Interlocking Concrete Pavers

[vmirat]

We do a lot of projects, mostly parking lots, using concrete pavers. The manufacturers love to tout the fact that their pavers are so much stronger than cast in place concrete. I'm aware that there is an ASTM requirement to meet or exceed 8,000 psi in strength. My question is, why do they have to be so much stronger than plain concrete pavement?

 

[concretemasonry]

The specified paver strength is 8,000 psi, but most are in the area of 10,000 and I have seen 12,000 psi. The major factor in the strength in not cement, but the aggregate gradation/particle shape. Pavers are dry or zero slump product made in a factory and poured concrete strengths are just a measure of the strength of the mix delivered. A portion of the difference is the due to the ASTM testing methods and the size and h/t of the meaningful sample.

The use of the materials are totally different as are the uses. Poured in place concrete is a rigid reinforced concrete pavement. Paver installations are a flexible pavement, relying more on the soil as part of the pavement system.

I have seen 20 to 40 acre paver (100 mm thick)installations for extremely heavy loads (40,000# plus) of 4 wheel straddle loaders, because they were the only products to provide smooth roadways and storage area at just a few feet above sea level. In that case, high compressive strength would not be an aid to provide a usable surface.

There is no correlation in the different testing methods or philosophies and uses of the materials (pavers and poured concrete) have different engineering applications and technologies.

I hope this provides some clarity to what seems to be a paradox at first glance.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

vmirat...to add to Dick's comments...

One important reason for the higher strength of pavers is for durability. Durability of concrete is directly related to compressive strength. Pavers have a rather unique exposure as compared to cast-in-place concrete. They move around under load, creating interface abrasion. They take concentrated loads, acting as short beams. The are subjected to surface abrasion from traffic loads. They are subjected to almost constant wetting and drying action on multiple sides, and they are subject to freeze-thaw conditions....all of which are only resisted by the durability of the individual pavers, thus their compressive strength must be relatively high.

 

[bimr]

The load distribution and failure modes of flexible asphalt and interlocking concrete pavement are very similar: permanent deformation from repetitive loads. Unlike asphalt, concrete pavers do not rely on continuity of their material for structural integrity.

By definition, concrete pavers have a minimum compressive strength of 8,000 psi (about 3 times stronger than regular poured concrete) and a maximum water absorption rate of 5%.

This provides concrete pavers with greater resistance to deicing salts and freeze thaw cycles than conventional paving materials due to high cement content, strength, density, and low absorption.

 

[hokie66]

I think Ron nailed it. Concrete pavers have to interlock, and this causes high stress at the edges. You don't want little bits breaking out, so the higher the strength, the better.

 

[vmirat]

I called the Interlocking Concrete Pavement Institute (didn't know such a thing existed).
- Concrete pavers are made without air entrainment, so they need the strength for durability.
- The pavers are made with a zero slump mix to facilitate manufacturing. They place the mix into the mold and then lift the mold right away to make the next brick. As a result of the very low w/c ratio, they inherently have a high compressive strength.
- Pavers can be a minimum of 2-3/8" thick (i.e. driveway pavers). This is the thickness as tested under ASTM C936. Since they can be this thin, they need the strength. They use the same mix regardless of thickness of paver.

 

[concretemasonry]

Pavers can easily be made to strengths higher than 8000 psi and it not really a cost factor. Iknow of pavers made that were made on a routine basis that were 10,000 to 12,000 psi.

It is a common error to take the compressive strength as the major factor affecting durability. The measurements for failure were the the number of cycles to lose a pre-determined amount of material.

During an industry research durability peoject using pavers from many producers across the country gave some interesting results. the factors looked at were the testing procedure (freash or saline water), number of cycles, and the physical properties (compressive strength, absorption, density were the main items). Unfortunately, due to the magnitude of the programs, number of samples and time required, the practical testing capacity in the U.S. was stretched, so it took over 18 months of actual testing. The results provided some interesting information.

In some cases the fresh water showed poorer unit durability than the saline for some units.

Compressive strength was generally a good guide, but not always true. There were son non-spec pavers with a 6000 psi compressive strength that had better durability than 8,000 to 10,000 psi pavers with similar absorption.

Absorption was also good general guide to durability, but there were exceptions. This could be due to the aggregate properties.

The density was not as critical, because some of the very durable pavers used lighter aggregates and made very durable pavers.

A paver manufacturer can be a magician since pavers have been made using the current technology world-wide for decades. Several manufacturers submitted several sets of 8000 psi pavers. These generally used normal graded aggregate and gap graded aggregate from the same source and the gap graded pavers had lower densities higher absorption (according to the test used) and similar strengths. In these cases, the gap graded pavers had better durability.

The test program did provide some valuable information to set standards for product performance and testing methods in addition to provide information of the complex relationship between the factors and the possiblities (a 6000 psi paver with high absorption was one of the more durable samples tested).

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

Dick...has the study been published in a peer-reviewed journal? If so, please provide a link if you have it.

 

[concretemasonry]

Ron -

I don't believe it was published and subjected to a peer review process, since it was one of the many industry studies that are done for education and determining the possible properties and results.

As I recall, it was lead by the National Concrete Masonry Association (NCMA), who have engineers active and voting members on the various ASTM committees (It took me 15 years of membership to be a voting member on some product stanards).

The producers were generally American and Canadians, and I think there were also some samples from European manufacturers that were regular or international memebers. I do not know if the Interlocking Concrete Paving Institute (ICPI) was a formal partner at the time, but many producers and equipment manufacturers had joint memberships in both NCMA and ICPI. It was a good study because it provided information for all and opened doors to technical improvement.

It was similar to some of the joint studies conducted between the NCMA and various state DOTs and engineering departments on segmental retaining wall units (SRWs) and property requirements to establish product standards. The resulting standards come out of ASTM that is based on a balanced voting membership.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.