Load Testing of Existing Concrete Structures (Part 2)-Load Testing of a Parking Structure Due to Low Concrete Compressive Strengths (PDF)

Load Testing of a Parking Structure Due to Low Concrete Compressive Strengths (PDF)

Pdf Summary

The presentation at the 2017 ACI Fall Convention by F. Dirk Heidbrink, PE, and Robert D. Kuykendall, PE, addressed concerns related to low concrete compressive strengths during the construction of a mid-rise office and parking structure. The concrete involved in Pours 2 and 3 of Level P2 was rejected due to low strength results from 56-day cylinder tests, and subsequent investigations found high slumps and possible curing and handling issues.<br /><br />The structure, designed per ACI 318-11, uses cast-in-place concrete with unbonded post-tensioning, featuring typical slabs, beams, and girders. Evidence of low strength arose from quality assurance cylinders, coring, and issues with cylinder curing and preparation.<br /><br />Physical testing included compressive core tests, rebound hammer tests, and petrography. Core testing results showed significant variability, and strength levels were sometimes notably below expected values, reaching minimums as low as 1740 psi.<br /><br />To determine compliance and ensure structural integrity, load testing was performed following ACI 318-11 guidelines. This involved a monotonic loading protocol to evaluate deflection and cracking under specified load conditions, aiming to verify that the structure's performance met the necessary criteria despite the low concrete strength issue. Tests demonstrated no significant deficiencies, satisfying deflection and other acceptance criteria stipulated by ACI 318-11.<br /><br />While structural safety was confirmed via load testing, durability concerns due to low compressive strength were emphasized. Decreased abrasion resistance, increased permeability, and carbonation rates remained concerns, potentially requiring remedial actions like resurfacing or coatings to mitigate long-term risks.<br /><br />In conclusion, while the load testing results alleviated immediate safety worries, addressing durability issues remains crucial for the structure's future performance.

Keywords

ACI 318-11

low compressive strength

post-tensioning

concrete testing

structural integrity

durability concerns

load testing

cylinder tests

mid-rise structure

concrete curing