Concrete may look hard and finished within hours of being placed, but the process of gaining strength continues well after the surface has hardened. That is why concrete testing does not end on the day of the pour.

One of the most common methods used to evaluate concrete strength is compressive strength testing of concrete cylinders. Cylinders are prepared from samples of fresh concrete, cured under specified conditions, and then tested by applying increasing compressive force until the specimen fails.

Two of the most familiar testing ages are 7 days and 28 days. But why are these particular dates important, and what can the results tell contractors, engineers, and project owners?

What Is a Concrete Cylinder Test?

During concrete placement, a representative sample of fresh concrete may be collected and used to prepare cylindrical specimens. Depending on the project’s testing requirements, the fresh concrete may also be evaluated for properties such as slump, air content, and temperature.

The cylinders are then stored and cured according to applicable testing procedures before being transported to a laboratory for compressive strength testing.

During the strength test, a cylinder is placed in a compression testing machine and subjected to an increasing load. The resulting compressive strength is typically reported in pounds per square inch (psi).

These results provide important information about whether the concrete is developing strength as expected.

Why Test Concrete at 7 Days?

Concrete generally develops a substantial portion of its strength during the first several days after placement. A 7-day cylinder test therefore provides an early indication of how the concrete is progressing.

The 7-day result can help the project team identify whether strength development appears to be generally on track.

For example, unexpectedly low early results could prompt the contractor, engineer, or testing laboratory to review factors such as:

• Concrete mix information
• Placement conditions
• Curing conditions
• Sampling and testing procedures
• Weather and temperature
• Additional available test specimens

However, a 7-day result should not automatically be treated as the final determination of whether concrete satisfies its specified compressive strength. The project’s specifications and applicable standards determine how strength results are evaluated.

Why Is 28-Day Concrete Strength So Important?

The 28-day test is commonly used as a benchmark for evaluating the specified compressive strength of concrete.

Concrete continues to hydrate and develop strength over time, and 28 days has long been used as a standard age for evaluating compressive strength in conventional concrete construction.

If project documents specify concrete with a particular compressive strength, such as 3,000 psi, 4,000 psi, or another value, that specified strength is commonly associated with a designated testing age—often 28 days unless the project requirements state otherwise.

The 28-day results therefore provide the engineer and project team with important data for evaluating whether the concrete meets applicable project requirements.

Does Concrete Stop Getting Stronger After 28 Days?

No. Concrete does not suddenly stop gaining strength once it reaches 28 days.

Provided conditions support continued hydration, concrete can continue gaining strength beyond the 28-day mark. The rate of strength gain generally slows over time, but later-age strength can still increase.

The importance of 28 days is not that it represents the maximum possible strength. Rather, it provides a standardized point at which concrete strength can be evaluated and compared with project requirements.

What If the 7-Day Concrete Test Is Low?

A low 7-day result can be concerning, but it does not necessarily mean the concrete will fail to reach its required strength at the specified age.

Concrete strength development can be affected by numerous factors, including the concrete mixture, curing temperatures, moisture conditions, admixtures, and cementitious materials.

The later test results are therefore important.

When early results are lower than anticipated, the project team may pay particularly close attention to the remaining cylinders and review the circumstances surrounding the concrete placement and testing.

What Happens If the 28-Day Concrete Test Is Low?

A lower-than-expected 28-day result may require further evaluation.

The appropriate response depends on the project’s specifications, the magnitude and pattern of the test results, and the judgment of the responsible design professionals.

Possible next steps may include reviewing batching, sampling, curing, and testing records; testing additional cylinders if available; evaluating the concrete in place; or performing additional testing when directed by the appropriate project professionals.

A single test result should be considered in the context of the project’s applicable specifications and testing standards rather than interpreted in isolation.

Why Are Extra Concrete Cylinders Sometimes Made?

You may notice that more cylinders are prepared during a concrete pour than are needed for the standard 7- and 28-day tests.

Extra cylinders can provide flexibility when additional strength information is needed.

Depending on the project, additional specimens might be tested at another age to evaluate later strength development or provide information needed for a particular construction decision.

The number of cylinders, testing ages, and procedures should be determined by the project’s specifications and applicable standards.

Proper Sampling and Curing Matter

Accurate strength testing begins long before a cylinder reaches the compression testing machine.

How the concrete is sampled, how the specimens are prepared, their initial storage conditions, transportation, laboratory curing, and testing procedures can all influence the reliability of the results.

That is one reason professional construction materials testing is an important part of quality assurance during construction. Testing should follow established procedures so that engineers, contractors, and owners receive meaningful information about the materials being incorporated into a project.

Concrete Testing Provides More Than a Number

A concrete cylinder breaking at a particular strength provides a measurable result, but the larger purpose of testing is to help the project team verify that construction materials are performing according to project requirements.

Testing at different ages provides information about strength development over time. The 7-day test offers an early look at that development, while the 28-day test commonly serves as an important benchmark for evaluating specified compressive strength.

When results are properly collected, documented, and interpreted, concrete testing can help identify potential concerns early and provide valuable quality-control information throughout construction.

Construction Materials Testing From Quality Testing & Engineering

Quality Testing & Engineering provides construction materials testing services to help contractors, engineers, developers, and project owners verify the quality and performance of materials used in construction.

From fresh concrete testing and concrete cylinder testing to soil and other construction materials testing, our team provides reliable testing and documentation throughout the construction process.

If your upcoming project requires concrete testing or other construction materials testing services, contact Quality Testing & Engineering to discuss your project’s testing requirements.

Buildings, parking lots, roadways, sidewalks, and other structures all depend on the soil beneath them for support. When that soil moves or compresses after construction, the result can be settlement that affects the performance and condition of the structure above.

Some settlement is expected and can be accounted for during design. Excessive or uneven settlement, however, can contribute to cracked concrete, uneven pavement, foundation problems, drainage issues, and other costly repairs.

Understanding why soil settlement occurs is an important part of evaluating a site before and during construction.

What Is Soil Settlement?

Soil settlement occurs when the ground beneath a structure decreases in volume or shifts downward under a load. This movement can happen gradually over months or years, or it can become noticeable relatively soon after construction.

One of the biggest concerns is differential settlement, which occurs when one portion of a structure settles more than another. Uneven movement can place additional stress on foundations, slabs, pavement, utilities, and other components.

Several site and construction conditions can contribute to settlement.

Poorly Compacted Fill

Improperly compacted fill is a common cause of settlement on construction sites.

When soil is placed to raise a site or replace unsuitable material, it generally needs to be installed in controlled layers and compacted to meet project requirements. If fill is placed too thickly, contains excessive moisture, or does not receive adequate compaction, voids can remain within the soil.

Over time, the weight of a building, pavement, traffic, or even the soil itself can compress these loose areas.

Field density and compaction testing can help verify whether the placed material meets the requirements established for the project.

Changes in Soil Moisture

Water can significantly affect the behavior of soil.

Some soils expand when they absorb moisture and shrink as they dry. Repeated wetting and drying cycles can cause movement beneath foundations and pavement. Other soils may lose strength when they become saturated, increasing the potential for settlement under a load.

Moisture changes can result from several sources, including:

• Poor site drainage
• Plumbing or utility leaks
• Changes in groundwater conditions
• Heavy rainfall
• Improper grading
• Downspouts discharging near foundations

Proper drainage and an understanding of the site’s soil conditions can help reduce these risks.

Weak or Compressible Native Soils

Not every site naturally contains soil capable of supporting the planned development.

Loose soils, soft clays, organic material, undocumented fill, and other unsuitable materials may compress significantly when subjected to the weight of a building or pavement system.

A geotechnical exploration performed before construction can help identify these conditions. Depending on the findings, recommendations may include removing unsuitable material, improving the soil, using engineered fill, modifying foundation design, or considering other site-specific solutions.

Consolidation of Clay Soils

Certain fine-grained soils can experience consolidation settlement.

When a new load is placed on saturated clay, water within the soil pores may gradually be squeezed out. As this happens, the soil particles move closer together and the soil layer compresses.

Unlike settlement caused by immediately compressing loose fill, consolidation can take place over an extended period. This makes understanding subsurface conditions particularly important for projects involving significant structural loads or thick layers of compressible soil.

Erosion and Loss of Supporting Soil

Settlement can also occur when soil is physically removed from beneath a structure.

Water moving through or across a site can erode supporting material, creating voids beneath pavement, sidewalks, slabs, or foundations. Poor drainage systems, leaking pipes, failed stormwater infrastructure, or concentrated runoff can all contribute to erosion.

Signs may include sinking pavement, voids beneath concrete, cracking, or localized depressions.

Improper Subgrade Preparation

Even suitable soil can create problems when the subgrade is not properly prepared before construction.

Before pavement, slabs, or foundations are installed, the underlying soil may need to be graded, moisture-conditioned, compacted, or otherwise prepared according to the project specifications.

Soft or unstable areas identified during construction may need additional evaluation before work continues. Addressing these conditions before concrete or pavement is placed can help prevent more difficult repairs later.

Increased Loads on Existing Soil

Soil conditions that adequately supported an undeveloped site may behave differently once substantial loads are introduced.

Buildings, retaining walls, heavy equipment, pavement, storage areas, and new fill all place additional stress on the underlying soil. If those loads exceed what the soil can adequately support, settlement may occur.

This is one reason geotechnical engineers evaluate both the subsurface conditions and the anticipated loads associated with a proposed development.

How Can Soil Settlement Be Reduced?

Reducing settlement risk begins with understanding the site.

Geotechnical exploration can provide information about soil types, groundwater conditions, strength, density, and other subsurface characteristics. Engineers can use this information to develop recommendations appropriate for the proposed construction.

During construction, materials testing and observation can help verify that earthwork is being completed according to project requirements. This may include monitoring fill placement, testing soil moisture and density, and evaluating questionable subgrade conditions.

Testing does not eliminate every potential construction problem, but it provides valuable information that allows project teams to identify issues and make informed decisions before they become larger problems.

Watch for the Warning Signs of Settlement

Settlement may become visible through cracking or movement in the completed structure. Potential warning signs include uneven floors, foundation cracks, doors or windows that begin sticking, depressed pavement, cracking around utility structures, or areas where water begins collecting unexpectedly.

Not every crack or uneven surface indicates a serious settlement problem. However, significant or progressive movement should be evaluated to determine the underlying cause.

Start With a Better Understanding of the Ground

The performance of a building or pavement system begins below the surface. Understanding the site’s soil conditions before construction—and verifying earthwork as the project progresses—can help reduce uncertainty and identify potential problems early.

Quality Testing & Engineering (QTE) provides geotechnical exploration, construction materials testing, environmental testing, and related engineering services for projects throughout the St. Louis Metro and surrounding areas.

If you’re planning a commercial, industrial, municipal, or residential development, contact QTE to learn how geotechnical and construction testing services can help you better understand the conditions beneath your project.

Construction schedules depend on many moving parts. Crews, materials, equipment, inspections, and subcontractors must all be coordinated carefully. When required testing is delayed, the effects can extend well beyond a single task and prevent other phases of the project from moving forward.

Construction materials testing and engineering services help confirm that work meets project specifications and quality requirements. Planning for these services early can reduce downtime, prevent scheduling conflicts, and help keep a project on track.

Here are five common testing delays that can disrupt a construction schedule.

1. Scheduling Testing Too Late

Waiting until the last minute to request testing is one of the most preventable causes of delay. Testing agencies coordinate technicians, equipment, travel, and laboratory capacity across multiple projects. A same-day request may not always be possible, especially during busy construction periods.

Project teams should review the plans and specifications before work begins to identify when testing will be required. Common examples include:

• Soil compaction testing
• Concrete testing
• Asphalt testing
• Masonry testing
• Structural steel observation
• Special inspections

Providing advance notice allows the testing agency to coordinate with the superintendent and arrive when the work is ready. It is also important to communicate schedule changes as soon as possible. If a concrete placement is moved or canceled, the testing agency needs enough notice to adjust its schedule.

2. The Site Is Not Ready When the Technician Arrives

A technician may arrive at the scheduled time only to find that the area is inaccessible or the work is not ready for testing. Excavation may still be underway, concrete trucks may be delayed, or a lift of fill may not be complete.

This can create unnecessary standby time and may require another visit. Depending on technician availability, rescheduling could prevent the contractor from proceeding as planned.

Before requesting testing, verify that:

• The test area will be accessible
• Required grading or placement work will be complete
• Equipment can safely reach the testing location
• The correct area has been identified
• A site representative will be available
• Necessary safety requirements have been addressed

Clear communication between the project manager, superintendent, subcontractors, and testing agency can reduce the likelihood of a technician arriving before the site is ready.

3. Required Project Information Is Missing

Testing personnel need accurate information to perform their work and evaluate the results. Missing or outdated documents can prevent a technician or engineer from confirming whether construction meets the project requirements.

Depending on the service, the testing agency may need:

• Approved construction drawings
• Project specifications
• Geotechnical reports
• Concrete mix designs
• Soil compaction requirements
• Approved material submittals
• Relevant change orders
• Current testing and inspection schedules

For example, a soil density result cannot be evaluated properly without knowing the required compaction percentage and the laboratory moisture-density relationship for the material being placed. Similarly, concrete test results must be matched to the correct mix design and placement location.

Providing current documents before testing begins helps technicians perform the correct tests and report results without unnecessary follow-up.

4. Failed or Noncompliant Test Results Require Additional Work

Testing does not always produce a passing result. Soil may not reach the required density, concrete strength may fall below expectations, or an installed material may not match the approved specifications.

When a result does not comply with project requirements, work may need to stop while the issue is evaluated. Corrective action could involve:

• Reworking and recompacting soil
• Adjusting the material’s moisture content
• Removing and replacing unsuitable material
• Collecting additional samples
• Performing supplemental testing
• Consulting the project engineer
• Developing an approved repair

Retesting may also be required before construction can continue. This can affect subsequent work, including foundation installation, paving, structural construction, and utility placement.

Not every unexpected result means the material or completed work must be removed. However, the issue should be communicated promptly so the appropriate project professionals can determine the next step. Fast reporting and clear documentation are particularly important when other work depends on the result.

5. Weather and Site Conditions Interrupt Testing

Construction testing frequently takes place outdoors, making it vulnerable to changing weather and site conditions. Heavy rain can saturate soil, flood excavations, prevent equipment access, or make a test area unsafe. Freezing temperatures and extreme heat can also affect construction materials, placement procedures, and testing conditions.

Weather-related delays can be especially disruptive when a project has a narrow window for concrete placement, earthwork, or paving.

Although the weather cannot be controlled, project teams can prepare by:

• Monitoring the forecast
• Protecting exposed soil and construction materials
• Maintaining safe access routes
• Using proper drainage and erosion-control measures
• Preparing contingency dates
• Communicating cancellations promptly
• Allowing time for wet areas to dry before retesting

If site conditions could affect the accuracy or safety of a test, additional preparation or rescheduling may be necessary.

Planning Can Help Prevent Testing Delays

The best way to reduce testing-related disruptions is to include testing in the construction schedule from the beginning. Testing should be treated as part of the work rather than as an activity arranged after construction is already underway.

Before each phase begins, confirm what services are required, who is responsible for requesting them, how much notice is needed, and which results must be received before work can continue. Regular communication among the contractor, owner, design team, and testing agency can help identify potential conflicts before they affect the schedule.

Coordinate Construction Testing With QTE

Timely testing helps project teams verify material quality, address potential concerns, and move confidently into the next phase of construction. Quality Testing & Engineering provides construction materials testing and engineering services for projects throughout the St. Louis metropolitan area and Southern Illinois.

Contact QTE to discuss your project requirements and coordinate the testing services needed to help keep your construction schedule moving.

Wetlands can have a significant impact on how a property can be developed or used. Before beginning construction, land development, utility installation, or other projects that disturb the ground, it may be necessary to determine whether wetlands are present on the site.

A wetland delineation helps identify and map potential wetland areas so property owners, developers, engineers, and contractors can better understand environmental considerations before moving forward with a project.

What Is a Wetland Delineation?

Wetland delineation is the process of evaluating a property to determine whether wetlands are present and, if so, identifying their boundaries.

Wetlands aren’t always obvious. While some contain standing water or wetland vegetation, others may appear relatively dry during certain times of the year. This is why a professional field evaluation is often necessary.

A wetland delineation generally evaluates three primary characteristics:

• Vegetation: The presence of plants commonly associated with wet environments.
• Soils: Soil characteristics that indicate prolonged saturation or wet conditions.
• Hydrology: Evidence that water is present at or near the surface for sufficient periods.

These observations help determine where potential wetlands begin and end.

Why Are Wetlands Important to Development?

Wetlands provide important environmental functions, including storing floodwater, improving water quality, controlling erosion, and providing wildlife habitat.

Because of these functions, certain wetlands and waterways may be regulated by federal, state, or local agencies. Development activities that affect regulated wetlands may require permits or additional environmental review.

Identifying wetlands early allows a project team to incorporate them into site planning rather than discovering potential restrictions after engineering or construction has already begun.

When Does a Property Need a Wetland Delineation?

A wetland delineation may be recommended when a property contains low-lying areas, drainage features, streams, ponds, frequently saturated soil, or vegetation associated with wet conditions.

It may also be needed when planning projects such as:

• Commercial or residential developments
• Roads and infrastructure improvements
• Utility installations
• Industrial developments
• Agricultural improvements
• Grading or excavation
• Drainage improvements
• Land purchases involving undeveloped property

Even if a property does not appear to contain wetlands, an evaluation may be worthwhile when environmental conditions or regulatory requirements are uncertain.

Why Conduct Wetland Delineation Early?

Waiting until construction is underway to address wetlands can result in unexpected delays and additional costs.

Conducting a wetland delineation during the early planning stages can help developers and property owners understand site constraints before making major design decisions.

With this information, engineers may be able to adjust building locations, roads, utilities, detention areas, or other improvements to minimize potential wetland impacts.

Early identification can also provide additional time to address permitting requirements when impacts cannot be avoided.

What Happens During a Wetland Delineation?

The process typically begins with a review of available information about the property, which may include maps, soil information, aerial imagery, and other environmental data.

Environmental professionals then perform a field investigation to evaluate vegetation, soil conditions, hydrology, and other site characteristics.

When potential wetlands are identified, their boundaries can be marked in the field and documented for mapping and project planning purposes. Depending on the project and regulatory requirements, additional coordination with the appropriate agencies may also be necessary.

Wetland Delineation for Property Development

Wetlands do not necessarily mean that a property cannot be developed. However, knowing their location can be critical to designing a project appropriately and determining whether additional regulatory review or permitting may be required.

For developers, engineers, municipalities, contractors, and property owners, identifying environmental constraints early can help reduce uncertainty and keep a project moving forward.

Wetland Delineation Services in Southern Illinois and the St. Louis Metro Area

Quality Testing and Engineering (QTE) provides environmental testing and consulting services to support construction and development projects throughout Southern Illinois and the St. Louis metropolitan area.

Our team can evaluate site conditions, identify potential wetland areas, and provide the information needed to help property owners and project teams make informed decisions before development begins.

Planning a project and unsure whether wetlands could affect your property? Contact QTE to discuss wetland delineation and environmental consulting services for your site.