limestone powder and silica fume resulted in apparent lower RCP values for these mixtures. For the modified concretes containing silica fume and limestone, the RCP values were lower than those of the plain concretes even though the pore structure parameters were either comparable or higher.
For Pavement Durability, 50:50 is Optimal for RCA, Virgin Materials
For durable concrete pavements, 50 percent substitution of recycled concrete aggregates (RCA) for virgin aggregates is the optimal dose, say Jitendra Jain, Kho Pin Verian, Jan Olek, and Nancy Whiting, Purdue University School of Civil Engineering, in their paper, Durability of Pavement Concretes Made with Recycled Concrete Aggregates.
In this study, recycled concrete aggregate obtained from crushing old concrete pavement were used as coarse aggregates at 0, 30, 50 and 100 percent replacement levels (by mass) for natural virgin aggregates (NVA).
Concrete mixtures were designed and produced to meet the concrete pavement requirements for air content, slump, and flexural strength stipulated by Indiana DOT. All mixtures were produced from fly ash concrete in which 18.5 to 20.0 percent of mass of cement was replaced by ASTM C 618 fly ash. The physical and mechanical testing involved evaluation of slump, air content and development of both flexural and compressive strengths. In addition, the durability was assessed using the freeze-thaw test, scaling test, rapid chloride permeability (RCP) test, and non-steady state migration (NSSM) test.
“The optimal dosages for replacing natural virgin aggregates with RCA for concrete pavements were found to be 50 percent, based on fresh concrete properties and the results of strength and durability tests,” Jain, Verian, Olek and Whiting say.
- The air contents of about 6.5 percent, typically required for good freeze-thaw (FT) resistance, can be achieved in pavement concretes with different levels of RCA (30, 50, or 100 percent of replacement of NVA) with minor modifications in the dosages of air-entraining and water reducing admixtures, even in presence of fly ash.
- The compressive and flexural strengths for concretes with RCA are not much different than concretes with NVA only. In fact, the flexural strength of the fly ash concrete with 50 percent RCA was higher (after 7, 28 and 56 days of moist curing) than that of concrete with only NVA.
- The freeze thaw resistance of concretes with RCA at all replacement levels studied in paper show relative dynamic modulus of elasticity (RDME) values of 90 percent or higher after 300 FT cycles. This signifies very good FT resistance for concretes with RCA. The mass changes are also found to be low for these mixtures. Based on FT tests, 50 percent replacement levels of NVA with RCA is optimal.
- The scaling resistance of concrete mixtures studied was good and the visual ratings after 50 FT cycles were either 1 or 0. This indicates good performance concretes with RCA as it is comparable with the performance of the plain concretes.
- The RCP test indicated moderate chloride ion penetrability for concrete mixtures with 30 and 50 percent RCA in spite of the influence of heat generated during the application of high potential of the specimens during the test and presence of alkalis and chlorides in the mortar attached to the particles of RCA. The NSSM test, which reduces heating effects due to lower applied potential (30V for this study) was used to compare different concretes. The NSSM coefficient was similar for concretes with 30 and 50 percent RCA.
- It is recommended to use RCA as coarse aggregate to replace 50 percent NVA as this level of replacement did not generate any negative effects with respect to such properties as air content, compressive and flexural strengths, and durability parameters in terms of FT, chloride penetration, and scaling resistance.
- Electrical impedance spectroscopy successfully can be used to assess potential chloride penetration resistance of concretes with RCA.
There’s a Limit to RCA Use in Pervious Concrete
Recycled concrete aggregate (RCA) can increase compressive strength of environmentally sustainable pervious concrete, but there’s a limit to how much can be used in place of virgin coarse aggregates, say Bradford M. Berry, Mark J. Suozzo, Ian A. Anderson, and Mandar M. Dewoolkar, The University of Vermont, in their paper, Properties of Pervious Concrete Incorporating Recycled Concrete Aggregate.
Their work investigated using RCA in pervious concrete, specifically the effects on the density, strength and permeability. Cylindrical specimens of pervious concrete with different percentages of RCA and conventional aggregate were cast. The coarse aggregate was substituted with 0, 10, 20, 30, 50, and 100 percent RCA. As percent RCA increased, both compressive strength and permeability generally decreased.
The strength and hydraulic characteristics of mixes examined in this study compared generally well with other studies, on pervious concrete without RCA, found in the literature. The results indicate that up to 50 percent substitution of course aggregate can be used in pervious concrete without compromising strength and hydraulic conductivity significantly. Further testing evaluating freeze-thaw durability is necessary if pervious concrete with RCA is to be used in cold weather climates.
Conventional paving surfaces prevent water from entering the subsoil beneath them. “These impervious surfaces increase runoff, cause flooding and contribute to siltation and other water pollution,” the authors say. “Pervious surfaces allow storm water to infiltrate into the ground, recharging the water table, and thus reduce the amount of runoff. This reduction in storm water runoff also lessens resulting environmental pollution. For these reasons the use of pervious concrete is among the Best Management Practices recommended by the Environmental Protection Agency.”
Ready-mix concrete is normally specified in accordance with the requirements of ASTM C94, Standard Specifications for Ready-Mixed Concrete, they say. For concrete in parking areas a minimum compressive strength of 24 MPa (3,500 psi) is recommended; however, in areas where freeze-thaw durability is a concern a minimum compressive strength of 28 MPa (4,000 psi) is advised. But typical pervious concrete mixtures can develop compressive strengths in the range of 3 MPa to 28 MPa (500 to 4,000 psi). For this reason pervious concrete is generally limited to low-traffic areas where reduced compressive strength is often acceptable. Applications include parking lots, bike paths, and pedestrian footpaths.
“The ecological benefits of pervious concrete can be taken a step further by incorporating recycled concrete aggregate (RCA) into the mix design,” the authors write. “Concrete recycling has gained importance because it minimizes the need for disposal by reducing dumping at landfills and it protects the natural environment by reducing gravel mining of virgin aggregate.”
The U.S. DOT reported that 38 states use RCA as an aggregate base and 11 states recycle concrete into new portland cement concrete. The states that do use recycled concrete as an aggregate source for new concrete report that concrete with RCA has performed as well as concrete with virgin aggregates. Additionally, the quality of concrete with RCA depends on the quality of the recycled material used.
“From an environmental and economic standpoint the optimum RCA content replacement of virgin aggregates will be 100 percent,” Properties of Pervious Concrete authors note. “It is likely RCA will not be able to replace virgin aggregate completely unless the minimum compressive strength and hydraulic conductivity criteria are met. Recycled concrete aggregates contain not only the original aggregates, but also low density hydrated cement paste, resulting in lower density compared to virgin aggregate.”
Their lab study examined some of the effects of incorporating varying amounts of RCA on the strength and hydraulic properties of pervious concrete. The experiments included compressive strength and hydraulic conductivity testing on specimens of varying mix designs substituting coarse aggregate with RCA, ranging from 0 to 100 percent.
- Density values were generally similar with increasing RCA content; however, 100 percent replacement of RCA resulted in lower density values. There are several factors, including differing aggregate densities and angularities which could be the cause of this difference.
- Increasing RCA generally decreased compressive strength, with 100 percent RCA content still providing strength values above 10 MPa (1,400 psi).
- Hydraulic conductivity measurements indicated that increasing RCA content decreased hydraulic conductivity. Nonetheless, all mixes yielded acceptable hydraulic conductivity and,
- Angularity of the RCA likely resulted in a more inefficient distribution of pore spaces, resulting in fewer pore spaces for water to flow.
“The relationship of compressive strength to hydraulic conductivity showed that pervious concrete with RCA display a similar relationship to pervious concrete with conventional aggregates, and falls within an expectable range to be considered an adequate substitute,” the authors say. “Based on the results of this study the particular RCA can be substituted up to 50 percent and provide strength and hydraulic conductivity values similar to the control mix design.”
Pervious portland cement concrete pavement permits down-drain of water, charging subsurface water supplies and keeping heavy metals from entering bodies of water.