International Concrete Abstracts Portal

Advances in concrete technology have led to the development of a new class of cementitious composites with improved mechanical and durability properties, named high performance concrete (HPC). Along with improved performance of HPC there is high cement consumption in the production of this type of concrete which leads to certain increases in CO2 emissions. Ecological and environmental benefits support the use of waste glass powder as supplementary cementing material by decreasing the necessity for landfills, by the reduction of non-renewable natural resource consumption, by the reduction of energy demand for cement production (less cement is needed), and by reducing the greenhouse gas emissions. The present research is focused on design of an HPC using different glass waste cullet ground along with sand into powders which have the most promising effect on the properties of concrete and the effectiveness of application of new generation poly-phosphonic superplasticizers blended with PCE based superplasticizer for HPC concrete. Portland cement is substituted at a level of 20% by mass with glass waste powder which gives the improvement of workability and mechanical properties of the concrete what makes glass powder a valuable Portland cement substitute.

To investigate the relationship between the alkali content of concrete and the expansion caused by alkali-silica reaction, several hundred concrete prisms containing reactive natural aggregate, were regularly measured over a period of ten years. These prisms contained between 0 and 70% slag cement in combination with portland cements, and had concrete alkali contents between 4.5 and 11 kg/m3 (0.3 and 0.7 lb/ft3). The alkali content of the Portland cements ranged from 0.54 to 1.15% and that of the slag cements from 0.58 to 0.83%. Prisms were moist-stored at 20°C (68 °F) and at 38°C (100°F). Storage at the higher temperature accelerated the rate of expansion, and slightly increased the ultimate expansion. The correlation between the two temperatures was very good in terms of classifying mixtures as either ‘expanding’ or ‘non-expanding’. It is concluded that storage at 38°C (100°F) is an accelerated test that can be used to reliably predict what would happen at ‘normal’ temperature. The mixtures containing slag cement, tolerated much greater alkali contents in the concrete, without expansion. This effect was more pronounced for higher proportions of slag cement.

During the early stages of hydration, cement paste develops a structure which will ultimately lead to setting through a depercolation process. This structuring process is reversible until setting, meaning that it can be destroyed by imposing a mechanical shear stress but it will rebuild with approximately the same kinetics. The driving force for this process lies in the attractive forces acting among the cement particles in the course of hydration. A wealth of organic admixtures is used to modify the interparticle forces and consequently altering the fresh state properties of cement paste, and we will concentrate in the following on superplasticizers and retarders. Both these classes of admixtures modify the microstructure of cement paste and yield different mechanical properties and different kinetics of the structuring process. The evolution of mechanical properties is followed by rheological measurements in the oscillatory mode, which enables to determine the storage and loss modulus of cement paste. These properties can be interpreted along a modeling scheme referring to a heterogeneous composite material, similarly to what has been done with nanoindentation and ultrasonic measurements. It is shown that the difference between superplasticizers and retarders lies only in a different grading of the same basic interaction with the hydrating surfaces, and in fact it is possible to devise molecular structures yielding a dispersing retardant or a slump-retaining superplasticizer, i.e., the intermediate member between the two families. In a similar way, it is possible to tailor the interaction of the organic molecules with the hydrating surfaces in order to develop more robust admixtures with regards to variations of the interstitial solution chemistry.

Noncontact Splices for Column Bars: I’m a construction manager on a job where a note on the design drawings states that “All column splices shall be Class B tension lap splices.” Does this mean that the splices must be full contact splices to meet code requirements? The contractor prefabricated the reinforcing bar cages for the columns, and when the cages were set in place, the vertical embedded dowels didn’t match up with the vertical bars in the column cage, but they were within 3 in. Minimum Average Compressive Stress for Prestressed Concrete: When prestressing steel is used for shrinkage and temperature reinforcement, Section 7.12.3.1 of ACI 318-02, “Building Code Requirements for Structural Concrete,” requires that tendons be proportioned “to provide a minimum average compressive stress of 100 psi.” Section 18.12.4 in the prestressed-concrete chapter of ACI 318-02 says to provide a minimum average prestress of 125 psi for slab systems. Why does Section 18.12.4 require an additional 25 psi? And what is the basis for the two numbers?

The ability of lithium-based compounds to suppress deleterious expansion due to alkali-silica reaction (ASR) in mortar and concrete was first demonstrated over 50 years ago. Lithium nitrate solution is now marketed in North America as a chemical admixture for inhibiting ASR; the product currently available is a 30% solution of LiNO3. The purpose of the study reported here was to determine to what extent a chemical admixture based on lithium nitrate influences the properties of fresh and hardened concrete when used at the dosage levels required to suppress expansion due to ASR. This was done by comparing the fresh and hardened concrete properties of a number of mixtures with and without the admixture. The constituent materials and concrete mixture proportions used in the study were based on those currently approved for the Class AA Structural Concrete for a major highway project in Albuquerque, New Mexico. These materials included a Type II portland cement, Class F fly ash, a reactive aggregate, and water-reducing and air-entraining admixtures. The water-to-cementitious-material ratio of the concrete was in the region of W/CM = 0.35. All of the concretes tested had either no air entrainment, or air contents in the range of 5% to 7%, slump values between 100 mm to 125 mm and strengths in the region of 32 MPa to 36 MPa. The use of lithium nitrate solution, at the levels of addition necessary to effectively control expansion due to ASR, appeared to have no adverse effect on the properties of fresh and hardened concrete, even at dosages in excess of 10 litres per cubic metre. Changes in the measured slump, air content and setting time of plastic concrete were generally insignificant. Tests on sawn concrete samples confirmed that the use of lithium had no impact on the hardened air-void parameters. Physical testing of hardened concrete showed that the use of lithium had little significant or consistent effect on the concrete strength or its durability as defined by the resistance of specimens to the penetration of chlorides or cyclic freezing and thawing.