Concrete Cylinder Testing

The concrete cylinder is the predominant specimen type for assessing concrete compressive strength. While alternative methods for evaluating concrete strength may offer cost efficiencies and potentially superior insights, concrete cylinder testing remains the enduring standard for acceptance.

Crafting well-formed concrete cylinders is a straightforward and economical endeavor. Typically, a lone technician stationed at the discharge or placement site is adequate for sampling and testing concrete batches, assessing parameters such as slump, air content, unit weight, and cylinder integrity. Although technicians must adhere to established protocols and often obtain certification in their respective regions, the requisite training is neither extensive nor intricate.

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What Insights Do Concrete Cylinders Provide?

It is imperative to delineate the essence of concrete cylinder testing. The concrete cylinders cast on-site do not primarily represent the in-situ strength of the concrete within the structure. Instead, the compressive strength testing of concrete mainly serves as a performance assessment of the concrete mix delivered to the project site. Consequently, most field-cast cylinders are formulated for quality control and product acceptance. This testing convention adheres to the "standard curing" procedure outlined in ASTM C31/AASHTO T 23, wherein samples are subjected to controlled and protected initial curing conditions before being transitioned to final laboratory curing within a 48-hour. Meanwhile, ASTM method C192/AASHTO R 39 governs testing specimens fabricated in laboratory settings, ensuring meticulous control over material selection, preparation, and mixing to evaluate mix design attributes.

In instances necessitating the monitoring of strength development under ambient conditions at the job site or within formwork, "field curing" provisions are outlined in ASTM/AASHTO practices. Field-cured cylinders undergo curing conditions mirroring those of the structural concrete, ostensibly reflecting the evolution of the concrete's in-place strength. This discourse predominantly focuses on standard-cure cylinders tailored for compressive strength acceptance evaluations.

Requisite Equipment for Fresh Concrete Sampling

The equipment essential for sampling and molding concrete cylinders is minimal:

• A receptacle such as a wheelbarrow for aggregating a composite sample of fresh concrete from various mixer types, as stipulated in ASTM C172. Certain project specifications may necessitate sampling at the discharge end of a concrete pump.
• A suitably sized scoop facilitates the collection of a representative sample, which is wieldable yet manageable for fresh concrete placement.
• The prescribed tamping rod varies in length and diameter for 4x8in and 6x12in cylinders, respectively.
• For stiff mixes boasting slumps of 1in (25mm) or less, using a concrete vibrator is permissible for consolidation and mandated.

ASTM C31/AASHTO T 23 mandates that molds be positioned on a stable surface and filled with the appropriate quantity of fresh concrete. Molds for 4x8in specimens are filled in two equal layers, while those for 6x12in specimens necessitate three equal layers. Each layer undergoes 25-rod strikes, followed by mold tapping with a rubber mallet to consolidate the sample. When employing a vibrator, both cylinder sizes are filled in two equal layers, with the vibrator inserted once for 4x8in molds and twice for 6x12in molds. A trowel or tamping rod achieves a flat, uniform surface finish.

Traditional concrete mix designs hinge on meeting strength benchmarks at 28 days of age. Given the predictable nature of strength development, early cylinder tests facilitate estimations of final strengths. Casting multiple test cylinders from the same fresh concrete sample enables prompt identification of strength discrepancies. While the exact number of cylinders in a set or the timing of earlier tests largely falls within the specifier's purview, a standard practice typically encompasses 3 to 5 cylinders. The controlling test at 28 days entails averaging the strengths of two cylinders, complemented by additional tests at 3 or 7 days to gauge strength development, along with a "hold" cylinder in instances of 28-day test disparities.

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Initial Curing: A Prudent Selection

During this critical phase, samples progress through the concrete mix's initial and final setting phases, commencing strength development through hydration. Given their heightened vulnerability to impact, displacement, and vibration, the designated area for initial curing necessitates meticulous selection.

Molding the concrete cylinders at their initial curing site streamlines operations minimizes effort and time expenditure, and mitigates sample disturbance. Should this option be unfeasible, immediate relocation of molded and finished cylinders to their initial curing locale upon completion becomes imperative.

An optimal initial curing locale must fulfill three pivotal criteria:

• Ambient temperature maintenance within the range of 60 to 80°F (16 to 27°C), with temperatures ranging between 68° to 78°F (20° to 26°C) for mixes boasting design strengths exceeding 6000psi (40 MPa). A cursory review of the weather forecast facilitates determinations regarding the necessity for insulated containers for heating or cooling. It is paramount to factor in the heat generated by the natural hydration of concrete, as cylinders curing within enclosed spaces can engender surprising heat levels. Cooling measures encompass water circulation, ice application, or electrical cooling devices.
• ?Prevention of moisture loss from specimens, achievable through snug-fitting plastic lids on single-use cylinder molds. As delineated in the specifications, alternative strategies include partial or complete immersion in water saturated with calcium hydroxide or maintaining elevated humidity levels via damp sand or wet burlap.
• Protection against jarring and vibration necessitates avoiding equipment-induced or foot traffic-generated impacts. The subtle yet potentially harmful effects of vibration, often overlooked, warrant commensurate consideration. Vibration emanating from nearby machinery or transmitted through the flimsy flooring of construction trailers may culminate in imperceptible damage and compromised test strengths.

Freshly molded samples must remain undisturbed within this controlled environment throughout the stipulated period, which may extend up to 48 hours. The ASTM/AASHTO standards proffer multiple viable options for controlling the initial curing milieu, with meticulously designed curing boxes emerging as the epitome of efficacy. Diverse models furnish heating, cooling, or both, supplemented by provisions for moisture retention via water, damp sand, or wet burlap. Some models integrate min/max thermometers to document temperature fluctuations throughout the curing duration.

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Transporting Concrete Cylinders

The timeframe stipulated by C31/T 23 for retrieving concrete samples spans from 8 hours post-final set (ascertained via ASTM C403) to 48 hours post-molding. Conventional mixes are conventionally assumed to be safe to handle samples after 16 to 24 hours. During transportation, cylinders necessitate cushioning against jarring and safeguarding against freezing and moisture loss. Transporting cylinders within their molds augments protection, preventing the practice of unsecured placement in exposed environments, such as the bed of a pickup truck.

Manual carriage of individual cylinders, especially 6x12in specimens, proves laborious and inefficient. Cylinder carriers and lifting handles afford the practical handling of two cylinders concurrently, whether in field or laboratory settings. Concrete cylinder transport racks present a secure and convenient means of transporting specimens across diverse vehicle types. Incorporating discarded neoprene capping pads at the rack's base bolsters protection during transit.

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Final Curing in Laboratory Settings

The ultimate phase of concrete cylinder curing centers on furnishing controlled and consistent moisture and temperature conditions to optimize strength development. Upon arrival at the testing laboratory, cylinders are promptly logged into the concrete sample registry, extracted from molds, and expeditiously transferred To the final curing milieu to preempt moisture loss and physical degradation.

Two viable options for final concrete cylinder curing environments, compliant with ASTM C511/AASHTO M 201 specifications, exist:

Option 1: Moist-curing rooms, ensconced enclosures outfitted with water-atomizing apparatuses, perpetually sustain condensed moisture on the concrete samples. Control panels integrate atomizing methodologies and temperature regulation to ensure alignment with specifications. Although optional, measuring and recording humidity levels via humidity meters augment the efficacy of this approach. Moist-curing rooms afford optimal space utilization for accommodating large sample volumes via rack or pallet storage.

Option 2: Curing tanks, replete with lime-saturated water and equipped with heaters and circulators, facilitate the curing of concrete cylinders and beams. Curing tanks prove cost-effective for temporary installations or moderate sample quantities.

The previous discourse on Concrete Curing delves into a comprehensive examination of the advantages and drawbacks inherent in various final curing methodologies.

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Capping Concrete Cylinders

The terminal preparation of each test cylinder ensures uniform distribution of applied forces and guarantees genuine axial compressive loads. Two predominant methods for end preparation are delineated:

Sulfur capping compound, a conventional technique outlined in ASTM C617/AASHTO T 231, entails heating a blend of sulfur, fly ash, and mineral filler until flowable. The molten material is subsequently poured into a capping mold, encapsulating the concrete cylinder end. Upon cooling, the resultant cap exhibits a smooth surface perpendicular to the axis. While widely adopted and safe, this method necessitates careful handling of exceedingly hot materials.

Neoprene Pads, synonymous with unbonded caps or pad caps, consist of neoprene discs installed on both cylinder ends immediately before testing. Confined by a steel retaining ring, these pads uniformly distribute loads over the cylinder surface, sans the need for hot sulfur material handling. Specifications governing this method are expounded in ASTM C1231.

Concrete cylinder end grinders offer a direct avenue to ensure square ends, facilitating accurate compression testing of concrete cylinders. Versatile across design strengths, these grinders reign as the preferred method for concrete mixes surpassing 7,000psi (48.3mPa). Laboratories handling substantial volumes of higher-strength cylinders stand to benefit most from the cost-effectiveness inherent in this approach.

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Concrete Compressive Strength Testing

Compression testing of concrete cylinders represents the culminating phase of this process and adheres to the standard test method ASTM C39/AASHTO T 22. The prepared concrete test cylinders are positioned within a concrete compression machine and subjected to axial loading at controlled rates until failure.

Concrete compression machines emerge as premier loading devices in construction materials testing, characterized by their formidable capacity and straightforward functionality. However, discerning features warrant scrutiny when selecting a concrete "break machine," as elucidated in a Gilson blog post offering insights into choosing the ideal concrete compression machine for laboratory operations.

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Crucial Steps for Compressive Strength Test Procedure

Concrete cylinders earmarked for compressive strength evaluation must undergo testing while still moist from the curing room or tank. Essential procedural steps encompass:

• The cylinder is placed atop the bottom plate and meticulously aligned with the compression machine's loading axis.
• Zeroing the compression machine's loading indicator.
• Manually aligning the spherically seated upper platen with the cylinder's top surface.
• Initiating load application. If utilizing unbonded caps, verification of sample perpendicularity via a validation device is recommended before load application reaches 10% of the anticipated strength.
• The initial half of the anticipated load may be applied at an accelerated controlled rate if desired. Subsequently, half of the expected load must be applied at 0.25 ± 0.05 MPa/s [35 ± 7 psi/s].
• Continue load application until the load indicator steadily declines and the cylinder fractures visibly.
• Recording the maximum load and computing compressive strength.

?This meticulously orchestrated process, encompassing specimen preparation, curing, and testing, underpins the robustness and reliability of concrete compressive strength assessments, thereby contributing to the overall quality and integrity of construction endeavors.