The curious case of pour 1B: Investigation of concrete strength and durability issues

The project
Construction of a large cast-in-place concrete structure is divided into segments with each pour sized and sequenced by how much concrete can be placed and finished at one time. Pour 1B occurred early in the construction of a five-level parking garage located in the Midwest, with a grade slab at the lowest level and the remaining four levels consisting of post-tensioned elevated slabs. The grade slab was poured in January and February with no significant issues, using a different concrete mix than elsewhere on the project. Following the grade slab, the columns and elevated post-tensioned slabs were poured using a 40 MPa (6000-psi) concrete mix. The post-tensioned slab pours were on the order of 300 to 1000 m² (3000 to 10,000 sf), requiring 70 to 250 m³ (100 to 350 cy) of concrete per pour. Pour 1A was placed in early March and was the first slab pour to use the 40 MPa mix. Five days later came pour 1B: approximately 550 m² (6000 sf) requiring some 150 m³ (200 cy) of concrete.

The equipment setup for pour 1B was typical for pours on the project. Concrete was delivered to the site via truck-mounted rotating drum mixers, discharged from the mixers into the hopper of a truck-mounted pump and boom, and then pumped from the truck to the pour 1B area. From there, the boom pump hose led directly to a placement boom consisting of a central pivot with an articulated boom made up of solid pipe segments. The placement boom was essentially a horizontal pipe extension, which had no pump of its own, and allowed flexibility in concrete placement.

Field testing
It is unclear from available records how many testing agency field technicians were present onsite, or if a representative from the ready mix plant was present for some or all of pour 1B. Specified field testing of the fresh concrete consisted of:

• sampling per ASTM C172, Standard Practice for Sampling Freshly Mixed Concrete (every batch);
• concrete temperature per ASTM C1064, Standard Test Method for Temperature of Freshly Mixed Hydraulic-Cement Concrete (every batch);
• slump per ASTM C143, Standard Test Method for Slump of Hydraulic-Cement Concrete (every batch);
• air content per ASTM C231, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method (every batch);
• unit weight per ASTM C138, Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete (every batch);
• water content per AASHTO T318, Water Content of Freshly Mixed Concrete Using Microwave Oven Drying (every 46 m [50 y]/five batches); and
• concrete cylinders were to be collected and standard cured per ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, for later compressive strength testing per ASTM C39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens (every 46 m/five batches).

Documentation
The 40 MPa mix used for pour 1B had a specified air content of 6.5 percent, with a +/- 1.5 percent tolerance allowed by ACI 318, Building Code Requirements for Structural Concrete and Commentary. The testing agency field report for pour 1B noted “gaining air through the pump,” implying air testing was being performed on samples taken both before and after the pump, but only one value for air content was reported. The field report also noted two of the batches arrived with low air, and air-entrainment admixture was added onsite. According to the field report, “air was high throughout the pour,” yet only three batches were reported as having air content higher than specified:

• batch 5: 8.5 percent;
• batch 9: 8.5 percent; and
• batch 12: 9.2 percent.

These were accepted and placed.

The 28-day compressive strength tests for the previous pour 1A using the 40 MPa mix had mixed results: the first two tested batches had results in the 49 to 50 MPa (7100 to 7200 psi) range, whereas the last three batches were in the 39 to 41 MPa (5600
to 5900 psi) range, slightly under the following acceptance criteria provided in ACI 318:²

5.6.3.3 Strength level of an individual class of concrete shall be considered satisfactory if both the following requirements are met:
(1) Every arithmetic average of any three consecutive strength tests equals or exceeds f’c.
(2) No strength test falls below f’c by more than [3.5 MPa] 500 psi if f’c is [35 MPa] 5000 psi or less; or by more than 0.10f’c if f’c exceeds 5000 psi.

For pour 1B, the 28-day compressive strength tests ranged from 32 to 36 MPa (4600 to 5280 psi), with an average of 34 MPa, well under the ACI acceptance criteria, and low enough to trigger an investigation into the cause(s). Project delays followed, and subsequent pours were re-sequenced in the event some or all of the slabs needed to be removed and replaced. Complicating matters were the five additional pours that had occurred in the 28-day period between placement and testing of pour 1B. Fortunately, the 28-day breaks for those pours met the acceptance criteria. So, the problem was apparently isolated to pour 1B and possibly the latter portion of pour 1A. The remainder of the project was poured using a revised 40 MPa mix with improved aggregate grading without issue. However, the matter of pour 1B was yet to be resolved.