This article is the third in a series of articles in ESCSI Lightweight Design
eNews which address common myths or misconceptions about lightweight aggregate or lightweight concrete. These articles specifically address issues related to concrete made using expanded shale, clay, or slate (ESCS) lightweight aggregate that has been manufactured using a rotary kiln process at temperatures around 2000 deg. F. When the particles reach these high temperatures, the material softens and bubbles are formed which remain as pores when the aggregate cools. The pores within the vitrified ceramic aggregate particles give the lightweight aggregate its reduced density and other unique properties.
The first two articles in the Engineer’s Corner series addressed common myths or misconceptions about lightweight aggregate or lightweight concrete. Future articles will continue this theme by addressing other misconceptions about lightweight aggregate and lightweight concrete.
The following myth will be addressed in this article:
Myth #3 – Lightweight concrete always has a reduced tensile strength compared to normal weight concrete of the same compressive strength, so aspects of design related to tensile strength, such as shear and development length of reinforcement, must be adjusted.
It is commonly assumed that lightweight concrete has reduced tensile strength compared to normal weight concrete with the same compressive strength. With a reduced tensile strength, it is appropriate to modify aspects of design related to tensile strength, such as shear design and the development length of reinforcement. Approaches to computing such reduction factors have been included in design specifications like the AASHTO LRFD Bridge Design Specifications for many years.
However, LRFD Specifications Article 5.4.2.8 provides a seldom-used option that allows the concrete density modification factor λ to be computed based on a specified splitting tensile strength of concrete fct. If the splitting tensile strength of lightweight concrete is at least equal to the splitting tensile strength of normal weight concrete, which is assumed to be 0.213√f’c, then λ can be taken as 1.0, with no modification for shear design or development length for lightweight concrete. While some lightweight concrete may indeed have a reduced splitting tensile strength, other mixtures have been shown to have at least the same tensile strength as normal weight concrete with the same compressive strength.
Rather than fully developing these concepts in an article in this newsletter, a link is provided to an article on the topic that was published in the Spring 2019 issue of ASPIRE magazine. The article discusses the option to specify the splitting tensile strength for lightweight concrete, then presents two sets of splitting tensile strength data, one of which uses lightweight aggregates from three different sources, that demonstrate that lightweight concrete can have a splitting tensile strength equal to or greater than the value expected for normal weight concrete. Since the lightweight concrete test results show splitting tensile strengths greater than the expected normal weight concrete value computed using 0.213√f’c, the potential exists for a designer to use λ = 1.0, or at least a value greater than what would be computed using LRFD Eq. 5.4.2.8-1, which provides values of λ ranging from 0.75 to 1.0 based solely on the unit weight wc of lightweight concrete.
Using the option permitted by the LRFD Specifications to specify the splitting tensile strength, designers can specify fct for lightweight concrete to be equal to the expected splitting tensile strength for normal weight concrete when shear or development lengths are important aspects of design. This allows the design of a lightweight concrete element to be the same as for a normal weight concrete element except for differences in dead load and the effect of a reduced modulus of elasticity. If desired, designers could also specify a splitting tensile strength for lightweight concrete that is somewhat less than the expected splitting tensile strength for normal weight concrete, which would result in an intermediate value of the concrete density modification factor λ that would be less than 1.0 but greater than the value computed based on the unit weight of the lightweight concrete computed using LRFD Eq. 5.4.2.8-1.