The process of concrete testing on today's projects is a routine and essential procedure. Monitoring the plastic properties (slump, air content, temperature) of concrete during placement and casting the required cylinders for compressive strength testing still provide critical information. These procedures enable the design/construction team to judge compliance of the concrete to the project specifications and structural requirements. However, as more and more construction projects are planned with ever-shrinking schedules (often referred to as hyper-track projects), methods providing immediate on-site concrete strength information are an important addition to the construction management process and are more in demand than ever.
There are several ways to monitor the strength gain of concrete at a project site. Probably the two most common methods are field-cured cylinders (stored next to the placement under similar conditions) and cast-in-place-punch-out cylinders (known as CIPPOC, which are formed into the concrete structure for subsequent removal). However, both of these methods have definite drawbacks regarding field predictions for fast-track construction. Field-cured cylinders do not reflect the actual internal curing temperatures of the concrete and underestimate the actual in-place strength. They are also susceptible to disturbance by construction activities. CIPPOC, or inserts, provide good strength indications, but there is always the potential of using up all test specimens before reaching the desired strength and they may represent an unwanted penetration through a slab. Both methods result in some delay associated with obtaining CIPPOC or field-cure cylinders from the site and returning them to the laboratory for testing.
To help meet client schedule requirements, American Engineering Testing (AET) has recommended the Maturity Method for on-site strength predictions. While temperature monitoring and determining the strength of in-place concrete using the Maturity Method is not a new idea, it hasn't enjoyed widespread use in the past. In midwestern United States, it was initially a method for cold weather concreting operations. Recently it has gained wide acceptance for use on projects requiring immediate on-site strength information to meet tight construction schedules.
The Maturity Method (ASTM C1074) measures the age and temperature of concrete, then uses these parameters to calculate a maturity factor. The first step is developing the relationship between the maturity factor and concrete strength for a specific mix design. When this relationship is established, remote thermocouples placed in strategic locations within the concrete measure the maturity factor for various time intervals, which can be read directly from the meter. A strength factor is obtained from the established curve at the site.
AET recently used this method on two projects with compressed construction schedules. The first project was Allianz Life/Life USA Corporate Campus in Golden Valley Minnesota. Here, the owner needed completion of their new corporate headquarters to avoid renewing existing leases. The resulting schedule required the form work for each placement be cycled in two days, yet a minimum strength of 4,000 psi was required. The Maturity Method was selected to provide in-place strength information of the concrete to facilitate form removal and post-tensioning of high-strength support cables. During construction, the maturity factor could be read directly from the Maturity Meter, eliminating delays associated with obtaining CIPPOC or field cure cylinders from the site and returning them to the laboratory for testing. Further, by reviewing the rate of increase of the maturity factor stored in the meter, an accurate time estimate of reaching the required concrete strength was used to efficiently assign manpower at the site.
The second project was a post-tensioned (PT) parking ramp for Hawthorne Transportation Center in Minneapolis. The microsilica concrete had a 3,000 psi tensioning requirement and a twenty-eight-day design strength of 6,000 psi. Project specifications also dictated PT cables be tensioned (at least partially) with-in three days of concrete placement. The construction team's goal was full tensioning of the post-tensioned cables within this time allotment. By using the Maturity Method, AET obtained accurate strength predictions which in turn helped the contractor schedule manpower as well as optimize the form work turnaround time.
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