Concrete Strength Testing with Schmidt Hammers
By: Isaak Tsalicoglou, Proceq
Source: Proceq
Concrete is the most widely used construction material in the world, thanks to its high compressive strength, durability, long life, and fire-resistant properties. Every year, millions of tons of concrete are used in large construction projects including dams, bridges, buildings, and roads.

In this interview with David Corbett, Product and Application Expert, we learn about the challenge of estimating the strength of concrete, why the economics and insights of destructive testing of concrete structures don’t scale, and of alternatives that are more cost-effective, efficient, and robust.

Dave, tell us about concrete strength - why is it important?

David Corbett: The compressive strength of concrete is a critical property for the safety of concrete structures and the general public. It is one of the key parameters specified for new constructions to ensure the designed service life. As such, evaluating in-situ strength is an important measure of concrete quality. Structures built with concrete of insufficient quality are more susceptible to corrosive agents that eventually lead to deterioration or even collapse, with catastrophic consequences.

What determines the strength of concrete?

The concrete strength is determined primarily by the "mix design" – this is the specification of the components it contains and under which conditions it will perform as designed. While generally we can assume that this carefully designed "recipe" will be used correctly by competent engineers, this is only half of the story. The other half is the actual execution on the construction site. For example, was more water added on site to ease the workability of the concrete? How well was the concrete compacted after pouring? I could go on, but you get the general idea: the final in-situ compressive strength is dependent on the mix design and the working practices of the construction crew. Because the latter can be highly variable, quality assurance of concrete strength is absolutely important for the safety of the structure and the public.

And what exactly does the compressive strength imply for the quality of the concrete?

Assessing the in-situ strength ensures that the concrete meets the design specification, i.e. that it possesses the required strength for the final structure to be safe, durable, and meet any required regulations. This assessment is not only possible for newly constructed structures, but also for older concrete structures or for those undergoing modifications.

How has the compressive strength of concrete structures been estimated traditionally?

The traditional method is the destructive testing of concrete cores. Coring involves cutting cylinders of concrete – the "cores" – from various locations of the structure. The compressive strength of the cores is then tested using a compression testing machine in a lab, off-site.

And what is so bad about coring?

I wouldn’t say coring is bad per se, though it does feature significant drawbacks. To start off, coring is in fact a precise method of evaluating the compressive strength of a concrete structure, when done correctly. That’s because, after all, you are removing parts of the actual structure and testing them destructively in the lab. However, by its very nature, coring is also a labor-intensive and messy activity – and, if done excessively, it would leave a structure locally weakened, with a compromised performance.

Does this mean that coring can only be used to a limited extent?

Indeed! Plainly put: extracting cores on-site and crushing them off-site doesn’t scale, neither in terms of effort, nor in terms of the extent to which it can be done to a structure. Conducting a complete assessment of concrete strength using coring alone is impossible; not only would the structure – or what remains of it – look like Swiss cheese afterwards, but it would also take forever and cost too much to do so.

Still, could you do this traditional testing while on-site to save time?

Not really. After extraction, the cores need to be taken to a lab with a compression testing machine. In other words: with traditional destructive methods you can’t just walk up to a concrete structure and have a testing result, an insight of its condition, in a matter of minutes or even seconds.

So what’s a way out of this problem, then?

The pragmatic, clever solution is to complement or even replace concrete coring with non-destructive testing methods for assessing in-situ strength. In fact, using such methods can aid engineers in selecting their coring patterns and performing assessments in a way that is more efficient and cost-effective. That’s why this "conditional coring" approach is recommended by major standards and guidelines institutions.

Which non-destructive methods do you have in mind?

The two most popular non-destructive methods are rebound hammer testing and ultrasonic pulse velocity testing. Among these, rebound hammer testing is the non-destructive method that has established itself as the most widely used best complement to coring and crushing, as it is the fastest and most economical test to carry out. And by rebound hammer testing, I mean Schmidt hammers.

How so? What makes Schmidt hammers so popular?

Schmidt hammers are popular and established in estimating concrete strength, because they are affordable, easy to use, relatively quick – and, of course, non-destructive. Beyond that, in their latest "Live" iteration they are versatile, connected, and help increase on-site productivity thanks to automating and mistake-proofing the testing process.

As I understand it, by crushing a core you measure the compressive strength of the concrete sample directly. How do Schmidt hammers measure strength?

There are two types of Schmidt hammers, both invented by Proceq. The first one is the Original Schmidt, which is an R-value hammer – it measures the rebound distance upon impact on the surface. The second one is the SilverSchmidt, which is a Q-value hammer – it measures the rebound velocity before and after impact. Each type has its benefits, but both types measure the surface hardness, which is correlated to compressive strength of the concrete.

Can I use a Schmidt hammer by itself, without also using coring?

Sure – in fact, using Schmidt hammers as a screening method is a popular application. By "screening" I mean that Schmidt hammers are used to check the uniformity of concrete across a section of the structure, and thus identify areas that are weaker. This uniformity testing is, however, exactly what makes Schmidt hammers so useful in combination with coring!

How so?

Imagine that you don’t have a Schmidt hammer at your disposal and must rely only on coring. Well, one question then is, where on this structure should you extract cores? And how can you minimize the number of cores you will extract? In other words, how can you be sure you are obtaining representative results from a very small sample set? And how can you minimize the damage caused to the structure by coring? Without a Schmidt hammer, you don’t have prior knowledge to select coring locations.

I think I know what you’re getting at – ideally, you would only core locations that actually deserve coring…?

Precisely! The core testing should produce results which are representative of the structure. If you choose the locations randomly, you may by chance only choose weak areas, or only choose strong areas – and end up with a lop-sided result. A Schmidt hammer survey allows the engineer to take cores over the whole range of strengths in the structure and obtain a good correlation with a minimum number of cores.

Can you really save so much time by doing so?

Absolutely. Remember: coring is messy, expensive, and labor-intensive. On top of that, you need to wait for the samples to get crushed off-site in the lab. Therefore, anything you do to cut down on the number of cores will have an immediate impact on your costs. It will also directly reduce the waiting time until you can conclude whether the structure is sound.

Is rebound hammer testing in the end only about saving time and costs?

At first sight, it might seem so, because the time and costs savings are significant, especially as the size of the structure and the uncertainty of its strength grows. However, Schmidt hammers also help to improve the quality of the overall structural assessment. That’s because, by its very nature, coring is limited in scope – and once a good correlation has been established, the rebound hammer test can provide a comprehensive assessment of the structure with far less effort and at far less cost.

Destructive and non-destructive testing methods, together?

Definitely! In fact, you are hinting at something very clever. Destructive testing provides more certainty with far higher effort – but NDT allows faster and easier testing even in areas that are inaccessible to destructive tests. Moreover, rebound and core testing data can indeed be compared and correlated to provide an overall assessment of the entire structure. This means that the validity of engineers’ insights regarding in-situ concrete strength are themselves strengthened when engineers combine coring with Schmidt hammer testing.

Quite impressive; any parting advice for civil engineers?

All civil engineers are already familiar with both Schmidt hammers and core testing, but many consider either the one or the other method in isolation. Our most advanced customers already understand the value of Schmidt hammers as a complement to coring. So, as parting advice, I invite engineers to consider and try out the combination of coring and either Original Schmidt Live or SilverSchmidt. Compared to just coring by itself, engineers can reap massive benefits in terms of cost, effort, ease of use, and timeliness and quality of insights.