Cement Mortar Drying Shrinkage Test: Standards, Procedures, and Applications

The cement mortar drying shrinkage test quantifies the length change of hardened mortar prisms exposed to controlled drying conditions, providing a critical indicator of a cement's volume stability and its potential to cause cracking in concrete structures. Performed by accredited third-party laboratories in accordance with standards such as JC/T 603 and ASTM C596, the test is widely used for product conformity assessment, quality control, and materials research. This article explains the test principle, sample preparation, equipment, procedural steps, calculation methods, and typical engineering applications.

Technical Overview

Drying shrinkage is the reduction in volume of hardened cement mortar caused by the loss of physically adsorbed and interlayer water from calcium silicate hydrate (C-S-H) phases when the material is exposed to an environment with lower relative humidity than its internal moisture state. In cement mortar, this moisture migration generates capillary tension and disjoining pressure changes within the pore structure, resulting in measurable contraction of the specimen.

Excessive drying shrinkage is one of the principal causes of cracking in concrete pavements, floors, walls, and thin-section elements. Such cracks reduce structural serviceability, provide pathways for aggressive agents such as chlorides and carbon dioxide, and accelerate reinforcement corrosion. The cement mortar drying shrinkage test therefore serves as a fundamental screening tool for evaluating the volume stability of cementitious binders before they are used in full-scale construction.

The test principle is straightforward: standard mortar prisms are cast with reference studs or gauge marks at their ends, moist-cured to a specified initial age, measured with a precision comparator to establish the initial length, and then stored in a controlled drying chamber. Length changes are recorded at prescribed intervals, and the shrinkage strain is calculated as the change in length relative to the effective gauge length, typically expressed as a percentage or in millimeters per meter (mm/m).

The most commonly applied standards and reference documents include:

• JC/T 603-2004 — Standard test method for drying shrinkage of cement mortar (Chinese building materials industry standard)
• ASTM C596 — Standard Test Method for Drying Shrinkage of Mortar Containing Hydraulic Cement
• ASTM C157 — Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete (reference method for length-change measurement)
• GB/T 17671 (equivalent to ISO 679) — Test method for cement mortar strength, which defines the standard sand, planetary mixer, and molding practice also used for shrinkage specimens
• GB 13693 — Standard for road Portland cement, which sets a maximum 28-day drying shrinkage limit of 0.10% for that cement type

Testing Samples

Standard Mortar Prism Specimens

Under JC/T 603-2004, the test specimen is a 25 mm × 25 mm × 280 mm prism with an effective gauge length of 250 mm between the inner faces of the two reference studs. ASTM C596 uses a similar 25 mm × 25 mm × 285 mm prism, also with a 250 mm gauge length. Each test typically requires a minimum of three replicate prisms, and the result is reported as the mean of the individual measurements.

Cement and Constituent Materials

The test can be applied to virtually all hydraulic cement types, including ordinary Portland cement, road Portland cement, masonry cement, expansive and shrinkage-compensating cements, and blended cements incorporating fly ash, slag, or other supplementary cementitious materials. For the Chinese method, the mortar is prepared from 500 g of cement, 1000 g of ISO-graded standard sand, and 250 g of water, corresponding to a cement-to-sand ratio of 1:2 and a water-cement ratio of 0.50. ASTM C596 specifies its own mortar proportions using graded sand conforming to ASTM C778.

Specimen Curing Prior to Testing

Immediately after casting, molds are held in a moist cabinet or room at high relative humidity. Specimens are demolded after approximately 24 hours and then submerged in saturated lime water or clean water at controlled temperature until the initial measurement age, which is 3 days in JC/T 603-2004 and 72 hours in ASTM C596. Consistent curing before the initial reading is essential, because the shrinkage value is computed relative to this baseline length.

Testing Items

The core deliverables of a cement mortar drying shrinkage test program typically include:

• Initial comparator length of each prism at the prescribed baseline age
• Length-change readings at specified drying ages, commonly 7, 14, 21, and 28 days, with optional extension to 56 or 90 days for long-term evaluation
• Drying shrinkage ratio at each age, expressed as a percentage or in mm/m
• 28-day drying shrinkage value for conformity judgment against product standards such as GB 13693 for road Portland cement
• Comparative shrinkage data for evaluating mineral admixtures, shrinkage-reducing admixtures, cement fineness, or sulfate optimization
• Where applicable, expansion measurements for expansive or shrinkage-compensating cements to characterize their deformation behavior under drying

Testing Methods

Apparatus

A complete test setup comprises the following equipment, all calibrated and verified at defined intervals:

• Planet-type mortar mixer and 25 mm × 25 mm × 280 mm (or 285 mm) three-gang prism molds with embedded reference stud assemblies
• Comparator-type length measuring instrument with a resolution of 0.01 mm or finer, together with a certified reference rod for zero verification before each set of readings
• Constant-temperature water bath maintained at 20 ± 1 °C for pre-measurement curing
• Drying chamber or controlled environmental room maintained at 20 ± 3 °C and 50 ± 4% relative humidity per JC/T 603-2004, or 23 ± 2 °C and 50 ± 4% relative humidity per ASTM C596
• Balances, timing devices, and hygrometers for verifying environmental conditions throughout the drying period

Specimen Preparation

The mortar is batched to the exact proportions specified by the governing standard, mixed in the planetary mixer according to the prescribed sequence, and compacted into the prism molds in layers with appropriate tamping or vibration to ensure uniform density. The molds are covered and placed in the moist cabinet. After 20 to 24 hours, the specimens are demolded, numbered, and immediately transferred to the water curing bath.

Measurement Procedure

At the baseline age, each prism is removed from the water, wiped to a saturated-surface-dry condition, and placed on the comparator with the reference studs seated on the anvil and spindle. The comparator is first zeroed against the certified reference rod, after which the specimen length is read and recorded as the initial value. The prisms are then transferred to the drying chamber. At each subsequent prescribed age, specimens are removed briefly, wiped of any dust, and re-measured in the same orientation to eliminate systematic bias. The drying chamber is only opened for the minimum time necessary to preserve its temperature and humidity stability.

Calculation and Reporting

The drying shrinkage ratio at a given age is calculated as the difference between the initial length and the length at that age, divided by the effective gauge length of 250 mm, and multiplied by 100 to obtain a percentage. Results are reported to the nearest 0.001%, together with the individual and mean values for all replicate specimens, the curing and drying history, and the environmental conditions maintained during the test. When shrinkage values are negative (that is, the specimen has expanded), this is noted explicitly in the report.

Factors Influencing Results

Accurate and reproducible shrinkage data depend on strict control of the following variables:

• Water-cement ratio and consistency of mortar batching
• Temperature and relative humidity stability of the drying chamber
• Timing precision of the initial reading and all subsequent age measurements
• Cement characteristics, including fineness, C3A content, sulfate optimization, and alkali content
• Mineral and chemical admixtures in the binder system
• Operator technique in seating specimens on the comparator and verifying the zero point

Applications

Cement mortar drying shrinkage test data are used across the cement and construction materials industry for several important purposes:

• Product conformity assessment, including verification of the 28-day shrinkage limit for road Portland cement under GB 13693 and acceptance requirements for masonry and special cements
• Factory quality control during cement production, where deviations in clinker chemistry, gypsum dosage, or grinding fineness are reflected in shrinkage performance
• Research and development of blended cements, shrinkage-reducing admixtures, expansive agents, and internal curing technologies
• Crack-risk evaluation for concrete pavements, industrial floors, bridge decks, canal linings, and other restraint-sensitive elements
• Comparative benchmarking of competing binders or suppliers during project material selection
• Input data for deformation modeling and joint spacing design in concrete construction

Because the test uses standardized mortar rather than a project-specific concrete mixture, its results isolate the intrinsic shrinkage contribution of the cement itself, making it an effective tool for comparing binders on a common basis. For project-specific predictions, mortar shrinkage data are typically complemented by concrete ring tests, restrained shrinkage tests, or length-change tests on concrete specimens in accordance with ASTM C1581 or ASTM C157.

FAQ

What is the difference between drying shrinkage, autogenous shrinkage, and carbonation shrinkage?

Drying shrinkage results from moisture loss to a lower-humidity external environment, whereas autogenous shrinkage occurs in sealed conditions due to internal self-desiccation during cement hydration, and carbonation shrinkage arises from chemical reactions between hydration products and carbon dioxide. The cement mortar drying shrinkage test specifically measures the moisture-loss-driven component under controlled drying at approximately 50% relative humidity.

What are typical drying shrinkage values for cement mortar?

For ordinary Portland cement mortars tested under standard conditions, 28-day drying shrinkage commonly falls in the range of approximately 0.05% to 0.15%, depending on cement composition and fineness. Road Portland cement is limited to a maximum of 0.10% at 28 days under GB 13693, and values for specialized low-shrinkage cements can be lower still.

How long does the test take?

A standard test sequence, from mortar mixing through the 28-day drying measurement, takes approximately one month. If extended ages such as 56 or 90 days are required by the specification or client request, the schedule lengthens accordingly, and specimen handling must remain consistent throughout.

Why is the humidity controlled at approximately 50% relative humidity?

A drying environment of about 50% relative humidity provides a reproducible and moderately severe moisture gradient that produces measurable shrinkage within a practical timeframe. Standardized humidity ensures that results from different laboratories remain comparable and that the measured deformation reflects the cement's behavior rather than uncontrolled ambient conditions.

Can the test be used to evaluate concrete shrinkage directly?

No. The mortar test isolates binder behavior using standardized sand and mix proportions. For actual concrete mixtures, which contain coarse aggregate that restrains paste shrinkage, length-change testing should be performed on concrete prisms in accordance with ASTM C157 or equivalent national standards, with the mortar test serving as a complementary binder-level indicator.