FINITE ELEMENT ANALYSIS FOR MORE SUCCESSFUL PATCHES? A HYPOTHETICAL STUDY

by PAT MCCORMICK, P.E., S.E.
ACI 302.1 STATES:

“Bonding of Two Course Floors, floors with a base slab and topping or overlay, is a highly critical operation requiring the most meticulous attention to the procedure described. Even so, such bonding has not always been successful.”

FINITE ELEMENT ANALYSIS FOR MORE SUCCESSFUL PATCHES?-

A HYPOTHETICAL STUDY

INTRODUCTION

Toppings of two course floors are routinely repaired using portland cement concrete or proprietary repair materials. Typically a material is chosen because of its compressive strength or ease of installation, then the repair is made by a contractor that has extensive experience in that type of repair. Yet, frequently those repairs fail, particularly when the patch is located in a heavily traveled lift truck aisle. Time and time again poor surface preparation is blamed for the failure, but when the contractor makes a reasonable attempt at surface preparation and the failures still occur, it is essential that we look at other factors that may be causing the failures.

One factor is: Are the repair materials that are chosen really compatible with the requirements of the repair?

PROCESS AND PURPOSE

Topping repairs can involve small areas of floors (patches) or large areas of floors. Primarily we see the need for those repairs in lift truck aisles on elevated slabs. Many elevated industrial floors were constructed with two-course flat slab systems, though we also have encountered many two-course one-way slabs supported on structural steel framing. The intent of the two-course slabs was to have an integral bond between the base slab and the topping. Experience has shown that those two-course slabs are very susceptible to loss of bond between the base slab and the topping.

To clarify one point- When we talk about the loss of bond, it might leave the impression that the failures always occur at the interface between the topping material and the base slab. Actually, the failures we have seen have not always occurred at the interface, but rather have occurred either in the topping or in the base slab in the vicinity of the interface.

Most of the two-course one-way floor slabs we encounter in older industrial facilities are typically 6″ to 7″ thick, which includes the base slab and topping. With integral toppings or patches, once the bond between the base slab and topping or patch is lost, the topping has failed and no longer contributes to the load carrying capacity of the slab. With the initial failure of a portion of the topping in lift truck aisles, the cyclic nature of the loading propagates the failure to the surrounding topping. When a large area of topping fails in a lift truck aisle, frequently the result is excessive cracking and deterioration of the base slab because of the loss of effective depth. Reestablishing and maintaining the integral bond between the base slab and topping in lift truck aisles, while difficult, is essential to maintain the integrity of those older floor slabs.

SURFACE PREPARATION:

Consider the surface preparation needed.

The preparatory work recommended by ACI and most material manufacturers is extensive. Most contractors who do repair work have extensive experience and know the surface preparation requirements of ACI and most material manufacturers, and they make reasonable attempts to follow those requirements.

“Even so,” as stated in ACI 302, “such bonding has not always been successful”. The truth is, it is very difficult to bond a patch or topping to an existing floor and successfully maintain that bond for the expected service life of the floor when that floor is in a lift truck aisle even, when the contractor does a reasonable job of surface preparation.

The question is, “WHY DON’T SOME OF THE REPAIRS WORK?”

When choosing a repair material, what does one look for?

There are numerous types of materials available for repairing toppings, and the physical characteristics of those repair materials vary widely. For instance, the compressive strength of proprietary repair materials can vary from 4000 psi to 12,000 psi at 28 days. ACI recommends that the topping or overlay of a two-course floor have a 28 day compressive strength of 5000 to 8000 psi.

One question that comes to mind is, are the strain characteristics of the repair materials being used for patches sufficiently compatible with those of the base concrete?

With the wide range of repair materials available, the effect of the different characteristics of those materials on the performance of the slab system needs to be investigated. One significant characteristic that needs to be considered is the modulus of elasticity, of both the base slab concrete and the repair material.

In ACI 318, the formula 57000x(f’c) is used to determine the modulus of elasticity of concrete. Using that formula, concrete with a compressive strength of 3000 psi would have a modulus of elasticity of 3.1×106 psi, while concrete with a compressive strength of 8000 psi would have a modulus of elasticity of 5.1×106 psi. The modulus of different proprietary materials used for topping repairs can vary from 3×106 psi up to as much as 5.5×106 psi. With base slab concrete of older buildings having a compressive strength as low as 3000 to 4000 psi, there would be a big difference between the modulus of the base slab concrete and that of the 5000 to 8000 psi topping concrete or of some of the proprietary materials that could be used.

With the differences in physical characteristics between the topping material and the concrete base slab, we felt it would be interesting to see just how stresses are distributed in a repaired slab, so we performed finite element analyses of four sections of floor slab. The finite element models we used were all 6′ long, 6″ deep simply supported sections of slab. The models were all made up of two-dimensional plate elements.

Model 1 was a full-depth concrete slab. That model was analyzed to serve as reference for stresses developed in the other three models.

Model 2 was a 5″ deep base slab with a full length 1″ deep overlay. Model 2 was analyzed to determine the stress distribution within a typical two-course floor.

Half of the length of the third model was a full depth slab, while the other half was 5″ deep base slab with a 1″ deep overlay that ended at midspan of the model. Model 3 was analyzed to determine the effect of terminating a topping within the compression zone of a slab.

The fourth model was similar to Model 3.

In each case, the modulus used for the base slab was 3.1 x 106 psi, which corresponds to a compressive strength of 3000 psi according to the ACI formula. The modulus used for the toppings of models 2 and 3 was 5.5×106 psi, which is a typical high-end value noted in literature for some proprietary materials, and the modulus used for model 4 was 1.0 x 106 psi which was used in an attempt to replicate topping that is not completely bonded or has cracked. The loads used corresponded to a 150 pound per square foot uniform live load on the full length of the model.

We used Algor, a proprietary finite element analysis program, to analyze the models. The software produces a graphic representation of the stress distribution within the model, with different colors representing different stress contours. The results of the analyses were then used to produce stress diagrams illustrating the stresses at midspan of each of the models.

The stress contours for model 1, which was the full depth slab, were typical of the bending stresses produced in a homogenous material, with the maximum bending stresses equal on both the tension and compression sides of the model. Also, the stress diagram for the first model has a classic shape, with the compression zone above the neutral axis mirroring the tension zone below the neutral axis, with the maximum compressive and tensile stresses both being 168 psi .

The stress contours for model 2, which was the 5″ deep base slab with a full length 1″ deep topping, were different than those for Model 1. The bending stresses developed were higher on the compression side, which is the topping material side, than on the tension side, which is the base slab.

The stress diagram for Model 2 differed from that for Model 1, in that the shape of the compression zone was different than the shape of the tensile zone. The maximum compressive stress in the topping material was 220 psi, and the maximum tensile stress in the base slab was 147 psi. Because the model assumed that there would be no relative movement between the topping and the base slab, the difference between the modulus of the topping and that of the base slab resulted in an abrupt reduction in compressive stress at the interface between the topping and the base slab. Also the neutral axis of the section moved upward, above the middepth point of the slab.

The stress contours for Model 3, which was half full depth slab and half 5″ thick base slab with a 1″ thick topping ending at midspan of the model, were similar to those of Model 2 for the topping side of the model, with the bending stresses developed being higher on the compression, or topping side, than on the tension, or base slab side, in the portion of slab containing the topping.

The stress diagram for Model 3 was somewhat similar to that for Model 2, except that there was actually an increase of compressive stress in the topping at the topping/base slab interface. We believe that increase of compressive stress was related to the difference in modulus of elasticity between the base slab and the topping. The stress developed in the topping was being resisted by the adjacent base slab which strains more due to the stress level in the topping, resulting in more stress being transferred at the horizontal interface by the stiffer topping.

The stress contours for Model 4 showed the high compressive stresses occurring in the base slab below the lower modulus topping material. There was an increase in compressive stress in the base slab at the interface just below the edge of the topping. That stress increase was caused by the discontinuity in the base slab caused by the lower modulus topping and demonstrated the reason topping failures occur so readily in lift truck aisles.

CONCLUSION

The analyses we performed indicated that the stress distribution at the interface between the base slab and topping changes abruptly when the physical characteristics between the two materials, primarily the modulus of elasticity, are significantly different. As was demonstrated with Models 3 and 4, that condition is particularly apparent at the edges of a topping when it ends in the compression zone of a slab.

These results are an indication that topping repairs, when located in lift truck aisles must be analyzed carefully to make sure that the materials chosen for those repairs will have the stress/strain characteristics required. In some cases those repairs may need additional help to ensure that the integral connection between the base slab and topping or patch is maintained.

The results of the analyses clearly show that we as structural engineers must take the repair of slabs seriously and look at all the factors that affect the performance of those repairs. Historically, successful repairs of floor slabs in lift truck aisles have been very difficult to achieve. If the variables involved are adequately analyzed, and the right materials chosen, those repairs can be successful.