Fresh Concrete Tests
Testing concrete during placement provides immediate information about quality and workability. , and proper compressive strength testing procedures should be verified during constructionThe slump test, performed according to ASTM C143, measures the consistency of fresh concrete by filling a cone-shaped mold with concrete in three layers, each rodded 25 times, then lifting the mold and measuring the vertical settlement. The slump indicates the water content and workability of the concrete mix, with typical structural concrete having a slump of 3 to 5 inches. A slump significantly different from the design value indicates a change in water content that will affect strength and durability.
Air content testing by the pressure method measures the volume of air voids in fresh concrete. Entrained air in the range of 5 to 8 percent is required for concrete exposed to freeze-thaw cycles. The air content affects concrete durability, workability, and strength. Each 1 percent of entrained air reduces compressive strength by approximately 5 percent, so the air content must be carefully controlled to provide freeze-thaw protection without excessive strength loss. The pressure meter test is quick and reliable when performed according to ASTM C231.
Concrete temperature measurement ensures that the concrete is within acceptable limits for the prevailing weather conditions. Hot weather limits are typically 90 degrees Fahrenheit maximum, while cold weather limits require concrete temperature above 50 degrees Fahrenheit at placement. Unit weight measurement by the gravimetric method verifies that the concrete density matches the design assumptions. The temperature and unit weight tests are simple but important elements of the quality control program.
Compressive Strength Testing
Compressive strength is the primary acceptance criterion for structural concrete. Standard 6 by 12 inch cylinders or 4 by 8 inch cylinders are cast from fresh concrete samples taken at the point of placement. Understanding concrete air content testing is essential for achieving quality results in this aspect of construction. Understanding statistical quality control is essential for achieving quality results in this aspect of construction.Understanding non-destructive testing methods is essential for achieving quality results in this aspect of construction.ASTM C31 specifies the procedures for sampling, molding, and initial curing of test cylinders. Field cylinders must be stored in controlled conditions, protected from vibration, temperature extremes, and moisture loss during the first 24 hours after casting.
After initial curing, cylinders are transported to the testing laboratory and stored in standard conditions of 73.5 degrees Fahrenheit and 100 percent relative humidity until tested. The 28-day compressive strength is the standard acceptance criterion, though early-age tests at 3, 7, or 14 days provide indications of strength development. The testing machine applies load at a specified rate until failure, and the maximum load divided by the cross-sectional area gives the compressive strength. A minimum of two cylinders from each test represents the strength of the concrete placed.
Non-Destructive Testing Methods
Non-destructive testing methods evaluate concrete quality without damaging the structure. The rebound hammer measures surface hardness by recording the rebound of a spring-driven hammer impacting the concrete surface. The rebound number correlates with compressive strength but is affected by surface moisture, carbonation, and aggregate type. Rebound hammer testing is useful for comparative assessments and identifying areas of potential weakness but should not be relied upon for precise strength determination.
Ultrasonic pulse velocity testing measures the time required for a sound wave to travel through concrete. The pulse velocity correlates with concrete quality, with higher velocities indicating better quality and fewer internal defects. This method is effective for detecting internal voids, cracks, and deterioration. Combining rebound hammer and ultrasonic pulse velocity measurements in the SonReb method provides more reliable strength estimates than either method alone.
Statistical Quality Control
The statistical evaluation of concrete test results provides a rational basis for acceptance decisions. The average strength of all test results must exceed the specified design strength by a margin that accounts for normal variability. ACI 318 requires that the average of three consecutive tests exceed the specified strength and that no individual test falls more than 500 psi below the specified strength. The standard deviation of test results measures the consistency of concrete production, with typical values ranging from 400 to 600 psi for well-controlled production.
When test results fall below acceptance criteria, the engineer must evaluate the structural adequacy of the affected concrete. Core tests drilled from the structure provide direct strength measurements of the in-place concrete. The location and number of cores should be determined by the engineer based on the nature and extent of the low-strength concrete. If core tests confirm inadequate strength, the engineer must evaluate whether the structure can safely support design loads or whether remedial measures such as strengthening or replacement are required.
Design Standards and Building Code Requirements
All construction work must comply with the applicable building codes and industry standards that establish minimum requirements for structural safety, fire protection, accessibility, and energy efficiency. The International Building Code provides the comprehensive framework for building design and construction in most jurisdictions. The code requirements for each building element depend on the occupancy type, the building height, the type of construction, and the seismic design category. The designer must review all applicable code provisions during the design phase to ensure that the design complies with every requirement. The permit review by the building department verifies that the design documents demonstrate compliance with the applicable codes before construction begins.
The material standards published by ASTM International, the American Concrete Institute, the American Institute of Steel Construction, and other organizations provide the specifications for material properties, testing methods, and quality control procedures. These standards ensure that the materials used in construction meet the minimum quality requirements for the application. The reference standards are incorporated into the building codes by reference, making them legally enforceable requirements. The contractor must verify that all materials meet the applicable standards through mill certifications, test reports, and product labeling. The quality control testing during construction verifies that the installed materials achieve the specified properties.
Construction Methods and Installation Procedures
The proper installation of construction materials and systems requires adherence to the manufacturer’s instructions and industry best practices. The installation procedures for each product are developed through testing and field experience to achieve the specified performance. The contractor must ensure that the installation crew is properly trained and qualified for the work. The quality of the installation is verified through inspections at each stage of the work. Any deviations from the specified procedures must be approved by the designer before proceeding. The documentation of the installation process provides the record of compliance for future reference.
The sequencing of construction activities affects the quality and efficiency of the work. The work must be planned so that each activity is performed in the correct order and with adequate time for preparation and curing. The protection of completed work from damage by subsequent activities is essential for maintaining quality. The coordination between different trades working in the same area requires careful scheduling and communication. The site conditions including weather, temperature, and humidity affect the installation procedures and must be considered in the planning. The contingency plans for adverse conditions ensure that the work can proceed safely and efficiently under varying conditions.
Quality Control and Inspection Requirements
The quality control program for construction includes the inspection of materials upon delivery, the observation of work in progress, and the testing of completed work. The inspector must verify that the materials meet the specifications and are properly stored. The observation of the work identifies any deficiencies that must be corrected before the work is concealed. The testing of the completed work verifies that the installed materials achieve the specified performance. The documentation of the inspection and testing results provides the quality record for the project. The non-conformance report documents any deficiencies and tracks the corrective action to completion.
The special inspections required by the building code for seismic and wind resistance must be performed by qualified inspectors. The special inspection program identifies the elements and systems that require continuous or periodic inspection during construction. The inspector must document the results of each inspection and report any non-compliance to the building official. The structural observations by the licensed design professional verify that the construction conforms to the design intent. The completion of all required inspections and tests is documented in the certificate of occupancy application.