How to Read Block Strength Classes for Reconstruction Projects

Block strength classes for reconstruction projects are read on four lines, not one. A class states the compressive strength value, the area basis it was measured on, the curing age at which that value applies, and how many units were tested. Two suppliers can both write 10 MPa on a quotation and still deliver blocks that behave differently in a wall, because an average of 6 units on gross area at 28 days is not the same claim as a single unit on net area at 7 days. Read those four lines before you sign, and write them into the purchase order.

What does a block strength class actually state?

The class is a claim about compressive strength and nothing else. It says that a block of a given size and mix carries a stated force per unit of area when it is crushed flat in a press. It does not promise durability, low absorption, low shrinkage or accurate dimensions, because all of those are separate requirements written on other lines of the same sheet. A block that meets its class and absorbs too much water will still crack in a wall exposed to rain and salt.

The two words that decide everything else are nominal and characteristic. A nominal value is usually the arithmetic average of the units tested, which means roughly half of the production can sit below it. A characteristic value is a level that a defined high proportion of the units is expected to exceed, commonly 95 percent, which makes it the lower and the safer of the two. When a sheet gives one number with no label, ask which one it is.

How do you read a class notation on a spec sheet?

A class notation is only usable when five items travel with it. Use this list to check any sheet that reaches your desk.

If any one of those six items is missing, the number cannot be compared with anything else. Ask for it in writing before the order is placed rather than after the first delivery, because a supplier who cannot state the basis today will not be able to defend the result in three months.

• The value and its unit: 10 MPa, 15 MPa, or a class name that hides the number behind a letter.
• The area basis: gross area as the block sits, or net area with the voids taken out.
• The test age: 7 days, 14 days or 28 days, and which of them the delivery will be checked against.
• The sample size: how many units were tested, and whether the individual results appear next to the average.
• The moisture condition at test, dry or saturated, because it moves the number.
• The standard and its edition, because two editions can differ on capping and on loading rate.

Which class do you need for a load bearing wall?

The class comes from the structural design, not from a catalogue. As a rough orientation, non load bearing partitions and infill commonly sit in the 3.5 to 7 MPa range, load bearing walls in the 7.5 to 15 MPa range, and heavily loaded or slender walls above that. Treat those bands as orientation only, because wall thickness, mortar, grout fill, reinforcement and the load path decide the real requirement, and only the design engineer can fix the number.

Two traps appear on almost every reconstruction site. The first is a class letter, such as class A or class 1, quoted with no MPa value anywhere on the sheet. The second is a strength copied from another project's specification without checking the block size, the hollow ratio or the wall thickness. Both are settled with one question that is easy to ask and hard to dodge.

Why do two labs report different strengths for the same class?

Because the reported number depends on the procedure as much as on the block. Curing differs first: a unit taken from dry yard storage and a unit that was water cured or steam cured are the same material at different ages. Capping differs next: a sulfur cap, a plywood pad and a ground face each transfer the load in their own way. The loading rate differs as well, and a press run fast reports a higher value than the same press run slowly. The moisture condition and the number of units tested complete the list. One unit is a spot reading; six units are a sample.

The practical rule is to compare only results produced by the same method, at the same age, on the same area basis. When you change laboratory, or change the age at which you test, run the old and the new method side by side on one batch of blocks, so the shift between them is measured rather than assumed.

What should a test report contain so you can defend it?

A report that survives scrutiny carries the production date and the batch or mix identifier, the test date and the age, the number of units, the individual results as well as the average, the dimensions and the area basis, the moisture condition, the capping method, the loading rate or machine class, the standard and its edition, and the identity of the laboratory. When the blocks contain rubble or recycled material, ask for the source note of the aggregate as well, because a strong result from one load says nothing about the next dispatch.

Keep one reference sample from every mix version in the yard, marked with its batch number. When a client or an engineer disputes a result six months later, that sample is the only thing that can settle the argument.

How do the machine and the mix set the class you can actually reach?

The class you reach comes from the combination of mix, compaction and curing, not from the press figure on its own. A machine that holds 25 MPa of working pressure, such as the QST6-15 at 34.4 kW and about 30 kW of site power, gives reliable compaction for the medium classes. Higher classes come from stable grading, enough cement, correct moisture and full compaction held through the whole cycle. Where large units and higher output are required, the QST1200 works a 1,350 × 880 × 25 mm pallet at 31.5 MPa and 57.6 kW, and the QST1800 adds 280 kN of vibration force and 104 kW of installed power on a 1,400 × 1,400 × 30 mm pallet.

Send us the class you have to certify, the block size and hollow pattern, your aggregate test results, and your site power supply. We will confirm which configuration and which mould set fits that class, and what the documentation set will contain. Our factory is in Quanzhou, Fujian, China, and we have been building block machines since 2008.