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Comprehesive and Tensile Splitting Test: Comprehensive Concrete Strength Determination
Comprehesive and Tensile Splitting Test
This comprehensive guide covers concrete strength determination through compressive testing of cubes and cylinders. You’ve learned proper specimen preparation techniques, standard testing procedures, strength development timelines, and critical differences between cube and cylinder methods. The article emphasizes Kenya-specific practices, KEBS requirements, and British Standards compliance while providing practical insights for construction professionals.
Picture this: You're standing on a construction site in Nairobi, watching workers pour concrete for a multi-story building. How can you be sure that this concrete will support the loads it's designed for? The answer lies in comprehensive concrete strength testing—a critical process that determines whether your structure will stand the test of time or become a costly failure.
Concrete strength testing isn't just a regulatory checkbox. It's your insurance policy against structural failures, legal liabilities, and catastrophic accidents. Every year, buildings in Kenya face scrutiny due to inadequate quality control, and proper testing could have prevented many of these issues.
What is Compressive Strength Testing and Why Does It Matter?
Compressive strength represents concrete’s ability to resist crushing forces applied from above. Think of it as measuring how much weight concrete can bear before it crumbles. This property matters because concrete primarily works in compression—supporting columns, beams, and slabs that push downward.
In structural engineering, compressive strength serves as the foundation for every design calculation. When an engineer specifies Grade 25 concrete (25 N/mm² or 25 MPa), they’re stating the minimum compressive strength the concrete must achieve at 28 days. Miss this target, and your entire structure’s safety is compromised.
The test works by applying controlled loads to concrete specimens until they fail. The maximum load divided by the specimen’s cross-sectional area gives you the compressive strength. Simple in concept, critical in execution.
Why Concrete is Strong in Compression but Weak in Tension
Concrete’s microstructure explains this paradox. The cement paste binds aggregate particles together under compression, creating a solid mass. Under tension, however, microscopic cracks propagate easily through the cement matrix. This is why concrete is approximately 10 times weaker in tension than compression—a fact that drives the need for tensile testing alongside compressive testing.
What are Concrete Test Cubes and When Should You Use Them?
Test cubes are the most common specimen type in Kenya and across Commonwealth countries. British Standards (BS) specify two standard cube sizes:
100mm × 100mm × 100mm cubes: Used when maximum aggregate size doesn’t exceed 20mm. These smaller cubes are practical for routine testing and require less material.
150mm × 150mm × 150mm cubes: Required when aggregate sizes reach 40mm. These larger cubes provide more representative results for concrete with coarser aggregates, which is common in structural concrete for buildings and bridges.
The choice between sizes isn’t arbitrary. Smaller aggregates fit proportionally better in 100mm cubes, while larger aggregates need the extra space of 150mm cubes to avoid edge effects that could distort results. Using the wrong size can yield misleading strength values—potentially 10-15% variance in either direction.
Standard Cube Dimensions and Applications
Most Kenyan construction projects use 150mm cubes as the standard. You’ll find these specified in projects ranging from residential housing in urban apartment developments to major infrastructure works. The Kenya Bureau of Standards (KEBS) primarily references BS standards, making cube testing the norm rather than the exception.
What are Concrete Test Cylinders and How Do They Differ?
While cubes dominate in Kenya, cylinders are the international standard in countries following American practices. ASTM standards specify cylindrical specimens with height-to-diameter ratios of 2:1:
100mm diameter × 200mm height: Suitable for aggregates up to 25mm
150mm diameter × 300mm height: The most common standard cylinder, used for typical structural concrete
Cylinders more closely resemble actual structural elements like columns. Their shape promotes more uniform stress distribution during testing, giving results that better predict in-situ performance. Research shows that cube strength typically reads 1.25 times higher than cylinder strength from the same concrete batch—a critical conversion factor when comparing international specifications.
How to Make Concrete Test Cubes: Step-by-Step Procedure
Creating reliable test specimens starts with proper technique. Here’s the complete procedure that meets BS standards and KEBS requirements:
Equipment You’ll Need
- Steel cube molds (100mm or 150mm)
- Tamping rod (16mm diameter, 600mm length, bullet-pointed end)
- Steel trowel or float
- Vibrating table (optional but recommended)
- Clean, non-absorbent platform
Detailed Molding Procedure
Step 1: Prepare the Molds
Clean molds thoroughly and apply a thin coat of release oil. Place them on a level, stable surface protected from direct sunlight and rain. A shed with controlled temperature works best.
Step 2: Sample the Concrete
Take samples randomly from the concrete batch. For large pours, sample from different truck loads. The concrete should represent what you’re actually placing in the structure. Test the slump first—if it’s outside specification, don’t bother making cubes from that batch.
Step 3: Fill in Layers
Pour fresh concrete into molds in layers approximately 50mm deep. For 150mm cubes, you’ll need three layers. For 100mm cubes, two layers work better.
Step 4: Compact Each Layer
You have two options:
Manual compaction: Strike each layer with 35 strokes for 150mm cubes or 25 strokes for 100mm cubes. Distribute strokes evenly across the surface. Let the tamping rod penetrate slightly into the previous layer to ensure good bonding.
Mechanical vibration: Place the mold on a vibrating table and vibrate until the surface becomes smooth and air bubbles stop rising. Don’t over-vibrate—this causes segregation with coarse aggregates settling to the bottom.
Step 5: Finish the Surface
Level the top surface with a trowel. The finish should be smooth and exactly level with the mold top. Remove any excess concrete from around the molds without disturbing the specimens.
Step 6: Initial Curing
Cover the molds with damp burlap or plastic sheeting to prevent moisture loss. Leave them undisturbed for 24 hours at ambient temperature (ideally 20-27°C).
Step 7: Demolding and Marking
After 24 hours, carefully remove the cubes from molds. Mark each cube clearly with:
- Date of casting
- Concrete grade
- Project identification
- Testing age (3, 7, or 28 days)
Step 8: Water Curing
Immediately immerse the cubes in clean water or saturated lime solution. Maintain water temperature at 27°C ± 2°C. Store them submerged until just before testing. Replace the water weekly to maintain quality.
Common Mistakes That Compromise Results
Inadequate compaction: Air voids reduce measured strength by 5-10% per 1% of voids. You’ll see a spongy appearance when cubes break.
Delayed casting: Don’t make cubes from concrete that has started setting. Aim to fill molds within 15 minutes of sampling.
Poor curing: Dry curing reduces 28-day strength by 20-30%. Water immersion is non-negotiable for accurate results.
Rough handling: Dropping or jarring specimens before testing introduces microcracks that reduce measured strength.
How to Make Concrete Test Cylinders: American Standard Method
Cylinder preparation follows similar principles but with specific differences:
Cylinder Molding Steps
Step 1: Position Cylindrical Molds
Place plastic or metal cylindrical molds on a flat, rigid surface. These molds split vertically for easy removal—ensure they’re properly sealed at the base to prevent water loss.
Step 2: Fill in Three Layers
Unlike cubes, standard practice divides cylinders into three equal layers regardless of size. Each layer should be approximately one-third the cylinder height.
Step 3: Rodding or External Tapping
For rodding: Use 25 strokes per layer, distributing them uniformly. Rod penetrates slightly into the previous layer.
For external tapping: Strike the mold sides with a rubber mallet 10-15 times per layer. This method works well for flowing concrete mixes with slumps above 75mm.
Step 4: Level and Cure
Strike off the top surface level with the mold rim. Cover and cure identically to cubes. The same 24-hour demolding applies.
Step 5: Capping
Cylinder ends must be perfectly flat and perpendicular to the axis. Use sulfur mortar capping or grinding to achieve this. Improper end preparation causes 15-20% strength variations.
Testing Age Requirements
Standard testing ages are:
- 3 days: Early strength check, typically 40% of 28-day strength
- 7 days: Critical milestone, usually 65% of final strength
- 14 days: About 85% of final strength (optional)
- 28 days: Design strength reference, considered 100%
Some projects also test at 56 or 90 days for mass concrete pours where late-age strength is crucial.
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What is the Standard Compressive Strength Test Procedure?
Testing requires precision and adherence to standards. Here’s what happens in a KEBS-certified laboratory or any properly equipped facility:
Pre-Test Preparation
Remove specimens from curing water 30 minutes before testing. Wipe surface water off with a damp cloth but don’t let them dry out completely. Specimens should be in a saturated surface-dry condition.
Inspect for defects: Check for honeycombing, excessive air voids, or damage. Specimens with obvious defects should be rejected or noted in the test report.
Measure and weigh: Record exact dimensions and weight. Calculate density (weight/volume) as an additional quality indicator. Normal concrete density ranges from 2,200-2,500 kg/m³.
Compression Testing Machine Setup
The machine must be:
- Calibrated within the last 12 months (KEBS or equivalent authority)
- Capable of loads up to 3,000 kN for standard specimens
- Equipped with spherical seating for the upper platen to accommodate slight irregularities
Loading Procedure
Step 1: Cap Cylinder Ends (cylinders only)
Apply sulfur mortar capping to create perfectly flat, parallel surfaces. For cubes, capping isn’t typically required if the mold produced good surfaces.
Step 2: Center the Specimen
Place the cube or cylinder on the lower platen. The load must apply to opposite cast faces for cubes—never test on the finished top surface.
Align the specimen’s center with the machine’s center of thrust. Misalignment causes non-uniform stress distribution and reduced measured strength.
Step 3: Apply Load Continuously
Rotate the upper platen down gently until it just contacts the specimen. Zero the load indicator.
Apply load continuously at 140 kg/cm²/minute (approximately 1.4 MPa/minute). This translates to roughly:
- 2,000-4,000 kg per second for 150mm cubes
- 800-1,600 kg per second for 100mm cubes
Modern digital machines maintain this rate automatically. Older manual machines require operator skill.
Step 4: Record Maximum Load
Continue loading until the specimen fails. The machine records the maximum load just before complete failure. Most specimens produce a distinctive cracking sound as they approach failure.
Step 5: Note Failure Pattern
Document how the specimen broke. Satisfactory failures show cone-shaped fracture patterns. Unsatisfactory failures (splitting vertically, shattering) suggest testing problems rather than concrete issues.
Calculating Compressive Strength
The formula is straightforward:
Compressive Strength (N/mm²) = Maximum Load (N) / Cross-sectional Area (mm²)
For a 150mm cube:
- Area = 150 × 150 = 22,500 mm²
- If maximum load = 900,000 N
- Strength = 900,000 ÷ 22,500 = 40 N/mm² (Grade 40 concrete)
Always test at least three specimens per age and take the average. If one result varies by more than 15% from the average, discard it and report the other two.
How Does Concrete Strength Develop Over Time?
Concrete isn’t just “done” when it’s poured. Strength development follows a predictable curve driven by the chemical hydration of cement:
Strength Development Timeline
1-day strength (approximately 15%): Concrete achieves minimal strength. Formwork must remain in place. Walking on slabs is possible but heavy loads are prohibited.
3-day strength (approximately 40%): Significant strength development occurs. Some lightweight formwork can be removed with engineering approval. Early-age testing at 3 days helps predict 28-day strength and allows correction if the mix is off-target.
7-day strength (approximately 65%): A critical milestone. Most formwork removal occurs around this point. The National Construction Authority regulations often reference 7-day strength for construction activity decisions.
14-day strength (approximately 85%): Rarely tested unless specified. Useful for early-strength mixes or when project timelines are tight.
28-day strength (100% – design reference): The standard design strength. All structural calculations assume concrete achieves this minimum strength. This is your pass/fail threshold.
Beyond 28 days: Concrete continues gaining strength for months or years. Mass concrete structures may be designed based on 56-day or 90-day strength to account for slower cement hydration in large pours.
Factors Affecting Strength Development
Temperature: Higher temperatures accelerate hydration. Concrete at 30°C gains strength faster than at 20°C. However, excessive heat (above 40°C) can cause rapid moisture loss and reduced final strength.
Cement type: Ordinary Portland Cement (OPC) provides standard development. Rapid-hardening cement achieves 28-day strength in 7 days. You’ll find various portland cement types available in Kenya that affect this timeline.
Water-cement ratio: Lower ratios mean higher strength but slower early development. The optimal balance depends on your specific requirements.
Curing quality: Continuous moisture availability is critical. Dry curing stops hydration, permanently limiting strength. This is why proper water immersion matters so much.
What are the Key Differences Between Cube and Cylinder Tests?
Understanding these differences prevents misinterpretation when comparing results:
Strength Value Differences
Cubes test higher than cylinders from identical concrete. The conversion factor averages 1.25:
Cube strength = 1.25 × Cylinder strength
For example:
- If cylinder strength = 32 MPa
- Expected cube strength = 32 × 1.25 = 40 MPa
This difference stems from geometry. Cubes’ smaller height-to-width ratio creates more lateral restraint from the testing platens, increasing apparent strength.
Regional Standard Preferences
Commonwealth countries (Kenya, UK, India, Australia): Predominantly use cubes following BS EN standards. KEBS certifies laboratories for cube testing primarily.
Americas (USA, Canada, parts of Latin America): Exclusively use cylinders following ASTM standards. Most international projects reference ASTM C39.
Which Test Better Represents In-Situ Strength?
Cylinders more accurately predict actual structural behavior. Their shape resembles columns and other vertical elements. Stress distribution during testing better matches real-world conditions.
Cubes are easier to make, require less material, and tolerate more variation in specimen preparation. They’re practical for routine quality control where relative comparison matters more than absolute accuracy.
For critical structures or international projects, consider testing both. Many contractors in Nairobi now maintain both cube and cylinder capabilities to meet diverse client requirements.
Converting Between Standards
When you must compare cube and cylinder results, use the 1.25 factor conservatively. Remember that other variables (specimen size, capping, machine calibration) also affect results. For forensic work or critical decisions, retest using the appropriate standard rather than relying solely on conversions.
Related Questions:
Q: What happens if my concrete cubes fail the 28-day test?
Failure triggers an investigation. First, verify the test itself wasn’t flawed. Check specimen preparation, curing conditions, and machine calibration. If testing was correct, core samples from the actual structure assess in-situ strength. Often, in-situ concrete performs better than test cubes due to better curing conditions. The structural engineer decides whether remedial action (additional reinforcement, demolition) is necessary.
Q: Can I test concrete cubes before 28 days?
Yes. Early testing at 3 or 7 days provides advance warning of problems. If 7-day strength is only 50% of design strength instead of the expected 65%, you know the mix requires adjustment for future pours. However, early tests don’t substitute for 28-day tests—they supplement them.
Q: How much concrete do I need to make test cubes?
Plan for approximately:
- 100mm cubes: 1 liter per cube
- 150mm cubes: 3.4 liters per cube
For a typical batch test (3 cubes at each of 7 and 28 days), budget 25-30 liters of concrete. This seems wasteful but represents less than 0.5% of a typical structural pour.
Q: What’s the difference between Grade 20, 25, and 30 concrete?
The number indicates the characteristic compressive strength in N/mm² (or MPa) at 28 days:
- Grade 20: 20 N/mm² – Used for blinding, minor structures
- Grade 25: 25 N/mm² – Standard for most residential slabs and beams
- Grade 30: 30 N/mm² – Common for columns and heavy-duty floors
You can learn more about these specifications in ready mix concrete supply standards across Kenya.
Q: Why do concrete test results vary even from the same batch?
Natural variability in materials causes this. Aggregate distribution differs slightly between specimens. Compaction effectiveness varies. Minor temperature differences during curing affect hydration. This is why you test multiple specimens and average results. Properly prepared specimens should vary less than 15% from each other.
Q: Do I need different cube sizes for different concrete grades?
No. Cube size depends on maximum aggregate size, not concrete grade. You can test Grade 20, 30, or 40 concrete using the same 150mm cubes if aggregate size is consistent. The machine capacity must handle the expected loads—higher grade concrete simply requires higher maximum loads.
What is Tensile Splitting Strength and Why Does It Matter?
While compressive strength gets most attention, tensile splitting strength reveals concrete’s resistance to forces that try to pull it apart. This property matters more than most engineers realize. Concrete fails in tension at roughly one-tenth its compressive strength—a vulnerability that drives cracking in slabs, beams, and roads.
Tensile strength accounts for only 7-15% of compressive strength, yet it determines when and where cracks appear. These cracks compromise structural integrity, allow water penetration, accelerate corrosion of reinforcement, and reduce service life. Roads and airport runways particularly depend on tensile strength since they lack reinforcement and must resist flexural stresses directly.
The splitting tensile test—also called the Brazilian test—provides a practical, reliable method to measure this critical property. Unlike direct tension tests that struggle with alignment and gripping issues, the splitting test applies compression diametrically across a cylinder, inducing uniform tensile stress along the vertical plane where failure occurs.
How Tensile Stress Develops in Structures
Restrained shrinkage creates tensile stress in concrete. As concrete cures and shrinks, restraint from foundations or adjacent elements prevents free movement. Temperature changes—daily cycles heating and cooling structures—generate expansion and contraction that produces tensile forces. Wind loads on tall buildings, seismic forces during earthquakes, and soil movements all introduce tension that concrete must withstand.
In foundation design for different Kenyan soils, understanding tensile capacity helps predict cracking patterns and guides reinforcement placement. Ground movements from expansive clays common in Nairobi and surrounding areas create particularly challenging tensile conditions.
How to Perform the Tensile Splitting Test: Complete Procedure
The splitting test requires the same cylindrical specimens used for compression testing, but the loading arrangement differs fundamentally.
Equipment Requirements
Compression testing machine: The same machine used for compressive testing works, provided it can apply loads at controlled rates of 0.7-1.4 MPa/minute.
Bearing strips: Two plywood or hardboard strips, each 3.2mm thick and 25mm wide. These strips must be at least as long as the cylinder length—typically 300mm for standard specimens. Never reuse strips; fresh strips for each test prevent error from crushing damage.
Supplementary bearing bars (if needed): When the machine’s upper or lower bearing surface is smaller than the cylinder length, steel bars (50mm wide minimum) distribute load across the entire specimen length.
Measurement tools: Vernier calipers for accurate diameter and length measurement. Digital instruments reading to 0.1mm precision are preferable.
Specimen Preparation Before Testing
Step 1: Retrieve from Curing
Remove cylinders from water storage 30 minutes before testing. Surface water must be wiped off, but specimens remain in saturated surface-dry condition. Don’t let them dry out completely.
Step 2: Inspect and Measure
Examine for visible defects—honeycombing, large voids, significant surface irregularities. Document any issues. Measure and record:
- Actual diameter (D) at three locations along the length
- Actual length (L) between ends
- Weight
Calculate average diameter from the three measurements. Variation shouldn’t exceed 2%.
Step 3: Mark the Loading Lines
Draw two diametrically opposite lines along the cylinder length using a straightedge. These lines guide specimen positioning and ensure load applies along a consistent diameter. Mark them clearly on both circular faces.
Testing Procedure Step-by-Step
Step 1: Position the Lower Bearing Strip
Place one plywood strip on the machine’s lower platen, aligned with the anticipated position of the specimen. The strip must lie perfectly flat without warping or kinking.
Step 2: Place the Specimen Horizontally
Lay the cylinder on its side on the bearing strip. Align it so the marked diametral lines are exactly vertical—one at top, one at bottom. The cylinder axis must be perpendicular to the direction of applied load. Use a small level to verify horizontal alignment if needed.
Center the specimen on the lower platen. Equal distances from cylinder ends to platen edges ensure uniform loading.
Step 3: Position the Upper Bearing Strip
Carefully place the second bearing strip on top of the cylinder, aligned directly above the bottom strip. The load path should be perfectly vertical through both strips and the cylinder diameter.
Step 4: Bring Down the Upper Platen
Lower the machine’s upper platen gently until it just contacts the top bearing strip without applying significant load. Adjust the specimen if necessary to ensure perfect alignment. Once satisfied, proceed immediately to testing.
Step 5: Apply Load Continuously
Apply load at a rate within 0.7-1.4 MPa/minute for splitting tensile stress. Most modern machines allow rate programming. For manual machines, estimate the rate based on specimen size and expected failure load.
For a 150mm × 300mm cylinder with expected tensile strength around 3 MPa:
- Target load rate ≈ 100-200 N/second
- Time to failure ≈ 2-4 minutes
Never pause or reverse loading once started. Maintain steady, continuous increase until failure.
Step 6: Record Maximum Load
The specimen will fail suddenly with a distinct crack propagating along the vertical diameter, splitting the cylinder into two halves. The machine records the maximum load (P) just before failure.
Step 7: Examine Failure Pattern
Satisfactory failure shows a relatively straight vertical crack through the cylinder center. The two halves should be fairly even. Unsatisfactory patterns include:
- Diagonal cracking
- Failure at the bearing strips (crushing)
- Multiple fragmentation
Unsatisfactory failures indicate testing problems—specimen defects, misalignment, or bearing strip issues. Discard these results.
Common Testing Errors to Avoid
Misalignment: The most frequent error. Even 5-degree misalignment reduces measured strength by 10-15%. Take time positioning specimens carefully.
Reusing bearing strips: Crushed or damaged strips create stress concentrations. Use fresh strips for every test—they’re cheap insurance for accurate results.
Excessive loading rate: Too-fast loading increases apparent strength artificially. Maintain rates between 0.7-1.4 MPa/minute as specified.
Testing wet specimens immediately: Some surface moisture is good, but water pooling at bearing strips affects results. Wipe thoroughly but don’t dry completely.
What is the Splitting Tensile Strength Calculation Method?
The calculation converts measured load to tensile stress using cylinder geometry:
The Standard Formula
T = 2P / (π × L × D)
Where:
- T = Splitting tensile strength (MPa or N/mm²)
- P = Maximum applied load at failure (N)
- L = Length of cylinder (mm)
- D = Diameter of cylinder (mm)
- π = 3.14159
Worked Example
A 150mm diameter × 300mm long cylinder fails at a maximum load of 190,000 N.
T = 2 × 190,000 / (3.14159 × 300 × 150)
T = 380,000 / 141,371
T = 2.69 N/mm² (or 2.69 MPa)
This represents typical splitting tensile strength for Grade 25 concrete (compressive strength 25 MPa).
Typical Tensile Strength Values
For common concrete grades in Kenya:
Grade 15 concrete: 1.5-2.0 MPa tensile strength
Grade 20 concrete: 2.0-2.5 MPa tensile strength
Grade 25 concrete: 2.5-3.0 MPa tensile strength
Grade 30 concrete: 3.0-3.5 MPa tensile strength
Grade 40 concrete: 3.5-4.2 MPa tensile strength
Correlation with Compressive Strength
The general relationship between splitting tensile strength and compressive strength follows the equation: fc = 0.56 × λ × √f’c (SI units). The factor λ adjusts for lightweight aggregates; for normal-weight concrete, λ = 1.0.
This correlation allows engineers to estimate tensile strength when only compressive data exists. However, splitting cylinder test gives values about 1.05-1.15 times those obtained from direct tension test, meaning it slightly overestimates true tensile capacity.
Where Can You Get Concrete Tested in Kenya?
Quality testing requires accredited laboratories with calibrated equipment and trained personnel. Kenya offers several options across different regions.
Kenya Bureau of Standards (KEBS)
KEBS operates a Civil Engineering Laboratory equipped with modern state-of-the-art combined compression and flexural machines capable of handling most strength requirements. Located on Popo Road, off Mombasa Road in Nairobi, KEBS provides the gold standard for concrete testing in Kenya.
Advantages:
- Government-backed credibility
- Internationally recognized certifications
- Comprehensive testing scope beyond just concrete
- Reports accepted by all regulatory bodies
Contact: P.O. Box 54794-00200 Nairobi, Kenya
Certified Private Laboratories
Several private facilities offer KENAS-accredited testing services. The Kenya Accreditation Service (KENAS) maintains a directory of accredited testing laboratories throughout the country.
MATLAB Engineers Ltd: Based in Kilimani, Nairobi, MATLAB specializes in site lab services for geotechnical engineering, including concrete testing. They offer mobile laboratory services—a significant advantage for remote project sites.
GeoIssa: Operating from Nairobi, GeoIssa maintains a fully equipped material testing laboratory providing testing for concrete, aggregates, soils and rocks.
Votex Limited: Located in Kiambu Town, Votex provides concrete cube testing services with on-site sampling and collection services. Their approach streamlines the process for contractors who lack transportation for specimens.
Spectra Lab: A full-service analytical laboratory in Nairobi serving multiple sectors including construction. They perform chemical tests on concrete according to BS 1377 standards.
University Testing Facilities
Several Kenyan universities offer testing services:
University of Nairobi: The Institute of Nuclear Science and Technology and Department of Civil Engineering maintain testing equipment. Primarily serves research but accepts commercial testing with advance arrangements.
JKUAT (Jomo Kenyatta University of Agriculture and Technology): Located in Juja, JKUAT’s Civil Engineering department operates testing facilities accessible to the construction industry.
Choosing the Right Laboratory
Consider these factors:
Accreditation status: KENAS accreditation to ISO/IEC 17025 ensures technical competence. Only accreditation to the ISO/IEC 17025:2017 version remains valid after November 2020. Verify current accreditation before engaging any laboratory.
Turnaround time: Standard testing typically takes 1-2 days after specimens reach testing age. Rush services may be available at premium rates.
Location and logistics: Transport costs for specimens can be significant. Laboratories offering collection services save money and hassle.
Scope of accreditation: Verify the lab is accredited specifically for the tests you need—compressive strength, tensile strength, etc. General accreditation doesn’t guarantee capability for all test types.
Cost: Prices vary significantly. Get quotes from multiple laboratories but don’t sacrifice quality for marginal savings.
For a comprehensive list of accredited facilities, visit the certified materials testing laboratories guide.
What Standards Apply to Concrete Testing in Kenya?
Kenya’s construction industry references multiple standard systems, creating complexity but also flexibility.
British Standards (BS)
The legacy of colonial-era construction means British Standards remain deeply embedded in Kenyan practice. Key standards include:
BS EN 12390-3: Testing hardened concrete – Compressive strength of test specimens
BS EN 12390-6: Testing hardened concrete – Tensile splitting strength of test specimens
BS 1881: The older standard still referenced in many Kenyan specifications, though largely superseded by EN standards
These standards specify specimen sizes, testing procedures, curing requirements, and reporting formats. Most Kenyan engineers trained before 2010 learned BS standards exclusively.
Kenya Standards (KS)
KEBS has developed Kenya Standards that generally align with international practice:
KS EAS 18-1: Specification for Portland cements—defines cement grades and properties
KS ISO series: Various standards adopted from ISO for testing methods
Kenya Standards often directly adopt or adapt BS or ISO standards with minor modifications for local conditions.
ASTM Standards
American Society for Testing and Materials standards appear increasingly in international projects and donor-funded infrastructure:
ASTM C39: Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens
ASTM C496: Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens
Projects funded by World Bank, African Development Bank, or involving American contractors typically specify ASTM standards. The shift toward cylinders reflects ASTM influence.
Which Standards Should You Follow?
For government projects: Follow specifications in the tender documents. Most reference BS or KS standards. The National Construction Authority regulations provide guidance on acceptable standards.
For private projects: Agree with your structural engineer. Document the chosen standard clearly in specifications to avoid disputes.
For international projects: Follow the project-specific requirements, usually ASTM for American-funded work, BS/EN for Commonwealth projects.
The key is consistency. Don’t mix standards within a single project—cube strengths and cylinder strengths aren’t directly comparable without conversion factors.
What are Common Testing Errors and How Can You Avoid Them?
Even experienced technicians make mistakes that compromise test validity. Recognize these patterns and implement preventive measures.
Specimen Preparation Errors
Inadequate compaction: Creates air voids that reduce measured strength dramatically. Every 1% air content by volume reduces strength by approximately 5%. Vibrate thoroughly but avoid segregation. Look for a smooth, glossy surface finish as compaction completes.
Wrong aggregate size for mold: Using 40mm aggregate in 100mm cubes creates edge effects—large stones can’t distribute properly in the small space. Match mold size to maximum aggregate size following standard specifications.
Surface finish problems: Rough, uneven mold tops cause non-uniform loading during testing. Take time finishing surfaces properly. Consider using capping compound for cylinders if surfaces aren’t perfectly flat.
Delayed casting: Making specimens from concrete that has begun setting yields unreliable results. Sample and mold within 15 minutes of concrete production or delivery. Don’t try to “refresh” concrete by adding water.
Curing Condition Errors
Insufficient moisture: Drying during curing stops hydration permanently. Strength continues developing only while moisture remains available. Concrete cured dry for just 7 days loses 30-40% of potential strength compared to continuously wet-cured specimens.
Temperature extremes: Curing at 10°C slows hydration dramatically—28-day strength might take 56 days. Curing above 35°C accelerates early strength but reduces ultimate strength through microcracking. Maintain 20-27°C for standard test conditions.
Inconsistent water quality: Changing water or using contaminated water affects results. Use clean, fresh water or saturated lime solution. Replace weekly as specified.
Premature demolding: Opening molds before 24 hours risks damaging specimens, especially corners and edges. Be patient.
Testing Procedure Errors
Machine calibration lapse: Uncalibrated machines produce meaningless results. Verify calibration certificates annually. Compression testing machines should be calibrated on an annual basis.
Incorrect loading rate: Too-fast loading increases apparent strength; too-slow loading reduces it. Program digital machines correctly. For manual machines, practice estimating correct rates.
Testing wrong faces: For cubes, never test on the finished (troweled) surface. Load must apply to cast faces for standard results. Mark specimens clearly during molding.
Averaging invalid results: When one specimen shows significantly different strength (±15% from others), discard it rather than averaging. Investigate why it differed—was it damaged, improperly cured, or incorrectly tested?
Poor record-keeping: Mixing up specimens, losing identification marks, or incomplete documentation ruins expensive testing programs. Implement systematic labeling and documentation procedures.
Quality Control Best Practices
Standardize procedures: Write clear standard operating procedures (SOPs) for your team. Include photos showing correct techniques.
Train personnel: Invest in proper training. KEBS and some universities offer courses in concrete testing. Untrained staff make expensive mistakes.
Conduct regular audits: Have experienced personnel periodically observe testing to catch emerging bad habits before they become systematic errors.
Compare with ready-mix suppliers: When using ready-mix concrete suppliers, compare your test results with supplier’s results from the same batch. Significant differences indicate problems in your procedures or theirs.
How Much Does Concrete Testing Cost in Kenya?
Testing costs vary by location, laboratory, and test type. Budget appropriately to avoid compromising quality control.
Typical Laboratory Charges (2024-2025)
Compressive strength test – single cube: KES 500-800 per specimen at private labs, KES 600-1,000 at KEBS. Prices are higher in Nairobi than upcountry.
Compressive strength test – set of 3 cubes: KES 1,200-2,000 per set. Most labs offer per-set pricing rather than per-specimen.
Tensile splitting test: KES 800-1,200 per cylinder. Less common than compressive testing, so fewer labs offer competitive pricing.
Core testing (drilled from existing structures): KES 1,500-2,500 per core, plus drilling charges if the lab provides that service.
Slump test (on-site): KES 200-500 if included with sampling services.
Rush testing: Premium of 50-100% for same-day or next-day results.
Additional Costs to Consider
Transportation: Delivering specimens to laboratories costs money. For projects in Nairobi testing at KEBS, allow KES 500-1,500 for transport depending on specimen volume and distance.
Molds: Steel cube molds cost KES 4,000-8,000 for 150mm size, KES 2,500-4,000 for 100mm. Plastic cylinder molds cost KES 1,500-3,000 each. Budget for 6-12 molds per project depending on testing frequency.
Sampling services: Some contractors hire laboratories to come to site, sample concrete, and make specimens. This service costs KES 3,000-8,000 per visit depending on location and number of specimens.
Report fees: Most laboratories include basic reports in testing fees. Comprehensive reports with interpretation and recommendations may cost extra.
Cost-Saving Strategies
Bulk testing agreements: Negotiate package rates for large projects. Testing hundreds of specimens over several months often qualifies for 20-30% discounts.
Own molds and equipment: Making specimens yourself and delivering to labs saves money. Initial investment pays off quickly on medium-to-large projects.
Strategic sampling frequency: Test strategically rather than arbitrarily. For routine work, test one set of cubes per 50-100 m³ of concrete or per day’s pour, whichever is more frequent. Critical elements (columns, beams) warrant more intensive testing.
Choose local laboratories: Transport costs can exceed testing fees for distant facilities. Local KENAS-accredited labs often prove more economical despite possibly higher per-test charges.
Remember that testing represents less than 0.5% of typical structural concrete costs. Reducing testing to save a few thousand shillings risks million-shilling problems if defective concrete enters your structure.
You can review regional pricing variations in our detailed concrete grade contractor rates guide.
Frequently Asked Questions
How many concrete cubes should I make per pour?
Make at least three specimens for each testing age (7 days, 28 days) from each concrete batch or pour day. For large projects (over 50 m³ per day), increase frequency to one set per 50 m³ or per major structural element. Critical structures may warrant testing every 20-30 m³.
Q: Can I test concrete cubes at home without specialized equipment?
No. Accurate compression testing requires calibrated machines capable of applying loads up to 3,000 kN. These machines cost hundreds of thousands of shillings. DIY testing methods lack accuracy and validity. Always use accredited laboratories for reliable results
What should I do if my 28-day concrete test results fail?
First, verify the test itself was valid—check specimen preparation, curing, and testing procedures. If the test was correct, commission core samples from the actual structure to assess in-situ strength. Often, structure concrete performs better than test cubes due to superior curing. Consult your structural engineer to determine if remedial action is necessary. Options include additional reinforcement, structural strengthening, or in extreme cases, demolition and reconstruction.
Is there a difference between 7-day and 28-day concrete strength requirements?
Yes. Seven-day strength typically reaches 60-65% of 28-day strength for normal Portland cement. Some specifications set minimum 7-day strength requirements (often 60-70% of design strength) to allow early formwork removal. However, 28-day strength remains the definitive pass/fail criterion. Seven-day tests provide early warning of problems, not final acceptance.
Can I use concrete immediately after pouring, or must I wait for test results?
You can proceed with construction following standard curing periods, but test results provide confirmation. Most specifications allow formwork removal based on time (typically 7 days for beams/slabs, 24-48 hours for columns with proper propping). However, if tests subsequently fail, remedial action may be required even on completed work. Risk-averse projects wait for test confirmation before proceeding to dependent activities.
Why do concrete strengths vary even when using the same mix design?
Natural material variability causes this. Aggregate moisture content fluctuates, affecting water-cement ratio. Ambient temperature varies. Mixing time differs slightly between batches. Compaction effectiveness varies. Transportation time from plant to site affects workability. These factors combine to create ±10-15% strength variation even from nominally identical concrete. This is why we test multiple specimens and average results.
What's the acceptable variation between three test cubes from the same batch?
Individual results should fall within ±15% of the average. If one cube varies by more than 15%, discard it and average the other two. If two or more vary significantly, the test set is invalid—something went wrong in preparation, curing, or testing. Investigate and retest.
Do I need to test both compressive and tensile strength for every project?
Most routine projects test compressive strength only. Tensile testing applies to specific situations: roads and airfield pavements (where flexural strength matters critically), mass concrete dams (where thermal cracking concerns exist), or research and development of new mix designs. Consult specifications to determine if tensile testing is required. When in doubt, test compressive strength at minimum.
How long can I store cured specimens before testing?
Test at the specified age ±2 hours for best accuracy. If unavoidable delays occur, test within 24 hours of target age. Beyond that, strength continues increasing, invalidating comparison with standard strength development curves. Plan logistics so specimens reach laboratories on schedule. Some labs offer collection services that eliminate this concern.
Can weather conditions during concrete placement affect test results?
Absolutely. Hot weather (above 32°C) accelerates setting, increases water demand through evaporation, and can reduce ultimate strength if moisture loss occurs before initial set. Cold weather (below 10°C) dramatically slows hydration—28-day strength might take 56 days. Rain during placement dilutes surface concrete. Strong wind increases evaporation rates. Adjust procedures for weather extremes and document conditions in testing records.
What is the difference between cube crushing strength and characteristic strength?
Cube crushing strength is the actual measured strength from testing specimens. Characteristic strength is a statistical concept—the strength below which no more than 5% of test results fall. Design calculations use characteristic strength with additional safety factors. For acceptance, average cube strength must exceed characteristic strength by approximately 3-4 MPa to account for normal variation.
How accurate are concrete testing results?
Well-conducted tests using proper procedures and calibrated equipment achieve ±5% accuracy. Poor technique can introduce ±15-25% error. This is why accredited laboratories matter—they follow quality procedures that minimize variability. Multiple specimens per test set improve statistical reliability.
Can I salvage concrete that fails testing?
It depends on how badly it fails and where it's located. Minor failures (10-15% below specification) in non-critical elements might be acceptable with engineering evaluation and possible load restrictions. Significant failures (over 20% below specification) in critical structural elements usually require remediation. Options include external post-tensioning, adding structural steel plates or jackets, increasing member size with additional concrete, or demolition and replacement. A structural engineer must evaluate each situation.
What information should concrete test reports include?
Complete reports document: specimen identification (project, element, date cast), concrete grade specified, specimen dimensions and weight, age at testing, curing method, testing machine details and calibration date, failure load, calculated strength, failure mode description, and technician signature. Photos of failure patterns add value. Incomplete reports lack legal and technical validity.
Are there alternatives to destructive testing like cube crushing?
Yes. Non-destructive testing (NDT) methods include rebound hammer tests, ultrasonic pulse velocity tests, and penetration resistance tests. These provide estimates of in-situ concrete strength without damage. However, NDT methods are less accurate than destructive testing and require correlation with actual crush test data. Use NDT for initial surveys or when core drilling is impractical, but validate with destructive testing when critical decisions depend on results.
What is the minimum concrete strength for different building elements?
Foundations in stable soils: Minimum Grade 15 (15 MPa), though Grade 20 is more common. Suspended slabs and beams: Grade 25-30. Columns: Grade 25-30 for low-rise, Grade 30-40 for high-rise. Water tanks and swimming pools: Grade 30 minimum for watertightness. Roads and pavements: Grade 25-30 depending on traffic loads. Always follow structural engineer's specifications—these are minimums, not universal standards.
