The Crown Castle Tower Project

Test

Petrographic Examination of Hardened Concrete, ASTM C 856

Petrographic analysis involves the optical examination of concrete specimen under low and high power magnification. Detailed instructions on conducting a petrographic examination of hardened concrete/mortar can be found in ASTM C856, “Standard Practice for Petrographic Examination of Hardened Concrete”. For our examination, a representative random sample of concrete near the surface was impregnated with blue dye under vacuum. The impregnation under vacuum causes the dye to permeate every crack, micro crack and all pores, including micro pores in the concrete sample. The impregnated concrete is cut and placed on a glass plate, ground and polished to a thickness of about 30 microns.

Findings

The concrete appeared to have been formulated with generally acceptable ingredients. The estimated cement content was approximately 5 ± ½ sacks/yd³, with approximately 25% fly ash by weight of cement. The water-to-cementitious ratio was estimated at approximately 0.48 ± 0.02.

Petrographic examination identified two distinct mortar conditions within the concrete: extensive areas of hard-set mortar and smaller areas of normally set mortar.

The coarse aggregate consisted primarily of fine-grained silica-type gravel, while the fine aggregate was siliceous sand. The concrete was not air-entrained, and the cement paste appeared well hydrated. No deleterious reaction products were identified.

Cracking was associated with areas of relatively softer, normally set mortar, which provided weaker paths through the concrete.

Outcome

C3S, Inc. determined that the differential setting and hardness of the mortar was the most significant feature associated with the observed cracking. The softer mortar areas provided preferential paths where stresses could initiate and propagate cracks.

One probable cause identified was re-tempering, in which water is added to hardening concrete and the mixture is re-mixed to improve workability. Another potential contributor was ineffective mixing, including hardened concrete buildup around ready-mix truck mixing blades.

The petrographic investigation provided the tower owner with a technical basis for understanding the cracking mechanism and assessing the condition of the existing concrete.

Featured Service:

• Petrographic Examination of Hardened Concrete (ASTM C856) 
• Forensic Concrete Investigation
• Crack Cause Evaluation

Photomicrographs from the petrographic analysis are shown

DISCUSSION OF RESULTS

The concrete appears to be well formulated with acceptable ingredients of concrete. What stands out in the evaluation is relatively small areas of normal set mortar (blend of cement, fly ash and sand mixed with original water) and areas of hard set mortar. Areas of normal set mortar present relatively low resistance to stresses in the concrete and break initiated at any such area tend to propagate along lines of least resistance. See Figure A-4 in the Appendix.

Several factors can contribute to the differential hardness in concrete. The often cited cause is re-tempering of the concrete. Re-tempering is the addition of water to a hardening concrete and re-mixing it for a while to render the concrete easy to pour and place. Depending on the duration of the mixing, the amount of water added and the degree of hardness attained by the original concrete any number of differential hardness can develop in a particular concrete.

Another probable cause of the differential hardness of the mortar system in concrete is the ineffectiveness of the mixing in the ready mix truck including hardened concrete build-up around the mixing blades.

APPENDIX