Quality control and commissioning
Strength gain comparison between a xe "standard"standard concrete mixture and with increasing percentages of fly ash (adapted from data from Headwaters Resources ). (Note: 1 MPa = 145.03 psi.)
Quality control through testing is important to ensure that the materials specified meet their performance requirements. A significant reduction in a project’s carbon footprint can be achieved by simple adjustments in the concrete requirements specified by the designer. This chapter discusses several ways that the designer can influence the economy, durability, and lowered carbon footprint of a structure through testing requirements.
The designer should address durability and strength requirements with an eye for performance and with an understanding of the relationship between the two; durability and strength in concrete are not completely independent of one another. Sampling and testing focuses on minimum strength but can lead to mixture proportions that may be significantly higher cement content than needed and an increase the associated carbon intensity. For specific applications, there may be a need for early strength gain so that the construction schedule is not compromised. On most projects, however, a significant portion of the structure can achieve the specified design strength at a later age without compromising the construction schedule or safety. Careful consideration of the degree of overdesign and age needed to meet acceptance requirements (ACI 318-08) can achieve the intended quality control, without undue excess.
Consider the following example to illustrate the benefit of extending the acceptance age from 28 to 56 days for a concrete footing. Assume the concrete has a design compressive strength of 3000 psi (20.7 MPa) and contains fly ash. Assume the variability of the producer is moderate, resulting in a standard deviation of 450 psi (3.1 MPa) for this mixture. The required average compressive strength, f′cr, is 3603 psi (24.8 MPa), derived from formulas prescribed previously in the chapter. Assuming a modest cementitious efficiency of 7 psi/lb per yd3 (0.14 MPa/kg per m3) of concrete, the mixture needs approximately 514 lb/yd3 (305 kg/m3) of cementitious material. Given a 10% increase in strength from 28 to 56 days, the mixture would deliver an average compressive strength of 3969 psi (27.4 MPa). If the acceptance criteria are changed to a required compressive strength of 3603 psi (24.8 MPa) at 56 days, cementitious material could be reduced by approximately 50 lb/yd3 (30 kg/m3), which is a significant reduction.
Concrete is not used directly for the types of systems that are typically commissioned (such as HVAC and lighting), but commissioning is briefly discussed here because it is an important part of the construction process. A sustainable building should be built with a whole-building approach to be truly successful, and commissioning is part of this process. Commissioning verifies that the systems in the building are functioning as they were intended. This includes verification of proper design, installation, calibration, and operation. The commissioning authority for a project should be a third party that is not involved directly in the design or construction of the building. The scope of the commissioner’s responsibility is wide, and includes, but is not limited to:
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