International Concrete Abstracts Portal

An investigation was performed on the drying shrinkage and tensile drying creep characteristics of a nonproprietary ultra-high-performance concrete (UHPC) mixture. The mixture was formulated using metakaolin as the supplementary cementitious material (SCM) and limestone powder as the mineral filler. Cylindrical specimens with dimensions of 52 x 400 mm (2.05 x 16 in.) were fabricated and loaded at 7 and 11 days from casting to various stress levels for 90 days. Additional specimens were fabricated from a proprietary mixture with a silica fume-ground quartz formulation to study the effects of mixture composition. Simultaneous free drying shrinkage measurements were recorded in accompanying specimens placed in the same room environment. Attention was given to the effect of the casting orientation, age at loading, and mixture composition on the drying shrinkage and drying creep behavior of the samples. These tests show that the metakaolin-limestone powder mixture has significantly lower drying shrinkage and specific drying creep than the silica fume-ground quartz mixture. Additionally, the age at loading influences primary creep behavior while not affecting secondary creep at the same stress level. It seems that fiber orientation plays a significant role in the drying creep behavior of UHPC and that cracked UHPC under constant tensile stress undergoes a significant amount of fiber slip.

Ultra-high-performance concrete (UHPC) is a cement-based material whose superior mechanical and durability features are ascribed to its enhanced microstructure. Nonetheless, large amounts of cement and other expensive components are usually necessary to achieve that particular microstructure, leading to higher costs and carbon footprint than standard concretes. Even though several pieces of research have focused on the employ of mineral admixtures as partial replacement of cement and silica fume while maintaining a compressive strength of over 150 MPa, there is a lack of knowledge on how these additions affect other properties of UHPC. This research paper aims to fill that gap in the Colombian case by analyzing the mechanical properties (compressive strength, modulus of elasticity) and durability (ultrasonic pulse velocity, volume of permeable voids, and chloride ion penetration) of two optimized, cost-efficient, and eco-friendly UHPC dosages containing locally available and low-cost mineral admixtures, encompassing two economically viable milled waste glass powders with an average particle size of 7 and 28 microns, and limestone powder. The results were compared with those obtained for a control mixture, which contained high amounts of cement and silica fume as cementitious materials. The conclusions showed that limestone and recycled glass powders reduce both the high-range water-reducing admixture (HRWRA) and the time of the mixing procedure, leading to a relevant decrease in the production costs. Cost-efficient and eco-friendly UHPCs’ mechanical and durability properties were minor but in the same range as the reference mixture. However, the UHPC dosage incorporating limestone and 28-micron glass powders implied a reduction in cement by 31% and material costs by 32.5% from the reference. In comparison, the dosage in that addition contained 7 microns of glass powder which led to a lessening of 29% in cement and 33.5% in final costs. These outcomes endorsed using limestone and recycled glass powders to develop low-cost and eco-friendly UHPC in Colombia.

Relatively limited work has been performed to quantify how internal curing influences curing specifications. This paper examines the performance of internally cured mixtures (made using fine lightweight aggregates) compared to conventional concrete cured with wet burlap and curing compounds. Mortar mixtures were prepared using ordinary portland cement (OPC), fly ash, and silica fume (SF) with water-cementitious materials ratios (w/c) of 0.35 and 0.45. Neutron radiography (NR) was used to determine the nonevaporable water content as a function of curing time and distance from the exposed surface. The curing-affected zone (CAZ) was determined using the nonevaporable water profiles. The CAZ was used to develop equivalent curing durations for conventionally cured and internally cured samples. Internally cured mixtures reduced the depth of the CAZ, especially in the samples with limited external curing durations (reduction up to 15 mm [0.6 in.]). The application of internal curing in all mixtures reduced the duration of external curing by 50 to 60%, except for the internally cured SF samples, which showed a slight reduction. This dramatically impacts the construction schedule.

Nowadays, the application of ultra-high-performance concrete (UHPC) has had a significant proliferation worldwide. However, its high carbon footprint and costs, due to its high cement, silica fume, quartz dust, and fiber contents, can be mentioned among its main drawbacks. It is the purpose of this review paper to summarize and analyze the contributions of the research performed in Colombia in the last 2 years on the optimization of UHPC dosages to render its costs and carbon footprint a minimum. The results of this review demonstrated that it is possible to obtain an eco-friendly and low-cost UHPC cementitious matrix by using mineral admixtures available in the Colombian market. Furthermore, the referenced pieces of research also showed that they could achieve excellent ductility parameters by adding less than 2% of total volume fiber to the optimized UHPC matrix.

The reactions of supplementary cementitious materials (SCMs) in concrete can be pozzolanic, hydraulic, or a combination of both. This paper focuses on the pozzolanic reactivity test (PRT) for SCMs that are blends of reactive aluminous and siliceous phases. The PRT quantifies reactivity by measuring heat release (Q) and calcium hydroxide (CH) consumption, which are interpreted using thermodynamic modeling. The robustness of the PRT is examined by experimentally varying the CH-to-SCM ratio, solution-to-solid ratio, sulfate content, alkali type (Na versus K), and alkali content. This paper also assesses similarities and differences between the PRT and the R3 test (ASTM C1897). It was found that sulfates, which are used in the R3 test, did not impact the siliceous reactions; however, they led to the preferential reaction with aluminous phases to form monosulfoaluminates and ettringite. A generalized relationship for the degree of reactivity is proposed as a function of Q and CH consumption.