Use of Post-Consumer Corrugated Fiberboard as Fine Aggregate Replacement in Controlled Low-Strength Materials

This study was conducted to investigate the use of post-consumer corrugated board in controlled low-strength material (CLSM) applications. Corrugated fiberboard (termed corrugate), which constitutes a significant fraction of the municipal solid waste stream in the United States (approximately one third by weight), was used as a partial replacement for fine aggregate in CLSM at aggregate replacement ratios ranging from 0 % (i.e., control) to 6 %. The corrugate was fiberized (i.e., repulped) in a blender prior to being mixed with other constituents in the CLSM. The density, air content, and flow consistency of the fresh CLSM were determined, and bleeding was qualitatively assessed. Also, the unconfined compressive strength was determined for the resulting mixtures at different test ages. As the corrugate content increased, air content and water demand increased, density and compressive strength decreased, and some mixtures exhibited excessive bleeding. Corrugated fiberboard was determined to be effective as a fine aggregate replacement to produce mixtures with 28-day compressive strengths within the range for excavatable CLSM. Introduction and Background Controlled low-strength material (CLSM) is a self-leveling, self-compacting cementitious material used primarily in lieu of compacted sand/soil backfill. Alternative terms for CLSM include flowable fill, unshrinkable fill, controlled density fill, flowable mortar, and soil-cement slurry, among others [1]. ACI Committee Report 116 [2] defines CLSM as a cementitious material that is fluid at placement and which results in a long-term compressive strength of 8.3 MPa (1200 psi) or less. Applications for CLSM include structural fills beneath buildings, backfill behind retaining walls, pavement base, conduit bedding, void filling, and bridge reclamation [1]. Materials used in conventional CLSM mixtures are described in Table 1. Various alternative materials have been used to produce satisfactory CLSM, including high fines (greater than 20 %), silty sands, and local soils. Soils with high clay contents are avoided, as they can have deleterious effects on mixture properties, such as increased water demand, increased shrinkage, and mixture heterogeneity from incomplete mixing of the clay. [Table 1] CLSM mixtures must have the ability to fill desired spaces and voids with a minimum need for manual labor during the placement effort. The properties of both fresh and hardened CLSM, including bleeding and density (fresh) and unconfined compressive strength (hardened), provide an indication of the field performance. The engineering properties of CLSM that have relevance to mixture proportioning and performance include flowability, bleeding, density, and unconfined compressive strength as described below. Flowability refers to the ability of a CLSM to self-compact and readily fill voids. Good flowability as determined using a flow consistency test (ASTM D6103 [10]) allows for placement of the CLSM without the need for conventional compaction equipment or labor. The bleeding of water from a fresh CLSM mixture indicates settlement of the freshly placed CLSM, and typically low bleeding is desirable. The volume of CLSM placed for a given application may need to be increased to account for this settlement and to obtain a desired final surface elevation. The density of CLSM is affected by mixture materials and amounts. Low density CLSM mixtures are used when low overburden stresses are required (e.g., weak underlying soils or insulating fill for thermal or shock isolation), whereas high density mixtures may be acceptable for cases with relatively high strength requirements. Unconfined compressive strength is the most commonly used requirement for CLSM mixtures. Several factors influence the requirements for compressive strength, including the application and the likelihood of future excavation. For a given project, a minimum strength, maximum strength, or both may be specified. The requirements and specifications used for CLSM mixtures are relatively limited compared to those for conventional concrete mixtures. Typical specifications for CLSM include provisions for the proportioning of ingredients (e.g., limits on grain size distributions and mixture proportions), plastic properties (e.g., flowability, segregation), and in-service properties (e.g., compressive strength of the mixture) [1]. Riggs and Keck [11] conducted a survey of CLSM specifications used by six state transportation agencies. For the states surveyed, the only mechanical property specified was the unconfined compressive strength. Several of the states surveyed waived any gradation requirements for aggregates. Some applications for CLSM require unconfined compressive strengths of less than 2.1 MPa (300 psi) to allow for future excavation [1]. Limited requirements given in specifications for CLSM mixtures allow for ease of incorporation of byproducts in CLSM mixtures [e.g., Ref 1]. Broad initiatives have been underway since the 1980s to divert materials from landfills by recovering materials for alternative use in CLSM (e.g., Ref 12). When materials are diverted from landfills to construction activities, the costs of both disposal and virgin material are avoided. Many engineering applications have been at the forefront of such reuse due to the high volumetric quantities involved in construction. For such operations, even small replacement ratios translate into significant diversion quantities. CLSM mixtures provide a viable application for incorporating post-consumer byproducts in construction because the performance requirements for CLSM could be less rigorous compared to those for the use of such materials in other cement-based materials such as concrete. Byproducts previously used in CLSM include shredded rubber tires, crushed glass, spent foundry sands, coal combustion products, pulp and paper mill residuals, incinerated sewage sludge ash, and similar materials [12-22]. Byproducts have been used to replace both cementitious materials (e.g., coal combustion byproducts) and aggregates (e.g., recycled glass, foundry cupola-slag, recycled concrete, and scrap tire rubber) in CLSM mixtures. A summary of byproduct materials incorporated in CLSM is presented in Table 2. Several of the byproducts included in mixtures have been materials with relatively high organic or carbon contents (e.g., high carbon content ash, pulp, and paper mill residuals). The addition of byproduct materials may require the adjustment of mixture proportions due to changes in the specific gravity and other properties of the byproducts. Poor mixture proportioning of CLSM can lead to excessive bleeding [14,18,19] and fluidity issues [15]. This investigation was conducted to evaluate the innovative use of corrugated fiberboard (referred to as corrugate) as an aggregate replacement for CLSM mixtures. Paper and paperboard constitute the highest fraction by weight and volume of municipal solid waste generated (33 % by weight) and disposed in landfills (22 % by weight) in the United States [23]. In 2007, the total amount of paper and paperboard generated in the United States was 83 x 10 tons, and the amount disposed of was 37.8 x 10 tons [23]. Due to its high strength to weight ratio, corrugated packaging is poised to be the leading choice for transport packaging in the United States [24]. Approximately 80 % of the paper-based packaging used is corrugated fiberboard shipping containers [24]. Corrugated fiberboard, which is widely used in the manufacturing of corrugated boxes and shipping containers, is a paper-based material consisting of a fluted containerboard sheet and at least one flat linerboard sheet. The use of recovered paper in the manufacturing of containerboard has remained nearly stable (at approximately 16 x 10 tons) since 1997 [25]. Therefore, a practical limit for incorporating waste paper into containerboard has been reached according to the paper industry. The beneficial reuse of corrugated fiberboard in geotechnical engineering and construction applications is not common [26]. Innovative reuse applications (beyond the packaging industry) need to be investigated in order to promote the beneficial reuse of paper products. [Table 2] Experimental Test Program Materials, Mixture Proportions, and Mixing Procedures The CLSM mixtures were prepared using cementitious materials (cement or cement and fly ash), fine aggregate, water, and fiberized (i.e., repulped) corrugate. Details of the materials used in the test program are provided in Tables 3 and 4. The baseline mixture design was based on a sample mixture design provided by the Ohio Department of Transportation [1]. The cementitious materials content was identical to those of the sample mixture design, and the fine aggregate content was adjusted to account for differences in the specific gravity of aggregates [American Concrete Institute (ACI) versus local source]. An assumed air content of 3 % was used for batch calculations [36]. Mixtures with variable cement to cementitious materials (c/cm) ratios were prepared for the test program. The mixtures had c/cm ratios of 0.29 (baseline), 0.65, and 1.0 (entirely portland cement with no fly ash). Fine aggregate was replaced with fiberized corrugate at specified replacement rates (ranging between 0.25 % and 6 % on a dry weight basis). A photograph of a sample of fiberized corrugate (i.e., pulp) is presented in Fig. 1, together with a microscopic image indicating the typical aspect ratios of individual fibers. The corrugated fiberboard was characterized according to Technical Association of the Pulp and Paper Industry (TAPPI) standards (Table 4). Results of the tests conducted on the corrugated fiberboard are provided in order to thoroughly document the materials used in the test program for potential future comparisons. [Table 3] [Table 4] [Figure 1] A total of 21 CLSM mixtures was tested in the investigation (Table 5). Aggregate material quantities are reported for saturated surface dry conditions in the table. The mixtures were assigned designations for reference: the number preceding the letter C is the percentage of fine aggregate replaced by corrugate, and the number following the hyphen is the c/cm ratio expressed as a percentage. For example, in mixture 0.5C-65, 0.5 % of the fine aggregate was replaced with corrugate, and 65 % of the cementitious material was cement (the remaining 35 % is fly ash). The water quantities provided in Table 5 represent all water in the mixture beyond the saturated surface dry condition of the fine aggregate. The amount of water that was held by the fiberized corrugate versus the amount that was available for the hydration of cement was not quantified. The amount that is held by the fiberized corrugate is variable as a function of confining stress conditions as the fibers represent a compressible solids fraction of the mixtures. [Table 5] The first round of testing was conducted using batches with an approximate volume of 0.057 m (2.0 ft), termed full-scale batches. The majority of tests were conducted using batches of approximately 0.014 m (0.5 ft), termed small-scale batches. The actual volumetric batch sizes varied due to the effects of decreased specific gravity of the corrugate as compared to the fine aggregate. Adjustments to the water content were made based on aggregate absorption and moisture content. Water masses equal to those of the baseline mixtures were used as a starting point for mixtures containing corrugate. Water was added as needed to reach equivalent flowability as compared to the baseline. The amount of added water was recorded to calculate the actual batched proportions (reported in Table 5). A similar approach was used previously by Cheung et al. [19] to prepare CLSM mixtures with shredded rubber tires used as a fine aggregate replacement. Prior to the preparation of the test batches, individual materials were weighed and sealed in buckets in order to prevent changes in their moisture content. After the corrugate was fiberized, the pulp was allowed to drain in order to remove some of the excess water. The mass of the corrugate and the total pulp mass were recorded to determine the effective water content of the pulp. Some but not all of this water was assumed to be available as free water in the CLSM mixture. Materials were mixed in a Multiquip rotary drum mixer (MC64PE) for full-scale batches and in a mortar mixer for small-scale batches. Materials were added in a manner consistent with the order provided by the ACI [1]. First, approximately half of the aggregate and approximately 75 % of the mix water were added. After mixing for several revolutions the cementitious materials were added, followed by the remaining aggregate and the balance of the mixing water. For mixtures with corrugate, the water content of the pulp was close to, if not in excess of, the baseline water content. For these mixtures, the addition of the fiberized corrugate-water blend was adjusted so as to follow the procedure described by the ACI [1].