Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

Determining the cause of a crack

Q. We need to repair a 1962 vintage, elevated, interior parking deck which was designed as a cast-in-place pan/joist system. Joist webs are 5" wide and 10" deep at 25" O.C. The slab is 2½" thick reinforced with welded wire fabric. About 20% of the joists have a horizontal crack at the joist/slab interface as if the joist is separating from the slab. Do you have any ideas, or can you point me in the proper direction, to determine the cause of such cracking?


A. To properly determine the causes of the cracks, preliminary evaluation and assessment of the existing structure is required. There are several questions that need to be answered before the causes of the cracks can be determined. Here are some of the questions that an engineer needs to investigate:

- To what standard was the slab built to (age, building code)?

- Are the original construction documents available?

- Has the slab been subjected to heavy loads during its service life?

- Where any repairs done to the slab system?

ACI has published the first repair code in the industry, ACI CODE-562, Assessment, Repair, and Rehabilitation of Existing Concrete Structures.  This standard provides direction and means to professional engineers to evaluate and assess existing concrete structures.  This standard will provide consistency to the repair industry and owners’ will appreciate the value that the new standard will provide in terms of what to expect when the structure is repaired and the life expectancy of the repaired structure.

ACI has also published, “Guide to the Code for Assessment, Repair, and Rehabilitation of Existing Concrete Structures”.  It is a guide that helps engineers better understand how to apply the new standard, presents more in-depth information, and provides real life examples of repair and rehabilitation of structures.  One of the examples included in this guide is the repair of precast concrete joist systems.


References: ACI CODE-562-21; MNL-3(20); ACI 318 Historical ACI Building Code Requirements (1908-2005)

Topics in Concrete: 318 Building Code; Cracking in Concrete; Repair of Concrete

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