Missing Control Joints in Slab

Exterior view of a community college building under construction, highlighting concrete slab areas evaluated for structural integrity and compliance.

Overview

A community college identified significant cracking and curling issues on the elevated concrete topping slab within an academic building. Concerns regarding concrete integrity and compliance prompted the management to commission a detailed forensic evaluation. The objective was to identify the causes of deterioration, assess compliance with design specifications, and recommend suitable remediation strategies.

Scope of Work

A comprehensive evaluation was performed involving visual inspections, and laboratory testings such as strength tests (ASTM C42) and petrographic analysis (ASTM C856). The investigation specifically targeted concrete quality, slab curling, cracking patterns, and reinforcement placement. Field evaluations documented crack locations, slab edge curling magnitudes, the presence and impact of radiant tubing, and the distribution and thickness of the sound control floor underlayment.

Key testing methodologies included:

Microscopic petrographic analysis of concrete core showing carbonation depth and cement paste characteristics. Identified areas highlight microcracking and air void distribution.
Detailed microscopic view of concrete sample displaying microcracks and aggregate bonding quality. Arrows indicate specific microcrack locations impacting structural integrity.

  • Microscopic analysis of concrete cores to determine aggregate composition, cement paste quality, carbonation depth, and air void structure.

  • Examination of microcracking and paste-to-aggregate bonding.

  • Detailed crack mapping to document crack patterns and slab edge curling.

  • Examination of slab reinforcement, radiant heating tubing placement, and presence or absence of control joints.

Blueprint detailing concrete slab crack locations in an academic building. Mapping illustrates patterns of structural distress due to lack of control joints.
Interior view showing radiant heating tubing placement within concrete slab reinforcement. Image emphasizes the structural layout affecting slab performance.
Concrete core samples prepared for laboratory compression testing. One core shows fiber reinforcement, making it unsuitable for testing and requiring additional samples.

  • Concrete core compression testing (ASTM C42); some initial cores contained fiber reinforcement.

  • Measurement of sound control floor underlayment thickness to verify compliance with specified requirements.

Key Findings

1. Concrete Composition and Quality:

  • Concrete Composition and Quality:

    • Lightweight concrete aggregates were well-distributed and adequately consolidated.

    • Compressive strength significantly exceeded the specified strength of 5,000 psi.

  • Cracking and Curling Issues:

    • Extensive slab cracking was primarily due to the lack of adequate control joints.

    • Curling up to ⅝-inch was primarily observed at slab corners, driven by drying shrinkage, thinner-than-specified topping slab thickness due to thicker sound control underlayment, and absence of control joints.

Measurement of sound control floor underlayment thickness, displaying a thicker-than-specified value. Thicker underlayment contributed to concrete slab curling issues.
Measurement of concrete slab edge curling, showing vertical displacement due to drying shrinkage. Curling magnitude significantly exceeded acceptable limits.

  • Construction Deficiencies:

    • The absence of control joints notably contributed to excessive cracking, particularly in areas with radiant heating tubing embedded in the slab.

    • Sound control underlayment measured between 17-18 mm, thicker than specified, causing reduced slab thickness and increased curling.

Radiant heating tubing manifold installation within concrete slab system. The radiant tubing potentially influenced slab cracking and curling.
Close-up view of a sound control floor underlayment material sample. The underlayment contributed to slab thickness issues due to exceeding specified dimensions.

Solutions and Recommendations

Corrective Measures:

  • Repair core holes by reinstalling acoustic underlayment plugs and filling with high-strength non-shrink grout.

  • Fill slab cracks using appropriate crack filler.

  • Grind curled slab edges to comply with ADA's maximum ¼-inch vertical offset requirement.

  • Level slab edges with epoxy or cementitious leveling materials if grinding is insufficient.

Concrete slab showing core holes drilled for analysis, alongside extracted core sample. Holes require repair using acoustic plugs and high-strength grout.
Close-up of a concrete slab showing significant edge curling measured using a leveling tool. Curling requires structural polyurethane foam injection to mitigate further displacement.

Curl Mitigation:

Inject structural polyurethane foam beneath the curled slab edges to improve support.

Close-up view inside a core hole drilled through the concrete slab.
Concrete slab surface condition shortly after hardening. Surface shows initial conditions before cracking and curling occurred.
Petrographic image showing significant slab crack and measured carbonation depth in concrete core sample. Crack depth indicates potential long-term structural issues.

Outcome

The detailed petrographic and structural evaluation clearly identified the causes of slab . Implementation of targeted recommendations effectively mitigated the slab's structural deficiencies, addressing compliance concerns and significantly enhancing slab performance and durability.

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