International Concrete Abstracts Portal

SP-218 This is a compilation of papers addressing “High-Performance Structural Lightweight Concrete” presented October 30, 2002 at the American Concrete Institute Fall Convention in Phoenix, Arizona. This symposium was sponsored by ACI Committee 213, Lightweight Aggregate and Concrete, to report on a wide range of global construction applications incorporating high-performance lightweight-aggregate concrete. This diverse symposium included papers that covered microstructural issues (autogenous shrinkage, internal curing), material and structural properties (transfer length, shear strength, seismic behavior), and applications in large civil structures (long-span balanced cantilever bridges, offshore platform, float-in navigational locks).

This paper outlines the testing program developed for the Raftsundet Bridge, the first bridge in Norway that utilized pumping for placement of lightweight concrete. It reports the results from parallel testing of both normalweight and lightweight concrete performed during construction of this bridge. This paper also presents a discussion of the economics of using high performance lightweight concrete on the Rugsund Bridge. It also describes the Sundoy Bridge, the second bridge in Norway to utilize pumping for lightweight concrete placement. These projects confirm that high strength lightweight concrete is an economical, efficient construction material for long span bridges. While lightweight concrete may cost more per cubic yard than normalweight concrete, the structure may cost less as a result of reduced dead weight and lower foundation costs (1).

This work is part of a much larger program to evaluate high performance concrete mixtures that can be used successfully in hot dry climates. In this research magnetic resonance imaging (MRI) was used to measure the effectiveness of extending the moist curing period by incorporating some saturated lightweight aggregates into a concrete mixture being placed in hot dry climatic conditions. A series of concrete mixtures were prepared and moist cured for either 0, 0.5, 1 or 3 days, or by using a curing compound, followed by air drying at 38°C and 40% relative humidity. To accomplish this, 11% by volume of the total aggregate content was replaced with lightweight aggregate. Type I white portland cement and quartz aggregate plus the lightweight aggregate were all selected for their low iron content to minimize adversely affecting the MRI measurements. The concrete mixtures were low strength concrete (W/C=0.60), self-consolidating concrete (W/C=0.33 containing 30% fly ash), and high strength concrete (W/ C=0.30 containing 8% silica fume). Specimens prepared with these mixtures were cast in triplicate. After curing, the specimens were dried in one direction in an environmental chamber at 38°C and 40% relative humidity. As the specimens were drying, magnetic resonance imaging was used to determine the evaporable water distribution. After the drying period, the specimens were conditioned in an oven at 105°C and water absorption tests were undertaken to determine their sorptivity. The profiles obtained during drying indicated a reduced moisture loss with increasing length of moist curing. Also the use of saturated lightweight aggregate does not eliminate the need to provide some external moist curing for a reduced period of time. The results from water uptake experiments indicated that the addition of lightweight aggregate particles substantially increases the sorptivity in low strength concrete while it has only a marginal effect in both self-consolidating and high strength concrete, when compared to the same concrete mixtures containing only normal-weight aggregate.

Structural lightweight concrete is defined as concrete made with low-density aggregate having an air-dry density of not more than 115 lb/ft3 (1850 kg/m') and a 28-day compressive strength of more than 2500 psi (17.2 MPa). This paper presents the test results of very low-density structural lightweight concrete mixtures developed in the laboratory for the purpose of finding a suitable mixture for use on a historic building rehabilitation project. Mixture parameters included a specified compressive strength of 3000 psi at 28 days and an air-dry density approaching 70 lb/ft3. Various constituent materials, mixture proportions and curing methods were examined. The result of this research exemplifies the feasibility of achieving very low densities with structural concretes.

The 3 main factors determining the efficiency of lightweight aggregate (LWA) as internal curing agents in concrete are discussed with reference to published papers: I) total amount of water in LWA, 2) LWA particle spacing factor and 3) the LWA pore structure. A desorption method is suggested to characterize factor 3) directly as the ability of the LWA to release water. The method is applied to two LWA types and the results demonstrate clear differences. The role of water in ordinary aggregates is discussed with reference to autogenous shrinkage measurements in concrete and the equivalent paste. It is concluded that the aggregate with 0.8% water absorption indeed serves as internal curing agent by reducing and delaying the autogenous shrinkage. Finally, it is shown that sealed curing does reduce and delay both the cement and pozzolanic reactions.