Constructing large capacity water storage tank slabs without construction joints is a complex proposition in Colorado’s low humidity and often windy climate. One solution that has been successful is to specify low-shrinkage concrete mixes and monolithic placement of the slab within a prescribed time period, based on ambient conditions, to reduce the probability of shrinkage cracking. However, even under the best of circumstances shrinkage cracking still occurred. It was felt that more could be done to improve concrete placing and finishing, further reducing shrinkage cracking and enhancing durability.
Based on research reviewed by the design team, Denver Water and Bates Engineering, Inc., it was decided to investigate the use of an Internally Cured Concrete (ICC) mix on the floor and roof slabs of the Denver Water 10-Million Gallon (MG) Lone Tree Tank No. 2 that Bates was currently designing. The Lone Tree tank floor and roof slabs are each about 61,000 square feet and would be monolithically placed. The slabs could be compared to other 10-MG floor and roof slabs designed by Bates and constructed within a few miles of the Lone Tree Project.
Trial batches were performed in the laboratory to determine plastic and hardened characteristics of the ICC as compared to traditional proportioned mix designs. Tests performed in the laboratory included: determination of compressive strength and drying shrinkage (ASTM C 157, modified 7-day saturation). Cure360™ was selected based on durability requirements including w/c ratio, shrinkage, and structural adequacy. Further verification of mix properties specific to the project were validated in the field.
Solution The design team conducted a feasibility study by way of constructing two small elongated slabs (10’ x 40’) to gain experience and to better understand the placing and performance of the Cure360™ mix and a control mix that had been used on other large monolithically placed tank slabs. Slabs were placed adjacent to one another, one with Cure360™ concrete and the other with traditional mix proportions. Tests performed on the concrete included: slump, air content (pressure and volumetric methods), fresh density, compressive strength, concrete maturity, modulus of elasticity, rapid chloride ion permeability (ASTM C 1202), water soluble chloride ions (ASTM C 1218) and measured equilibrium density per ASTM C 567-05a thru the first 28-days. No visible drying shrinkage cracking was observed on either slab. However, small micro-cracking was observed on the surface of the concretes, with the Cure360™ significantly outperforming the traditional concrete. Verification of hardened concrete properties and visual observation of the test slabs served as justification to move forward with the Lone Tree tank floor and roof slabs.
Given the early strength gain, workability, and minimal shrinkage cracking of the Cure360™ mix on the floor slab, Garney Construction submitted a request to use the mix for the remaining concrete gained to strip and cycle the wall forms more quickly, increasing the number of wall placements per week. Denver Water granted the request because of the success of the floor slab including reasonable quick strength gain. That fact that both 7-day and 28-day compressive strength test results were consistent along with little variation in the slump and air-entrainment also aided more rapid formwork cycling. Two shrinkage cracks were observed in the floor slab prior to stressing the slab tendons. This reduced approximately 80% of the number of cracks that would normally be observed compared to other similarly size tank slabs, according to Denver Water. Due to the compressive stress placed on the slab, it is not unusual for the shrinkage cracks to no longer be visually apparent, which is an advantage of a post-tensioned concrete slab. The project team is pleased with the success of the Cure360™, and this product will be specified for future work.
Future It is Denver Water’s opinion that concrete tanks using ICC (Cure360™) will have better performance with less leakage, require less maintenance and result in a longer service life. All of these contributing factors make for a higher quality sustainable facility for the rate payers of Denver.
Client: Denver Water
Designer: Bates Engineering
Contractor: Garney Construction
Product: Cure360™ ICC