Adam M. Knaack

Design of Concrete Mixtures with Recycled Concrete Aggregates

This paper investigates the workability, compressive strength, and elastic modulus of normal-strength concrete with recycled concrete aggregate (RCA) as replacement for coarse natural aggregate (for example, crushed stone, gravel). To represent the variability in material quality and properties, 16 different RCA sources are used in the experimental program. Three RCA concrete mixture design methods using varying amounts of direct volume aggregate replacement, direct weight replacement, and equivalent mortar replacement are compared. It is found that these different mixture design methods result in similar compressive strength and elastic modulus of RCA concrete; however, the workability of RCA concrete may be considerably reduced when using the equivalent mortar replacement method. While the effect of increased amounts of RCA on the concrete compressive strength is generally small for all three mixture design methods, the concrete elastic modulus is more significantly affected by RCA. A multiple least-squares regression analysis of the test results is conducted to develop predictive design relationships for the strength and stiffness of RCA concrete. The results suggest that the RCA water absorption and deleterious material content can be used to prequalify the material for selected concrete strength and stiffness performance objectives.

Design of Normal Strength Concrete Mixtures with Recycled Concrete Aggregates

This paper investigates the design of normal strength concrete mixtures that use recycled concrete aggregates as replacement for virgin natural aggregates. Three mix design methods utilizing direct weight replacement, equivalent mortar replacement, and direct volume replacement are compared based on concrete workability, compressive strength, and elastic modulus. A total of 42 mixes were made with different aggregate replacement amounts. It was determined that the concrete workability changes significantly depending on the replacement method used, with the direct volume and equivalent mortar methods resulting in the best and worst workability, respectively. The compressive strength and elastic modulus of the mixes with recycled concrete aggregates showed little variation from concrete with natural aggregates.

CREEP AND SHRINKAGE OF NORMAL STRENGTH CONCRETE WITH RECYCLED CONCRETE AGGREGATES

This paper describes an experimental investigation on the creep and shrinkage of normal-strength concrete with recycled concrete aggregates (RCA) as replacement for coarse natural aggregates (for example, crushed stone and gravel). Three RCA sources and two aggregate replacement levels were used; additional parameters were curing conditions, loading age, and axial stress level. It was found that RCA significantly increases the creep and shrinkage deformations of concrete, but this increase is smaller than the effect of RCA on the initial (that is, immediate mechanical) deformations. The creep compliance increases with increased loading age of RCA concrete, in a similar manner as for conventional concrete. Out of the three materials tested, the RCA producing the smallest concrete compressive strength resulted in the largest creep compliance. Adjustment factors were determined for code-based creep models to predict the creep deformations of RCA concrete. The use of these factors is intended to be similar to other code-based adjustment factors for “nonstandard” conditions (for example, loading age).

Compressive Strength Relationships for Concrete under Elevated Temperatures

This paper focuses on compressive strength relationships for the design of concrete structures under elevated temperatures from fire. The development of a database of previous experimental research on the temperature-dependent properties of unreinforced concrete is described. A comprehensive statistical analysis of the concrete strength data from this database is conducted using the method of multiple least-squares regression with coded variables. High-strength concrete (HSC) and normal-strength concrete (NSC) with normalweight and lightweight North American aggregates are considered in the investigation. The results are used to develop predictive relationships for the concrete strength loss under fire. Compared with existing strength loss relationships, the proposed relationships are based on a much larger data set, thus increasing statistical robustness. It is shown that a reasonable statistical fit is achieved with the available data, especially considering that the proposed relationships use relatively simple regression models suitable for design. The most significant parameters affecting the concrete strength loss with temperature are the concrete strength at room temperature, aggregate type, and heating test type. Through a critical evaluation of the current database, recommendations are presented for areas where future research should be directed. Recommendations are also made for presenting the results from future fire tests so that researchers can most effectively use this data.

Sustained Service Load Behavior of Concrete Beams with Recycled Concrete Aggregates

8 This paper describes an experimental investigation on the time-dependent sustained 9 service-load behavior of normal strength concrete beams with recycled concrete aggregates 10 (RCA) used as replacement for coarse natural aggregates (NA). Eighteen beams were tested 11 incorporating different aggregate replacement amounts, reinforcing details, concrete age at 12 superimposed loading, and beams with and without cracking upon immediate load application. It 13 is shown that increased amounts of RCA result in significant increases in both the immediate and 14 long-term deflections of concrete beams, but this effect is smaller for beams with increased 15 cracking. Greater creep and shrinkage deformations also cause a greater downward shifting of 16 the neutral axis (i.e., increase of the neutral axis depth) in RCA concrete beams as compared to 17 NA concrete beams. ACI 318 and Eurocode generally gave good design estimates for the 18 immediate deflections of the test specimens. In comparison, the long-term deflections were 19 significantly underestimated for the uncracked beams. For the cracked beams, the long-term 20 deflections were somewhat underestimated by ACI 318 and overestimated by Eurocode by 21 approximately similar percent errors. The ability of the design methods to predict the measured 22 deflections did not differ significantly between the NA concrete and RCA concrete beams. 23

Rheological and Mechanical Behavior of Concrete Mixtures with Recycled Concrete Aggregates

This paper investigates the workability, compressive strength, and elastic modulus of normal strength concrete mixtures that use recycled concrete aggregates (RCA) as replacement for virgin coarse natural aggregates. To provide a regional geographical representation of the variability in material quality and properties, a total of 144 mixtures with 16 different RCA sources are used in the experimental program. Three RCA concrete mix design methods utilizing varying amounts of direct volume aggregate replacement, direct weight replacement, and equivalent mortar replacement are compared. It is found that the RCA material properties do not affect the workability of RCA concrete designed using the direct volume replacement method. However, for the equivalent mortar replacement method, there is a significant reduction in workability even when the maximum recommended amount of water reducer is used. The compressive strength and elastic modulus of concrete are not significantly affected by the mix design method. Depending on the RCA source, the compressive strength of RCA concrete is generally comparable to the strength of concrete with virgin aggregates. However, there is a greater effect in the elastic modulus as the amount of RCA is increased. The results suggest that the RCA material properties that most affect the mechanical behavior of concrete are the aggregate water absorption and deleterious material content.

Stress-Strain Relationships for Concrete at Elevated Temperatures

This paper is on the compressive stress-strain behavior of unreinforced concrete under elevated temperatures from fire. Relationships are developed for the temperature-dependent compressive strength, elastic modulus, strain at peak stress, and ultimate strain of concrete by conducting a comprehensive multiple least squares regression analysis on the existing experimental data, including the effects of aggregate type, test type, and compressive strength at room temperature. These relationships are then used to produce predictive compressive stress-strain models for concrete under fire. High-strength and normal-strength concrete with normal-weight as well as light-weight North American aggregates are considered in the study. Unlike previous models, creep deformations are not included in the proposed models, resulting in baseline relationships to which time-dependent creep strains can be explicitly added in the future. It is shown that the relationships developed provide a good statistical fit to the available experimental data.