Written by: Erika Mae Buela, RCE
“The Philippines has some of the worst-quality concrete we’ve encountered.”
These are the exact words of one of JVAZCO’s international partners, and this claim cannot be dismissed lightly. A recent project highlighted the critical importance of onsite testing.
The team was called in to investigate cracking issues at a project they had previously worked on. Anticipating such issues, as blame is often placed on the last touch—in this case, the team— a drilled core test was conducted. The results revealed that, 397 days after placing and finishing the floor, the highest compressive strength was only 2040 psi—far below the 3000 psi specified for 28 days. To make matters worse, apart from the slump test, no other onsite tests were performed. This oversight turned into a ticking time bomb that exploded just one year later.
This experience underscores a broader issue: the significant role of proper onsite testing in ensuring concrete quality. Inadequate testing not only jeopardizes the integrity of a project but can also lead to costly and dangerous consequences. Addressing these testing gaps is crucial for improving construction standards and outcomes.
The Bare Minimum: Fresh Concrete Tests
When a truckload of ready-mix concrete arrives on-site, the pressure mounts. The driver hands over the delivery receipt, and without even scanning it, someone signs it. The technician performs a quick slump test (if you’re lucky), gets a nod from the project engineer, and in the blink of an eye, you hear the booming sound of pumpcrete discharging the concrete. This scene is all too familiar.
But slump is just one aspect. Temperature, density, and air content tests are equally crucial and non-negotiable. They are as important as the well-known compressive strength test. Inadequate testing procedures and mishandled samples can lead to poor strength results. These quality assurance checks not only determine if the mix is right for the job but also predict whether the concrete will perform as expected and endure over time. Yet, many construction sites in the Philippines dismiss these onsite tests as a “waste of time,” skipping them to save a few minutes and avoid project delays. This is their perfect alibi.
Common Issues with Onsite Tests
At a few project sites we visited for quality checks of RMC, I observed two common scenarios:
- No tests are conducted.
- Tests are conducted but not properly executed.
Let’s focus on the second scenario. Complacency is growing among owners, RMC suppliers, and project engineers, who often prioritize speed or cost over quality. A common issue is rushing or skipping tests. Here are key errors and bad habits to avoid:
Sampling
Before testing or molding specimens, a sample must be taken, and it needs to be representative of the entire batch. A common mistake is taking a sample from the first part of the batch as soon as the truck arrives at the job site. Instead, follow this rule: “Do not take samples before 10% or after 90% of the batch has been discharged.” In other words, samples should come from the middle of the batch. Learn more about ASTM C172/C172M.
Temperature Test
The temperature test is usually simple, but there are two common mistakes that raise concern. First, technicians often forget to keep the temperature measuring device (TMD) submerged for at least two minutes. Second, the TMD is sometimes removed from the concrete before taking a reading. Learn more about ASTM C1064/C1064M.
Slump Test
This is a pretty straightforward procedure too, but it’s prone to several errors. A slump cone needs to be filled in three layers, and for each layer, as tiring as it may be, it needs to be rodded with 25 strokes. Technicians also tend to wiggle or twist the cone when lifting it, instead of using the correct method: a steady, vertical motion without any lateral movement. Additionally, the result should be a true slump, not shear. Learn more about ASTM C143/C143M.
Density and Air Content Test
The density and air content tests are closely connected because the air content test happens right after the density test. If there’s a mistake in the density test, it affects the air content results too. A common mistake is not tapping the sides of the container properly with a mallet as each layer of concrete is added. This can leave air pockets and mess up both the density and air content readings. Another issue is not cleaning the flange or rim of the container, which can cause leaks and affect the results. Learn more about ASTM C138/C138M and ASTM C231/C231M.
So what? You ask. Here’s why.
Fresh concrete testing is your first-line of defense to ensure durability and safety of a structure.
Sampling
ACI 301 requires one test with two samples for every 100 cubic yards or part of it placed in a day. Meanwhile, ACI 318 requires one test with two samples for every 150 cubic yards, but at least one test must be done each day. Hence, these critical numbers should be truly representative. The standard 28 liters of sample represents only 0.025 to 0.037 percent of the total concrete placed. This tiny fraction must reflect the entire batch so any deviation can lead to a misleading strength result.
Temperature
Temperature determines the quality, setting time, and strength of concrete. Concrete with a high temperature might gain strength quickly in the early stages, but expect a lower strength later on. It also influences the types of curing and protection needed. Not monitoring temperature can lead to long-term issues like cracking.
Slump
The slump test is a quick and simple way to assess the consistency of fresh concrete. A low slump indicates a dry mix, while a high slump suggests a wet mix. If this test is done improperly or skipped, the concrete could end up too dry, leading to poor compaction and voids, or too wet, resulting in segregation and bleeding. Both scenarios sound like costly repairs.
Density
Concrete density affects its overall performance. If the density changes, it might mean there’s too much air or water, wrong mix proportions, or less cement. Low-density concrete can cause problems like reduced strength and poor performance in pumping, placing, and finishing. It’s a red flag for possible cracks, lower load-bearing capacity, and other durability issues.
Air Content
Air content in concrete is crucial for its strength, resistance to wear and chemicals, and how much it shrinks. Too much air can weaken the concrete. For example, if concrete is designed to reach 3,000 to 5,000 psi strength, a 1% increase in air content can reduce its strength by 3 to 5%. So, if the air content is supposed to be 5% but goes up to 10%, the concrete could lose 15 to 25% of its strength.
How to Avoid these Issues?
Think of it this way: Be very specific to avoid these traps. Project owners and RMC suppliers share accountability for quality assurance. By working with an established third-party testing organization, owners can increase their confidence. To reduce hazards, select a reliable RMC supplier with a solid (positive) reputation for providing high-quality materials. For instance, their ACI-certified Concrete Field Testing Technicians must follow ASTM C31/C31M guidelines when creating samples in accordance with ASTM C94. Don’t stop there; educate yourself on the testing procedures to avoid being in the dark. ACI also advises owners to conduct independent tests to verify results. Even better, certify your QA/QC staff, as the strongest defense is a well-trained workforce. At JVAZCO, we deploy certified technicians to witness every test, ensuring that no corner is cut and no test is skipped. It’s a small investment that can save millions in repairs, lawsuits, reputation, and lives.
References:
- ACI 301 – Specifications for Structural Concrete
- ACI 318 – Building Code Requirements for Structural Concrete
- ACI Concrete Field Testing Technician – Grade 1 Workbook
- ACI/ASCC Manual: The Contractor’s Guide to Quality Concrete Construction 4th Edition
- ASTM C94/C94M – Standard Specification for Ready-Mixed Concrete
- ASTM C31/C31M – Standard Practice for Making and Curing Concrete Test Specimens in the Field
- ASTM C143/C143M – Standard Test Method for Slump of Hydraulic-Cement Concrete
- ASTM C1064/C1064 – Standard Test Method for Temperature of Freshly Mixed Hydraulic-Cement Concrete
- ASTM C138/C138M – Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete
- ASTM C231/C231M – Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method
- ASTM C172/C172M – Standard Practice for Sampling Freshly Mixed Concrete
