Innovating Two-Stage Concrete with Improved Rheological, Mechanical and Durability Properties

Two-stage concrete (TSC), also known as preplaced aggregate concrete, is a special type of concrete that is produced using a unique procedure which differs from that of conventional concrete. TSC is distinguished by its high coarse aggregate content and exceptional placement technique, whereby aggregates are first pre-placed in the mold then injected with a special grout. The preplacement of aggregates saves substantial energy since only the grout needs mechanical mixing; the grout is self-leveling and needs no vibration and no mechanical compaction. However, TSC applications are still limited despite substantial advancement of modern concrete technology. Therefore, there is a need to explore new possibilities and applications for TSC through adjusting and improving its properties. The objective of this study is to advance the TSC technology through the use of supplementary cementitious materials (SCMs), fibre reinforcement, capturing its sustainability features to develop novel pavements with very high recycled content, and establishing models with predictive capability for its engineering properties. Therefore, the fresh and hardened properties of grout mixtures incorporating various SCMs, including fly ash (FA), silica fume (SF) and metakaolin (MK) were investigated. An attempt was made to identify the optimum water-to-binder (w/b) ratio and the high-range water-reducing admixture (HRWRA) dosages for grout mixtures that meet the recommended efflux time (i.e. 35-40 ± 2 sec) according to ACI 304.1. Moreover, the effects of various SCMs at different dosages on the development of TSC mechanical properties were investigated. Likewise, the performance of TSC made with single, binary and ternary binders exposed to different environments conducive to physical and chemical sulfate attack was explored. The negative influence of fibres on the workability of conventional concrete is eliminated in TSC since the coarse aggregates and fibres are preplaced in the formwork and then injected with a flowable grout. This allows using fibre dosages beyond the practical levels typically adopted in conventionally mixed concrete. Therefore, the mechanical performance of two-stage steel fibre-reinforced concrete (TSSFRC) made with different dosages of steel fibres having various lengths was explored for the first time. The high coarse aggregate content endows TSC with superior volume stability, making it an ideal contender for pavements and sidewalks, which typically suffer from shrinkage and thermal cracking. In this study, the preplaced material consists of recycled concrete aggregate and scarp tire rubber granules along with scrap tire steel wire fibres, while the grout uses high-volume fly ash. The performance of such a “green” TSC pavement construction technology was explored. Finally, the experimental results were used to create a database which was utilized for developing fuzzy logic (FL) models as a means of predicting the grout flowability (i.e. efflux time and spread flow) and the mechanical properties (i.e. compressive and tensile strength) of a variety of two-stage concrete (TSC) mixtures. Results indicated that grouts made with water-to-binder ratio (w/b) = 0.45 can achieve the recommended grout flowability for successful TSC production. Moreover, TSC grout properties highly depended on the type and dosage of SCM used. The grout flowability was significantly enhanced as the FA dosage was increased, while the compressive strength was decreased. Partially replacing cement with 10% SF or 10% MK reduced the grout flowability and enhanced its compressive strength. Moreover, the binder composition has a great influence on the TSC mechanical properties. Empirical relationships between the properties of the grout and those of the corresponding TSC were proposed, offering a potential tool for estimating TSC properties based on primary grout properties. Furthermore, the ease of using a high dosage of pre-placed fibres in TSSFRC allowed achieving exceptional engineering properties for the pre-placed aggregate concrete. Indeed, TSSFRC can easily be produced with 6% steel fibre dosage, which makes it an innovative option and a strong contender in many construction applications. Fully immersed TSC specimens incorporating FA or MK in sodium sulfate solution exhibited high sulfate resistance. Surprisingly, TSC specimens incorporating SF deteriorated significantly due to thaumasite formation. Under physical sulfate attack exposure, TSC specimens incorporating FA and/or SF incurred severe surface scaling at the evaporative front, while those made with MK exhibited high resistance to surface scaling. A novel eco-efficient technology for the construction of pavements and sidewalks was proposed. The results demonstrate the feasibility of TSC eco-efficient technology to produce durable and cost-effective sidewalks and pavements, offering ease of placement and superior sustainability features. Finally, the performance of the developed FL models was evaluated using error and statistical analyses. The results indicate that the FL models can offer a flexible, adaptable and reasonably accurate tool for predicting the TSC grout flowability and mechanical properties. The findings of this study should provide a leap forward in establishing the TSC technology as a strong contender in many construction applications. It contributes to taking the TSC from a basic technology to a more modern system that benefits from advancements in concrete technology through the use of SCMs, chemical admixtures and fibre reinforcement. In particular, in a new context that values sustainability and “green” construction technology, this study has proven TSC to be exceptional in its ability to use recycled materials without the drawbacks observed in normal concrete technology. These findings should contribute to enhancing the understanding of the TSC behaviour, paving the way for its wider implementation in today’s concrete industry

MAT-715: SUSTAINABLE HIGH-VOLUME FLY ASH GROUTS FOR TWO-STAGE CONCRETE

Two-stage concrete (TSC) is a special type of concrete in which coarse aggregates are pre-placed in the formwork and subsequently injected with a grout. Beneficiating fly ash in TSC grouts increases TSC sustainability through the ecological use of large quantities of fly ash, reduced carbon-dioxide emissions associated with cement production, and enhancement of resource productivity of the concrete industry. Limited research has explored the effects of using high volume of fly ash as partial replacement for cement in TSC grout mixtures. Therefore, the flowability of grout mixtures incorporating various fly ash addition rates (i.e. 0%, 30%, 50% and 70%) was evaluated using the flow cone method and spread flow test. Correlations between the efflux time and spread flow for the grout mixtures were developed. Results show that increasing the fly ash addition reduced the grouts efflux time while increasing its spread flow. The optimum high-volume fly ash dosage for achieving high flowability and acceptable TSC compressive strength was identified

MAT-716: GREEN SIDEWALKS USING SUSTAINABLE TWO-STAGE CONCRETE

Two-stage concrete (TSC) is a special type of concrete, which has a high potential for use in sidewalk construction owing to its high volume stability. TSC is characterized by its high coarse aggregate content. Hence, using recycled solid waste materials as a coarse aggregate will increase TSC sustainability, while providing a cost-effective alternative to natural aggregates. Aggregates are pre-placed in TSC. Hence, water absorption by recycled concrete aggregates and the associated rheology problems do not exist in TSC. This study explores the performance of green TSC sidewalks incorporating recycled concrete aggregates (RCA) and crumb rubber from scrap tires. Mechanical properties of the proposed green TSC including compressive strength, modulus of elasticity, flexural strength and toughness, as well as durability to freeze-thaw cycles were investigated. Results show a slight reduction in TSC mechanical properties due to the use of RCA. Moreover, incorporating tire particles reduced TSC mechanical properties significantly, while improving its toughness and freeze-thaw resistance. Addition of recycled tire steel wires allowed to overcome the negative effects on the mechanical properties induced by crumb tire rubber. Therefore, recycling solid waste materials in TSC sidewalks can be an effective strategy to beneficiate such waste materials