Masonry Mortar Property vs. Proportion Method

[BadgerPE]

On a current project (and all for that matter) we specified proportion method for mortar design (in accordance with MSJC standard). The reason I have always been given is that it is cheaper because the mix does not need to be tested, as is the case with property method. However, I was submitted a mix design that had been done in accordance with the property specification. Test results for the mix were included in the submittal.

I am inclined to approve this design even though it was not designed in accordance with the project specification because it seems like a viable alternative approach. Does anyone have any thoughts or comments on this issue? Are there any additional requirements in the field for property vs. proportion?

 

[concretemasonry]

There are really no acceptable test/sampling methods for controls in the field. The mortar tests people normally see are cubes mad the test laboratory methods to approve a new specific mix design or materials and to see that type of mortar it cam be classified as and what proportions are needed.

Strength is very rarely a critical in the use of masonry. If you look at Appendix Note 1 of ASTM C270, It states that is best to use the weakest mortar possible to carry the structural loads because of the benefits.

The strength of the masonry (f'm) is determined by prism test of two hollow block(8x8x16) high CMUs with no grout and a single bed joint. Testing has proved conclusively that the mortar has no real effect on the compressive strength of masonry and the CMUs are by far the controlling item. I had hollow prisms (2 high) made with 2200 psi mortar and the prisms tested over 4800 psi (minimum) with the remaining ones 10% to 20% higher with the same mortar.

Dick

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

 

[BadgerPE]

Dick,

So based upon what you are saying, you feel that due to the lack of good field testing methods and/or that strength rarely controls, property or proportion methods are acceptable?

In the contract documents I specified proportion method to eliminate lab testing requirements. However, the contractor submitted a "just add water" pre-mixed design. Attached to the submittal were lab results showing that the minimum compressive strength (as required by MSJC) were met. I feel like there is less room for error if water is the only ingredient to be added vs. adding a multitude of ingredients and that since the testing has already been performed, the property specification is an acceptable alternative to the proportion specification.

I know this is a relatively small issue, but I would like to have a solid handle on it moving forward. Also, if there is a reason to reject, I would like to have a leg to stand on.

Thanks for the help!

 

[concretemasonry]

The testing procedures are for laboratory prepared mortar samples, since the costs and problems with sampling representative mortar (mixed in small batches) and prepared on site, cured and transported to a lab are not practical. That is why it is suggested to use mortar testing for lab research only to determine the compliance or mortar classification. The mortar classification are really a gradation with a wide range of individual material/component ranges.

Cementitious materials -

1). There are pre-packaged materials (masonry cements)that are use with the addition of aggregate (sand) on site volume basis and water is added (not measured) to provide the desired workability, which is the most important property of mortar. - This may be the product that you see in the test reports provided, so a better description on the product proposed would be needed.

2.) There is also the ability to use Portland cement and lime (both measured by bulk volume). This is most traditional method and the type of mortar that has been used on the tests used for the historical basis of many of the codes. In all cases, the water is added as needed to provide the desired workability. In some areas, the cementitious materials (Portland cement and lime) are available pre-proportioned in 80-100 pound bags.

In rare cases the Portland cement, lime (and coloring pigments) and sand are available from site located silos that are mixed with adequate water for workability.

I suspect you are looking at test reports where the contractor prefers to use a pre-proportioned masonry cement with local sand instead of using Portland cement, lime and water with local sand to create a workable mortar.

The decision up to you is to decide how critical the proportioning is and how much you want to send on enforcing the specification. Mortar is imposable to split hairs on. It's closest relative is structural masonry grout that allows a range of slump from 8" to 1" that is also generally proportioned by volume. Many knowledgeable engineers even specify a maximum strength of the grout permitted.

Dick



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

 

[Ron]

I have long battled with the discrepancy between the proportion and property specs in ASTM C270.

I can see both points of view; however, I'm not convinced that low strength mortar is a good thing, except perhaps in repointing for historical applications.

There is a huge discrepancy between the compressive strength you will get between proportion and property specs. As an example, Type S mortar should have a compressive strength of 1800 psi. To do this, the sand content has to be lower than the proportion specs would indicate (or increase the cement content). When using pre-bagged mortar mix, you will almost always have lower with compressive strength if you use the bagged mix with the specified proportions. Bagged mortar mixes generally do not have the same cement content as would be required in the proportion specs for mortar.

I would like to see the ASTM committee charged with writing C270 get it together and address this issue as it has been going on for at least 35 years that I know of!

 

[concretemasonry]

Ron -

Your question is probably the second most asked about mortar. - The first is - "Does mortar hold units together or keep them apart?". This has been around for the centuries mortar has been used. Since modern structures may require different measures of performance the specifications have slowly revolved to reflect the performance of structures and some of the well documented testing of mortar, masonry units and combined results over the past 60 or 70 years.

One of the best references is the NCMA site regarding mortar that does a good job of making the history relevant to to the modern products and applications. To get the NCMA TEK note for mortar, go to the NCMA site. On the opening, click NCMA Solutions>E-TEK>Mortar grout and stucco>TEK09-01A, which covers the subject of mortar in a very good way, since it was written by many engineer members that have be active in the ASTM committees that write the ASTM standards and the ACI 530 and previous standards for engineered masonry.

Referring to ASTM C270, it clearly states that Table 2 - Property Specification Requirements only applies to laboratory prepared mortar ONLY. The tables does hang a number on the strength that is used to allow new mortar type/brands to establish a type of mortar category for that product for proportioning only. Obviously, many of the common bagged will test far beyond that minimum. - As an example in a the hollow 8x8x16 prisms I referred earlier to with an f'm of 4800+ psi were made by masons is a laboratory, so proper sampling and tested could be accomplished as a measure of control. The mortar was according to Proportion Specifications made using a Type S mortar that showed up as having a minimum of 1800 psi, but the lowest sample (of 20 samples) we found was slightly over 2200 psi. The face shell bedded 2-block high hollow prisms between 4800 psi and 5100 psi.

The bottom line for serious engineered masonry, just use the proportion specification to determine the f'm if you need a single approach because it is very usable and has a very long history.

The NCMA TEK 9-1A has very good summary on the last page: The ASTM C780 (the testing procedure) contains no requirement for minimum compressive strength of field mortar itself. The mortar strength in the wall will be much higher than field test (of mortar) because of the reduced water cement ratio due to the absorption of mix water and a greatly reduced shape factor in the mortar versus the mortar test cube. - ASTM recognizes this and states that the strength should not construed as being representative of the actual strength of the mortar. This is one of the cases where the water does provide a real benefit.

Incidentally, the cost of the cement in mortar is minor compared to productivity given to costly mason laying the units. Also, the wide range of aggregate in the specification is to allow for the actual, gradation and particle shape to crate a workable mortar for quality construction. Section 5.1 of ASTM does address the case of mortar being too strong by stating that "A mortar type of a known higher strength shall not be indiscriminately substituted where a mortar type of anticipated lower strength is anticipated lower strength is specified. - I saw this happen on a loadbearing complex of 12 to 20 story loadbearing buildings that had a central facility and had to create a separate facility to eliminate the the possible problem (the blocks were colored coded by strength at he plant before delivery or distribution).

I hope this helps to sort out the concepts somewhat and may help in the specifying mortar types and strengths.

Dick

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

 

[JedClampett]

OK, all that is well and good, but no matter how many times I specify the proportion method, I get batched mortar with testing paid for and provided by the batch plant.
I called up the head technician at the material company and he explained that his tested mortar meets code just as well as the proportioned method. And since all I care about is the end result (a good CMU structure) I accept the mix.
I suspect that the proportion method is so rich that contractors can save money using the batched mortar. Even if labor is the predominate cost, a buck saved is a buck in his pocket.
My gripe is that the code is really emphasizing (no testing, simple recipe) the proportion method, but it's not translating to field construction.

 

[darthsoilsguy2]

i'm going to go on a limb and say all packaged mortars, mortar cements, and masonry cements have laboratory testing done whether it is specified or not. Manufacturers routinely do this. If the stuff is made in USA, it is probably traceable back to the silo it was tested from. i say this because you said something about getting a cheaper mix that didn't need testing.... The testing is already done that allows them to sell it on the marketplace which usually requires manufacturing QC, so there is no cost benefit to be had. Now if you are talking about testing by a field tech... that has already been answered...

Specify the proportions and you've met code. The most important thing is to have a solid precon meeting and watch (from a distance) a few batches done during your periodic structural observations. observe the control of clean aggregate, protection of cements, batching equipment, mixing time, water source, control of waste, missing a few things here...., and don't let them shovel.... sure, you'll upset a few who know how to get it right with the shovel.... but we are living in a different era now and the batchman should not be assumed to be a craftsman on day 1. Any good experienced mason has worked plenty of jobs that won't let them shovel sand.